lowestlocant

Lowest locant is a rule used in IUPAC nomenclature to assign the numerical positions of substituents, functional groups, and unsaturation on a carbon skeleton in such a way that the resulting set of locants is as small as possible. The term is often described as the lowest set of locants rule (LSLR). When multiple numbering schemes are possible for a molecule, each scheme yields a different set of locants. These sets are arranged in increasing order, and the preferred scheme is the one whose locant sequence is smaller at the first point where the sequences differ.

A common way to apply the rule is to list the locants for each possible numbering, order them, and compare the sequences from left to right. The scheme with the smaller locant at the first difference is chosen. For example, a molecule that can be numbered to give substituents at positions 1 and 3 (set 1,3) is preferred over numbering that would place them at 2 and 4 (set 2,4), since 1 < 2 at the first difference.

In practice, the lowest locant principle often interacts with other naming rules. If a principal functional group (such as a carboxyl, carbonyl, or alcohol group) is present, its locant is typically given priority so that the group receives the lowest possible number. For cyclic compounds, numbering around the ring is chosen to minimize the set of locants, often considering both directions around the ring. In more complex cases, other criteria, such as alphabetical ordering of substituents or functional group precedence, may influence the final name.

See also: IUPAC nomenclature, locant rules, parent selection.