N
H
N
1H-Azepine
2H-Azepine
Important aromatic heterocycles that contain a single hetero-atom include
pyridine, quinoline, isoquinoline, pyrrole, thiophene, furan and indole.
N
N
N
Pyridine
.. 1
2
3
4
5
6
1
2
3
4
5
6
7
8
Quinoline
1 2
3
4
5
6
7
8
:
Isoquinoline
..
N
H
S
O
N
H
..
..
..
..
..
Pyrrole
Thiophene
Furan
Indole
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
6
7
..
Derivatives of these heterocyclic compounds are named in the same way as
other compounds, by adding the name of the substituent, in most cases as a
prefix to the name of the heterocycle, and a number to indicate its position
on the ring system, e.g. 2-methylpyridine, 5-methylindole and 3-phenylthiophene.
N
CH 3
N
H
C
H 3
S
Ph
2-Methylpyridine
..
5-Methylindole
..
..
3-Phenylthiophene
Heterocyclic aromatic compounds can also have two or more hetero-atoms.
If one of the hetero-atoms is a nitrogen atom, and the compound has a fivemembered system, their names all end in -azole, and the rest of the name
indicates other hetero-atoms. For example, pyrazole and imidazole are two
isomeric heterocycles that contain two nitrogen atoms in the ring, thiazole
has a sulphur atom and a nitrogen atom in the ring, and oxazole contains an
oxygen atom and a nitrogen atom. In imidazole and oxazole, two heteroatoms are separated by a carbon atom, whereas in their isomers, pyrazole
and isoxazole, the hetero-atoms are directly linked to each other. The sixmembered aromatic heterocycles with two nitrogens can exist in three
isomeric forms, the most important being pyrimidine.
N
N
H
N
N
H
S
N
O
N
O
N
Pyrazole
1 2
3
4
5
1
2
4
5
1
2
4
5
1
3
4
5
1
2
4
5
3
3
3
2
..
..
..
..
..
.. :
:
Imidazole
Thiazole
Oxazole
Isoxazole
..
..
..
..
..
4.7 HETEROCYCLIC COMPOUNDS AND THEIR DERIVATIVES
145