students able to get knowledge on heterocyclic compounds nomenclature

1. UNIT III
HETEROCYCLIC
COMPOUNDS
Presented by
P.SOWMIYA
Assistant Professor
Dept. of Pharmaceutical Chemistry
SVCP

2. INTRODUCTION
 Heterocyclic compounds are those cyclic compounds in
which one or more of the ring carbon are replaced by
another atom.
 The non-carbon atoms in such rings are referred to as
Hetero atoms.
 Most common hetero atoms are
➢ Nitrogen
➢ Oxygen
➢ Sulphur
➢ Boron
➢ Phosphorous
➢ Silicon

3. IMPORTANCE OF HETEROCYCLIC
COMPOUNDS
 Heterocyclic compounds are very widely distributed in nature and
are particularly important because of wide variety of physiological
activities.
 Important compounds containing heterocyclic rings are
❖ Vitamins
❖ Alkaloids
❖ Chlorophyll
❖ Amino acids
❖ Dyes
❖ Drugs
❖ Enzymes
❖ DNA
❖ Genetic material

4. NOMENCLATURE
 There are Two systems for naming Heterocyclic compounds
NOMENCLATURE
I. TRIVIAL/ COMMON
NAMES
II. SYSTEMATIC/
IUPAC NAMES
A) Hantzsch
Widman system
B) Replacement
method

5. I. Trivial or common names
 They convey little or no structural information but it still widely
used.
 Eg: O
Ethylene oxide
NH
Pyrrole
O
Furan
S
Thiophene
NH
N
Imidazole
N
Pyridine
NH
Indole

6. II. Systematic/ IUPAC NAMES
A) HANTZSCH-WIDMAN METHOD
 According to IUPAC System of nomenclature
(International Union of Pure and Applied Chemistry), the
names of heterocyclic compounds are made up of three
parts:
i. Stem indicates ring size
ii. Prefix describes Hetero atom
iii. Suffix denotes the degree of unsaturation

7. Prefixes for hetero atoms
Hetero atom Valence Prefix
O 2 Oxa
N 3 Aza
S 2 Thia
Se 2 Selena
Te 2 Tellura
P 3 Phospha
As 3 Arsa
Si 4 Sila
Ge 4 Germa

8. Stems and Suffixes for naming
Heterocyclic compounds
Ring size Stem Unsaturated
compound
Saturated
compound
With N Without N With N Without N
3 -ir- -irine -irene -iridine -irane
4 -et- -ete -ete -etidine -etane
5 -ol- -ole -ole -olidine -olane
6 -in- -ine -in - -ane
7 -ep- -epine -epin - -epane
8 -oc- -ocine - -ocin -ocane

9. Rules for Systematic nomenclature
1) IUPAC Name = Prefix + stem + suffix
Combine the prefix and suffix together and drop the first
vowel if two vowels came together.
Eg:
Oxa + irane
Oxirane
O

10. N
H NH
N

11. 
NH
Aza + etidine
Azetidine
Aza + ole
Azole
Aza + ine
Azine

12. O
O
NH
O
O

13. O
O
NH
O
O
Oxa + ine
oxine Oxa + ane
oxane
Oxa + epane
oxepane
Aza + epine
Azepine
Oxa + epin
oxepin

14. 2) In the numbering of heterocyclic rings, the hetero atom is
always given number 1
3) When two or more heteroatoms of same type are present
in a ring, prefixes like di, tri, tetra etc., are used before the
hetero atom name.
NH
1 2
3
5
4
Pyrrole
5
O
1
4
2
O
3
1,3- dioxolane
N
H
NH
NH
1,2,3- triazole
1
2
3 4
5

15. 4) When two or more different heteroatoms are present in a same
ring, prefixes of hetero atoms are combined in the order of priority.
(order – O, S, N, P, Si)
5) Numbering is done around the ring in that direction which gives the
lowest number to the substituents.
O
1 2
3
5
4
NH2
6
3-amino-1-oxazole
O
1 2
N
3
5
4
Oxazole

16. 6) The state of hydrogenation is indicted by prefix like
dihydro, tetrahydro or by prefixing H preceded by a number
indicating the position of saturation.
7) In case of benzofused heterocyclic compounds, prefix the
word benzo followed by position of fusion in square brackets
and name of heterocyclic ring.
O
b
c d
e
a
Benzo[b]furan
S
a
b
c
d e
Benzo[c]thiophene
NH
1 2
3
5
4
1-H Azole/
1-H Pyrrole
O
Tetrahydrofuran

17. 8) Numbering of fused polycyclic heterocyclic
compounds follows the numbering system of
carbocyclic system.
N
Quinoline
1
2
3
4
5
6
7
8 9
10

18. B) REPLACEMENT NOMENCLATURE
 In this method, the heterocycle's name is composed of
corresponding carbocycle’s name and prefix for the hetero atom
is introduced.
S
Cyclohexane Thiocyclohexane
N
Naphthalene 1-Azanaphthalene

19. CLASSIFICATION OF
HETEROCYCLIC COMPOUNDS
Depending on
I. Size of heterocyclic ring
II. Number of rings present
III. Aromaticity