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Timothy J. Mason, Mircea Vinatoru
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Timothy J. Mason, Mircea Vinatoru
Sonochemistry
Volume 2: Applications and Developments

Authors
Prof.Dr.em.TimothyJ.Mason
FacultyofHealthandLifeSciences
CoventryUniversity
PrioryStreet
CoventryCV15FB
UnitedKingdom
[email protected]
Dr.MirceaVinatoru
FacultyofChemicalEngineeringandBiotechnology
UniversityPOLITEHNICAofBucharest
Spl.Independenteinr.313
060042Bucharest
Romania
[email protected]
ISBN978-3-11-099990-7
e-ISBN(PDF)978-3-11-099993-8
e-ISBN(EPUB)978-3-11-098972-4
LibraryofCongressControlNumber:2022941982
BibliographicinformationpublishedbytheDeutscheNationalbibliothek
TheDeutscheNationalbibliothekliststhispublicationintheDeutscheNationalbibliografie;
detailedbibliographicdataareavailableontheinternetathttp://dnb.dnb.de.
©2023WalterdeGruyterGmbH,Berlin/Boston
Coverimage:NiPlot/iStock/GettyImagesPlus
Typesetting:IntegraSoftwareServicesPvt.Ltd.
Printingandbinding:CPIbooksGmbH,Leck
www.degruyter.com

Preface for Volume 2
This two-volume book“Sonochemistry”is not written in the style that might be ex-
pected of such a comprehensive history of the subject. This is because the authors,
Tim Mason and Mircea Vinatoru, were active participants in its development from
the 1990s to the present day and the text reflects their experiences. In the early
days it was used mainly in the field of chemistry but within a few years the subject
had begun to extend into other disciplines including environmental protection, the
extraction of natural materials, food technology and medicine.
In his opening address at the 17
th
meeting of the European Society of Sono-
chemistry in Jena in August 2022 Tim Mason used a term to describe his entry into
the subject as a case of“Serendipity”which is defined as the occurrence and devel-
opment of events“by chance”and“in a happy way”. It certainly applied when he
gained his first permanent teaching post at Coventry Polytechnic because it corre-
sponded exactly in time to the appointment of another chemist, Phil Lorimer, in the
same department. It was these two who were to go on and establish the Sonochem-
istry Centre in Coventry. There are many other examples of serendipity recounted in
the book, one of which was the arrival of an unsolicited letter from Mircea Vinatoru
to Tim Mason in July 1990 which asked for some guidance on sonochemistry. This
led to the two scientists meeting in Bucharest and resulted in a long-lasting friend-
ship. Many years later and after continued research collaboration it led to the writ-
ing of this book.
An important source of information for the authors was the paperwork that Tim
Mason had collected from the very start of his time in Coventry. He had remained in
the same building for the whole of his 40 years there and amassed a wealth of ma-
terial, the earliest parts of which were not stored electronically amongst which were
some significant but faded faxes that have now become very difficult to read. Cov-
entry University closed the Sonochemistry Centre in 2018 and some years later the
whole building within which it had been housed was demolished. The collected his-
torical material was saved and transferred in several filing cabinets to Tim’s garage
at home.
Volume 2“Applications and Developments”contains 6 chapters which detail
the developments of sonochemistry in fields which continue to attract considerable
research and development interest from academia, medicine, and industry. The au-
thors have made contributions to all of these fields and they have approached the
content in a way which they hope will prove to be understandable to readers whose
expertise is not primarily in the individual topic. Each of the applications and de-
velopments described here help to illustrate the diverse nature of sonochemistry
but also the unifying theme of the effects of acoustic energy on a wide range of
technologies.
https://doi.org/10.1515/9783110999938-202

Contents
Preface for Volume 2 V
Chapter 5
Sonochemical synthesis 1
5.1 Historical introduction 1
5.1.1 Mechanistic aspects 2
5.1.2 Synthetic aspects 5
5.2 Sonochemical synthesis in Coventry 13
5.2.1 The Ullmann reaction 13
5.2.2 Halogenation of aromatics using CuBr
2supported on
alumina 14
5.2.3 O-Alkylation of hindered phenols 15
5.2.4 O-Alkylation of 5-hydroxychromones 18
5.2.5 Ultrasonic effects on metal powders and sonochemical
catalysis22
5.3 Sonochemical synthesis in Romania 35
5.3.1 Charge transfer complexes 35
5.3.2 Self-assembly membranes 49
5.3.3 Attempts to cause automerization of C
13
-labelled
naphthalene 51
5.3.4 Ultrasound-assisted esterification using enzymes51
5.3.5 Sonochemical preparation of catalysts 54
5.4 Concluding remarks 56
References56
Chapter 6
Surface coating, metallurgy and materials technology 63
6.1 Introduction 63
6.2 Electroplating with ultrasound 63
6.2.1 Introduction 63
6.2.2 Electroplating in Coventry 67
6.3 Electroless plating with ultrasound 73
6.3.1 Electroless nickel 74
6.3.2 Electroless copper 75
6.4 Printed circuit board technology 77
6.4.1 Surface preparation 78
6.4.2 Electroless plating on PCBs 82
6.4.3 Improved solder joints in PCBs 84
6.5 Production of nanoparticles using pulsed
sonoelectrochemistry86

6.5.1 Introduction 86
6.5.2 Metal nanoparticle synthesis in Coventry–the SELECTNANO
project88
6.6 Metallurgy 90
6.6.1 Introduction to light metal casting92
6.6.2 Preliminary work at Coventry 93
6.6.3 1996 Coventry group visit Moscow 96
6.7 The joint venture company Industrial Applications for Ultrasonics
(IUS)97
6.7.1 Ultrasonic treatment of molten and solidifying aluminium98
6.7.2 Ultrasonic impact treatment of metal surfaces100
6.7.3 Electric arc welding with ultrasonics100
6.7.4 Ultrasonically assisted metal on metal coating101
6.7.5 Ultrasonics for Al–Pb antifriction composites101
6.8 Polymer science 102
6.8.1 Polymer degradation 102
6.8.2 Radical polymerization 104
6.8.3 Emulsion polymerization 105
6.8.4 Electroinitiated polymerization 106
6.9 Small projects with industry 107
6.9.1 Ultrasonically assisted spray coating107
6.9.2 Encapsulation 109
6.9.3 Crystallization–the synthesis of zeolites110
6.10 Concluding remarks 111
References112
Chapter 7
Therapeutic ultrasound 117
7.1 General introduction 117
7.2 Low-frequency ultrasound 20 –100 kHz 118
7.2.1 Cutting and drilling in dentistry and surgery118
7.2.2 Emulsification for removal of tissue119
7.2.3 Ultrasonic thrombolysis for the removal of blood clots119
7.2.4 Synthesis of microcapsules for drug delivery120
7.3 High-frequency ultrasound 1 –5 MHz 121
7.3.1 Non-therapeutic applications of high-frequency ultrasound121
7.3.2 Therapeutic applications of high-frequency ultrasound123
7.4 The Sonochemistry Centre and therapeutic ultrasound 127
7.4.1 Conferences involving sonochemistry and therapeutic
medicine 128
7.4.2 Dentistry 132
7.4.3 Transdermal drug delivery and enhanced cell permeability134
VIII Contents

7.4.4 The links between HIFU in Chongqing and the Sonochemistry
Centre in Coventry138
7.4.5 Microcapsules for targeted drug delivery 146
7.4.6 Research collaboration with Wu Wei 153
7.5 Concluding remarks 155
References155
Chapter 8
Power ultrasound in food technology 161
8.1 Historical introduction 161
8.1.1 Mechanical effects of ultrasound 161
8.1.2 Chemical and biological effects of ultrasound173
8.2 Food technology at Coventry: links with industry 178
8.2.1 Leatherhead Food Research Association (LFRA) 179
8.2.2 Campden and Chorleywood Food Research Association
(CCFRA)182
8.2.3 Mars Foods 186
8.2.4 Unilever 189
8.2.5 Kraft Foods 190
8.3 Ultrasound and food technology at Coventry: academic links 193
8.3.1 1996 food conference and the bookUltrasound in food
processing193
8.3.2 Review articles from the Coventry group 194
8.3.3 International collaboration via research exchanges with other
university groups198
8.4 Food research in Romania 202
8.4.1 Sonicated champagne research project 202
8.4.2 Extraction of natural sweeteners from Stevia203
8.5 Concluding remarks 204
References205
Chapter 9
Textile and leather processing 213
9.1 Introduction 213
9.2 Production processes in the textile industry 213
9.2.1 Fibre production 213
9.2.2 Yarn production 216
9.2.3 Fabric production 219
9.3 Fabric treatment 221
9.3.1 Washing 223
9.3.2 Scouring 226
9.3.3 Carbonizing 227
Contents IX

9.3.4 Sizing 227
9.3.5 Desizing 228
9.3.6 Mercerization 228
9.3.7 Bleaching 229
9.3.8 The use of enzymes 229
9.4 Final treatment of fabrics 230
9.4.1 Dyeing 230
9.4.2 Biocidal treatment 232
9.5 Sonochemical production of antimicrobial fabrics 233
9.5.1 The SONO project for antimicrobial fabrics234
9.5.2 Mechanism for the production of metal oxide nanoparticles in the
SONO process 235
9.5.3 Impregnation of metal oxide nanoparticles into the fabric in the
SONO process 236
9.5.4 The pilot plant installations237
9.5.5 Biocidal efficiency of the treated fabrics238
9.6 Developments in the impregnation of fabrics with biocidal
nanoparticles after the SONO project239
9.6.1 Modification of the Viatech system 239
9.6.2 Developments in Coventry 240
9.6.3 Developments in Bucharest 241
9.7 Production processes in the leather industry 242
9.7.1 Historical 243
9.7.2 Production processes in the leather industry244
9.7.3 Leather processing in Coventry 244
9.8 Further developments in leather processing 246
9.8.1 Developments in tanning 247
9.8.2 Developments in dyeing 248
9.9 Leather processing in Bucharest 249
9.10 Concluding remarks 250
References251
Chapter 10
Ultrasonically assisted biodiesel synthesis 257
10.1 An introduction to biofuels 257
10.1.1 First-generation biofuels 257
10.1.2 Second-generation biofuels 257
10.1.3 Third-generation biofuels 258
10.2 A general introduction to diesel fuel 258
10.3 The history of biodiesel 260
10.3.1 The first synthetic biodiesel fuel261
X Contents

10.3.2 Thefirstreferencetothechemicaltransesterificationofa
glyceride262
10.4 Ultrasonicallyassistedbiodieselsynthesis(UABS) 263
10.4.1 ThechemistryinvolvedinUABS 264
10.4.2 Ultrasonicallyinducedoilandmethanolemulsification267
10.5 TheworkofMirceaVinatoru(MV)onUltrasonicallyAssisted
BiodieselSynthesis(UABS) 268
10.5.1 MVandUABS–Japan 268
10.5.2 MVandUABS–Romania(Part1)278
10.5.3 MVandUABS–Canada 279
10.5.4 MVandUABS–Texas 280
10.5.5 MVandUABS–Romania(Part2)289
10.6 Somecommentsonthescale-upofUABSforuseasan
agriculturalfuel298
10.7 SomecommentsonUABSproduction301
References302
Index 307
Contents XI

Chapter5
Sonochemicalsynthesis
5.1Historicalintroduction
Thereweretwomaininterestsintheuseofultrasoundinchemistryfromthebegin-
ningofresearchinthisfield:oneinvolvedtheuseoflow-powerultrasoundfor
analysisandtheotherwithchemicalchangesthatcouldbeaffectedbyhigh-power
ultrasound.ThiswasidentifiedbyWeisslerinhisseminalpaper“Ultrasonicsin
chemistry”whichwaspublishedin1948[1].Intheintroduction,hestatedthat:
Therearetwomainfieldsinwhichultrasonicscontributesvaluableinformationtochemistry.
Oneoftheseistheinvestigationofmolecularpropertiesoffluidsbymeasurementoftheveloc-
ityofweakultrasonicwaves;theotheristhestudyofchemicalreactionswhicharecausedor
acceleratedbyintenseultrasonicirradiation.
Ultrasoundusedforchemicalanalysisisnotoneofthetopicsofthisbookbutisa
researchfieldthathasattractedalotofinterest.Thiswaspresentedinthefirstso-
nochemistrysymposiumin1986[2]butsinceitinvolveslowpower,usuallyhigh-
frequencymeasurementofvelocity,attenuationandscatteringofultrasounditfits
moresquarelywithnon-destructiveevaluationofmaterialsandacoustics.Never-
theless,atmuchhigherultrasoundpowers,theremustbeaconnectionbetween
thewayinwhichsoundwavesinteractwithamediumandthecreationofacoustic
cavitation.Thiswaswhatfirstbroughttheattentionofchemiststoanewbranchof
chemistry–sonochemistry–atermthatwasfirstusedbyWeyl[3]andWeissler[4]
inthe1950s(seeVolume1,Chapter1).
In1986,TimMasonpublishedashortreviewontheusesofultrasoundin
chemicalsynthesis[5].Inthispaper,hechampionedtheuseoftheterm“sono-
chemistry”:“Anewwordhasrecentlyappearedinthechemicalliteraturetocover
thisrapidlyexpandingfield,theuseofultrasoundinchemistrywhichisnowgenerally
referredtoassonochemistry.”Healsomadethepredictionthat:
Sonochemistrymaybeasimportantatopicwithinchemistryasphotochemistry,thermochem-
istryorhigh-pressurechemistry.Itmightevenbearguedthatitcouldbecomemoreimportant
becauseofitsgreatergeneralapplicability.
TogetherwithJimLindley,acolleaguefromCoventryUniversity,over100referen-
cesonthesyntheticaspectsofsonochemistryweregatheredtogetherandreviewed
inthefollowingyear,1987[6].
Itisouropinionthat1986shouldbeconsideredtobetheyearwhichsawthe
renaissance(rebirth)ofsonochemistry.Duringthatyear,thefirst-everinternational
symposiumonasubjectidentifiedassonochemistrywasorganizedatWarwickUni-
versity,UnitedKingdom,aspartoftheAutumnMeetingoftheRoyalSocietyof
https://doi.org/10.1515/9783110999938-001

Chemistry [7]. This meeting signified the beginning of serious interest in the uses of
ultrasound in chemistry, which now spreads across almost all possible areas of
chemical sciences and beyond.
5.1.1 Mechanistic aspects
Many researchers who became involved in sonochemistry began asking questions
about how sound energy could cause changes in chemical reactions. It had been rec-
ognized from the very beginning that there could not be a direct interaction between
ultrasound and the bonds holding together atoms in molecules but, despite this, ul-
trasound could influence chemical reactions. In 1927, Richard and Loomis had con-
sidered the direct effect of acoustic vibrations observing that the frequencies of
ultrasonic waves are much lower than the vibrations of molecular bonds [8]. The
words that they used in their paper were:
A third possible effect should be mentioned, although it cannot be treated in detail in this
communication, namely, the effect of the vibration frequency of the sound wave itself on an
unstable molecule, apart from its local kinetic effect upon molecules collectively. Although
the frequencies used in the work described below (289,000 per second unless otherwise
stated) were of a magnitude far below that of molecular vibration, certain effects, to be dis-
cussed later, seems to substantiate such an hypothesis.
Scientists began asking deeper questions about the reasons for the interaction be-
tween sound and chemistry. This included delving into the energies evolved during
cavitation bubble collapse particularly in terms of sonoluminescence [9]. The most
accepted explanation emerged from the idea that acoustic bubbles generated by the
passage of ultrasound through a solution of chemicals would be subject to collapse
through normal cavitation processes. Such cavitation bubble collapse can produce
high local temperatures and pressures around each bubble, and this was identified
by Fitzgerald et al., in their paper in which“hot spot”chemistry was introduced to
scientists for the first time [10]. The authors raised the question about why do any
chemical reactions occur when a system is irradiated with high-intensity ultrasound?
They explored the influence of different gases upon the cavitation threshold of
liquids and the outcomes below and above that threshold. The conclusion was that:
Since the threshold of cavitation is strongly affected by so many factors, we would like to em-
phasize that studies of chemical effects of ultrasonics must always include a measurement of
the threshold of cavitation for the particular experiments being undertaken.
It is our opinion that cavitation threshold is indeed an important factor in sono-
chemistry. There are, however, two problems associated with this measurement.
Firstly, the values are normally obtained for very pure solvents (and chemistry sel-
dom uses materials of such purity), and secondly, chemical reactions almost always
2 Chapter 5 Sonochemical synthesis

involve mixtures and these also change the cavitation threshold of a liquid. It is an area
of research which traditionally belongs to the physicist or physical chemist but maybe
some new research should be opened to investigate“A list of liquids (common laboratory
solvents) and their cavitation threshold as a function of ultrasonic frequency and power”.
When sonochemistry became the subject for conferences, there were some ques-
tions about whether sonochemical effects were mainly mechanical rather than chemi-
cal. This was a reasonable point given that cavitation collapse could produce effects
similar to high shear mixing:
–extremely good mixing,
–emulsification,
–powder deaggregation and dispersal,
–particle size reduction,
–surface cleaning,
–mass transfer to surfaces.
Some scientists were not happy that sonochemistry might be considered simply to be
the result of some form of super mixing. In the 1990s, there were attempts made to
predict the effect of power ultrasound on reactions themselves and to try and formulate
rules governing such predictions. It was Jean-Louis Luche who made the most con-
certed effort to introduce some order in this part of chemistry [11]. He suggested that
sonication promotes reactions proceeding through radical pathways [12, 13] and began
to examine the chemical effects of ultrasound and defined any accompanying mechan-
ical effects as“False sonochemistry” . He went on to suggest that“True sonochemistry”
could occur either in homogeneous or heterogeneous systems through processes in
which the reactive intermediate was a radical or a radical ion since the production of
such species could be stimulated by cavitation. He developed three rules covering so-
nochemical reactions which were written in the following terms in a book published in
1996 entitledChemistry Under Extreme or Non-Classical Conditions[14].
–Rule 1applies to homogeneous processes and states that those reactions which
are sensitive to the sonochemical effect are those which proceed via radical or
radical-ion intermediates. This statement means that sonication is able to affect
reactions proceeding through radicals and that ionic reactions are not likely to
be modified by such irradiation.
–Rule 2applies to heterogeneous systems where a more complex situation oc-
curs, and here reactions proceeding via ionic intermediates can be stimulated
by the mechanical effects of cavitational agitation. This has been termed“false
sonochemistry”although many would argue that the term“false”may not be
correct, because if the ultrasonic irradiation assists a reaction, it should still be
considered to be aided by sonication and thus“sonochemical”. In fact, the
right test for“false sonochemistry”is that similar results should, in principle,
be obtained using an efficient mixing system in place of sonication. Such a
comparison is not always possible.
5.1 Historical introduction3

–Rule 3applies to heterogeneous reactions with mixed mechanisms, that is, radical
and ionic. These will have their radical components enhanced by sonication, al-
though the general mechanical effect from Rule 2 may still apply. There are two
situations that can occur in heterogeneous systems involving both of these mecha-
nistic paths: (a) When the two mechanisms lead to the same product(s), which we
will term a“convergent”process, in this case the result is an overall rate increase
(b) if the radical and ionic mechanisms lead to different products, then sonochem-
ical switching can take place by enhancing the radical pathway only. In such“di-
vergent”processes, the nature of the reaction products is actually changed by
sonication.
The study of kinetics in sonochemistry led to some insights into the possible mecha-
nisms of such reactions. In 1967, Chen and Kalback reported that the hydrolysis rate
of methyl acetate using hydrochloric acid increased with increasing sonic amplitude
but varying the frequency had only a negligible effect [15]. In the following year, Fo-
gler and Barnes attributed the increase in reaction rate caused by ultrasound in this
reaction to the high temperatures reached within the cavitation bubbles [16]. They
also noted that the yield did not increase indefinitely with increasing power applied
to the transducer, but instead reached an optimum. The optimum power changed
with temperature because the collapse time increases with increasing temperature.
This was the direct result of the change in solvent vapour pressure with the tempera-
ture of the reaction.
Solvolysis was the subject of the first research work on sonochemistry in Coven-
try (see Volume 1, Chapter 1). We chose the homogeneous hydrolysis of 2-chloro-2-
methylpropane (t-butyl chloride) in aqueous alcoholic media (Scheme 5.1) because it
is one of the classic examples of a unimolecular nucleophilic displacement reaction
(termed S
N1). The reactions were monitored by conductance changes due to the liber-
ated HCl.
The reactions were performed at 25 °C and ultrasound was introduced by dipping the
reaction vessel into an ultrasonic cleaning bath (45 kHz). An increase in the alcohol
content of the solvent led to slower reactions but the rate ratio (k
ultrasonic/k
silent)in-
creased up to a maximum of 2 fold [17]. Later results suggested that there was a re-
gion of maximum structure in the binary solvent mixture [18]. Similar results were
found for the solvolysis in aqueous isopropanol and tert-butyl alcohol. Detailed stud-
ies of the aqueous ethanol system led to the following main conclusions:
CH
3
C
CH
3
ClH
3C
aqueous ethanol
CH
3
C
CH
3
OHH
3C
+ HCl
Scheme 5.1:The hydrolysis of 2-chloro-2-methylpropane in aqueous alcoholic media.
4 Chapter 5 Sonochemical synthesis

–The effect of ultrasound increased with increased ethanol content and decreased
temperature giving rate enhancements up to 20-fold at 10 °C in 60% w/w.
–At ethanol concentrations of 50% and 60%, the actual rates of reaction under
ultrasonic irradiation increased as the temperature was reduced from 20 to 10 °C
(by factors of 1.4 and 2.1, respectively).
–A maximum effect of ultrasound appeared to occur at a solvent composition of
around 50% w/w at 25 °C.
There are two factors which contribute to these conclusions. Firstly, there is the ef-
fect of increasing cavitational collapse energy via a lowering in vapour pressure as
the temperature is reduced (see earlier). This does not adequately explain the effect
of the change in solvent. The primary process is unlikely to occur inside the cavita-
tion bubbles and a radical pathway should be discarded. The most likely explana-
tion is that the disruption induced by cavitation bubble collapse in the aqueous
ethanolic media is able to break the weak intermolecular forces in the solvents.
This will alter the solvation of the reactive species present. Significantly, the maxi-
mum effect is found in 50% w/w ethanol/water composition–a composition very
close to that which contains the maximum hydrogen bonded structure.
Some years later, in 1997, this explanation was supported by Tuulmets [19]. After
a thorough analysis of the experimental data he concluded that the idea that the ap-
plication of ultrasound had led to a perturbation of the bonding within the reacting
system was justified and that the effect on the kinetics was a direct result of this per-
turbation. The work of Tuulmets group was mainly based on the concept of the per-
turbation of the solvation of reacting particles by the application of ultrasound. This,
in principle, was similar to the one that we had used. However, he added a note of
caution that the detailed mechanism of the action of ultrasound remains to some
extent uncertain.
The development of rules for sonochemistry continues to this day and recently
the authors of this book launched a discussion paper [20]:“Can sonochemistry take
place in the absence of cavitation?–A complementary view of how ultrasound can
interact with materials” . It was intended to invite researchers to consider (or recon-
sider) the way in which ultrasound could intervene in chemical reactions. Further
discussion of sonochemical mechanisms can be found later in this chapter and else-
where in this book.
5.1.2 Synthetic aspects
In the early years of sonochemistry, there were many publications in the field of
synthesis which were not labelled as sonochemistry since that terminology did not
exist. In this section, we will try and draw together what might be considered signif-
icant publications during that period.
5.1 Historical introduction5

The pioneering work of Richard and Loomis published in 1927 seems to be the
first recorded instance of the effect of ultrasound on chemical reactions [8]. The exam-
ples chosen were not strictly from synthesis but included the influence of ultrasound
on the hydrolysis of dimethyl sulphate in the presence of sodium hydroxide and the
iodine“clock”, a classical oscillating reaction first reported in 1886 by Landolt [21].
These are examples of ultrasound influencing the rate of chemical reactions, but it is
difficult to find synthetic uses from this early period. The acceleration of solvolysis
reactions remains an important and wide-ranging application of sonochemistry, oscil-
lating reactions such as the iodine clock have not been examined in detail except by
Margulis and Maximenko [22].
From the very beginning of sonochemistry, hydrogen peroxide production was
observed as a product of the sonication of water. This was reported in 1929 by
Schmitt et al. [23]. Weissler and Henglein separately observed the formation of hy-
drogen peroxide when water containing oxygen was irradiated with ultrasound [24,
25]. The formation of H
2O2is a consequence of the homolytic split of water mole-
cules into radical species and subsequent reactions (Scheme 5.2).
The oxygenated radicals HO
•
and HO
2
•are powerful oxidizing agents and provide
the means by which organic pollutants in water can be destroyed by sonication, a
topic which is explored in Volume 1, Chapter 4 of this book dealing with environmen-
tal protection.
The formation of hydrogen peroxide in the presence of KI is the basis of a dosi-
meter now widely used in sonochemistry whose sensitivity can be enhanced when car-
bon tetrachloride (CCl
4) is added to the solution. Sonication of water containing CCl
4
produces molecular chlorine, which reacts quickly with iodide ions in solution to lib-
erate molecular iodine. In 1950, Weissler investigated this reaction“Chemical Effect
of Ultrasonic Waves: Oxidation of Potassium Iodide Solution by Carbon Tetrachlo-
ride”anditnowcarrieshisname[26].Weisslerwasoneofthepioneersofsonochem-
istry and produced a review“Sonochemistry: The Production of Chemical Changes
with Sound Waves”in 1953 [4]. In this paper, he attempted to summarize the existing
knowledge on the chemical, physical and biological effects of ultrasound. He also re-
ferred to some of the work of Moriguchi who is perhaps better known for his early con-
tributions to electrochemistry in the 1930s. However, in 1933, Moriguchi had
H2O
HO + H
..
H
.
+O2 HO2
.
2HO
.
H2O2
.
2HO2
H2O2+ O2
))))
Scheme 5.2:Decomposition of water under sonication.
6 Chapter 5 Sonochemical synthesis

beguntopublishaseriesofpapers(inJapanese)ontheeffectsofultrasoundon
chemical phenomena. In the first of these dealing with heterogeneous reactions,
he observed that the reaction of zinc with hydrochloric acid is accelerated as was
the reaction of calcium carbonate with sulphuric acid [27]. The enhanced dissolu-
tion of metals and solids would later become an important consideration in the
mechanical effects of ultrasound used in synthesis.
Probably the first application of ultrasound in a catalytic reaction involving
gases came in 1950 with the publication of a patent entitled“Ammonia synthesis”
(Scheme 5.3) [28]. The overall method is similar to the well-known Haber process dat-
ing from the early twentieth century. The patent claims:“A process for the synthesis of
ammonia from its constituent elements which comprises subjecting a gaseous mixture
of nitrogen and hydrogen carrying a finely divided catalyst in suspension therein to
ultrasonic vibrations of a frequency greater than twenty thousand cycles per second.”
This conversion took place at lower pressures and temperatures than any established
process at that time.
What makes this application of ultrasound particularly interesting is the way in which
it was applied to a gaseous mixture of nitrogen and hydrogen together with a finely
divided catalyst because sonochemistry is almost exclusively employed in a liquid me-
dium However, there is no evidence of this procedure has been used in industry.
At that time, the effect of ultrasound on nitrogen fixation in aqueous conditions
was an active area of research. One of the first to publish in this field was the Finn-
ish scientist Artturi Ilmari Virtanen who was the 1945 Chemistry Nobel Laureate. He
had received his award for his work on the biological fixation of nitrogen and the
preservation of fodder in agriculture, and their importance to human nutrition. To-
gether with Nils Ellfolk, he investigated the oxidative fixation of nitrogen in water
exposed to the atmosphere at a frequency of 300 kc/s and a radiating intensity of
10 W/cm
2
[29]. They attributed the cavitation energy to electrical discharges on bub-
ble collapse. The products included NO
2¯ and NO
3¯ ions but when hydrogen and car-
bon monoxide were bubbled through the solution nitrogen fixation was inhibited.
In a subsequent study, volatile organic substances belonging to homologous series
of aliphatic fatty acids, aldehydes, alcohols, aromatic hydrocarbons, and amines
were also found to inhibit fixation [30]. On the basis of the results, the most likely
reason suggested for this is the influence of the volatile substances on surface activ-
ity and the resultant energetic changes in the cavitation. Diffusion of substances
into the cavitation bubbles was not thought to be a sufficient explanation.
Sonochemical nitrogen fixation was further explored in one of the earliest books
involving sonochemistry written by Isaak El’piner entitledUltrasound: Physical,
H2+N2+Fe2O3+Al2O3+K2O+ultrasound!2NH 3
Scheme 5.3:Ammonia production under sonication.
5.1 Historical introduction7

Chemical, and Biological Effects[31]. The original text was published in 1963 in the
Russian language and a year after translated into English. He makes an interesting
comment in the book concerning the fixation of nitrogen in an ultrasonic field and
the formation of biologically important substances. When water containing nitrogen
and hydrogen was irradiated with ultrasound (in a sealed glass tube), ammonia was
formed up to 12.5μg/mL in around 6 h. Introducing carbon monoxide in the gaseous
mixture did not inhibit the ammonia formation in sonicated water. El’piner also de-
scribes in his book some of his research in which ultrasonic irradiation of water satu-
rated with nitrogen and hydrogen containing organic fatty acids (in the absence of
oxygen). The nitrogen is fixed by the organic aliphatic acids, resulting in the forma-
tion of several amino acids (citation 56 in chapter IV of his book). The significance of
such results is that one can consider the conditions which existed in the very early
stages of the birth of our planet Earth. Then the natural conditions that existed in-
cluded vibrations, UV light, electrical discharges as well and radioactive decay of
some elements. It seems likely that such a combination of conditions in the presence
of very simple chemicals could provide enough energy to trigger the synthesis of
amino acids. These are the building blocks for the construction of living organisms,
thus perhaps primeval conditions, especially in water, might be similar to the ex-
tremes developed during acoustic cavitation and lead on to the development of early
forms of life on our planet. Conditions might be particularly beneficial for such reac-
tions in the“black smokers”or deep-sea hydrothermal vents found on the seabed at
great depths where high pressures, heat and bubbles of gas mix.
5.1.2.1 Organic reactions with ultrasound
It is not an easy task to find references to the very first organic reaction activated by
ultrasound. However, there is a section in El’piner’s book, chapter V, pages 79–115
[31], in which he describes how the early stage of research was mostly related to irra-
diation of aqueous solutions of organic compounds with ultrasound at different fre-
quencies. This type of research was aimed mostly at studies of the decomposition of
organic compounds. It is clear that these transformations occur as a consequence of
water dissociation (Scheme 5.2). An example published in 1955 is the case of benzene
sonication in the presence of water and atmospheric air [32]. In this paper, Robert,
Prudhomme and Grabar reported that they had detected in the products phenol, res-
orcinol, diazotizedp-nitroaniline as well as compounds having aldehydes in the
structure. This is a clear indication that hydroxyl radicals are generated by ultra-
sound which then react with benzene. The results showed the formation of similar
products to those resulting from X-ray irradiation [33] despite the fact that the latter
is an ionizing radiation, whereas ultrasound is not.
The presence of nitrogen-containing compounds in the products resulting from
the sonication of benzene in air provides further evidence of oxidative nitrogen fixa-
tion previously observed by Virtanen (see earlier) [29]. He also noticed that argon and
8 Chapter 5 Sonochemical synthesis

oxygen gases enhanced nitrogen oxidation [34], while some volatile compounds in-
hibited the process [35].
In 1965, Prakash and Pandey investigated the behaviour of saturated aqueous
solutions of iodoethane, iodobenzene and 1,2-dichlorobenzene under ultrasonic ir-
radiation [36]. They used an ultrasonic bath (1 MHz) and found that aromatic com-
pounds containing halogens generated the halogen in the form of acids, whereas
aliphatic compounds liberated both acidsand free halogens. This was an early paper
in the field, and it suggested that generally only aqueous systems support sonochemi-
cal reactions, which do not take place in pureorganic liquids. They attributed this to
theuniquepropertiesofwaterwhichareresponsible for the abnormal high release of
energy from aqueous cavitation bubbles.
5.1.2.2 Organometallic reactions with ultrasound
Some of the early uses of ultrasound in chemical synthesis have been somewhat
overlooked since they did not explicitly mention ultrasound. Such was the case in
one of a series of papers entitled“Electron donor and acceptor complexes with aro-
matic systems”. In 1957 Part 4 appeared entitled“An improved method of preparing
metal addition complexes with aromatic systems”this included a diagram of an ap-
paratus involving an ultrasound probe for sodium activation [37]. Three types of
preparation were compared for the reaction of sodium with benzoquinoline:
1. Direct reaction of sodium wire with benzoquinoline dissolved in diethylether
2. Direct reaction of sodium with the benzoquinoline in boiling dimethoxyethane
or dioxan
3. Ultrasonic activation of a sodium cube immersed in a solution of the benzoqui-
noline in diethylether or dimethoxyethane.
C
D
B
G
F
A
E
N
2
Figure 5.1:Schematic apparatus for the
ultrasonically activated reaction of sodium with
benzoquinoline, whereAis the 25 kHz
magnetostrictive transducer,Bis a stainless steel
probe,Cis a stainless steel screw-on basket,Dis
the side arm for nitrogen as protective gas,Eis the
condenser,Fis a rubber gasket andGis a
polyethylene disc, protecting the rubber gasket.
5.1 Historical introduction9

The ultrasonic method proved to be the most effective and the apparatus looked re-
markably similar to what might be used today (Figure 5.1). A nitrogen atmosphere
was used, and a Mullard magnetostrictive 25 kHz transducer (A) was attached to a
stainless steel probe (B) which was adjusted to give maximum output to a sodium
metal cube contained in a stainless steel basket (C) attached to the base of the probe.
In 1980, serious interest began in organometallic sonochemistry. In that year, a
paper from Luche and Damiano described the effects of ultrasound on a modified
Barbier reaction (Scheme 5.4). He reported the direct, in situ formation of alkyl and
aryl lithium (3) by the reaction of an organic halide (2) with lithium wire (or lithium
with 2% sodium sand) in ether immersed in an ultrasonic bath. The organolithium
reacted with the carbonyl compound present (1) to lead, after work up to product
(4). The technique avoided the use of activating reagents (e.g. I
2) and afforded a sig-
nificant amelioration of the reaction both by increasing reactivity and removing the
induction period which is often involved in this type of reaction [38]. These synthe-
ses are largely free from side reactions such as reduction and enolization which are
common using conventional methodology.
The work of this group on ultrasound in organometallic synthesis continued with a
series of some 20 papers involving different metals such as lithium, potassium, copper,
magnesium, zinc, nickel and mercury. A further advantage when using ultrasound is
that such reactions can be performed in damp, technical-grade tetrahydrofuran (THF),
a potential boon for large-scale industrial operations. Almost all of these papers were
summarized along with attempts to rationalize their mechanisms in a book chapter
from Luche and Cintas in 2007 [39].
Synthetic applications of the effects of ultrasound (using an ultrasonic cleaning
bath) on the coupling reactions of organic halides using lithium metal in THF were
published as a series from 1981 by Boudjouk and Han [40]. Several organic halides
such as chlorobenzene, bromobenzene, iodobenzene,p-bromotoluene,p-iodotoluene,
m-bromotoluene, benzyl chloride, benzoyl chloride and 1-chloropropane in THF solu-
tion were sonicated in the presence of lithium wire with results as shown in Table 5.1.
The group also investigated the coupling of chlorosilanes under similar conditions [41].
In 1982, Repičdescribed a modification of the Simmons–Smith cyclopropanation
reaction using diiodomethane and sonochemically activated zinc which avoided the
sudden exotherm normally associated with this type of reaction [42]. Up to this time,
most methods for this reaction relied upon activation of zinc by using zinc-silver or
zinc-copper couples and/or the use of iodineor lithium. In the sonochemical procedure
R
1
O
R
2
R
3X+ +
R
1R2
C
OHR
3
MetalR
3Metal
Scheme 5.4:The Barbier reaction.
10 Chapter 5 Sonochemical synthesis

no special activation of the zinc was required, indeed equally good–and reproduc-
ible–yields were obtained using zinc in the form of dust, foil, mossy or metallic rod.
The ultrasonic source employed wasa small laboratory cleaning bath.
The method using solid zinc metal and ultrasound was successfully scaled up to
achieve the cyclopropanation of methyl oleate in 0.5 kg quantities (Figure 5.2) [43].
The reactor consists of a 22-L four-necked flask immersed in a 50-gallon ultrasonic
bath (3,000 W, 80 kHz). Two zinc cones were suspended in the reaction mixture and
could be withdrawn if the reaction became too violent. The metal was not reactive
Table 5.1:Ultrasound-induced coupling of organic halides with lithium.
RX Product Time (h) Yield (%)
C
HCl C H–CH  
C
H
Br C
H
–C
H
  
C
H
IC
H
–C
H
  
p-CH
C
H
Br (p- CH
C
H
)
  
p-CH
C
H
I( p-CH
C
H
)
  
m-CH
CHBr (m-CH CH)  
C
HCHCl C HCHCHCH  
a
CHCOCl C HCO–COC H  
CH
CH
CH
Cl CH
(CH
)
CH
  
a
Yield by NMR. All others are isolated yields (>95% pure).
addition
funnel
suspension
wires
stirrer
nitrogen
inlet
condenser
stirrer
thermocouple
zinc cones
heating
coil
Ultrasonic
bath
Figure 5.2:Large-scale cyclopropanation reactor.
5.1 Historical introduction11

until the ultrasound was turned on, and several advantages over more traditional
procedures were claimed:
–Foaming of the solution is reduced.
–The exotherm of the reaction is more evenly distributed over the reaction period
as only a small area of fresh zinc surface is continuously being exposed to the
reaction.
–The zinc can be raised out of the reaction mixture at any time that the exotherm
and the reflux become too vigorous.
–Any excess zinc can be easily recovered as the remaining part of the solid metal
block.
One of the most common laboratory applications of ultrasound is the initiation of a
reluctant Grignard reaction (Scheme 5.5). The quantitative effects of ultrasound on
the induction times for the formation of a Grignard reagent in various grades of
ether are given in Table 5.2 [44].
The term“crushed”in the table refers to the methodology commonly employed to acti-
vate magnesium–mechanical crushing of the metal to expose fresh surface to the re-
agents. In each of the three solvents, there was no significant difference in yield of the
alkyl magnesium bromide with or without sonication, the figures being 65%, 55% and
55%. Significantly prior sonication of the metal in ether had no effect on the induction
time when the subsequent preparation was performed under non-ultrasonic condi-
tions. This clearly eliminates simple surface cleaning as the source of the effect and it
was suggested that during sonication adsorbed water was removed from the metal sur-
face, kept clear while irradiation continued, but was re-adsorbed on switching off the
CH3CH2CHBrCH3+Mg!CH 3CH2CH MgBrðÞ CH 3
Scheme 5.5:Grignard reaction.
Table 5.2:The preparation of butan-2-yl magnesium bromide in ether in an
ultrasonic bath.
Type of diethyl ether used Method Induction time
.% water Stirred –min
.% ethanol Sonicated < s
.% water Stirred –h(“crushed”)
.% ethanol Sonicated –min
% saturated Stirred –h(“crushed”)
.% ethanol Sonicated –min
12 Chapter 5 Sonochemical synthesis

bath. When all the water dislodged from the surface has reacted with the newly formed
Grignard reagent, the system was essentially“dry”and reaction continued normally.
5.2 Sonochemical synthesis in Coventry
5.2.1 The Ullmann reaction
In the Coventry sonochemistry group, we had begun our interests in synthesis within
the field of organometallic chemistry. We chose the Ullmann coupling reaction since
that was an area of interest for Jim Lindley who was an inorganic chemist within our
department of chemistry [45]. The Ullmann coupling of aryl halides to yield biaryls is
widely used in aromatic chemistry and is normally carried out by heating the halide
(with or without solvent) in the presence of copper powder. Classical Ullmann condi-
tions involve high temperatures, long reaction times and a large excess of copper. At
the time that we did this work, the best reported yield (99.6%) at the lowest tempera-
ture (60 °C) for such a reaction was the conversion of 2-iodonitrobenzene to 2,2′-
dinitrobiphenyl over a period of 48 h using a 10-fold excess of copper [46], and so
this system was chosen as a suitable model for study. Using a probe system (20 kHz),
the product 2,2′-dinitrobiphenyl (75%) was obtained in 2 h at 62 °C with only a 4-fold
excess of copper–a 50-fold increase in reactivity compared with non-ultrasonic con-
ditions [47]. We presented further results on Ullmann phenyl ether reactions at the
Sonochemistry Symposium at Warwick University in 1986 [48].
The Ullmann condensation (Scheme 5.6) is a related organometallic reaction
involving copper or copper compoundsas a catalyst or reagent in the nucleo-
philic substitution of aryl halides and is of industrial importance. The reaction
of 2-bromonitrobenzene with potassium phenoxide in sulpholane in the pres-
ence of copper bronze under anhydrous conditions at 63 °C showed no signifi-
cant improvement in the rate of formation of phenyl nitrophenyl ether when
subjected to sonication. A 65% yield was obtained after 2 h, and no other prod-
ucts were detected by gas–liquid chromatography (GLC; Table 5.3).
However, the preparation of anhydrous phenoxide for the reaction is experimentally
inconvenient. An alternative is to produce the phenoxide in situ from phenol and
potassium hydroxide. Under these conditions normal stirring gave a substantially
OX
+
Br
NO
2
base, Cu
sulpholane
O
O
2N
X = K or H
Scheme 5.6:Ullmann condensation (X = H or K).
5.2 Sonochemical synthesis in Coventry 13

reduced yield of 21% but using sonication this could be increased to 63%, which is
very similar to that obtained using pre-prepared phenoxide [48]. The improvement
was considered to be due to the presence of the small amount of water from the
neutralization of phenol by potassium hydroxide. This then reacted with the surface
of copper under the influence of ultrasound to generate reactive oxides which were
catalysts for the reaction.
5.2.2 Halogenation of aromatics using CuBr
2supported on alumina
In 1988, Kodomari et al. reported that aromatic hydrocarbons could be halogenated
by supporting a copper(II) halide on alumina in a reaction subsequently named
after him [49]. This prompted us to extend our research into copper promoted or-
ganic reactions into the effects of ultrasound on the Kodomari reaction itself and on
its effect during the preparation of the supported reagent [50].
Sonication using a Sonics and Materials’ VC600 Sonicator (42 W/cm
2
) led to a
40% increase in initial rate and a 44% reduction in overall time of conversion of
naphthalene into products. This result was thought to arise mainly from improve-
ments in mass transport due to acoustic microstreaming as cavitation effects are
unlikely to be significant at such a high temperature in a high vapour pressure sol-
vent such as CCl
4(Table 5.4).
Table 5.4:Bromination of naphthalene using CuBr
2/Al
2O
3at 76 °C in CCl
4.
Preparation of supported
reagent
% conversion
(min)
%-Br %,-Br

CuBr/AlOprepared by
method of Kodomari
Silent   
As above Sonicated   
Aqueous suspension of
CuBr
and AlOafter
sonication forh and drying
Silent   
Table 5.3:Effects of ultrasound on the condensation of phenol and
2-bromonitrobenzene using Cu in dry sulpholane at 63 °C.
X Base Conditions % yield after ( h)
K – Stirred .
K – Sonicated .
H KOH Stirred .
H KOH Sonicated .
14 Chapter 5 Sonochemical synthesis

Ultrasound was shown to have the most beneficial effect when applied to the
preparation of the supported reagent. A suspension of alumina in aqueous copper
(II) bromide was sonicated for 1 h prior to drying. This treatment produced a mate-
rial that in the absence of sonication gave an 80% increase in initial rate and a 67%
reduction in time for total conversion to products.
Particle size analysis showed that the reagent which was subjected to ultrasound
during its preparation had a significantly smaller particle size and narrower size dis-
tribution (5–85μm, SD 5.05μm) compared with the reagent prepared under silent
conditions (14.56μm, SD 15.39μm).Electronmicroscopyrevealedfairlysmoothcrys-
tal faces for the normally prepared material, whereas sonication appears to have gen-
erated agglomeration to give particles with a sponge-like appearance containing
large cavities.
It is believed that the mechanism involves the initial formation of aromatic hy-
drocarbon radical cations at electron acceptor sites at the alumina surface. Support
for this was provided by the retardation of the reactions with the addition of the
radical scavenger DPPH (2,2′-diphenyl-1-picrylhydrazyl).
5.2.3 O-Alkylation of hindered phenols
The O-alkylation of hindered phenols under heterogeneous conditions using K
2CO
3
inN-methylpyrrolidinone (NMP) is normally a sluggish reaction when performed
under conventional conditions but it is considerably enhanced by sonication. We
presented some preliminary results at an Ultrasonics International Conference in
1987 (Table 5.5) [51]. Ultrasound (20 kHz) was introduced via a Heat Systems W225
probe system operating at 40% power immersed in the reaction mixture contained
in a modified rosette cell (see volume 1, figure 2.9).
Perhaps the most obvious effect of ultrasonic irradiation on this O-alkylation reac-
tion is the substantial acceleration in rate giving almost complete reaction in about
1.5 h for all three reactions. A second significant feature is the falloff in reaction
which takes place under normal conditions indicating a reduction in reactivity as
the reaction proceeds. Since it is known that sonication is capable of reducing parti-
cle size in reactions involving powders, the possibility that simple particle size
Table 5.5:Yield of ether from the alkylation of 2,6-dimethylphenol.
Alkyl halide Yield % (glc) after.h
Silent Sonicated
Iodomethane (afterh) 
-Iodopropane (afterh) 
-Bromoprop--ene  
5.2 Sonochemical synthesis in Coventry 15

reduction (i.e. increase in surface area) of the potassium carbonate might explain
the sonochemical enhancement was investigated. The reaction was followed under
normal (stirred) conditions using K
2CO
3powder which had been pre-sonicated for
1 h during which the initial agglomerates of some 100–300 µm were broken down
to particles with a fairly even size distribution of 3–5 µm. Although this material
gave a very much faster initial rate of reaction of 2,6-dimethyl phenol with iodopro-
pane, the reaction tailed off at about 87% completion but could be rapidly forced to
completion on further sonication. Such results support the hypothesis that sonica-
tion has a 2-fold effect on heterogeneous reactions: (a) a reduction in particle size
affording a far larger surface area for reactivity and (b) much more efficient reagent
mixing than can be achieved by normal stirring.
The heterogeneous reaction between 2,6-dimethyl phenol and iodomethane was
investigated further in terms of the effects of changes in solvent and volume of reac-
tion on the yield of this type of reaction. The results were presented in 1989 at the
next UI conference in Madrid [52]. The sonochemistry experiments were performed in
a modified rosette cell immersed in a constant temperature bath. The probe system
used was a Sonic Systems W8603 unit operating at 20 kHz with a probe tip diameter
of 0.5 inches (~1.3 cm) at a power setting of 15 W. The heterogeneous reaction mixture
was 2,6-dimethylphenol (5 g, 0.041 mol), 1-iodopropane (13.96 g, 0.082 mol) and po-
tassium carbonate (11.32 g, 0.082 mol) in different solvents (each 100 cm
3
). For com-
parison purposes, the silent (classical) reaction mixture was mechanically stirred in a
250 cm
3
round-bottomed flask immersed in a constant temperature bath at 45 °C.
Some conclusions that arose from this study in terms of choice of solvent and
changes in particle size, reaction volume and ultrasonic power are generally appli-
cable to a wide range of synthetic sonochemistry and are summarized below.
5.2.3.1 The effects of solvent and particle size
The effect of solvent in a sonochemical reaction is closely related to its ability to sup-
port cavitation. This in turn is inversely related to its vapour pressure. Volatile sol-
vents would be expected to“boil”during the rarefaction cycle of an acoustic wave
producing bubbles saturated in vapour and incapable of violent cavitational collapse.
Only at low temperatures could such solvents support efficient cavitation and so it is
common practise in sonochemistry to either choose solvents of low vapour pressure
and/or to use solvents which have high boiling points, i.e. low vapour pressures.
In heterogeneous reactions of this type, it has been shown that a significant pro-
portion of sonochemical enhancement is due to particle size reduction, in this case it
is the powdered K
2CO
3base [53]. Comparison was made of particle size reductions of
16 Chapter 5 Sonochemical synthesis

the solid in three common organic solvents butan-2-one (methyl ethyl ketone (MEK),
bp 79.6 °C), 4-methylpentan-2-one (MIBK, bp 117 °C) and NMP (bp 202 °C). The results
revealed that it was only in NMP where significant particle size reduction occurred
which was in accordance with the differences in vapour pressures. In addition, it was
only when using NMP that the reaction itself was found to be sonically enhanced.
For this reaction it seemed that the sonochemical enhancement was the result of a
combination of both particle size reduction and solvent effect. This was demonstrated
by the use of decalin as solvent. Particle size reduction in decalin (bp ca 190 °C) was
shown to be of a similar level to that achieved in NMP but no appreciable alkylation
was achieved in this solvent with or without sonication. This reflects the important ad-
ditional feature of NMP as a solvent for this alkylation; it specifically solvates the po-
tassium ion, thus affecting the basicity of the carbonate and nucleophilicity of the
phenoxide intermediate [54].
5.2.3.2 Change of the reaction volume
Alteration to the overall reaction volume is an approach to the problem of determin-
ing the effect of power density on a reaction of fixed molar concentrations as the re-
action volume is decreased, and the yield is enhanced. A limit to the extrapolation
does occur, however, when the total volume of solvent is reduced to the extent that it
is too small to couple the ultrasonic vibrations from the probe tip efficiently to the
reaction mixture.
A very important point which arises from these results is that in order to monitor
the reaction progress the removal of an aliquot from a sonochemical reaction mixture
will affect the overall volume and consequently the ultrasonic power entering the re-
action. Hence, the smaller the sample taken, the more reliable are the results.
5.2.3.3 Change in ultrasonic power
There is always the temptation in sonochemistry to believe that the maximum sono-
chemical effect is to be obtained by applying the maximum ultrasonic power avail-
able. The reaction was monitored at constant temperature but under differing power
settings on the probe. Results showed that under the conditions used there was an
optimum power input of 15 W, and further increases in power did not result in in-
creased reactivity [52].
5.2 Sonochemical synthesis in Coventry 17

5.2.4 O-Alkylation of 5-hydroxychromones
5-Hydroxychromones are important intermediates in the drug industry, and Fisons
Pharmaceuticals based in Loughborough was interested in finding methods of O-
alkylation of these compounds. Fisons had supported several of our sonochemistry
research students in those early years.
The O-alkylation of 5-hydroxychromones is difficult, and the cause of this lack of
reactivity is thought to be electronic rather than steric hindrance as a consequence
of hydrogen bonding between the carbonyl and OH groups on the adjacent rings
(Scheme 5.7). This also results in some dispersion and reduction of the negative
charge on O of the OH group due to resonance stabilization. As a result of this, the
alkylation to 5-alkoxychromones, for example (R = alkyl) (Scheme 5.8) is normally
performed as a two-step process. First, 2,6-dihydroxyacetophenone is converted
into 2-hydroxy-5-alkyloxyacetophenone followed by a Claisen condensation of this
material with diethyl oxalate in sodium ethoxide to give the required chromone.
The direct alkylation of ethyl-5-hydroxychromone-2-carboxylate with 1-iodopropane
in MEK produces none of the alkylated product (R = propyl) even after 5 h at 60 °C
(Scheme 5.8). Neither the addition of TDA-I as phase transfer catalyst nor prolonged
sonication of the reaction mixture (Heat Systems W225 Sonicator operating at
O
OOH
CO 2Et
O
O
CO 2Et
O
-
M
+
B
-
M
+
Scheme 5.7:Suggested reason for electronic hindrance to the O-alkylation of 5-hydroxychromones.
O
OOR
CO 2Et
COMe
OHHO
RX
K
2CO3 - MEK
COMe
ORHO
EtO
-
COOEt
COOEt
O
OOH
CO
2Et
RX
K
2CO
3 - NMP
))))
Scheme 5.8:O-Alkylation of ethyl 5-hydroxychromone-2-carboxylate.
18 Chapter 5 Sonochemical synthesis

20 kHz) under the same conditions were found to be effective [53]. However, when
the solvent was changed to NMP and sonication was applied at 65 °C to a mixture
of ethyl 5-hydroxychromone-2-carboxylate, 1-iodopropane and K
2CO
3(mol ratio
1:2:2), the O-alkylated product was formed in 100% yield (glc) after 1.5 h. In the ab-
sence of ultrasound, a yield of only 28% (glc) was observed under otherwise identi-
cal conditions. A likely explanation for the effectiveness of NMP, rather than MEK,
as a solvent for this reaction is that the former is an aprotic solvent of the type used
for selective solvation of cations. In the O-alkylation of chromones in MEK, the
anion and cation produced by the action of the bases (K
2CO
3) may be a tightly asso-
ciated ion pair, whereas in NMP, partial solvation of the potassium ion occurs, per-
mitting easier nucleophilic attack on the halogenoalkane by the oxygen anion.
Since it is known that sonication is capable of reducing particle size in reactions
involving powders, the possibility that simple particle size reduction (i.e. increase in
surface area) of the powdered base might explain the sonochemical enhancement
was investigated in the same way as Section 5.2.3 with similar results. When commer-
cial K
2CO
3is subjected to sonication in NMP at 65 °C for 30 min, the average particle
size of the base is reduced from ca. 300 to 3 µm. Using this pre-sonicated base under
the reaction conditions described, but with mechanical stirring rather than further
sonication 90% (glc) (R = propyl) was produced in 90 min after which the reaction
slowed. The initial reactivity in this case was similar to that obtained using commer-
cial K
2CO
3under continuous ultrasonic irradiation which suggests that sonochemical
enhancement to reactivity is predominantly a particle-size effect. If sonication was
applied when the reaction approached its limit (near 90% conversion), a complete
reaction was achieved. This supported our previous suggestion that the additional ef-
fect is likely to be related to sonication producing much more efficient reagent mixing
and mass transfer than can be achieved by normal stirring.
The scope of the reaction was expanded by using a range of halogenoalkanes
for the alkylation. In all cases, sonication resulted in considerably improved O-
alkylation (Table 5.6).
Table 5.6:Sonochemical alkylation of 5-hydroxychromone
(Scheme 5.8).
a
Compound
R=
Time (min) Yield (glc %)
ultrasound silent
i-Pr   
PhCH
   
Me   
n-Bu   
Allyl   
a
A 2-fold excess of halogenoalkane and K2CO3over
chromone (1) (5 g) in 100 cm
3
NMP at 65 °C.
5.2 Sonochemical synthesis in Coventry 19

In 1989, I was invited as a Visiting Professor of Chemistry to the University Paris
Sud, France. I joined the laboratories of Georges Bram and Andre Loupy where they
were involved in early studies of microwave (MW) chemistry and in particular with
the effects of MWs on solid-supported reactions. This field was chosen because the
use of solvents in chemical reactions carried out in MW ovens often resulted in the
generation of elevated temperatures and consequently high pressures which might
lead to dangerous situations. They had reported their findings on the use of“dry
media”MW irradiation for the pinacol rearrangement (on montmorillonite) or acetate
alkylation (on alumina or silica gel) [55].
During my stay, they were also investigating phase transfer reactions on
solid supports in the absence of solvent. Together we chose to compare the im-
provement in the O-alkylation of 5-hydroxy-chromone-2-carboxylate using sono-
chemistry in N-methylpyrrolidinone (NMP) and enhanced phase transfer catalysis
using Aliquat 336 in the absence of solvent [56]. In order to maintain a reasonable
comparison between the two techniques, a controlled thermal reaction using K
2CO3,
as the common base, and a ratio of substrate:haloalkane:base of 1:2:2 was used
throughout (Table 5.7).
Both the PTC and sonochemical routes are more effective than conventional heat-
ing. However, the PTC route employs higher temperatures and requires longer reac-
tion times than corresponding sonicated reactions. In practical terms, the PTC route
is much simpler because the workup of the reaction mixture to isolate the product
only requires simple filtration.
Table 5.7:O-Alkylation using reactive bromo compounds.
a
Bromide Method Time (min) Isolated yield (%)
Benzyl
a
Thermal  
Benzyl
a
Ultrasonic  
Benzyl
b
PTC ( % Aliquat)  
Allyl
a
Thermal  
Allyl
a
Ultrasonic  
Allyl
b
PTC ( % Aliquat)  
a
65 °C /N-methylpyrrolidinone;
b
100 °C, no solvent.
O
(1) (2) (3) (4)
OOH
CO
2Et
R1X
O
OOR
1
CO
2Et
O
OOH
CO
2R
1O
OOR
1
CO
2R
1
Scheme 5.9:Mixture of products obtained when using less reactive 1- and 2-bromobutane for the
O-alkylation of a chromone.
20 Chapter 5 Sonochemical synthesis

When alkyl bromides (1-bromobutane and 2-bromobutane) are used in place
of more reactive bromo compounds, the reactions are more sluggish and two
other products are formed (Scheme 5.9) [56]. Under thermal conditions, the pro-
duct mixture contains products which result from transesterification: the trans-
esterified product (3) and the doubly O-alkylated and trans-esterified product (4)
together with the O-alkylated product (2) (Table 5.8). Phase transfer catalysis
can produce either exclusive transesterified or O-alkylated products by simply
altering the catalyst used from Aliquat to crown ether (18-c-66), respectively
(Table 5.8).
A proposal to explain the dependence of product distribution on the experimental
technique used was based on the role of halide ion in the transesterification reac-
tion. A possible mechanism involves transesterification via a S
N2-type hydrolysis
mechanism in which the halide ion attacks the ethyl ester leading to alkyl cleavage
(Scheme 5.10).
The resultant ethyl bromide or chloride is volatile and is driven out of the reaction
mixture at the temperatures employed. A rapid re-esterification then occurs with
the excess of the higher boiling alkyl halide. When Aliquat 336 is used, there is ha-
lide ion available in the PTC from the outset of the reaction. In the case of thermal
Table 5.8:O-Alkylation using less reactive bromo compounds R
1Br Scheme 5.9).
R
 Method Time (h) Relative Yield % (glc)
(starting material)
-Bromo Thermal     
-Bromo Ultrasonic     
-Bromo PTC % Aliquat   
-Bromo Thermal     
-Bromo Ultrasonic     
-Bromo PTC % Aliquat     
-Bromo PTC %-c-     
RO
O
Cl
-
RO
-
O
+EtCl
Br
RO
O
Scheme 5.10:Proposed halide displacement S
N2 route to transesterification.
5.2 Sonochemical synthesis in Coventry 21

or ultrasonic reactions, the halide is produced during the reaction itself. The use of
Aliquat 336 leads to exclusive formation of the transesterified product. The use of
the catalyst 18-crown-6 in the absence of solvent can lead to a switching of path-
ways and the exclusive formation of O-alkylated products.
5.2.5 Ultrasonic effects on metal powders and sonochemical catalysis
5.2.5.1 Particle size reduction of metal powders
Quite early in the studies of catalysis, the Coventry group had developed an interest
in particle size reduction via sonication. We studied the effects of irradiation power,
frequency and other physical parameters on copper bronze suspensions. This was
presented at the first ESS meeting in Autrans in 1990 as a report and was later pub-
lished in 1992 in a paper entitled“Quantifying sonochemistry: Casting some light on
a black art”[57]. The investigations were performed using the Undatim: Sonoreactor
probe system, which had adjustable power output through separate interchangeable
probes making it possible to use several different frequencies. This instrument was
also equipped with an automatic transducer resonance frequency search device, en-
abling the power input to a system and the resonance frequency to be maintained
accurately throughout the reaction. At the time, it was the only instrument of its type
on the market and we were grateful to Eric Cordemans for the loan of a prototype
(Figure 5.3). The results showed how the ultrasonic energy entering a system is affected
by the frequency and power of ultrasound as well as the presence of a bubbled gas.
5.2.5.1.1 Effect of frequency
At that time in the development of sonochemistry, one of the problems associated
with operating at different frequencies is that the physical dimensions of the trans-
ducers and horns placed a real limit upon the power which they could deliver. A
comparison of the amount of energy (measured by calorimetry) dissipated in 100 cm
3
of water at 30 °C by three different horn systems (20, 40 and 60 kHz) is shown in
Table 5.9. The particle size reduction of a suspension of copper bronze in water at a
concentration of 2.0 g in 100 cm
3
(original particle size was 60 µm) was also exam-
ined. Samples were analysed after 1 h sonication at 30 °C. The particle size reduction
is greatest using the 20 kHz horn and smallest for the 60 kHz horn with 40 kHz in
between. This also corresponds to the order of ultrasonic intensities applied and
seemed logical because the process was a mechanical particle size reduction (commi-
nution) rather than a chemical process.
These results were obtained in 1992 and at the time we commented in the paper
that it should not be concluded that the optimum sonochemical effect could only be
obtained at high powers and low frequencies (here the most power was delivered at
20 kHz). We were aware that this type of study involving the effects of sonication
22 Chapter 5 Sonochemical synthesis

frequency was relatively new but there were indications in homogeneous reactions
and those involving emulsification that there might well be a significant frequency
dependence.
Figure 5.3:Undatim Sonoreactor with interchangeable horns.
Table 5.9:Effect of power generated by horns of different frequencies in water.
Nominal
frequency (kHz)
Power
setting
Tip area
(cm

)
Power
(W)
Intensity
(W/cm

)
Particle
size
   .   .  .
   .   .  .
   .   .  .
5.2 Sonochemical synthesis in Coventry 23

5.2.5.1.2 Effect of bubbled gas
Acoustic cavitation has two effects on dissolved gas in a fluid:
(a) Ultrasound can be used to degas a liquid, and this is the first stage of cleaning
using an ultrasonic bath. When the power is first turned on, cavitation will be
relatively easy and the power of bubble implosion and resultant jetting will not
be large. As the gas (air) is removed, cavitation becomes more difficult to pro-
duce i.e. requires more energy. When applied to sonochemical reactions, this
means that at the start cavitation will degas the mixture with little effect on the
reaction for some seconds but when the dissolved gas is removed cavitation in-
tensifies and sonochemistry begins.
(b) Small gas bubbles in a fluid act as nuclei for cavitation. In the early days, some
groups involved in sonochemistry deliberately bubbled a gas through a reaction
in order to maintain uniform cavitation. According to theory, the energy devel-
oped on collapse of these gas-filled bubbles will be greatest for gases with the
largest ratio of specific heat. For this reason, monoatomic gases (He, Ar or Ne)
should be used in preference to diatomic (N
2or air). Gases with more than two
atoms such as CO
2are the least suitable. This can be directly related to the indi-
vidual ratio of specific heats for these gases, the highest ratio (1.6 for mon-
atomic argon) giving the greatest cavitational effects (the value for nitrogen is
1.44). Background to this effect and relevant references are to be found in the
bookApplied Sonochemistry[58].
We investigated the use of bubbled gas in the particle size reduction of copper
bronze in water (Table 5.10). The original particle size of the copper bronze was
60 µm at a concentration of 2.0 g in 100 cm
3
of water. When nitrogen was bubbled
through the mixture, there was a small effect in reducing particle size compared
with sonication in air but entrained argon had a significant effect. Nevertheless,
even though bubbled argon provided an increase in sonochemical effect, this was
not adopted as common practice in sonochemistry.
Table 5.10:Effect of bubbled gas ultrasonically induced
reduction of particle size of copper bronze in water.
a
Gas used Particle size (µm)
No gas (open to the atmosphere) .
Nitrogen .
Argon .
a
Intensity of 65 W/cm
2
at 30 °C, at a frequency of
20 kHz for 1 h.
24 Chapter 5 Sonochemical synthesis

5.2.5.1.3 Effect of temperature
The effect of solvent temperature on particle size reduction was investigated using
the 20 kHz probe for the sonication of aqueous copper bronze at four temperatures
(Table 5.11). The concentration and original particle size were the same as used in
Section 5.2.5.1.2. The Undatim Sonoreactor power control knob operating at 20 kHz
was set at“7”for each of the four temperatures, and this resulted in four different
input powers due to the change in parameters of the water with temperature. In
terms of the effect on reducing the particle size of copper bronze, it is clear that
there is a greater effect on particle size reduction as the solvent temperature is
reduced.
In simple terms, this can be correlated with a change in vapour pressure because the
collapse of a cavitation bubble is directly affected by the amount of vapour entering
the bubble during its formation. The more vapour present, the more cushioned is the
collapse; thus, if the temperature of the system is raised, then ultrasonic power input
must be increased to maintain constant cavitational collapse energy. On the other
hand, at a constant power setting, a greater sonochemical effect will be achieved by a
low temperature. Therefore, a greater particle size reduction of copper bronze is pro-
duced at low temperature.
5.2.5.2 Simmons–Smith cyclopropanation
In 1982, Repičdescribed the ways in which ultrasound could improve a cyclopropa-
nation reaction involving zinc [42]. We chose to investigate this reaction through
the cyclopropanation of styrene using dihalomethane and zinc metal in the form of
powder. The study was part of the PhD thesis of Darren Bates entitled“The effect of
ultrasound and other physical parameters on the reactivity of powders and cata-
lysts”which included the work described in Section 5.2.5.1 [59] and was presented
in 1991 at ESS2 in Gargnano [60].
The conventional procedure involved a suspension of zinc powder (2.51 g) and
1,2-dimethoxyethane (40 cm
3
) stirred at 90 °C to which was added a mixture of diio-
domethane (10.3 g) and styrene (1 g) dropwise over 10 min. This was compared with
Table 5.11:Effect of water temperature on the
ultrasonically induced reduction of particle size of copper
bronze at 20 kHz for 1 h.
Temperature (°C) Intensity (W/cm

) Particle size (µm)
  .
  .
  .
  .
5.2 Sonochemical synthesis in Coventry 25

the sonicated process in which the zinc powder in 2-dimethoxyethane was first acti-
vated using a 20 kHz probe (Undatim Sonoreactor, 88 W) for 2 h at room tempera-
ture with bubbled nitrogen. After 2 h, zinc and 1,2-dimethoxyethane suspension
was transferred to a flask at 90 °C and diiodomethane (10.9 g, 1.39 mol) and styrene
(1 g, 0.34 mol) was added dropwise over 10 min under continued sonication and
bubbled nitrogen. A comparison of yield from these experiments showed 54.3%
conversion in 6 h by the conventional route compared with 94.3% after only 2.5 h
using ultrasound.
The importance of pre-sonication of zinc was determined by comparing the cy-
clopropanation reaction using as-supplied zinc powder with the pre-sonicated ma-
terial on yields after 2.5 h (Table 5.12). The pre-sonication step reduced the particle
size of the powder from 36 to 16 µm and increased the conventional reaction yield
from 27.46% to 65.05%. However, if the pre-sonicated zinc is then used for the so-
nochemical cyclopropanation reaction, the yield improves further to 95.1%.
5.2.5.2.1 Calculation of the“efficiency” of the Simmons–Smith cyclopropanation
reaction at different frequencies
A series of experiments was performed using the Undatim Sonoreactor as the source
of ultrasound (Figure 5.3) to determine the effects of frequency (20, 40 and 60 kHz)
on this reaction. We were fortunate to have this system, but it suffered from the dif-
ficulty that each of the probes has a different physical dimension and irradiating tip
area. This meant that each horn would deliver different powers into any given reac-
tion. We decided to compare the results obtained at each frequency using the con-
cept of some form of ultrasound dose in parallel with measurements for other types
of radiation dosage. It was first used in the thesis of Darren Bates [59]. Here it was
termed“Total energy input”and this is defined as follows:
Total energy input=calorimetric measured energy×time
Using this equation, it is possible to sonicate the three reactions for different times
to give the same total energy input (approximately 2,600 W
✶
min) at each frequency
(Table 5.13).
Now the efficiencies can be compared because the yield from approximately
the same total energy input from each probe can be measured. If we divide yield %
Table 5.12:The effect of sonicating the zinc powder
before use in cyclopropanation.
Using as-supplied zinc Using pre-sonicated zinc
Conventional Sonicated Conventional Sonicated
.  .  .  .
26 Chapter 5 Sonochemical synthesis

by the total energy input, this will give a measure of the efficiency of each fre-
quency from which we can conclude that:
–In terms of power consumption, the 60 kHz transducer is the most efficient but
takes 180 min to reach 54.81% yield.
–In terms of time, the 20 kHz transducer gives 30.67% yield in 30 min, but it is
the least energy efficient.
We used the same idea in our work with dosimetry using aqueous terephthalate ion
as a fluorescence monitor [61]. Here we used the term“ultrasound dosage”(D), in
parallel with other radiation dosages as the ultrasonic power entering the liquid
system (W) recorded by calorimetry multiplied by the time of exposure in seconds.
The units of dosage are W
✶
s, i.e. Joules. The fluorescence yield is then defined as
fluorescence intensity produced per unit ultrasound dosage (F/D) and has units of
J
−1
. As with the cyclopropanation work, it was shown that when using the Undatim
Sonoreactor at 20, 40 and 60 kHz, the greatest sonochemical efficiency was attained
at the highest of these frequencies (60 kHz).
5.2.5.3 Nickel-catalysed hydrogenation of oct-1-ene
In general, the catalytic activity of metals is enhanced by ultrasonic irradiation,
and this was the reason why it was one of the early targets for the improvement
of reactions such as catalytic hydrogenation. Simple nickel powder is a very poor
catalyst for the hydrogenation of alkenes; however, in 1990, Suslick et al. used
high-intensity ultrasound to activate nickel powder for use as a hydrogenation
catalyst [62]. Surface studies of the metal after sonication revealed dramatic
changes in morphology in that the surfaces were smoothed. Continued sonication
further reduced the particle size and they began to come together and form ex-
tended aggregates.
In Coventry, we decided to study the effects of ultrasound on the catalytic activ-
ity of three types of nickel powder (3 µm, sub-micrometre and Raney nickel) and
also 10% palladium on carbon. The hydrogenation study was also part of the work
of Darren Bates (see earlier) whose PhDprogramme was in conjunction with the
Harwell“Sonochemistry Development Club”(see Volume 1, Chapter 1). The idea
was that we could determine whether sonicating them before use (pre-sonicating)
Table 5.13:Frequency effect irradiating the same total energy.
Frequency
(kHz)
Energy
(watts)
Sonication Time
(mins)
Total Energy
(W
✶
min)
Yield
%
Efficiency
    , .  .
    , .  .
    , .  .
5.2 Sonochemical synthesis in Coventry 27

in ethanol under a nitrogen atmosphere would result in an increase in activity. The
model system chosen was the hydrogenation of oct-1-ene, and a Sonic Systems
20 kHz probe system was employed.
The results differed for each type of catalyst with perhaps the most unexpected re-
sult obtained with Raney nickel, where a reduction in activity was produced by pre-
sonication. The results are shown in Table 5.14 [63].
–3 µm nickel: The as-supplied powder was not a catalyst for this reaction. However,
an optimum in catalytic activity was obtained when 3 g Ni was pre-sonicated for 1
h at 10 °C using 34 W total power. With the catalyst prepared under these condi-
tions, 65% octane was obtained after 60 min hydrogenation at 25 °C. This increase
in activity was thought to be due to a combination of factors including the removal
of impurities from the surface of the nickel, a reduction in the thickness of the
oxide coating and reduction in the particle size of the material.
–Sub-µm nickel: As in the case of 3 µm nickel, the as-supplied powder was not a
catalyst for this reaction, but in this case, pre-sonication carried out as earlier had
a smaller effect. Only 15% octane was produced in the same time using the acti-
vated material which is somewhat contrary to what might be expected. Images of
this material after sonication indicated that large agglomerates were formed by
sonication, and these may have less activity than the sonicated 3 µm nickel.
–Raney nickel: In complete contrast to the earlier results, the sonication of Raney
nickel reduced its catalytic efficiency. The as-supplied material gave 100% con-
version after 30 min under conventional conditions compared to only 25% con-
version after 30 min following pre-sonication. Examination of the products
using GLC suggested that the sonicated catalyst promoted isomerization at the
expense of hydrogenation. Thus, after 30 min hydrogenation using untreated
Raney nickel, there were 100% octane and 0% oct-2-ene, whereas in contrast,
the use of pre-sonicated Raney nickel gave 37% octane, 20% oct-2-ene and 6%
unreacted oct-1-ene after 1 h. The commercial preparation of Raney nickel produ-
ces a porous catalyst with a large surface area free from any passivating layers
that are present on the two other nickel powders studied. The mechanical effects
of cavitation were thought to cause interparticle collision which for such a
Table 5.14:Hydrogenation of oct-1-ene using a nickel catalyst.
a
Catalyst used Yield (%)
Silent
Yield (%)
Pre-sonicated
µm nickel (min) (min)
Sub-µm nickel (min) (min)
Raney nickel I(min) (min)
%PdonC (min) (min)
a
20 kHz, 34 W, 25 °C, absolute ethanol.
28 Chapter 5 Sonochemical synthesis

material would cause an agglomeration and compression of the powder reducing
its porosity and thus its catalytic effect.
–10% Pd on carbon: Pre-sonication of this catalyst achieved a considerable en-
hancement in activity resulting in 100% conversion after only 30 min compared
with 66% in the same time using the as-supplied material. This enhancement
in activity appeared to correlate with an increase in the adsorptive uptake of
hydrogen on the catalyst.
5.2.5.4 Diels–Alder cyclization reaction
The Diels–Alder (4 + 2) electron cycloaddition reaction between conjugated dienes and
reactive alkenes is an important reaction in synthetic organic chemistry but despite
the widespread use of ultrasound to assist organic synthesis in the early 1990s, there
were very few reports of sonochemically assisted Diels– Alder reactions [64, 65].
We were interested in the synthesis of drugs used to combat psoriasis, and
there was a report in the literature about a one-pot synthesis of lonapalene using a
Diels–Alder reaction [66]. Like many syntheses involving Diels– Alder reactions,
this involved a long reaction time. The initial step between 3-chloro-1-methoxy-1,3-
butadiene (1) and 2,3-dimethoxy-2,5-cyclohexadiene-1,4-dione (2) in dichlorome-
thane at room temperature in an inert atmosphere was 24 h which was followed by
the addition of
D,L-camphorsulfonic acid and continued stirring for an additional
12 h to produce lonapolene (5) (Scheme 5.11).
Cl
OCH
3
O
O
OCH
3
OCH
3
+
benzene, toluene, or CH
2Cl
2
OCH
3
OCH
3
Cl
O
O
OCH3
OCH
3
Cl
OCOCH
3
OCOCH3
OCH
3
OCH3
OCH
3
Cl
OCOCH
3
OCOCH3
N
(CH
3CO)
2O
+-camphor sulphonic
acid
(CH3CO)2ON
12
3 4 5
Scheme 5.11:Synthesis of lonapalene using a Diels–Alder reaction.
5.2 Sonochemical synthesis in Coventry 29

This was the synthesis which we chose to investigate using sonochemistry
(20 kHz Sonics and Materials VC600) under the same general conditions used in
the one-step silent reaction (Scheme 5.11) [67]. The resulting Diels–Alder cyclo-
addition reactions between substituted 1,3-butadienes with substituted 2,3-
dimethoxycyclohexadiene-1,4-diones afforded a variety of bicyclo[4,4,0] fused
ring systems (naphthalene derivatives) inhighyield(Table5.15).Inourhands,
the naphthalene derivatives that were isolated using these conditions in meth-
ylene chloride were significantly increased under sonication and lonapalene it-
selfwasobtainedin86%yieldin2hatroomtemperaturecomparedwith
56.4% under silent conditions over 41 h.
When this procedure was repeated using two different solvents (benzene and tolu-
ene), the yields were also improved by sonication, with the yields in the latter sol-
vent larger than in methylene chloride.
At the time that this study was published (1996), we thought this to be the only
reported example of conventional homogeneous Diels–Alder reaction promoted by
ultrasound. Later, in the same year, a review which mentioned our work, written by
nine authors including Jean-Louis Luche entitled“The Diels-Alder cycloaddition,
an intriguing problem in organic sonochemistry”was published [68]. The authors
claimed that most of the significant results obtained up to then were obtained when
quinones are the dienophiles. They suggested that a possible mechanism should in-
clude a redox process between the diene and dienophile, giving radical-ion intermedi-
ates. These findings were related to the Luche ideas on the rules of sonochemistry (see
Section 5.1.1) because the conclusion indicated that in cases where an initial redox
step was unlikely, ultrasonic irradiation would have little or no effect.
The original and well-documented rules governing Diels–Alder reactions in-
volved a theoretical approach using the concept ofthe conservation of orbital sym-
metrywithorbital correlation diagrams[69], predicting those reactions that could
take place thermally or photochemically. With the publication of the review in 1996
Table 5.15:Isolated yields of adduct under normal and sonicated conditions.
Product Reaction time
(h)
Conditions Solvent and yield (%)
methylene chloride benzene toluene

. Silent .
a
. .
. Sonicated . . .

. Silent .
b
. .
. Sonicated . . .

. Silent .
c
. .
. Sonicated . . .
a,b,c
Yields quoted in the literature (a) 72, (b) 93 and (c) 58 [66],
30 Chapter 5 Sonochemical synthesis

[68], there was the suggestion that ultrasound might act on some reactions of this
type via a different pathway through redox reactions. In 2004, an example was pro-
duced on the way in which ultrasound could change the course of such reactions
[70]. It involved the Diels–Alder reaction of a furan derivative with dimethyl acet-
ylene-dicarboxylate(DMAD)at80°C,orinthepresenceofLewisacidstheadduct
1 was obtained in a small yield (ca 5%), while under ultrasound it was the exclu-
sive compound (Scheme 5.12):
The authors wrote:“We reason that this dramatic effect may be attributed to the so-
nochemical (kinetic) activation of the furan nuclei (oxa-cyclic diene) bypolarization
induced electronic excitationorpolarization-facilitated (single) electron trans-
ferto the dienophile (a redox process).”This result also lends support to our hy-
pothesis on theordering effectof ultrasound–via developing solid-like charged
structures within a sonicated liquid, facilitating a much easier transfer of electrons
from one molecule to another (see Volume 1, Chapter 1, section 1.11.2) [20].
In a review of Puri et al., published in 2013 [71], they wrote:
It was found that the Diels-Alder reaction cannot be accelerated by ultrasound except when
SET (single electron transfer) or free radical processes are promoted, the rectified diffusion
during cavitation cannot be responsible for the acceleration of reactions, and the sonochemi-
cal acceleration of polar homogeneous reactions takes place in the bulk reaction medium. This
implies the presence of a“sound-field”sonochemistry besides the“hot-spot”sonochemistry.”
A statement that again points to the fact that sonochemistry, perhaps through an
ordering effect, can also involve those reactions in which electrons could easily be
mobilized to travel (transfer) from one molecule to another.
5.2.5.5 Friedel–Crafts reactions
An important group of non-steroidal anti-inflammatory agents are theO-arylpropanoic
acids of which ibuprofen 2-(4-isobutylphenyl)-propanoic acid (4) is a typical example
(Scheme 5.13). We undertook a sonochemical synthesis based on the Friedel–Crafts
(FC) alkylation reaction between a methanesulphonate ester (mesylate) and isobutyl-
benzene using aluminium chloride as catalyst.
O
MeO
2C
CO
2Me
R
O
R
O
R
CO
2Me
CO
2Me
DM AD DM AD
US ))))80oC
30oC
1
Scheme 5.12:The use of sonochemistry to change the course of a Diels–Alder reaction.
5.2 Sonochemical synthesis in Coventry 31

Piccolo et al. had reported the synthesis of (S)-methyl-2-phenylpropanoate in
good chemical and optical yields of 50–80% and≥97%, respectively, with optically
active lactic acid derivatives [72]. Somewhat surprisingly, although ultrasound had
been used to promote FC acylation [73] to our knowledge, there were no examples
of its use in this type of alkylation reaction published at that time. The FC alkylation
of isobutyl benzene (2) with (S)-2-mesyloxypropanoate (1a) in the presence of alu-
minium chloride afforded the corresponding (S)-methyl-2-phenylpropionate (3) in
15% yield after 2 h at 40 °C (Scheme 5.13). In an ultrasonic bath (Kerry Pulsatron 55,
38 kHz) and otherwise identical conditions, this was raised to 19% [74].
The yield for the subsequent hydrolysis of esters in aqueous NaOH was over 95%.
As an alternative synthesis of ibuprofen (4), we studied the direct reaction of (2)
with (1b) which, under the same conditions, afforded 17% in 2 h at 40 °C. This yield
was improved to 50% under sonication in an ultrasonic bath for 2 h at the same
temperature, which represents a 3-fold improvement in yield over the one-pot syn-
thesis and a 3.5-fold improvement on the two-stage process (Table 5.16).
H
3CCH
OSO
2Me
CO
2R
a: R = CH
2CH
3
b: R = H
+
CH
3
CO
2CH
2CH
3
CH
3
COOH
aq. NaOH
D, 3 h
12 3
4
)))
Scheme 5.13:Friedel–Crafts synthesis of ibuprofen.
Table 5.16:Influence of ultrasound on some Friedel–Crafts reactions.
Compound Reaction Yield (%)
Intermediate Silent 
Sonicated (bath) 
Sonicated (probe) 
One-pot direct synthesis
of(ibuprofen)
Silent 
Sonicated (bath) 
Sonicated (probe) 
32 Chapter 5 Sonochemical synthesis

Subsequently, we explored the reaction using the higher energy input of an ultra-
sonic probe system (Vibracell VC50, 20 kHz, 5 mm tip) [75]. These reactions were per-
formed at a controlled temperature of 10 °C for 2 h and provided substantially greater
yields of (3) (44%) and (4) (64%) (Table 5.16). Thus, it was shown that ultrasound
mediation has a small but favourable effect on the FC alkylation with mesylate ester
(1a), but a much more significant effect on the one-pot synthesis with (1b).
Under the high-energy conditions involved in probe sonication, slight erosion
of the probe tip can occur. The tip is made of titanium alloy which, in the presence
of AlCl
3, may allow some titanium to enter the reaction as TiCl
3,aLewisacid,
which could itself act as a promoter in FC reactions. To investigate this possibility,
a catalytic amount of TiCl
3was deliberately added to the mixture for reactions per-
formed in an ultrasonic bath and with theprobe.Intheformercase(ultrasonic
bath), where there was no possibility of contamination with TiCl
3through erosion
during the reaction, the addition of TiCl
3resulted in a small increase in yield. When
TiCl
3was added to the reaction under probe sonication again a small increase in
yield was obtained but by no means large enough to support the possibility that
probe sonication had itself produced enough TiCl
3in the medium to catalyse the FC
reaction. These results suggested that it was sonication itself rather than the intro-
duction of TiCl
3which was responsible for promoting the reactions involving the
probe.
Another significant result from this work was the change in isomer distribution
(analysed by GLC and GC/MS) obtained in the case of the one-pot synthesis of ibupro-
fen. The isomer ratio under sonication was ortho (24%), meta (17%) and para (59%),
whereas the distribution under classical reaction conditions was ortho (22.8%), meta
(37.2%) and para (40.0%). The optically pure para-isomer, (S)-2-(4-isobutylphenyl) pro-
pionic acid [α]
25
D
+ 59.5º (ethanol 95%), was identical to an authentic sample of ibupro-
fen. The precise reason for the change to a greater proportion of para-substitution was
not clear.
In view of the ease of the reaction with optically pure lactic acid, the one-pot
synthesis represents a valuable approach to the preparation of a range of optically
active 2-arylpropionic acids and esters in moderately high yields. To our knowl-
edge, the isomeric distribution of ibuprofen synthesis has not previously been re-
ported and appeared to have some potential for a commercial process, although the
reaction yields had not been optimized.
In summary, the direct one-step sonochemical process affords a mixture of iso-
mers of ibuprofen (4) in approximatively 50% yield with a regioselectivity of 59%
for the para-isomer. This represents an increase in yield of the target isomer from
7% obtained under silent conditions to 30%. Hence, the overall chemical yield of
the pharmaceutically important para-isomer is increased by a factor of 4 using
sonochemistry.
5.2 Sonochemical synthesis in Coventry 33

5.2.5.6 Sonochemical switching and the reaction of lead tetraacetate
with styrene
In 1996, Jean-Marc Leveque came to the Coventry group to continue and extend his
PhD studies with Takashi Ando and Jean-Louis Luche involving the reactions of lead
tetraacetate (LTA) with styrene in acetic acid. In 1991, the Ando group had identified
reactions like this as further examples of sonochemical switching 1991 [76].
The addition of LTA to styrene may follow either an ionic or a radical pathway,
affording the products shown in Scheme 5.14.
–Reaction (i) is an ionic pathway through dissociation of the LTA oxidant, fol-
lowed by an electrophilic attack on the alkene bond to yield (2).
–Reaction (ii) probably involves a preliminary one-electron oxidation of styrene
to a radical cation leading to (3).
–Reaction (iii) is a radical pathway to (4) via the addition to the styrene double
bond of a methyl radical formed by homolyses of LTA.
One of the features of sonication is that it tends to favour radical mechanisms [12]. The
oxidation of styrene by LTA under ultrasonic irradiation would thus be expected to
proceed with an increased selectivity in favour of compound 4 (and also possibly 3).
The results are shown in Table 5.17, where sonication generates (4) and some (3),
Ph
i
ii
iii
Ph
Pb(OAC)3
+
Ph
OAc
OAc
1
2
Ph
+.
Ph
OAc
OAc
3
+
Pb(OAC)
4
Ph
4
Ph
OAc
.
Scheme 5.14:Reaction pathways of LTA with styrene in acetic acid.
Table 5.17:Yield from reaction of styrene
with LTA in acetic acid (50 °C, 1 h).
Product 
Stirring  .
Sonication . . .
34 Chapter 5 Sonochemical synthesis

whereas with stirring only a small amount of the ionic route product (2) is formed [77].
This result confirms that this reaction is an example of sonochemical switching.
At that time we wrote:“A question is still open about the origin of the sono-
chemical effect”[77]. One possibility was that in a heterogeneous situation, where
the LTA was not fully dissolved, sonication might provide super-mixing which
would then induce rapid mass transfer and enhanced reactivity. However, this
could not be the case because the sonochemical effect was observed even if totally
homogeneous solutions were used.
Another consideration is that the change in reactivity due to sonication could
not relate to LTA because it is not volatile and so could not enter the cavitation bub-
ble. Therefore, it must be linked to the volatile reagent“styrene”. The work that
was done in Coventry confirmed the importance of sonochemical activation of the
overall reaction through ionization of the volatile reagent styrene inside the bubble
to the radical cation [77]. The styrene radical cation had been observed spectroscop-
ically in radiolysis experiments [78], and there were reported similarities in the
chemical effects of ultrasound and ionizing radiation [79].
5.3 Sonochemical synthesis in Romania
5.3.1 Charge transfer complexes
Our Romanian work in sonochemistry began in 1991 when Tim Mason visited the
C. D. Nenitzescu Institute of Organic Chemistry for the first time and brought with
him the gift of an ultrasonic bath (see Volume 1, Chapter 1). The bath arrived not
long after we had begun to study the reactions of triphenylchloromethane (TPCM) and
triphenylbromomethane (TPBM) in nitrobenzene (NB), both of which involved charge
transfercomplexesasintermediatesanddisplayedsomeunusualbehaviour[80].
As soon as the ultrasonic cleaning bath was in the laboratory, we started to in-
vestigatetheinfluenceofultrasoundonthese reactions. Part of the results were
published in the first issue of a brand-new journal–Ultrasonics Sonochemistry(see
the history of this journal in Volume 1, Chapter 1)–with the title“Ultrasonically
stimulated electron transfer in organic chemistry. Reaction of nitrobenzene with tri-
phenylmethane and its derivatives”[81]. The reaction of all of the triphenylmethane
derivatives with NB was extensively investigated in my PhD thesis and published
later in related papers [80–82].
5.3.1.1 Electronic absorption spectrum of triphenylmethyl halogen derivatives
with nitrobenzene
The absorption spectra of a mixture of TPCM with NB (molar ratio 1:1) in hexane
revealed a peak at 232 nm, this was thought to indicate the presence of a charge
5.3 Sonochemical synthesis in Romania 35

transfer complex (see Figure 5.4a). In the case of TPBM, there was no discrete peak
which could be assigned to a charge transfer complex. Similarly, our attempts to iden-
tify charge transfer complexes in mixtures of triphenyliodomethane, triphenylmeth-
ane and triphenylcarbinol with NB were not successful. Perhaps this was because the
charge transfer complexes, if formed, were too weak to register in an electronic ab-
sorption spectrum.
The formation of such a charge transfer complex is shown in Scheme 5.15.
Now that we had an ultrasonic bath I wondered if ultrasound would have any ef-
fects on further reactions of this type of complex. Charge transfer would correspond
to the first step in sonochemical activation according to the rules of J. L. Luche (see
Section 5.1.1) [83] and so we compared the thermal with the sonochemical path-
ways for this reaction.
a b
Figure 5.4:Electronic absorption spectrum of: (a) TPCM, NB and a mixture of TCBM/NB; and
(b) TPBM, NB and a mixture of TCBM/NB.
Ph
3CX
+ NO
O
+
_
Ph
3CX
_
+
NO
O
+
..
Scheme 5.15:Electron transfer between halogen derivatives and nitrobenzene.
36 Chapter 5 Sonochemical synthesis

5.3.1.2 The chemical reactions of triphenylmethyl halogen derivatives
with nitrobenzene
The first step of electron transfer (equilibrium reaction) yields a pair of radical ions:
triphenylmethyl halogen radical cation and NB radical anion (Scheme 5.15). Subse-
quently, the radical cation derived from the triphenylmethyl halogen has two ways
of splitting giving triphenylmethyl radical or triphenylmethyl cation, both relatively
stable intermediates (Scheme 5.16).
Once formed, these intermediates can then become involved in further reaction in
response to heat or ultrasound to produce a variety of products (see later). The reac-
tions were monitored by titrimetric measurement of the disappearance of TPCM or
TPBM (Figure 5.5):
Under thermal conditions (in refluxing NB at 210 °C) the bromine derivative was
more reactive than chlorine. TPCM shown an induction time of around 0.75 h, after
which the reaction was fast up until a conversion of 90%. After this point, the reac-
tion was rather slow (reaching 96% in 8 h). In the case of the bromine derivative, the
induction time was much shorter (0.25 h) and the subsequent reaction is faster (5 h)
(Figure 5.5).
.
+
Ph
3CX
Ph
3C
Ph
3C
+
.+ X
+ X
.
+
Scheme 5.16:Splitting possibilities for the triphenylmethyl halogen radical cation.
0
10
20
30
40
50
60
70
80
90
100
0510
TPCM conversion %
Time (h)
a
0
10
20
30
40
50
60
70
80
90
100
051 0
TPBM conversion %
Time (h)
b
Figure 5.5:Reaction progress for (a) TPCM and (b) TPBM.
5.3 Sonochemical synthesis in Romania 37

In the presence of ultrasound at the lower temperature (40 °C) both were much
slower. After 30 h of sonication, TPCM conversion was only 60%, while of TPBM
was 84%. An explanation for this slower reaction might be because triphenylmethyl
halogen derivatives are not volatile and so cannot easily enter into the cavitation
bubbles generated in NB under these conditions; therefore, the sonicated reactions
must take place in the bulk liquid. Hence, the reactions that occur must involve
species generated from NB itself. It is possible to produce cavitation bubbles in NB
but only at either (a) a higher ultrasonic power or (b) at a higher temperature than
40 °C [84].
Triphenylmethyl halogen derivatives would be expected to generate very stable
cations that can then behave as reducing agents in the presence of oxidizing com-
pounds such as NB. Based on the reactivity of the triphenylmethyl derivatives stud-
ied under thermal conditions, the expected order of reactivity is obtained which is
that the most reactive is the iodine derivative followed by bromine and then chlo-
rine. What is somewhat surprising is that under sonication the order is changed
with iodine most reactive and then bromine followed by chlorine, that is, the order
of the last two compounds is reversed as shown in Scheme 5.17, two other triphe-
nylmethyl compounds have been included in these comparisons [82]:
This suggested that sonochemistry plays an important role in how such compounds
react with nitrobenzene. The triphenylmethyl derivatives cannot enter cavitation
bubbles (see earlier) and so cannot undergo the type of thermal reactions expected
in these“hot spots”[10]. Nevertheless, some of products from sonication and ther-
mal activation are common to both reactions (see Tables 5.18 and 5.19).
In the traditional and hence well-known chemistry of nitrobenzene reduction,
the final compound is aniline, but it is also possible to stop the reduction process at
previous stages where azo-benzene or azoxy-benzene are formed as intermediates
leading to the formation of aniline (Scheme 5.18):
The presence of such compounds in the reaction products would provide some im-
portant support to the proposed redox reaction between triphenylmethyl derivatives
Thermal: Ph 3C−I>Ph 3C−Br>Ph 3C−Cl′Ph 3C−OH>Ph 3C−H
Sonochemical:Ph
3C−I>Ph 3C−Br>Ph 3C−Cl′Ph 3C−H>Ph 3C−OH
Scheme 5.17:Order of reactivity of triphenylmethyl compounds.
Ph NH
2
Ph N N Ph Ph N N Ph
O
Scheme 5.18:Compounds generated during the reduction of nitrobenzene.
38 Chapter 5 Sonochemical synthesis

and NB. Indeed, these compounds were detected in the products from thermal acti-
vation (Schemes 5.19 and 5.20) but only in trace amounts.
The thermal and the ultrasonic reactions of TPCM with NB were conducted
using the apparatus setup shown in Figure 5.6.
The thermal reaction was conducted at the boiling point of NB (210 °C) for 8 h,
under argon as protective gas, returning NB into the reaction flask. The ultrasonic
reaction was conducted at 40 °C, for 30 h, using a Langford Sonomatic cleaning
bath T175, working at 40 kHz and 180 W electrical power.
5.3.1.2.1 Products of reaction between triphenylchloromethane
and nitrobenzene
For TPCM, the possible reaction products from thermal and sonochemical activation
are shown in Scheme 5.19 and their molar distribution in Table 5.18.
There is a clear difference between the products from thermal and ultrasonic
activation. While thermal activation yields detectable quantities of reduction com-
pounds of NB, the sonochemical reaction produces only a trace of only one such
compound (7). The main two differences are highlighted in bold in the table:
1. In the thermal process,hydrochloric acidis evolved, while in the ultrasonic pro-
cessmolecular chlorineis formed.
(a)
Legend:
1. Condenser
2. Dean-Stark collector
3. Gas trap
4. Heating plate
5. Pressure valve
Legend:
1. Condenser
2. Ultrasonic bath
3. Ultrasonic transducer
4. Gas trap (with water)
5. Pressure valve
2
Argon
Argon
4
3
5
1
1
4
5
3
2
(b)
Figure 5.6:Equipment used for thermal (a) and ultrasonic (b) reactions.
5.3 Sonochemical synthesis in Romania 39

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tunnusti samalla sydämessään menettelevänsä väärin. Hän tunsi
halveksivansa itseään, mutta kysyi sittenkin:
"Missä on se metsäläinen, joka lähti teitä pelastamaan? Miksi hän
ei ole palannut mukananne?"
"En ymmärrä", vastasi Clayton. "Ketä tarkoitatte?"
"Samaa, joka on meidät kaikki pelastanut — joka minutkin pelasti
gorillan kynsistä!"
"Oho!" huudahti Clayton hämmästyen. "Hänkö siis teidät pelasti?
Ettehän vielä ole kertonut mitään omista seikkailuistanne. Kertokaa
nyt vihdoinkin!"
"Mutta ettekö todellakaan ole häntä nähnyt?" jatkoi tyttö
kysymyksiään. "Kun kuulimme viidakosta laukauksia, hyvin heikosti
ja kaukaa, lähti hän luotani. Olimme juuri päässeet tänne likelle
majaa, ja hän riensi taistelupaikalle. Tiedän varmasti, että hän tahtoi
teitä auttaa."
Hänen äänensä oli melkein rukoileva, ja näennäisesti tyynen
pinnan alla oli tukahdutettua liikutusta. Clayton ei voinut olla
panematta sitä merkille ja ihmetteli, miksi Jane oli niin kuohuksissa
— miksi hän niin kiihkeästi halusi tietää, missä tuo outo olento nyt
viipyi. Kuinka hän olisikaan voinut aavistaa totuutta!
Kuitenkin heräsi hänessä jokin epämääräinen kuvitelma
lähestyvästä surusta, ja hänen rinnassaan orasti hänen itsensäkään
sitä tietämättä mustasukkaisuus ja epäilys apinamiestä kohtaan, jolle
hän oli hengestään kiitollisuudenvelassa.

"Emme ole nähneet häntä", vastasi hän tyynesti. "Hän ei tullut
luoksemme." Sitten hän lisäsi hetken mietittyään: "Ehkä hän liittyi
omaan heimoonsa — siihen joukkoon, joka hyökkäsi kimppuumme."
Hän ei tiennyt, miksi näin sanoi, sillä hän ei uskonut sitä itsekään,
mutta rakkaus on oikullinen.
Tyttö katsoi häneen vähän aikaa silmät suurina.
"Ei!" huudahti hän sitten kiivaasti — liiankin kiivaasti Claytonin
mielestä. "Se on mahdotonta. Ne olivat neekereitä, mutta hän on
valkoinen — ja lisäksi kelpo mies."
Clayton näytti hämmästyvän. Mustasukkaisuuden vihreäsilmäinen
paholainen kiusasi häntä.
"Hän on merkillinen, puolivilli viidakon asukas, neiti Porter.
Me emme tiedä hänestä mitään. Hän ei puhu eikä ymmärrä mitään
Euroopan kieltä — ja hänen koristuksensa ja aseensa ovat samat
kuin
länsirannikon villeillä."
Clayton puhui nopeasti.
"Täällä ei ole monien satojen kilometrien alalla muita ihmisolentoja
kuin villejä, neiti Porter. Hänen täytyy kuulua niihin heimoihin, jotka
ahdistavat meitä, tai joihinkin muihin samankaltaisiin — voipa hän
olla ihmissyöjäkin."
Jane Porter kalpeni.
"Sitä en tahdo uskoa", väitti hän puoleksi kuiskaten. "Se ei ole
totta. Saattepa vielä nähdä", jatkoi hän kääntyen Claytoniin päin,

"että hän tulee takaisin ja näyttää teidän erehtyneen. Te ette häntä
tunne niinkuin minä. Minä tiedän, että hän on kelpo mies."
Clayton oli hyväsydäminen ja ritarillinen mies, mutta nuoren tytön
kiihkeys metsäläisen puolustamisessa yllytti hänet päättömän
mustasukkaiseksi, niin että hän hetkeksi unohti kaikki, mistä heidän
tuli kiittää tätä villiä, ja vastasi tytölle pilkallisesti hymyillen:
"Ehkäpä olette oikeassa, neiti Porter, mutta en sittenkään luule,
että kummankaan meistä tarvitsee huolehtia ystävästämme
raadonsyöjästä. Onhan mahdollista, että hän on jokin raaistunut,
hunningolle joutunut haaksirikkoinen, joka unohtaa meidät
nopeammin kuin me hänet. Hän on vain viidakon elukka, neiti
Porter."
Tyttö ei vastannut, mutta tunsi sydämensä ikäänkuin kutistuvan.
Rakastettuamme kohtaan osoitettu viha ja vaino terästää
mieltämme, mutta halveksiminen ja sääli tekee meidät vaiteliaiksi ja
hävettää.
Hän tiesi, että Clayton puhui vain, mitä ajatteli, ja ensi kerran hän
alkoi miettiä, mistä johtui hänen uusi rakkautensa, ja arvostellen
muistella pelastajaansa.
Hitaasti hän kääntyi ja palasi majaan. Hän koetti kuvitella
metsäläistänsä rinnallaan valtamerilaivan salongissa ja oli
näkevinään, kuinka mies söi käsistään, repi ruokaansa kuin villieläin
ja pyyhki rasvaisia sormiaan reisiinsä. Häntä värisytti.
Sitten hän kuvitteli esittelevänsä tuota villiä ystävilleen, kömpelöä,
sivistymätöntä raakalaista, ja se ajatus suorastaan kouristi sydäntä.

Nyt hän oli tullut omaan soppeensa ja istui saniais- ja
ruohovuoteensa reunalla, painaen kättään aaltoilevaa poveansa
vasten. Silloin hän tunsi Tarzanilta saamansa medaljongin puseronsa
alla.
Hän otti sen esiin, piti sitä hetken aikaa kämmenellään ja katseli
sitä kyyneleet silmissä. Sitten hän nosti sen huulilleen, hautasi
kasvonsa pehmeihin sananjalkoihin ja nyyhkytti:
"Eläinkö! Hyvä Jumala, tee sitten minutkin eläimeksi. — Ihminen
tai eläin, minä rakastan sinua…"
Sinä päivänä hän ei enää nähnyt Claytonia. Esmeralda toi hänelle
illallista, ja hän lähetti sanan isälleen, että tunsi itsensä rasittuneeksi
seikkailujensa jälkeen.
Seuraavana aamuna lähti Clayton varhain retkikunnan mukana,
jonka oli määrä etsiä luutnantti D'Arnotia. Tällä kertaa kuului
joukkoon kaksisataa aseistettua miestä, kymmenen upseeria, kaksi
välskäriä, ja muonaa otettiin mukaan viikon ajaksi.
Heillä oli myöskin huopapeitteitä ja riippumattoja; jälkimmäisissä
voitaisiin kuljettaa sairaita ja haavoittuneita.
Kaikkia elähdytti taistelunhalu ja vimma. Nyt ei menty ainoastaan
pelastamaan, vaan tahdottiin myös kostaa. Edellisen retkikunnan
taistelupaikalle saavuttiin vähän puolenpäivän jälkeen, sillä he
kulkivat tuttua tietä eivätkä menettäneet aikaa tiedusteluun. Sieltä
he marssivat elefanttien polkua myöten suoraan Mbongan kylälle.
Kello oli vasta kaksi, kun etujoukko pysähtyi raivatun alan reunalle.

Luutnantti Charpentier, joka komensi joukkoa, lähetti osan
miehistään viidakon läpi kylän vastakkaiselle puolelle. Toinen osasto
jäi portin kohdalle, ja hän itse pääjoukon kera asettui kylän
eteläpuolelle.
Sitä ennen oli sovittu, että pohjoispuolelle menevä joukko, joka
viimeksi ehtisi perille, aloittaisi taistelun, ja että heidän ensimmäiset
laukauksensa olisivat merkkinä yleiseen hyökkäykseen kaikilta
tahoilta, jotta kylä vallattaisiin heti väkirynnäköllä.
Puolisen tuntia sai luutnantti Charpentierin joukko odotella
merkkiä tiheässä viidakossa. Minuutit tuntuivat heistä tunneilta. He
saattoivat nähdä, kuinka alkuasukkaat puuhailivat pelloilla ja toiset
liikkuivat edestakaisin kylän portista.
Vihdoin kuului merkki — rätiseviä kiväärinlaukauksia, ja heti tuli
samanlainen vastaus viidakosta lännen ja etelän puolelta. Pelloilla
työskentelevät mustat heittivät työkalut käsistään ja ryntäsivät kuin
hullut varustuksen suojaan. Ranskalaiset luodit pyyhkäisivät heidät
maahan, ja matruusit juoksivat heidän ruumiittensa yli porttia kohti.
Niin äkillinen oli valkoisten hyökkäys, että he pääsivät portille,
ennenkuin kauhistuneet alkuasukkaat ehtivät sitä teljetä, ja
seuraavassa hetkessä vilisi kylän raitilla aseellisia miehiä hurjassa
käsikähmässä sekasortoisen mustan joukon kanssa.
Hetken aikaa jaksoivat alkuasukkaat pitää puoliaan, mutta
ranskalaisten revolverit, kiväärit ja miekat surmasivat heidän
keihäsmiehensä, ja mustat jousimiehet kaatuivat ennenkuin olivat
edes ehtineet sovittaa nuolia paikoilleen.

Pian kehittyi taistelu villiksi riehunnaksi ja sitten armottomaksi
verilöylyksi, sillä ranskalaiset matruusit olivat nähneet D'Arnotin
univormun palasia eräiden mustien soturien yllä. He säästivät lapset
ja naiset, joita ei tarvinnut itsepuolustukseksi tappaa, mutta kun he
vihdoin hengästyneinä, verentahraamina, hikisinä lakkasivat
työstänsä, johtui se vain siitä, ettei heitä vastassa enää ollut
ainoatakaan soturia koko Mbongan kylässä.
Huolellisesti he tutkivat joka majan ja sopen, mutta D'Arnotista ei
tavattu jälkeäkään. Merkkien avulla he tiedustelivat asiaa vangeilta,
ja vihdoin muuan matruuseista, joka oli palvellut Ranskan Kongossa,
huomasi voivansa saada heidät ymmärtämään erästä sekakieltä, jota
valkoiset ja rannikolla asustavat rappeutuneet heimot keskenään
käyttivät, mutta sittenkään he eivät saaneet mitään varmaa tietoa
D'Arnotin kohtalosta. Vastaukseksi kysymyksiin tuli vain kiihkeitä
liikkeitä ja pelon ilmeitä, ja vihdoin täytyi heidän uskoa, että nämä
merkit todistivat mustien paholaisten syyllisyyttä, ja että heidän
toverinsa oli kaksi yötä takaperin täällä teurastettu ja syöty.
Vihdoin he luopuivat kaikesta toivosta ja valmistautuivat
leiriytymään yöksi kylään. Vangit suljettiin kolmeen majaan, joissa
heitä pidettiin ankarasti silmällä. Teljetylle portille pantiin vartijoita,
ja sitten kylä vaipui uneen ja hiljaisuuteen, jota ei häirinnyt muu kuin
mustien naisten ruikutus heidän surressaan kuolleitansa. —
Seuraavana aamuna lähdettiin paluumatkalle. Ensin aiottiin polttaa
kylä, mutta siitä ajatuksesta luovuttiin, ja vangit saivat jäädä
kotikyläänsä itkemään ja valittamaan, kuitenkin katto yllään ja aitaus
turvanaan viidakon petoja vastaan.
Verkalleen retkikunta palasi edellisen päivän jälkiä myöten.
Kymmenen täyteen kuormitettua riippumattoa hidastutti kulkua.

Kahdeksassa niistä makasivat vaikeammin haavoittuneet, kahdessa
kannettiin kuollutta.
Clayton ja luutnantti Charpentier marssivat viimeisinä.
Englantilainen oli vaiti kunnioituksesta toisen surua kohtaan, sillä
D'Arnot ja Charpentier olivat poikuudesta asti olleet läheisiä
ystävyksiä.
Clayton saattoi hyvin ymmärtää, että ranskalaisen suru oli sitä
katkerampi, kun D'Arnotin uhraus oli ollut turha, koska Jane Porter
oli pelastettu, ennenkuin D'Arnot oli joutunut villien käsiin. Ja olihan
hän lisäksi menettänyt henkensä ulkopuolella varsinaista
velvollisuuttaan, uhrannut itsensä vieraan ja ulkomaalaisen vuoksi.
Mutta kun Clayton huomautti siitä luutnantti Charpentierille, pudisti
tämä päätänsä.
"Ei, hyvä herra", sanoi hän, "D'Arnot olisi itse halunnut kuolla tällä
tavalla. Suren vain sitä, etten saanut kuolla hänen sijastaan tai
ainakin hänen kanssaan. Toivoisin, että olisitte tutustunut häneen
paremmin. Hän oli todella upseeri ja herrasmies, jollainen nimitys
annetaan monelle, mutta jonka vain harvat ansaitsevat. Hän ei
kuollut turhaan, sillä hänen kuolonsa amerikkalaisen tytön vuoksi
rohkaisee meitä, hänen tovereitaan, urheasti kohtaamaan kuolemaa,
milloin tahansa se meidät yllättää."
Clayton ei vastannut, mutta hänessä heräsi yhä suurempi
kunnioitus ranskalaisia kohtaan, eikä se tunne hänestä sitten
milloinkaan haihtunut.
Oli jo hyvin myöhä, kun he saapuivat majalle. Yksi ainoa laukaus
ennen heidän tuloansa esille viidakosta oli ilmoittanut sekä leirissä
että laivassa olijoille, että retkikunta oli ehtinyt liian myöhään perille.

Oli nimittäin sovittu ennakolta, että kun he tulisivat kilometrin tai
parin päähän leiristä, yksi laukaus merkitsisi epäonnistumista, kolme
onnistumista, kaksi taas merkitsisi, etteivät he olleet löytäneet
jälkeäkään D'Arnotista eikä hänen mustista vangitsijoistaan.
Totinen oli siis se seura, joka odotti heidän tuloaan, ja vain
muutamia sanoja vaihdettiin, ennenkuin kuolleet ja haavoittuneet
varovasti sijoitettiin veneisiin, jotka ääneti soudettiin risteilijää kohti.
Clayton oli väsynyt viiden päivän rasittavista marsseista viidakon
puhki ja kahdesta ottelusta mustien kanssa, ja lähti majalle
saadakseen hiukan ruokaa ja päästäkseen verrattain mukavalle
ruohovuoteelle lepäämään, vietettyään kaksi yötä taivasalla.
Ovella seisoi Jane Porter.
"Kuinka on käynyt luutnantti-raukan?" kysyi hän. "Löysittekö
hänestä mitään jälkiä?"
"Tulimme liian myöhään, neiti Porter", vastasi Clayton alakuloisesti.
"Kertokaa! Mitä on tapahtunut?"
"En voi, neiti Porter, se oli liian kauheaa."
"Ette kai tarkoita, että he olivat kiduttaneet häntä?" kuiskasi tyttö.
"Emme tiedä, mitä he tekivät, ennenkuin tappoivat hänet", vastasi
Clayton kasvot vääristyneinä väsymyksestä ja siitä surusta, jota hän
tunsi ajatellessaan onnettoman upseerin kohtaloa. Sanalle
"ennenkuin" hän pani erikoisen koron.

"Ennenkuin tappoivat hänet! Mitä tarkoitatte? Eiväthän he…?
Eiväthän he sentään liene…?"
Hän muisti äkkiä, mitä Clayton oli sanonut metsäläisen
mahdollisesta sukulaisuudesta tämän heimon kanssa, eikä voinut
saada kamalaa sanaa huuliltaan.
"Niin, neiti Porter, he olivat — ihmissyöjiä", sanoi Clayton melkein
katkerasti, sillä hänenkin mieleensä oli tullut metsäläinen, ja
merkillinen järjetön mustasukkaisuus, jota hän oli tuntenut pari
päivää aikaisemmin, valtasi hänet jälleen.
Sitten hän jatkoi äkillisen töykeästi, mikä oli yhtä vähän hänen
tapaistaan kuin apinalta voi odottaa hienotunteisuutta.
"Kun teidän metsänhaltianne jätti teidät, oli hänellä varmaankin
kova kiire päästä mässäyksestä osalliseksi."
Hän katui sanojaan, ennenkuin oli edes ehtinyt lauseensa loppuun,
mutta sittenkään hän ei tiennyt, kuinka julmasti se loukkasi tyttöä.
Sillä oikeastaan hän katui kiittämättömyyttään sitä miestä kohtaan,
joka oli pelastanut hänet ja kaikkien hänen seuralaistensa hengen
eikä milloinkaan tehnyt heille pahaa.
Tyttö nosti päätään.
"Teidän väitteeseenne on vain yksi sopiva vastaus, herra Clayton",
sanoi hän jäätävän kylmästi, "ja minä surkuttelen, etten ole mies
voidakseni sen antaa". Samassa hän kääntyi nopeasti ja astui
majaan.
Clayton oli englantilainen ja siksi tyttö ehti näkyvistä, ennenkuin
hänelle kylliksi selvisi, miten mies olisi vastannut.

"Totta vieköön", mutisi hän onnettoman näköisenä, "hän sanoi
minua valehtelijaksi. Ja luulenpa sen hyvin ansainneeni", lisäsi hän
miettivästi. "Kuules, Clayton, vanha veikko, tiedän kyllä, että olet
väsynyt ja lamaantunut, mutta ei sinun silti tarvitse käyttäytyä
nautamaisesti. Parasta on, että nyt lähdet makuulle."
Sitä ennen hän kutsui Jane Porteria purjekangasseinän toiselta
puolelta, sillä hän halusi pyytää anteeksi, mutta yhtä hyvin hän olisi
voinut odottaa vastausta sfinksiltä. Silloin hän kirjoitti
anteeksipyyntönsä paperilipulle ja työnsi sen väliseinän alitse.
Jane Porter näki kirjelapun, mutta ei tahtonut olla siitä
tietääkseen, sillä hän oli hyvin vihastunut ja loukkaantunut. Kun hän
kuitenkin oli nainen, otti hän sen vihdoin kuin sattumalta käteensä ja
luki:
Hyvä neiti Porter.
Minulla ei ollut mitään syytä lausua sitä vihjaustani.
Ainoana puolusteluna voisi minulla olla se, että hermoni eivät
liene kunnossa, mutta sekään ei ole mikään syy.
    Olkaa hyvä ja koettakaa ajatella, etten ole sitä sanonutkaan.
    Olen hyvin pahoillani. Teitä olisin kaikkein vähimmin tahtonut
    loukata. Sanokaa, että annatte anteeksi.
W:m Cecil Clayton.
— Hän ajatteli sitä, sillä muuten hän ei olisi sitä sanonut, —
päätteli tyttö. — Mutta se ei voi olla totta — oh, minä tiedän, että se
ei ole totta!

Muuan kirjeen lauseista peloitti häntä: "Teitä olisin kaikkein
vähimmin tahtonut loukata."
Viikko takaperin olisi tämä lause täyttänyt hänet ilolla; nyt se teki
hänet vain alakuloiseksi.
Hän toivoi, ettei koskaan olisi tavannut Claytonia. Hän oli
pahoillaan, että ollenkaan oli nähnyt metsäläisen — eipä, hyvillään
hän siitä oli. Ja hänen mieleensä muistui toinen kirje, jonka hän oli
löytänyt ruohikosta majan edustalta päivää myöhemmin kuin oli
palannut viidakosta — Apinain Tarzanin rakkauskirje.
Kukahan tämä uusi kosija oli? Jos hän oli tämän kauhean
metsäseudun asukkaita, niin mihin hän ryhtyisikään saadakseen
hänet omakseen…
"Esmeralda! Herää!" huusi hän. "Minua ärsyttää, että sinä voit
nukkua noin rauhallisesti, vaikka tiedät, että maailma on surua
täynnä…"
"Ai taivas!" kiljahti Esmeralda nousten istumaan. "Mitäs nyt?
Petoko!
Missä on se, miss Jane?"
"Loruja, Esmeralda, ei hätää mitään. Nuku vain uudelleen. Siinä on
jo kylliksi kiusaa, kun nukut, mutta vielä pahempaa on kun olet
valveilla."
"Vai niin, kultaseni, mutta mitä varten tarvitsee olla niin
kummallinen kuin pikku neiti on ollut koko illan?"
"Oh, Esmeralda, minä olen niin pahalla päällä tänä iltana", vastasi
tyttö. "Älä nyt välitä minusta."

"Enkä välitä, mutta pankaa nyt vain maata. Hermot on pilalla, ja
kyllä tässä hermostuukin, kun ajattelee kaikkia niitä petoja ja
ihmissyöjiä, joista massa Philander on kertonut. Eihän olisikaan
mikään kumma, jos tulisimme kaikki kipeiksi."
Jane Porter naurahti, astui pikku huoneen poikki suutelemaan
mustaa uskollista poskea ja sanoi hänelle hyvää yötä.

KOLMASKOLMATTA LUKU
Ihmisten veljestyessä
Kun D'Arnot heräsi tajuihinsa, huomasi hän makaavansa
pehmoisella saniais- ja ruohovuoteella A:n muotoisessa teltassa, joka
oli kyhätty oksista.
Oviaukossa hän näki vihreän nurmikon ja vähän matkan päässä
viidakon tiheän seinän.
Hän oli perin nääntynyt, runneltu ja heikko ja tunsi vihlovaa tuskaa
monista haavoistaan, samalla kun kaikkia jäseniä ja lihaksia särki
kestetyn kidutuksen jälkeen.
Pelkkä pään kääntäminen tuotti hänelle sellaista kärsimystä, että
hän pitkän aikaa makasi hiljaa silmät ummessa. Hän koetti muistella
seikkailunsa yksityiskohtia, ennenkuin oli menettänyt tajuntansa,
voidakseen keksiä missä hän nyt oli — ystävienkö vai vihollisten
luona?
Vihdoin hänen mieleensä palasi hirvittävä näytelmä paalun luona
ja sitten myös se outo, valkeaihoinen olento, jonka käsiin hän oli

pyörtynyt.
D'Arnot pohti itsekseen, mikä kohtalo häntä nyt odotti. Hän ei
voinut nähdä tai kuulla mitään elonmerkkejä ympärillään. Viidakon
lakkaamaton pauhu — miljoonien lehtien suhina — lintujen ja
marakattien äänet tuntuivat sulautuvan ihmeelliseksi, nukuttavaksi
säveleksi, ikäänkuin hän itse olisi ollut kaukana tästä alinomaa
hyörivästä elämästä, jonka äänet vain heikkona kaikuna ulottuivat
hänen korviinsa.
Vihdoin hän vaipui rauhalliseen uneen, josta vasta iltapäivällä
heräsi. Taas hän tunsi samaa sekaannusta kuin ensi kertaa
herätessään, mutta pian hän sitten muisti selvästi, mitä oli
tapahtunut, ja katsoessaan ulos teltan oviaukosta hän näki miehen
istuvan kyyryssä edessään.
Leveä, vahvalihaksinen selkä oli häneen päin kääntynyt, ja vaikka
se olikin auringon paahtama, huomasi D'Arnot kuitenkin heti, että se
oli valkoisen miehen selkä, ja hän kiitti Jumalaa sydämessään.
Ranskalainen äännähti heikosti. Mies kääntyi, nousi ja astui
majalle päin. Hänen kasvonsa olivat hyvin kauniit — kauniimmat kuin
D'Arnot luuli koskaan nähneensä.
Outo mies kyyristyi, konttasi telttaan haavoitetun upseerin viereen
ja laski viileän käden hänen otsalleen.
D'Arnot puhui hänelle ranskaksi, mutta mies vain pudisti päätänsä
— surunvoittoisesti, kuten ranskalaisesta näytti.
Sitten D'Arnot koetti englantia, mutta vastaukseksi tuli nytkin vain
päänpudistus. Italialla, espanjalla ja saksalla ei saatu sen parempaa

tulosta. D'Arnot osasi muutamia sanoja norjaa, venäjää ja kreikkaa,
ja sitäpaitsi hänellä oli vähän aavistusta eräästä länsirannikon
neekerikielestä — mutta mies ei ymmärtänyt niistäkään mitään.
Tarkastettuaan D'Arnotin haavoja mies katosi teltalta. Puolen
tunnin kuluttua hän palasi tuoden hedelmiä ja vettä, jota hänellä oli
kurpitsan tapaisessa kuoressa.
D'Arnot joi ja söi vähäsen. Häntä kummastutti, ettei hänellä ollut
kuumetta. Taas hän koetti keskustella oudon hoitajansa kanssa,
mutta turhaa se oli.
Äkkiä mies kiirehti ulos teltalta ja palasi parin minuutin päästä
mukanaan kaarnanpalasia ja — voi ihmeiden ihmettä! — lyijykynä.
Kyyristyen D'Arnotin viereen hän kirjoitti kaarnan sileälle
sisäpinnalle muutaman sanan ja ojensi sen sitten ranskalaiselle.
D'Arnot hämmästyi nähdessään selvillä paino kirjaimilla piirrettyinä
seuraavat englanninkieliset sanat:
"Olen Apinain Tarzan. Kuka te olette? Osaatteko tätä kieltä?"
D'Arnot tarttui kynään, mutta pysähtyi sitten. Tämä vieras mies
kirjoitti englantia — ilmeisesti hän oli siis englantilainen.
"Kyllä", vastasi hän, "minä ymmärrän englantia. Puhun sitä
myöskin. Nyt voimme keskustella. Sallikaa minun ensiksi kiittää
kaikesta, mitä olette tehnyt hyväkseni."
Mies vain pudisti päätänsä ja osoitti lyijykynää ja kaarnanpalasta.

"Mon dieu!" huudahti D'Arnot. "Jos olette englantilainen, kuinka
ette osaa puhua englantia?"
Sitten hänen mieleensä välähti selitys tähän ilmiöön: tuo mies oli
mykkä, ehkäpä kuuromykkäkin.
Hän otti siis kaarnan käteensä ja kirjoitti englanniksi:
"Olen Paul D'Arnot, luutnantti Ranskan laivastossa. Kiitän teitä
siitä, mitä olette tehnyt hyväkseni. Olette pelastanut henkeni, ja olen
siis iäti teille kiitollisuudenvelassa. Saanko kysyä, mistä johtuu, että
kirjoitatte englantia, mutta ette puhu sitä?"
Tarzanin vastaus kummastutti D'Arnotia vielä enemmän:
"Puhun vain oman kansani kieltä — Kertshakin suurten apinain
kieltä, myös vähän Tantorin, elefantin, ja Numan, leijonan, ja
ymmärrän muiden viidakon eläjien puhetta. Kenenkään ihmisolennon
kanssa en ole milloinkaan puhunut, paitsi kerran merkkikieltä Jane
Porterin kanssa. Nyt keskustelen ensi kertaa oman rotuni jäsenen
kanssa kirjoitettujen sanojen avulla."
D'Arnot ei tiennyt, mitä tästä piti ajatella. Tuntui uskomattomalta,
että maan päällä oli täysikasvuinen mies, joka ei ollut milloinkaan
puhunut lähimmäisensä kanssa, ja vielä mahdottomampaa oli, että
sellainen mies osasi lukea ja kirjoittaa.
Hän katsahti taas Tarzanin kirjoitukseen… "Paitsi kerran
merkkikieltä Jane Porterin kanssa." Tämähän oli se amerikkalainen
tyttö, jonka gorilla oli vienyt viidakkoon.
Äkkiä alkoi asia selvitä D'Arnotille — tämä mies oli siis tuo "gorilla".
Hän tarttui kynään ja kirjoitti:

"Missä on Jane Porter?"
Ja Tarzan kirjoitti sen alapuolelle:
"Seuralaistensa luona Apinain Tarzanin majassa."
"Hän ei siis ole kuollut? Missä hän kävi? Mitä hänelle tapahtui?"
"Hän ei ole kuollut. Terkoz vei hänet ottaakseen hänet
vaimokseen, mutta Apinain Tarzan otti hänet pois Terkozilta ja tappoi
Terkozin, ennenkuin hän ehti tehdä pahaa valkoiselle naiselle.
Kukaan ei koko viidakossa voi tapella Apinain Tarzanin kanssa ja
jäädä eloon. Minä olen Apinain Tarzan — mahtava taistelija."
D'Arnot kirjoitti:
"Olen iloissani, että se tyttö on pelastunut. Tunnen kipua, kun
kirjoitan. Lepään vähän."
Tarzan taas:
"Niin, levätkää. Kun parannutte, vien teidät seuralaistenne luo."
Monta päivää D'Arnot makasi pehmeällä saniaisvuoteellaan.
Toisena päivänä tuli kuume; D'Arnot luuli, että se tiesi
verenmyrkytystä, josta olisi seurauksena kuolema.
Silloin hänen mieleensä juolahti muuan asia, ja häntä ihmetytti,
ettei hän aikaisemmin ollut tullut sitä ajatelleeksi.
Hän kutsui luokseen Tarzanin ja ilmoitti merkeillä haluavansa
kirjoittaa, ja kun Tarzan oli tuonut kaarnanpalan ja kynän, kirjoitti
D'Arnot:

"Voitteko mennä seuralaisteni luo ja tuoda heidät tänne? Panen
mukaanne kirjeen ja he seuraavat teitä tänne."
Tarzan pudisti päätään, otti kaarnan ja kirjoitti:
"Ajattelin sitä itsekin — ensi päivinä, mutta en uskaltanut. Suuret
apinat tulevat usein tänne päin, ja jos ne löytäisivät teidät täältä
haavoittuneena ja yksin, tappaisivat ne teidät."
D'Arnot kääntyi kyljelleen ja sulki silmänsä. Hän ei tahtonut kuolla,
mutta hän tunsi, että näin kävisi, sillä kuume yhä nousi. Seuraavana
yönä hän menetti tajuntansa.
Kolme vuorokautta hän houraili, ja Tarzan istui hänen vierellään,
kostutti hänen päätänsä ja käsiään ja pesi hänen haavojaan.
Neljäntenä päivänä kuume taukosi yhtä äkkiä kuin oli tullutkin,
mutta D'Arnot oli nyt vain varjo entisestään ja hyvin heikko. Tarzanin
täytyi kohottaa häntä hänen juodessaan kurpitsasta.
Kuume ei ollut tullut verenmyrkytyksestä, kuten D'Arnot oli luullut,
vaan oli sitä laatua, joka tavallisesti ahdistaa Afrikan viidakkoon
joutuneita valkoisia, joko vieden hengen tai hellittäen yhtä nopeasti
kuin nyt D'Arnotin oli käynyt.
Pari päivää myöhemmin D'Arnot jaksoi jo hoippuen kävellä ulkona
Tarzanin väkevän käden tukiessa ja estäessä häntä kaatumasta.
He olivat istuutuneet korkean puun siimekseen ja Tarzan haki
kaarnanpalasia, että voisivat keskustella.
D'Arnot kirjoitti ensin:

"Kuinka voin palkita teille kaiken sen, mitä olette tehnyt
hyväkseni?"
Ja Tarzan vastasi: "Opettakaa minulle ihmisten kieltä".
D'Arnot aloitti heti, osoittaen läheisiä esineitä ja sanoi niiden nimet
ranskaksi, sillä hän arveli olevan helpointa opettaa Tarzanille tätä
kieltä, jota hän parhaiten osasi.
Tarzanille se tietysti oli samantekevää, sillä hän ei osannut erottaa
toista kieltä toisesta. D'Arnot osoitti sanaa homme, jonka oli
painokirjaimin kirjoittanut kaarnanpalaselle, ja neuvoi hänelle, kuinka
se äännettiin, selittäen sen merkitsevän ihminen, ja samoin
opetettiin singe, apina, ja arbe, puu.
Tarzan oli hyvin innokas oppilas, ja parin päivän perästä hän jo
osasi sen verran ranskaa, että saattoi sommitella pikku lauseita,
kuten: "Tuo on puu", "Tämä on ruoho", "Minun on nälkä", ynnä
muuta sen tapaista; mutta D'Arnot huomasi pian, että hänelle oli
vaikeata opettaa ranskankielen lauserakennusta, kun hän oli saanut
alkutietonsa englanninkielestä. Ranskalainen kirjoitti hänelle
englanninkielellä pieniä lauseita ja pyysi häntä kääntämään ne
ranskaksi, mutta kun sananmukainen käännös tavallisesti oli hyvin
heikko, tunsi Tarzan peräti sekaantuvansa.
D'Arnot käsitti nyt tehneensä virheen, mutta liian myöhäistä oli
perääntyä, ryhtyä opetukseen alusta ja saada Tarzan unohtamaan
kaikki oppimansa, etenkin kun oli pian tulossa se aika, jolloin he
voisivat sujuvasti puhella keskenään.
Kolme päivää kuumeen hellittämisen jälkeen Tarzan kirjoitti ja
kysyi D'Arnotilta, tuntisiko tämä itsensä kyllin vahvaksi, jotta hänet

voisi kantaa takaisin majalle. Tarzan oli yhtä innokas lähtemään
täältä kuin D'Arnotkin, sillä hän kaipasi Jane Porteria.
Juuri tästä syystä hänen oli ollut hyvin vaikeata pysytellä
ranskalaisen luona kaikki nämä päivät, ja että hän sittenkin oli näin
uhrautunut, se oli vielä selvempi todistus hänen luonteensa
jaloudesta kuin hänen urotyönsä ranskalaisen upseerin
pelastamisesta Mbongan kynsistä.
D'Arnot oli hyvin innokas yrittämään retkeä ja kirjoitti:
"Mutta ettehän voi kantaa minua koko matkaa vaikeakulkuisen
metsän läpi."
Tarzan nauroi.
"Mais oui", vastasi hän, ja D'Arnot nauroi ääneensä kuullessaan
Tarzanin huulilta nämä sanat, joita hän itse niin usein käytti.
Niin he lähtivät, ja nyt sai D'Arnot, kuten ennen häntä Clayton ja
Jane Porter, ihmetellä apinamiehen merkillistä voimaa ja nopeutta.
Iltapäivällä he saapuivat aukealle alalle, ja kun Tarzan viimeisen
puun oksilta laskeutui maahan, sykki hänen sydämensä rajusti
ajatuksesta, että hän kohta saisi taas nähdä Jane Porterin.
Majan ulkopuolella ei näkynyt ketään, ja D'Arnot hämmästyi
nähdessään, etteivät risteilijä ja Arrow enää olleet ankkurissa
lahdella.
Yksinäisyyden tunne alkoi painostaa molempia miehiä heidän
lähestyessään majaa. Kumpikaan ei puhunut, mutta he aavistivat jo
ennen oven avaamista, mikä heitä sen takana odotti.

Tarzan nosti salvan ja työnsi ison oven auki. Asian laita oli, niinkuin
he olivat pelänneet. Maja oli autio.
Miehet silmäsivät toisiaan. D'Arnot totesi, että hänen
kansalaisensa luulivat häntä kuolleeksi, mutta Tarzan ajatteli vain sitä
naista, joka rakkaudesta oli suudellut häntä ja nyt oli lähtenyt hänen
luotansa sillä välin kun hän hoiti D'Arnotia.
Suuri katkeruus tulvahti hänen sydämeensä. Hänen teki mieli
lähteä tiehensä kauas viidakkoon ja taas liittyä heimoonsa. Koskaan
hän ei enää tahtonut nähdä oman rotunsa jäseniä eikä myöskään
enää käydä tässä majassa. Iäksi hän jättäisi hautomansa toiveet
löytää oma rotunsa ja päästä ihmiseksi ihmisten joukkoon.
Entä ranskalainen? D'Arnot? Kuinka hänen kävisi? Hän saisi tulla
toimeen, niinkuin Tarzaninkin oli täytynyt? Tarzan ei tahtonut häntä
enää nähdä. Hän tahtoi vain päästä irti kaikesta, mikä muistutti
hänelle Jane Porterista.
Tarzanin seisoessa kynnyksellä mietteissään oli D'Arnot astunut
sisään. Sinne oli jätetty yhtä ja toista. Hän tunsi monta kapinetta
risteilijältä — kenttäuunin, muutamia keittokaluja, kiväärin,
ammuksia, säilykkeitä, huopapeittoja, kaksi tuolia, riippumaton sekä
useita kirjoja ja aikakauslehtiä, enimmäkseen amerikkalaisia.
— Varmaankin he ovat aikoneet palata, — arveli D'Arnot.
Hän astui pöydän luo, jonka John Clayton oli niin monta vuotta
takaperin kirveellä valmistanut, ja näki sillä kaksi kirjettä, jotka oli
osoitettu Apinain Tarzanille.

Toinen oli kirjoitettu vahvalla miehekkäällä käsialalla ja
sinetöimätön. Toinen, naisen käsialalla kirjoitettu, oli sinetöity.
"Täällä on teille kaksi kirjettä, Apinain Tarzan", sanoi D'Arnot ja
kääntyi ovelle päin, mutta siellä ei enää näkynyt hänen toveriansa.
D'Arnot kiirehti ulos katsomaan. Tarzania ei näkynyt missään. Hän
huusi kovaa, mutta vastausta ei kuulunut.
"Mon dieu!" huudahti D'Arnot, "hän on jättänyt minut. Siltä
minusta tuntuu. Hän on palannut viidakkoonsa ja jättänyt minut
yksin tänne."
Sitten hän muisti Tarzanin kasvojen ilmeen, kun he huomasivat
majan tyhjäksi — saman ilmeen, jonka metsästäjä näkee
ilkivaltaisesti haavoittamansa hirven silmissä.
Tarzan oli pahasti loukkaantunut — D'Arnot tajusi sen nyt — mutta
miksi? Hän ei voinut ymmärtää.
Ranskalainen katseli ympärilleen. Kauhea yksinäisyyden tunne
alkoi rasittaa hänen hermojaan, jotka jo ennestään olivat
heikontuneet kidutuksista ja sairaudesta.
Jäädä tänne hirveän viidakon naapuruuteen — koskaan saamatta
kuulla ihmisääntä tai nähdä ihmiskasvoja — aina pelätä ja varoa
villieläimiä ja mitä villimpiä ihmisiä — autius ja toivottomuus ainoina
tovereina, se oli kauheaa!
Sillä välin Tarzan samosi puita pitkin kauas itään heimonsa luo.
Milloinkaan ennen hän ei ollut pitänyt niin kiirettä. Hänestä tuntui
kuin hän pakenisi omaa itseään, näin metsän läpi pelästyneen
oravan lailla syöksyen pääsisi eroon omista ajatuksistaan. Mutta

vaikka hän olisi kuinka kiirehtinyt, niin aina seurasivat häntä samat
ajatukset kuin varjo.
Hän sivuutti Saborin, naarasleijonan, jäntevän ruumiin, joka oli
matkalla vastakkaiseen suuntaan. — Majalle se menee, — ajatteli
Tarzan.
Mitä D'Arnot voisi Saborille — tai jos Bolgani, gorilla, hyökkäisi
hänen kimppuunsa — tai Numa, urosleijona, tai julma Shita?
Äkkiä Tarzan pysähtyi.
"Mikä sinä olet, Tarzan?" kysyi hän ääneen. "Apina vai ihminen?
Jos olet apina, niin teet kuin apinat — jätät heimolaisesi kuolemaan
viidakkoon, kun oikkusi käskee sinua menemään muuanne. Jos olet
ihminen, niin palaat suojelemaan omaa vertasi. Ethän voi karata
oman kansasi luota senvuoksi, että yksi heistä on lähtenyt sinun
luotasi." —
D'Arnot sulki majan oven. Hän oli hyvin hermostunut. Rohkeitakin
miehiä — ja D'Arnot oli rohkea — pelottaa toisinaan erämaan
yksinäisyys.
Hän latasi kiväärin ja pani sen ulottuvilleen. Sitten hän meni
pöydän luo ja otti käteensä Tarzanille osoitetun sinetöimättömän
kirjeen.
Hänen mielestään siinä voisi olla ilmoitus, että hänen väkensä oli
vain väliaikaisesti lähtenyt jonnekin, eikä hän näissä oloissa pitänyt
vääränä lukea tätä kirjettä. Hän otti sen siis kuoresta ja luki.
Apinain Tarzanille.

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Sonochemistry Voume 2 Applications And Developments Mason Tj

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