locant

Locant is a numeric indicator used in systematic chemical nomenclature to specify the position of a substituent, functional group, or unsaturation on a parent structure such as a chain, ring, or aromatic system. Each locant corresponds to a specific position along the main skeleton, typically a carbon atom in organic compounds.

In IUPAC naming, locants are attached to substituent names or placed to indicate where a feature occurs. Examples include 2-butanol, where the hydroxyl group is on carbon 2; 1,3-dichlorobenzene, indicating chlorine substituents at positions 1 and 3 on a benzene ring; and but-2-ene, where the double bond starts at carbon 2. For rings and arenes, locants are determined so that the substituents receive the lowest possible numbers; numbering can proceed in either direction around a ring, and the direction yielding the lowest set of locants is chosen. Locants are separated by commas and listed in ascending order; multiplicative prefixes such as di- and tri- do not affect the locant values themselves. If two numbering schemes produce the same locant set, tie-breaking may involve alphabetical order of the substituent names.

Limitations: not all naming uses explicit locants, especially in common or trivial names where positions are implied. The locant concept is a core element of the IUPAC system, designed to minimize ambiguity in chemical names by precisely locating functional groups, substituents, and multiple bonds.