regioisomer

Regioisomer refers to a type of isomer in which two or more compounds have the same molecular formula and the same overall connectivity of atoms, but differ in the positions of functional groups or substituents on the main framework. In contrast to stereoisomers, which differ in spatial arrangement, regioisomers differ in where substituents are attached along a chain or ring. Regioisomerism is a common feature in organic chemistry and is important for determining chemical reactivity and physical properties.

A classic example involves disubstituted benzene rings. Ortho-, meta-, and para- (o-, m-, p-) isomers have the same benzene ring and the same substituents, but the second substituent occupies different positions relative to the first. Electrophilic aromatic substitution on toluene shows regioselectivity, with directing effects of the methyl group favoring ortho and para positions, producing mainly o- and p-nitrotoluene over the meta isomer.

Regioisomerism also arises in reactions of unsymmetrical substrates, where different regiochemical outcomes can occur. In alkene additions, the Markovnikov rule governs the preferred placement of a new group, but alternative conditions (such as hydroboration-oxidation) can give anti-Markovnikov products, yielding different regioisomers. Recognizing regioisomerism helps in planning syntheses, predicting product distributions, and understanding how directing groups, catalysts, and reaction conditions influence outcomes.

Identification and separation of regioisomers rely on spectroscopic and chromatographic methods, and their distinct physical properties can be exploited in purification and applications, including pharmaceuticals and materials science.