Nomenclature of heterocyclic compound

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1. NOMENCLATURE OF
HETEROCYCLIC COMPOUND
Ravish Yadav

2. Nomenclature of heterocyclic compounds
•Therearethreesystemsfornamingheterocycliccompounds:
1)The common nomenclature: which convey little or no structural
information but it still widely used.
2)The Hantzsch-Widman (IUPAC or Systematic) method which in contrast
is designed so that one may deduce from it the structure of the
compound.
3)The replacement method

3. I-Common NomenclatureN
H
N
1
2
34
5
Eachcompoundisgiventhecorrespondingtrivialname(which
shouldbememorized,seethefollowingslides).Thisusually
originatesfromthecompoundsoccurrence,itsfirstpreparationorits
specialproperties.
If there is more than one hetroatom of the same type
numbering starts at the saturated one, e.g. imidazole.
Ifthereismorethanonetypeoftheheteroatoms,theringis
numberedstartingatthehetroatomofthehigherpriority(O>S>N)
anditcontinuesinthedirectiontogivetheotherhetroatomsthe
lowernumbersaspossible.

4. Common Nomenclature
•Ifsubstituentspresent,theirpositionshouldbeidentifiedbythenumber
oftheatomsbearingthemandthentheyshouldbelistedinalphabetical
order.O
N
1
2
3
4
5
Br
H
2N
5-Amino-4-bromoisoxazole H
N
1,2-Dihydro-pyridine
The wordsdihydro, ortrihydro, or tetrahydroare used if two or
three or four atoms are saturated. These words are preceded by
numbers indicate the position of saturated atoms as low as possible
and followed by the corresponding fully unsaturated trivial name.

5. Trivial namesO O N N
N
N
N
N
N
These are tautomers
Both are not aromatic
DNA/RNA bases
Common azines-six-membered aromatic nitorgrn heterocycles
2H-Pyran 4H-Pyran Pyridine Pyridazine Pyrimidine Pyrazine
1) 5-membered heterocycles with one or two heteroatoms
2) 6-membered heterocycles with one or two heteroatoms

6. Trivial NamesN
N
N
NH
O
NH
2
Guanine
3) Fused heterocycles

7. Trivial NamesO O
O
O
Coumarine
O
O
Ph
Flavone
Chromen-2-one
Chromen-4-one N
H
N
H
1
2
9H-Carbazole
3
4
5
6
7
8 9
9,10-Dihydro-acridine
1
2
3
45
6
7
8 9
10

8. Trivial NamesN
N
H
2N CH
3
O
NH
H
N
4) Saturated heterocycles
Exercise :
Give the common name of the following compounds:

9. II-Replacement nomenclature
•Inreplacementnomenclature,theheterocycle’snameiscomposedofthe
correspondingcarbocycle’snameandanelementalprefixfortheheteroatom
introduced(ifmorethanoneheteroatomispresenttheyshouldbelistedaccording
tothepriorityordershownin(table1).Accordingtothisnomenclature,
tetrahydrofuran,forinstance,iscalledoxacyclopentane.
Table
1
Atom
O oxa
Se selena
S thia
N aza
P phospha
Priority decreases
Prefix

10. II-Replacement nomenclatureO
N
O
N
S
N
O
N
N
O
N
H
Benzene
Cyclopentadiene
Cyclopentadiene
Cyclopropene
Cyclopentadiene
Cyclohexane
1,4-Diazabenzene
Oxacyclopenta-2,4-diene
1-Oxa-3-azacyclopenta-2,4-diene
Oxazacyclopropene
1-Thia-2-azacyclopenta-2,4-diene
1-Oxa-4-azacyclohexane
N
Naphthalene
O
Cyclopropane
Oxacyclopropane
2-Azanaphthalene

11. II-Replacement nomenclatureS
O
N
N
O
S
S
N
cyclohexane
naphthalene
naphthalene
cyclododecadiene
Thiacyclohexane
1,4-dithianaphthalene
1,7-Dioxa-2,8-diazacyclododeca-2,8-diene
4H-4a-azanaphthalene
1
2
3
4
4a
1
2
8a
1
2
3
45
6
7
8
4a
78

12. III-Hantzsch-Widman nomenclature (IUPAC)
•Hantzsch-WidmannomenclatureisnamedaftertheGermanchemists
ArthurHantzschandOskarWidman,whoproposedsimilarmethodsfor
thesystematicnamingofheterocycliccompoundsin1887and1888
respectively.
•Accordingtothissystemthreetoten-memberedringsarenamedby
combiningtheappropriateprefix(orprefixes)thatdenotesthetypeand
positionoftheheteroatompresentintheringwithsuffixthatdetermines
boththeringsize(dependingonthetotalnumberofatomsinthering)
andthedegreeofunsaturation(notethatfullysaturatedandfully
unsaturatedhavecertainrulesfornomenclaturewhilepartially
unsaturationwillbeindicatedincertainways).Inaddition,thesuffixes
distinguishbetweennitrogen-containingheterocyclesandheterocycles
thatdonotcontainnitrogen
•IUPAC name = locants +Prefix + suffix

13. Hantzsch-Widmanrulesforfullysaturatedandfullyunsaturatedheterocycles
1)Identify the hetroatom present in the ring and choose from (table 1 on
slide 9)the corresponding prefix (e.g. thia for sulfur, aza for nitrogen
and oxa for oxygen).N
CH
3
1
2
3
4
2)The position of a single heteroatom control the numbering
in a monocyclic compound. The heteroatom is always
assigned position 1 and if substituents present are then
counted around the ring in a manner so as to take the
lowest possible numbers.
For example:

14. Hantzsch-Widman rules1
2
3
4
N
N
H
5
3)A multiplicative prefix (di, tri, ect.) and locants are used
when two or more similar heteroatoms contained in the
ring( two nitrogen indicated by diaza) and the
numbering preferably commenced at a saturated rather
than an unsaturated atom, as depicted in the following
example: 1,3-diaza….
4)Ifmorethanonetypeofhetroatomspresentinthering
thenamewillincludemorethanoneprefixwithlocants
toindicatetherelativepositionoftheheteroatoms.
Atomprefixeshaveastrictorderofpriority(preference)in
whichtheyaretobelisted.Forexample,‘’Oxa’’(foroxygen)
alwayscomesbefore‘’aza’’(fornitrogen)inaname(see
table1).
Whencombiningtheprefixes(e.g.oxaandaza)two
vowelsmayenduptogether,thereforethevowelonthe
endofthefirstpartshouldbeomitted(oxaza).

15. Hantzsch-Widman rules
•Thenumberingisstartedfromtheheteroatomofthehighestpriorityin
suchawaysoastogivethesmallestpossiblenumberstotheother
heteroatomsinthering(thesubstituentsareirrelevant).Forexamplethe
prefixcorrespondingtothefollowingcompoundis4-Methyl-1,3-Thiaza….1
2
3
4
N
S
5
5)Choose the appropriate suffix from (table 2) depending
on whether or not nitrogen atom is present in the ring,
the size of the ring and presence or absence of any
double bonds
6)Combine the prefix(s) and suffix together and drop the
first vowel if two vowels came together.

16. Hantzsch-Widman rules
Unsat
Unsat
sat sat
N-present N-absent
irine iridine
irene irane
ete
ete
etidine
etane
ole olidine ole
olane
ine
a
in
ane
ocine
epin
ocane
Table 2
a
Ring size
epine
a
ocin
epane
9
10
onine
a
ecine
a
onin
ecin
onane
ecane
a: means use the prefix perhydro followed by the fully unsaturated name

17. Hantzsch-Widman rules
•Examples
•This ring contains (N) Prefix is aza
•The ring is 3-membered and fully saturated
suffix is iridine
•By combining the prefix and suffix, two vowels
ended up together(azairidine), therefore the vowel on the end of
the first part should be dropped. Thisgives the correct name:
AziridineH
N

18. Hantzsch-Widman rulesHN O
•Thisringcontains(O,N)and(o)hashigher
prioritythan(N)andbystartingnumberingthe
ringat(O) Prefixis1,2-Oxaaza,but
thefirstvowelmustbeomittedtogive
1,2-Oxaza
•Theringis4-memberedandfullysaturated
suffixisetidine
•Bycombiningtheprefixandsuffix,twovowels
endeduptogether(1,2-oaxazaetidine),therefore
thevowelontheendofthefirstpartshouldbe
dropped.Thisgivesthecorrectname:
1,2-oxazetidine

19. Hantzsch-Widman rules
•This ring contains (O) prfix1(oxa), andtwo (N)
prfix2diaza
•Locants, since (O) is higher priority than (N) so it is in position 1 by default and the
two (N) are therefore at positions 2 and 5, this gives thecombined prefixesas
1,2,5-oxadiaza (note that theain oxa is not dropped)
•It is 5-membered,fully unsaturated ring with (N)
the suffixisole
•By combining the prefixes and the suffix and dropping the appropriate vowels we
getthe correct name as
1,2,5-OxadiazoleN
O
N
1
2
34
5

20. Hantzsch-Widman rulesN
H
Theringis6-memberd,fullysaturatedwithN
Prefixperhydrofollowedbythenameoffullyunsaturated6-
memberdringwithnitrogen azine
Thusthefullnameisperhydroazine

21. Hantzsch-Widman rules
•Exercise:
Explain how can you name the following heterocycles.O S
N N
N
O
N S
Br

22. Hantzsch-Widman rules for partially unsaturated
heterocycles
Partial unsaturation in heterocyclic compounds can
beindicated by one of the following methods:
a)Thepositionofnitrogenorcarbonatomswhichbearextrahydrogenatoms
mustbeindicatedbynumbersanditaliccapitalH(e.g.1H,2H,etc.)followed
bythenameofmaximallyunsaturatedring.6
1
1
2
3
O
2H, 3H-Oxole
4
5
N
1H-Azepine
1
2
3N
S
O
4
5
5H-1,2,3-Oxathiazole
1
O
2
3
4
5
4H-Oxin
H

23. Hantzsch-Widman rules for partially unsaturated
heterocycles
b)The wordsdihydro, ortrihydro, or tetrahydroare used if two or
three or four atoms are saturated. These words are preceded by
numbers indicate the position of saturated atoms as low as
possible and followed by the corresponding fully unsaturated
Hantzsch-Widman name.1
2
3
N
H
1,2-Dihydroazine
N
H
1,4-Dihydroazine
Isomers have the same M.F.
but differ in the position of
the double bond
N
2,3,4,5-Tetrahydroazine
1
11
222
3
33
4
44
555
666
O
2,3-Dihydrooxole
4
5

24. Hantzsch-Widman rules for partially unsaturated heterocycles
c)Alternatively, the partially unsaturated 4 and 5 rings(i.e. rings contain one
double bond) are given special Hantzsch-Widman suffixes as in table 3and the
double bond is specified as ∆
1
, ∆
2
, ∆
3
, etc.. Which indicates 1 and ; 2 and 3; 3
and 4 atoms respectively have a double bond
(i.e.Name : ∆
x
+ Prefix + special suffix )
( x= locant of the double bond)
-etine -etene
With N Without N
-oline -olene
Table 3
Ring size

25. Hantzsch-Widman rules for partially unsaturated
heterocycles
•ExamplesHN HN

2
-Azetine
1 1
2
O
1
2

2
-Oxetene
N
H
O

3
-Azoline 
2
-Oxolene
1
2 2
3 34 4
5
5
1 S
NH
1
2
3
5

4
-1,3-Thiazoline
4
N
H
N
2
34
5
1

2
-1,3-Diazoline

26. Nomenclature of Fused Systems
Definitions:
Fusion: This term is used to describe the process of
joining two separate rings with the maximum number of
non-cumulative double bonds viatwo atoms and one
common bond.
Ortho-fusedrings: are those rings that have only two
common atoms and one bond, example; naphthalene
Ortho-and peri-fused rings: are those found in a
polycyclic compound with a ring that is ortho-fused to
different sides of two other rings that are themselves
ortho-fused together (i.e. there are three common atoms
between the first ring and the other two).Naphthalene

27. Nomenclature of Fused Systems1H-Phenalene
 Forexample:1H-phenaleneisconsideredasbeing
composedofthreebenzenerings,eachisortho-peri-
fusedtotheothertwo.
Polycycliccompoundsincorporatingoneheterocyclicring
orfusedheterocylicsystemfusedtobenzeneareknown
benzoheterocycles.
Alsobicycliccompoundswithtwofusedheterocyclicrings
arewellknown.
Bothtypescanbenamedaccordingtocertainrules

28. I-Nomenclature of Fused Heterocyles
A.Nomenclature of benzofused compounds:
•Unless listed as a trivially named heterobicycle (see slide 6), a benzene
ring fused to a heteromonocycle of five or more members or a
heterobicycle is named by prefixingthe word benzoto a letter
indicating the position of fusion in square brackets by the name of
heterocyclic ring(common or IUPAC or modified replacement name).
Name= Benzo[letter]name of heterocyclic ring
•For designating the position of fusion, the peripheral bonds of the
heterocyclic ring are consecutively assigned alphabetical letters staring
with the 1,2-bond as a side and the labeling is continued around the
ring to give the common bond the lowest order.

29. I-Nomenclature of Fused HeterocyclesO
1
2
3
4
5
a
b
Benzo[b]furan
S
1
2
3
4
5
6
7
a
b
c
d
Benzo[d]thiepine
N
H
1
2
3
4
5
a
b
Benzo[b]pyrrole
Indole
N
1
N
1
2
2
3
4
5
6
7
8
4a
8a
Benzo[b]pyidine
Qunioline
a
a
b
b
cd
ef
Benzo[f]qunioline
S
1
2
3
4
5
a
b
c
Benzo[c]thiophene
Examples

30. I-Nomenclature of Fused Heterocyles
B.Nomenclature of fused heterocylic compounds:
Naming a fused heterocyclic systems composed of two
monoheterocyclic units or benzoheterocycles (e.g. chromene)
fused with another hetrocycle ring is based upon considering
one system as the parent (base) and the second is considered
as substituent
The name is formed of :
name of minor ring [number,number-letter]name of major ring
The name of the minor ring is derived by writing a contracted
prefix for the substituent ring present
Furo from Furan
Imidazo from Imidazole
pyrido from Pyridine
Pyrimido from Pyrimidine
Thieno from Thiophene
Pyridazino from pyridazine
Pyrazino from pyrazine
Chromeno from chromene

31. I-Nomenclature of Fused Heterocyles
The numbersindicate which atoms in the minor ring are
common to the major ring (fusion sites in minor ring).
The order of the numbers indicates which atom of the
minor ring is encountered closest to atom 1 in the major
numbering system (i.e. these numbers may be written in
ascending or descending order e.g.2,3 or 3,2 )
The letterdefines the position of attachment of the minor
ring to the major ring (fusion sites in base component)
Finally a suffix indicate the name of the base ring is
written.
The numbering system for the whole fused system is not
the same as the numbers in the square brackets (i.e.
there are three numbering systems; one for minor ring,
one for major ring and the third is for the system as a
whole)
Priority order of component ring systems:
Selection of a parent component or attached component is
based on the following rules which are applied in order

32. I-Nomenclature of Fused HeterocylesO
N
O
N
Substituent ring
Chromeno
Base or parent ring
because it has N
pyrrole
1
2
3
1
2
3
4
5
a
b
c
Chromeno[2,3-c]pyrrole
Rule1:Aheterocyclicringcontainingtheheteroatomoccurring
earliestintheorderN,F,Cl.Br,I,O,S,Se,..
(i.e.ringcontainingNpreferredtotheringsdoesnotcontainN
orcontainingO,orS)

33. I-Nomenclature of Fused HeterocylesO
S
Substituent ring
Thiopyrano with one satrated C that take locant 7 when
the system is numbered as a whole (starting from O to
give the two heteroatoms locants 1,4 while starting from
S gives them locants 1,5)
Parent ring
O preferred to S
Furan
1
2
3
4
1
2
3
4
5
6
7
1
2
3a
b
S
O
7H-Thiopyrano[3,2-b]furan

34. I-Nomenclature of Fused Heterocyles
Rule2:Aheterocycliccomponentcontainingthelargest
possibleindividualringO
O
2H-Furo[3,2-b]pyran
(pyran [6] preferred to furan [5])
a
b
1
2
3
NumberingthewholesystemisstartedfromOinfuran
ringtogivethetwoheteroatomslocants1,4while
starting from O in pyran ring gives them locants 1,5, thus
the indicated H takes locant 2
Indicated H

35. I-Nomenclature of Fused Heterocyles
Rule3:Aheterocycliccomponentcontainingthegreater
numberofhetroatomsofanykindN
O
N
5H-Pyrido[2,3-d][1,2]oxazine
(Oxazine preferred to pyridine)
a
b
c
d
1
2
3
5
1
2
3
4
6
7
8
4a
8a
N.B.Thewholemoleculeisnumberedstartingfrompyridineringto
givethethreeheteratomsthelowestlocants(1,6,7),however,
statingfromoxazineringwillgivethemlocants(2,3,5)or(2,3,8).

36. NOMENCLATURE OF FUSED HETEROCYCLES
Rule 4: A heterocyclic component containing
the greater variety of hetroatoms
N.B.Thewholemoleculeisnumberedstarting
frompyrazoleringtogivethefour
heteratomsthelowestlocants(1,2,4,6).
Whilestartingfromoxazoleringgivethem
locants(1,3,4,5)or(1,3,5,6).O
N
N
NH
1H-Pyrazolo[4,3-d]oxazole
(O & N preferred to N only)
a
b
c
d 1
3
2
4
5

37. NOMENCLATURE OF FUSED HETEROCYCLES
Rule 5: A heterocyclic component containing
the greater number of heteroatoms most
preferred when considered in order F, Cl, Br,
I, O, S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn
Pb, B, HgO
N
N
S
[1,3]Thiazolo[5,4-d][1,3]oxazole
(N & O preferred to N & S)
a
b
c
d
1
2
34
5

38. NOMENCLATURE OF FUSED HETEROCYCLES
Rule 6: A heterocyclic component with the lower
locants for heteroatoms
Exercise: Name the following compound.N
N
N
N
Pyrazino[2,3-d]pyridazine
(pyridazine [2N-1,2] preferred to pyrazine [2N-1,4]
a
b
c
d
1
2
3 O
S
N
H

39. NOMENCLATURE OF FUSED HETEROCYCLES
Rule 7: If a position of fusion is occupied by a
heteroatom the name of the component rings
to be used are so chosen as both to contain
the heteroatom.Imidazo[2,1-b]thiazole
1
23
a
b
N
N
S

40. Orderofpreferencebetweenalternativenumberingsystemofthe
wholemolecule
• Numberingthewholefusedsystemshouldstartfromthefirstatomafterfusionin
anydirectiontofulfillthefollowingrulesinorder:
a)Givelownumbersfortheheteroatomsasaset
b)Givelownumbersforheteroatomsofhigherpriorityi.e.O,S,NH
N
N
O
1H-Furo[2,3-d]imidazole
(heteroatoms 1,3,4 is preferred to 1,3,6 or 1,4,6)
4
H
N
N
O
H
N
N
O
1
1
22
3
3
4
4
5
5
6
6
1
2
3
5
6 O
S
4,5-Dihydro-thieno[2,3-b]furan
1
2
34
5
6

41. Order of preference between alternative numbering system of the
whole molecule
c)Givelownumberstofusioncarbonatoms
d)Givelownumberstoindicatedhydrogenatom2
O
O

N
N
H

1
2
3
5
6
O
O

N
N
H

1
2H,4H-[1,3]dioxol[4,5-d]imidazole
Indicated hydrogens 2,4 not 2,6
4
3
5
6
4
Not 4
N
N
N
N
N
N
N
N
N
N
N
N
Imidazo[1,2-b][1,2,4]triazine
1
2
3
5
6
7
8
fusion C -4a is preferred to 8a
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
8a
8a4a
Not
Not

42. NOMENCLATURE OF HETEROCYCLES
•Exercise
Q1. Name the following compounds (a-d):N
N
N
H
N
(a) (b) (c)
N
N
S
S
O
N
(d)

43. Summary of Nomenclatures Rules
1.Is there only one ring which contains nitrogen?
(YES:. choose this as base component)
2. Are the two rings have the same heteroatoms but their size is different ?
(Yes: choose the larger one )
3.Arethetworingsofthesamesizebuthavedifferentheteroatoms?
(YES: choose the ring containing a heteroatom of the highest
priority i.e. O >S)
4. Are the rings of the same size but contain different numbers of
heteroatoms?
(Yes: choose the ring with the greater number )
5. Are the two rings of the same size and the same number of different
heteroatoms?
(Yes: choose the ring with the greatest variety of heteroatoms
7-Are the two rings have the same size and the same number and type of
heteroatoms?
(yes: choose the ring with the lower numbers for heteroatoms )
Scheme for deriving the base component of' a fused ring system