Stereoisomerie

Stereoisomerism refers to compounds that share the same molecular formula and the same connectivity of atoms but differ in the three‑dimensional arrangement of those atoms in space. These differences lead to distinct spatial arrangements that can give rise to different physical and chemical properties, even though the atoms are the same.

The two main classes are enantiomers and diastereomers. Enantiomers are non‑superimposable mirror images and often exhibit opposite optical activity, interacting differently with chiral environments such as biological systems. Diastereomers are stereoisomers that are not mirror images and typically have different physical properties and reactivities. A related concept is geometric isomerism, including cis/trans or E/Z isomerism, which describes different arrangements around double bonds or within cyclic systems where rotation is restricted.

Stereoisomers may include conformational isomers, or conformers, arising from rotation around single bonds. These interconvert rapidly at room temperature in many cases and are sometimes not isolated as distinct stereoisomers, though they can become distinguishable in certain contexts or at low temperatures.

Absolute configuration assigns stereocenters as R or S according to CIP priority rules. Molecules with chirality may exist as racemates (equal mixtures of enantiomers) or as enantiomerically enriched samples. Mesodisformations, such as meso-tartaric acid, are achiral despite having stereocenters due to internal symmetry.

Examples include lactic acid and tartaric acid, where different stereoisomers can have markedly different biological activities. In chemistry and pharmacology, control of stereochemistry is crucial for activity, selectivity, and safety. Techniques for determination and separation include polarimetry, chiral chromatography, and NMR with chiral shift reagents.