2isomer

2-isomer is a term used in organic chemistry to refer to a positional isomer in which a substituent, functional group, or multiple bond is located at carbon-2 of a parent chain or ring. The position of substituents is indicated by numbers in IUPAC nomenclature, and the set of possible positional isomers includes 1-, 2-, 3-, and so on. The label “2-isomer” by itself is ambiguous without specifying the parent compound and the substituent; more precise names such as 2-bromopropane or 2-butanol are used to identify the exact structure.

Examples illustrate the concept. For a three-carbon chain, 1-bromopropane and 2-bromopropane are positional isomers, as are 1-butanol and 2-butanol for a four-carbon alcohol. In cycloalkanes, substituents can also occupy different positions, so 1-chlorocyclohexane and 2-chlorocyclohexane are positional isomers of the same molecular formula.

Differences from stereoisomerism are important: 2-isomer refers to a change in the position of atoms or groups along the carbon skeleton (constitutional or skeletal isomerism), not to the spatial arrangement of atoms around a stereocenter. Therefore, 2-isomers are contrasted with other types of isomers such as geometric (cis/trans) or enantiomeric pairs, which involve stereochemistry rather than just positional differences.

In practice, chemists usually replace the generic term with the full name of the compound or with the broader designation “positional isomer” to avoid ambiguity.