stereoisomersmolecules

Stereoisomers are molecules that have the same molecular formula and the same connectivity of atoms but differ in the three-dimensional arrangement of those atoms in space. Because their shapes are different, stereoisomers can display distinct physical and chemical properties, especially in chiral environments. They are different from structural (constitutional) isomers, which have different connectivity.

There are several main types of stereoisomers. Enantiomers are pairs of non-superimposable mirror-image molecules that share many properties but interact differently with plane-polarized light and with biological systems. Diastereomers are stereoisomers that are not mirror images of each other and often have more widely differing properties. Geometric isomers, a subset of diastereomers, arise around restricted rotation about double bonds or within cyclic structures and include cis/trans forms. Some stereoisomers, called meso compounds, possess internal symmetry and are achiral despite having stereocenters.

Nomenclature and measurement reflect the arrangement of substituents around stereogenic elements. A stereocenter, often a carbon atom with four different substituents, is assigned an R or S configuration using the Cahn–Ingold–Prelog priority rules. For double bonds, the E/Z system indicates the relative positions of high-priority groups. Enantiomers exhibit optical activity, rotating plane-polarized light in opposite directions; a racemate is a 1:1 mixture of enantiomers.

Applications of stereochemistry are central to chemistry and biology. The two enantiomers of a drug, for example, can have very different efficacy and safety profiles. Methods to obtain or separate specific stereoisomers include asymmetric synthesis, chiral catalysis, and resolution. Analytical techniques such as polarimetry and chiral chromatography are used to characterize and quantify stereoisomeric composition.