Stereoisomer

A stereoisomer is a type of isomer whose molecules have the same molecular formula and the same connectivity of atoms but differ in the three-dimensional arrangement of those atoms. Stereoisomers are distinguished from constitutional (or structural) isomers, which differ in the order in which atoms are linked.

There are two main classes: enantiomers and diastereomers. Enantiomers are non-superimposable mirror images of each other and typically occur when a molecule contains one or more stereocenters. They have identical physical properties in achiral environments but rotate plane-polarized light in opposite directions and interact differently with chiral reagents and biological systems. Enantiomer pairs are assigned configurations such as R and S.

Diastereomers are stereoisomers that are not mirror images of each other. They arise when a molecule has two or more stereocenters or from restricted rotation around double bonds or ring systems. Geometric isomers such as cis/trans or E/Z are common diastereomers, often with different physical properties and reactivities. A meso compound is a special case of diastereomerism where the molecule is achiral despite having stereocenters.

Conformational isomers, or conformers, result from rotation about single bonds and can rapidly interconvert at typical temperatures; they are sometimes considered separately from true stereoisomers.

Notation and analysis: stereochemistry at stereocenters is described by the Cahn–Ingold–Prelog R/S system, while double-bond geometry is described by E/Z or cis/trans. Determining stereochemistry can involve optical rotation measurements, X-ray crystallography, or NMR with chiral reagents.