Naming heterocyclic compounds

Hantzsch-Widman system

The Hantzsch-Widman system is the most common system for naming heterocycles, especially aromatics, and is a hybrid between trivial and systematic names using standard prefixes and suffixes.

In this system the prefix indicates the type or types of heteroatoms that form the heterocycle. The suffix indicates the size of the ring, whether or not it has nitrogen and whether it is saturated or unsaturated.

Table 1
(1) The final letter "a" is eliminated when the prefix is followed by a vowel.

When two or more different heteroatoms are present the prefixes are placed in the order shown in Table 1. Two or more heteroatoms of the same type are indicated as di, tri, etc. To number them, start the numbering at the heteroatom shown in Table 1.

Table 2
No. of ring termsNitrogen ringsrings without nitrogen
(1) With the maximum no of non-cumulative double bonds, the heteroatoms having the valences of Table 1.
(2) It is expressed with the prefix perhydro attached to the name of the unsaturated compound Perhydroazine ...

Heterocyclic systems that are not saturated, but to a lesser degree than that corresponding to the maximum number of non-cumulative double bonds, are named using the prefixes: dihydro, tetrahydro, etc.

In the 4- and 5-membered rings, a special termination is used for structures with a single double bond ...

Table 3
No. of ring termsnitrogen ringsrings without nitrogen

When the same heteroatom occurs more than once in a ring, the numbering is done in such a way that the locators are as low as possible.

Trivial names of common ring systems

In the early days of organic chemistry, compounds were given names to identify them,

... always before their structure is known. This is still done today for newly discovered natural products, the names of the compounds are usually based on the source of the compound or one of its most characteristic properties.

... is called picoline based on the Latin word picatus, meaning tarry, because it was isolated from coal tar. Pyrrole comes from the Greek meaning fiery red. due to the color produced by the compound with a pine splinter dipped in hydrochloric acid.
These trivial names contain no information about the structure of the compound and their use is being abandoned.

Today, about 60 trivial names survive and are recognized by IUPAC. These names that are still in use are important because they are used as a basis for constructing other names.


Nomenclature of fused annular systems

This system is used for heterocycles containing two or more fused rings, some of which have recognized trivial names.

Most of the fused heterocycles have no recognized trivial name so the systematic name has to be used, for this purpose the common atoms are considered as belonging to both ring systems. The corresponding name is constructed by combining the names of the individual rings.

Basic guide to constructing names of merged annular systems

  • The names of the components of the merged systems are chosen, whenever possible, from the list of recognized trivial names. The major component with a recognized name is selected. For example, indole is better than pyrrole if the indole fragment is present. When a fragment has no recognized name, its systematic name is used.
  • In a fused system consisting of two or more individually named components, one of them is chosen as the base component for the compound name. For example, in (2) pyrimidine is the base component.
  • The second component is added as a prefix to the name of the base component. This prefix is derived by replacing the "o" in the name of the ring system by e.