8._Aromaticity,_Substitution_Reactions Nadir.pdf

Aromaticity

1. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 1
Aromaticity
•Theterm“Aromatic”wasinitiallyassociatedwithbenzene
•Nowlinkedwithcycliccompoundshavingchemicalpropertiessimilarto
benzene
•Thecharacteristicbehaviorexhibitedbyaromaticcompoundsiscalled
“Aromaticity”
•Propertyofsp
2
hybridizedplanarring
•p-orbitaloneachatomallowcyclicdelocalizationofπ-electrons
•Aromaticcompoundsexhibithighdegreeofunsaturationandundergo
substitutionreactionsratherthanaddition
•Haveextrastability(resonanceenergy)
Dr Ahmad Raza 1
Aromaticity
•Thearomaticsystempossessclosedsystemofπ-electrons
Huckle’srule
•GivenbyHucklein1931formonocycliccompounds
•Cyclicsystemhaving4n+2delocalizedπ-electronsexhibitaromaticcharacter
Wheren=0,1,2,3,4,5……etc.
•Ingeneralplanarcyclicstructureswith2,6,10,14,18,22etc.willbearomatic
n=0weget(4×0)+2=2 n=1weget(4×1)+2=6
n=2weget(4×2)+2=10 n=3weget(4×3)+2=14
Note:Thesystemcanalsobeanion
Dr Ahmad Raza 2
Aromaticity
The necessary conditions for aromaticity
•Foracompoundtobearomatic,itmustfulfillfollowingrequirements:
1.Musthavecyclicandplanarstructure
2.Eachatomofringmustbesp
2
hybridized
3.MustobeyHuckle’srule
Examples:HH
Non-aromaticH Aromatic
•Althoughcyclopropaneiscyclic,
planarandhave2pielectrons,yetis
non-aromatic
•Itdoesnothavedelocalizationofpi
electrons
CyclopropeneCyclopropenylcationDr Ahmad Raza 3
Aromaticity
Examples:…………
PyrroleN
H O FuranS ThiopheneN
Pyridine BenzeneH H
1,3-cyclopentadiene
Non-aromaticH
1,3-cyclopentadienyl cation
Non-aromaticH
1,3-cyclopentadienyl anion
aromatic
Dr Ahmad Raza 4

2. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 2
Aromaticity
Examples:…………
Cycloheptatriene
Dr Ahmad Raza 5
Aromaticity
Examples:…………
Naphthalene
aromatic
Cyclodecapentaene
Non-aromatic Not planar
cyclooctatetraene
Non-aromatic
Dr Ahmad Raza 6
Aromaticity
Annulenes
•monocycliccompoundscontainingalternatingringdoublebonds,
suchasbenzene,butofdifferentsizes.
•Accordingtosystematicnomenclature,cyclobutadieneis
[4]annulene,benzeneisa[6]annulene,andcyclooctatetraeneisan
[8]annulene
•Digitwithinbracketsindicateno.ofpielectrons
•[10]-Annulenewasexpectedtobearomaticbutisnon-aromaticdueto
ringstraine.g.Cyclodecapentaene
•Higherannulnesi.e.14,18etcarearomatic
Dr Ahmad Raza 7
Electrophilic substitution
•Theexposedpielectronsofbenzenearesusceptibletoelectrophilic(E
+
)
attack,henceundergoeselectrophilicaromaticsubstitution
•Thereactionsinwhichaproton(H
+
)ofaromaticringissubstitutedbyan
elctrophile(E
+
)iscalledaselectrophilicaromaticsubstitution
Mechanism
•Inthefirststep,anelectrophile(E
+
)attackstheπ-electronsofbenzenering
•Acarbocation(areniumionorσ-complex)isgeneratedwhichisdelocalized
overthering
•Thiscarbocationisnon-aromatic(has4π-electrons)
1.Step one
Dr Ahmad Raza 8

3. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 3
Electrophilic substitution
Mechanism…….
•E
+
maybepositiveionordipole
•Areniumionisresonancestabilizedandhybridofthreestructures
•Firststepisslowandisthereforeratedeterminingstep
Dr Ahmad Raza 9
Electrophilic substitution
Mechanism…….
2.Step two
•Inthesecondstep,areniumionloosesaprotontobasepresentinmixture
•TheprotonislostfromthecarbonwhichbearsE
+
•ThereleasedelectronpairofC-Hbondbecomespartofcyclicπ-system
andrestorethesixπ-electronstoregainaromaticcharacter
•Sincestabilityisgainedbythemoleculehencethisstepisfast
•Thedrivingforceofsubstitutionisrearomatization
Dr Ahmad Raza 10
Electrophilic substitution
Energy diagram
Dr Ahmad Raza 11
Electrophilicsubstitutionreactions
1.Nitrationofbenzene
•Commonlycarriedoutbyheatingbenzenewithmixtureofconc.HNO
3and
conc.H
2SO
4
•Thenitroniumion(NO
2
+
)isgeneratedaselectrophile(observed
spectroscopically)
Dr Ahmad Raza 12

4. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 4
Electrophilic substitution reactions
Nitration of benzene………
Mechanism
Dr Ahmad Raza 13
Electrophilicsubstitutionreactions
2.Sulphonationofbenzene
•Byheatingbenzenewithconc.H
2SO
4at80°C
Or
Fumingsulphuricacid(H
2SO
4+SO
3)at25°C
•AlthoughSO
3isneutralbutstillactasE
+
sinceSiselectrondeficient
Dr Ahmad Raza 14
Electrophilic substitution reactions
Sulphonationof benzene……….
Mechanism
1 = Generation of E
+
2 = Attack of E
+
3 = Restoration of aromaticity
by abstraction of proton form
sp
3
carbon by HSO
4
-1
4 = Proton transfer from
H
2SO
4to benzene sulphonate
ion
Dr Ahmad Raza 15
Electrophilicsubstitutionreactions
3.Halogenationofbenzene
•Thesubstitutionofhalogenatom(Cl,BrorI)inplaceofhydrogeniscalled
halogenation
•BenzenereactswithhalogensinthepresenceofLewisacid(FeCl
3orFeBr
3)
•Fereactswithalogentofromferrichalide
2Fe+3X
2 2FeX
3
25°C Heat
Dr Ahmad Raza 16

5. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 5
Electrophilicsubstitutionreactions
Halogenationofbenzene…………..
Mechanism
•Aromatichalogenationreactionsarecarriedoutintheabsenceoflightsince
sidechainsubstitutioniscatalyzedbylight
Dr Ahmad Raza 17
Electrophilicsubstitutionreactions
Halogenationofbenzene………… ..
•HoweverintheabsenceoflighttoluenereactswithCl
2orBr
2inthe
presenceofAlCl
3orFeCl
3togivemixtureofo-andp-derivatives
Dr Ahmad Raza 18
Electrophilicsubstitutionreactions
•DevelopedbyCharlesFriedelandJamesCraftsin1877
•Involvestheintroductionofalkyloracylgroupintobenzeneringinthe
presenceofanhydrousAlCl
3oranyotherLewisacidi.e.BF
3,FeCl
3etc.
Friedal-Craftsreactions
4.Friedal-Craftsalkylation
•Carriedoutbytreatingbenzenewithalkylatingreagentlikealkylhalide(R-
X),alcohols(R-OH),oralkenes
•Usedforintroducingalkylgroupinthebenzenering
•Reactioniscarriedoutininertsolventlikenitrobenzeneorcarbondisluphide
Dr Ahmad Raza 19
Electrophilic substitution reactions
Friedal-Crafts alkylation……
ExamplesCH
3Cl
AlCl
3
5
o
C
CH
3
HCl (CH
3)
2CHOH
BF
3
-10
o
C
CH
H
2O
H
3C
CH
3 CH
3-CH=CH
2
AlCl
3
HCl
CH
H
3C
CH
3
Dr Ahmad Raza 20

6. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 6
Electrophilic substitution reactions
Friedal-Crafts alkylation……
Mechanism
Dr Ahmad Raza 21
Electrophilic substitution reactions
Friedal-Crafts alkylation……
Lmitations
a)Polyalkylation
•Itisdifficulttostopthereactionwhenonealkylgrouphasenteredthering
•Oftenpoly-alkylatedbenzenesareproduced
•Forexample:BenzenewithexcessofCH
3Clgives1,2,4,5-tetramethyl
benzeneCH
3Cl
AlCl
3
CH
3
H
3C
H
3C
CH
3
Dr Ahmad Raza 22
Electrophilic substitution reactions
Friedal-Crafts alkylation……
Lmitations
b)Productsduetorearrangementofcarboniumionelectrophile(E
+
)
•Thegeneratedcarbocationofalkylhalidemayundergorearrangement
beforeattackingthebenzenering
•Thisrearrangementisintheorderprimary secondary tertiary
attributedtotheirrelativestability
Dr Ahmad Raza 23
•ThisrearrangementusuallytakesplacebytransferofH
-
orR
-
•Thereforereactionofbenzenewith1-chloropropane(propylchloride)in
thepresenceofAlCl
3producesisopropylbenzene(cumene)asmajor
productratherthanexpectedn-propylbenzene
Electrophilic substitution reactions
Friedal-Crafts alkylation……
LmitationsCH
3-CH
2-CH
2-Cl
AlCl
3
CH
CH
3
CH
3
H
2
C
H
2
C CH
3
Major product
Minor product
Dr Ahmad Raza 24

7. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 7
Electrophilicsubstitutionreactions
5.Friedal-Craftsacylation
•Carriedoutbytreatingbenzenewithacylatingreagentlikeacylhalide
(CH
3COX),oracidanhydride,orketenes
•Usedforintroducingacylgroup(RCO-)inthebenzenering
•Reactioniscarriedoutininertsolventlikenitrobenzeneorcarbon
disluphide
•Aromaticketoneisgenerated
Dr Ahmad Raza 25
Electrophilicsubstitutionreactions
Friedal-Craftsacylation……..
ExamplesAlCl
3
C CH
3
O
HClH
3CCCl
O AlCl
3
C CH
3
O
CH
3COOH
H
3CC
O
H
3CC
O
O AlCl
3
C CH
3
O
H
2C C O
Ketene
Acetophenone
Dr Ahmad Raza 26
Electrophilicsubstitutionreactions
Friedal-Craftsacylation……..
Mechanism
Dr Ahmad Raza 27
Electrophilicsubstitutionreactions
Friedal-Craftsacylation……..
•UnlikealkylgroupinFriedalcraftalkylation,acyldoesnotundergo
rearrangement,notitallowstheintroductionofanotheracylgroupin
thebenzenering
Advantage of Friedal crafts acylation
Dr Ahmad Raza 28
•Friedalcraftacylationcanbeappliedtopreparestraightchainalkyl
benzenes
•Acylationofbenzenegivesketonewhichisthenreducedton-
alkylbenzenesbyClemmensenreductionorWolfKishnerreduction

8. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 8
Electrophilicsubstitutionreactions
Friedal-Craftsacylation……..
Advantage of Friedal crafts acylation
Dr Ahmad Raza 29
Electrophilicsubstitutionreactions
5.Formylation
•Substitutionofformylgroup(aldehyde)-CHO-intoaromaticring
•Similartoacylation
•Cannotbecarriedoutwithformylchlorideorformicanhydride
•FormylchlorideisstableforshorttimeinCHCl
3at-40°C
Dr Ahmad Raza 30
•Formicanhydridedecomposesatroomtemperature
•Maybeaccomplishedby:
1.Gatterman-Kochreaction
2.Gattermanreaction
3.Formylationwithformylfluoride
Electrophilicsubstitutionreactions
1.Gatterman-Kochreaction
Mechanism
•Similar to Friedel-Crafts acylation
•CO and HClreact to form formyl chloride which then reacts with benzene
to form benzaldehyde
•Formyl chloride can be generated by dropping chlorosulphonicacid on
formic acid
Dr Ahmad Raza 31
Electrophilicsubstitutionreactions
2.Gattermanreaction
•Gattermanused HCN in place of CO (poisonous) as source of formylgroup
•Direct use of HCN can be avoided by using Zn(CN)
2
Dr Ahmad Raza 32

9. BS-Organic Chemistry-I 15-Jan-20
Dr Ahmad Raza (PhD, QAU Islamabad) 9
Electrophilicsubstitutionreactions
3.Withformylfluoride
•Benzene is treated with formyl fluoride (HOCF) in the presence of BF
3 ether
(solvent)
•Formyl fluoride is stable as compared to formyl chloride
Dr Ahmad Raza 33