Stereocentra

Stereocentra, or stereocenters, are atoms within a molecule where exchanging two substituents produces a stereoisomer. They are the source of molecular chirality. The most common stereocenters are tetrahedral carbon atoms bearing four different substituents, which yields two nonsuperimposable mirror images, or enantiomers. If a molecule has several stereocenters, the overall stereochemical relationship can be described as enantiomeric or diastereomeric pairs. A molecule may be meso if an internal plane of symmetry makes it superimposable on its mirror image despite having stereocenters.

Stereocenters can also occur at other elements that adopt tetrahedral geometry and bear four different substituents; common examples include sulfur atoms in sulfoxides, and certain phosphorus and silicon centers. In these cases, the center is designated R or S by the CIP rules just as for carbon stereocenters.

Configural assignment follows the Cahn–Ingold–Prelog priority rules; absolute configuration is labeled R or S, depending on the arrangement of substituents around the stereocenter. Optical activity—rotation of plane-polarized light—can provide experimental evidence of chirality, though some meso compounds are achiral.

In synthesis and analysis, isolating or counting stereocenters informs the number of possible stereoisomers, up to 2^n for n stereocenters, adjusted for meso forms.