stereocentrum

A stereocentrum, also called a stereogenic center, is an atom at which exchanging two substituents gives a stereoisomer. In organic chemistry, the prototypical stereocentrum is a tetrahedral carbon bonded to four different groups, making the molecule chiral and able to exist as non-superimposable mirror images (enantiomers). Stereocenters can also occur at other elements, such as sulfur, phosphorus, or silicon, when they bear four distinct substituents or configurations that break symmetry.

Determining the configuration of a stereocentrum relies on the Cahn–Ingold–Prelog (CIP) priority rules. Substituents are ranked by atomic number and priority of their atoms, and the arrangement of substituents around the center is described as R (rectus) or S (sinister) based on the order of decreasing priority when the lowest-priority group is oriented away from the observer.

The presence of one or more stereocenters often confers optical activity, with enantiomers rotating plane-polarized light in opposite directions. Molecules with multiple stereocenters can also form diastereomers, which are not mirror images of each other. Some compounds, however, are achiral despite possessing stereocenters, due to internal planes of symmetry (meso forms) or rapid inversion at certain centers.

Examples illustrate the concept: lactic acid has one stereocentrum; tartaric acid has two and can exist as a pair of enantiomers or as a meso compound. Stereocenters are central to stereochemistry, influencing properties such as reactivity, pharmacology, and physical behavior in many compounds.