Stereochemistry

Stereochemistry is the branch of chemistry that studies the spatial arrangement of atoms in molecules and how this arrangement affects properties and reactions. It deals with stereoisomers—compounds that have the same connectivity but differ in three-dimensional orientation.

Two broad categories are configurational and conformational stereoisomerism. Configurational isomers require breaking bonds to interconvert, and include enantiomers (non-superimposable mirror images) and diastereomers (not mirror images). Geometric isomerism, such as cis/trans or E/Z isomerism, is a common subset where rotation around a bond leads to different spatial arrangements. Conformational isomers, or conformers, arise from rotation about single bonds and may rapidly interconvert.

Chirality is a central concept: a molecule is chiral if it cannot be superimposed on its mirror image. Most chiral molecules contain one or more stereogenic centers, typically tetrahedral carbon atoms with four different substituents, though chirality can arise from other arrangements as well. The absolute configuration of a stereocenter is designated R (clockwise priority) or S (counterclockwise) according to the Cahn–Ingold–Prelog (CIP) rules.

Optical activity is a common property of chiral compounds; enantiomers rotate plane-polarized light in opposite directions, quantified as specific rotation. A racemate is a 1:1 mixture of enantiomers and may exhibit no net rotation. Enantiomeric excess measures the predominance of one enantiomer.

Stereochemical information influences chemistry and biology, including reaction outcomes (stereoselectivity and stereospecificity) and biological activity. Techniques such as polarimetry and chiral chromatography help analyze enantiomeric composition. Applications are prominent in pharmaceuticals, agrochemicals, and biochemistry, where one enantiomer can differ in efficacy or safety from its mirror image.