stereoisomerism

Stereoisomerism is the branch of chemistry that studies isomers which have the same molecular formula and connectivity but differ in the three‑dimensional arrangement of their atoms. Stereoisomerism is typically divided into configurational isomerism, in which isomers cannot interconvert without breaking bonds, and conformational isomerism, which arises from rotation about single bonds and interconverts readily at room temperature. Configurational stereoisomers are further classified as enantiomers and diastereomers.

Enantiomers are non-superimposable mirror images, usually arising from one or more stereogenic (chiral) centers. They have identical physical properties in achiral environments but rotate plane-polarized light in opposite directions and can interact differently with other chiral substances. Each stereocenter is assigned an R or S configuration, and a molecule with a single stereogenic center exists as a pair of enantiomers.

Diastereomers are stereoisomers that are not mirror images. They may differ at one or more stereocenters and often have different physical properties. Geometric isomers, a subset of diastereomers, occur when restricted rotation around a double bond or in a cyclic system produces cis (same side) and trans (opposite side) forms. Common examples include cis- and trans-2-butene and the geometric forms of maleic and fumaric acids.

Compounds with multiple stereocenters may exist as meso forms, which are achiral due to an internal plane of symmetry. For example, the meso form of tartaric acid is optically inactive despite having stereogenic centers.

Methods to distinguish and determine stereochemistry include measuring optical rotation, chiral chromatography or NMR with chiral solvating agents, and X-ray crystallography. In synthesis and pharmacology, stereochemistry is crucial because enantiomers and diastereomers can differ markedly in biological activity and pharmacokinetics; hence many processes aim to control stereochemical outcome using asymmetric synthesis, chiral catalysts, or chiral auxiliaries.