Hantzsch-Widman nomenclature

Hantzsch–Widman nomenclature, also called the extended Hantzsch–Widman system, is a type of systematic chemical nomenclature used for naming heterocyclic parent hydrides having no more than ten ring members. Some common heterocyclic compounds have retained names that do not follow the Hantzsch–Widman pattern.

Hantzsch–Widman nomenclature is named after the German chemist Arthur Hantzsch and the Swedish chemist Oskar Widman, who independently proposed similar methods for the systematic naming of heterocyclic compounds in 1887 and 1888 respectively. It forms the basis for many common chemical names, such as dioxin and benzodiazepine.

A Hantzsch–Widman name will always contain a prefix, which indicates the type of heteroatom present in the ring, and a stem, which indicates both the total number of atoms and the presence or absence of double bonds. The name may include more than one prefix, if more than one type of heteroatom is present; a multiplicative prefix if there are several heteroatoms of the same type; and locants to indicate the relative positions of the different atoms. Hantzsch–Widman names may be combined with other aspects of organic nomenclature, to indicate substitution or fused-ring systems.

The Hantzsch–Widman prefixes indicate the type of heteroatom(s) present in the ring. They form a priority series: If there is more than one type of heteroatom in the ring, the prefix that is higher on the list comes before the prefix that is lower on the list. For example, "oxa" (for oxygen) always comes before "aza" (for nitrogen) in a name. The priority order is the same as that used in substitutive nomenclature, but Hantzsch–Widman nomenclature is recommended only for use with a more restricted set of heteroatoms (see also below).

All of the prefixes end in "a": In Hantzsch–Widman nomenclature (but not in some other methods of naming heterocycles), the final "a" is elided when the prefix comes before a vowel.

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