A Translator’s Guide to Organic Chemical Nomenclature

XI. Aromatic compounds (continued)

Monosubstituted benzenes

Some examples of monosubstituted benzenes are listed below with their widely-used names. The name listed first is the most commonly used name.
 

Disubstituted benzenes

The 6-membered ring structure of benzene allows for the existence of three positional isomers of disubstituted benzenes. Locants are specified by either of two systems: The numbered system (1,2-, 1,3-, or 1,4-disubstituted) has its counterpart in ortho-, meta-, or para-disubstituted (abbreviated o-, m-, or p-disubstituted), respectively. Because the benzene ring is planar, no stereoisomers can exist.

The three dimethylbenzenes, for example, are:

o-Xylene	m-Xylene	p-Xylene

Many other disubstituted benzenes have commonly used trivial names. Among them are:
 

The number of known trisubstituted benzenes can readily be imagined from the short list above. Many have trivial names, but many do not. When in doubt of the translation of a trivial name, The Merck Index, Merck & Co., Inc., Rahway, NJ, USA, is an invaluable and relatively inexpensive first reference. The next stop of the author when still in doubt is an Internet search. Naturally tetra-, penta-, and hexasubstituted benzenes abound.

Although our coverage of aromatic compounds has only scratched the surface of this topic, it's time to move on to broaden our base of categories.

XII. Heterocyclic compounds

Three major methods exist for naming heterocyclic compounds:

1) The Hantzsch-Widman Suffix method, first proposed in 1887-8 and since modified;

2) The replacement method, also widely used;

3) Trivial names.

The Hantzsch-Widman system is clarified by a careful study of the tables below. In the replacement system, the rings are first named as if all ring members were carbon atoms. The hetero atoms are identified by prefixes from the table on the next page. The trivial system, of course, often defies explanation.

Hantzsch-Widman Suffixes

The tables below are reproduced from "Nomenclature of Organic Compounds," Advances in Chemistry Series #126, American Chemical Society, Washington, D.C., 1974.
 

Every suffix requires a prefix; the most common hetero atoms (i.e. non-carbon atoms) to be identified as part of a heterocyclic ring system are:
 

The final a- is often elided preceding a leading vowel in the suffix. The system can best be understood by examples. Let's first consider a very common hetero atom, nitrogen, and the most common ring size, six members. The name shown in bold typeface is the one most commonly encountered in the literature.
 

Hantzsch-Widman name:	Azine
Replacement name:	Azabenzene
Trivial name:	Pyridine
Hantzsch-Widman name:	1,4-Diazine
Replacement name:	1,4-Diazabenzene
Trivial name:	Pyrazine
Hantzsch-Widman name:	1,3-Diazine
Replacement name:	1.3-Diazabenzene
Trivial name:	Pyrimidine
Hantzsch-Widman name:	1,2-Diazine
Replacement name:	1,2-Diazabenzene
Trivial name:	Pyridazine
Hantzsch-Widman:	2,4,6-Triamino-1,3,5-triazine
Replacement:	2,4,6-Triamino-1,3,5-triazabenzene
Trivial:	Melamine
Hantzsch-Widman:	2,4,6-Trihydroxy-1,3,5-triazine
Replacement:	2,4,6-Trihydroxy-1,3,5-triazabenzene
Trivial:	Cyanuric acid

Hantzsch-Widman:	1,2-Dihydroazine
Replacement:	1-Azacyclohexa-3,5-diene
Trivial:	1,2-Dihydropyridine
Hantzsch-Widman:	1,2,3,6-Tetrahydro-1,3-diazine
Replacement:	1,3-Diazacyclohex-4-ene
Trivial:	1,2,3,6-Tetrahydropyrimidine
Hantzsch-Widman:	Perhydro-1,4-diazine
Replacement:	1,4-Diazacyclohexane
Trivial:	Perhydropyrazine

Part XX will continue the discussion of nitrogen heterocycles and will add the excitement of oxygen in the ring!