Regioisomerism

Regioisomerism is a form of structural isomerism in organic chemistry. It occurs when two or more compounds share the same molecular formula and the same carbon skeleton, but differ in the position at which a substituent or functional group is attached. In other words, the compounds are regioisomers because their substituents occupy different positions on the same framework.

Regioisomerism is a subset of constitutional (structural) isomerism rather than stereoisomerism. The key distinction is the location of groups within the molecule, not the spatial arrangement of atoms in three-dimensional space. Reactions that produce different attachment sites are governed by regiochemistry, and the preferred outcome is described by regioselectivity.

Common sources of regioisomerism include electrophilic aromatic substitution on unsymmetrical rings, where substituents direct new groups to certain positions (for example, methyl groups directing to ortho and para positions on a benzene ring), yielding ortho-, meta-, or para-regioisomers. Regioisomerism also appears in additions to alkenes, where reagents can add to different carbons of the double bond. A classic case is the addition of HX to propene, which can yield mainly 2-bromopropane (Markovnikov addition) or, under certain conditions, 1-bromopropane (anti-Markovnikov addition).

Understanding regioisomerism is important for synthesis and product planning, as different regioisomers often exhibit distinct physical properties and reactivities. Separation and selective formation of a desired regioisomer are common goals in organic synthesis.