Locants

Locants are numerals used in chemical nomenclature to indicate the position of a substituent, functional group, multiple bond, or ring position within a parent structure. They serve as position indicators and are essential for uniquely identifying a compound.

In IUPAC naming, the parent structure is chosen to include the principal functional group and to maximize the length or complexity of the parent. Locants are then assigned to substituents and features from the end of the parent that yields the lowest set of locants, according to the lowest-locant rule. When two possible sets are compared, the set with the smaller number at the first point of difference is preferred. Locants are written in ascending order, separated by hyphens from substituent names, and by commas between multiple numbers.

Special considerations include the locants for double and triple bonds; these must be chosen to give the lowest possible numbers, and in many cases they influence which end of the chain is numbered first. The presence of higher-priority functional groups can also affect parent selection and numbering.

Ring systems follow similar principles. For cycloalkanes, numbering begins at a substituent in a way that produces the lowest locant set around the ring; a single substituent is typically numbered as 1. For disubstituted rings, common patterns are 1,2- (ortho), 1,3- (meta), and 1,4- (para), used especially in aromatic rings such as benzene.

Examples include 2-bromobutane, indicating a bromine at carbon 2, and 1,3-dichlorobenzene, with chlorines at positions 1 and 3 on the ring. Locants provide precise, machine-readable, and unambiguous names.