Stereocenter

A stereocenter, or stereogenic center, is an atom in a molecule at which the interchange of two substituents changes the molecule’s stereoisomeric form. In practice, this most often refers to a tetrahedral carbon bonded to four different groups, creating a chiral center. If the four substituents are not all different, the atom is not a stereocenter and the molecule may be achiral.

When a molecule contains a stereocenter, it can exist as non-superimposable mirror-image forms called enantiomers. Related stereoisomers, called diastereomers, arise when a molecule has two or more stereocenters. Doable symmetry, such as an internal plane of symmetry, can render a molecule meso and achiral despite the presence of stereocenters.

Configuration around a stereocenter is commonly assigned using the R/S system according to the Cahn–Ingold–Prelog (CIP) priority rules. The designation R (rectus) or S (sinister) describes the arrangement of the four substituents. Double bonds are described separately as E or Z according to CIP priorities.

Although carbon is the most familiar stereocenter, other atoms can be stereogenic under suitable circumstances (for example, certain phosphorus, sulfur, silicon, or nitrogen centers). Nitrogen often rapidly inverts its configuration, so many amines do not constitute robust stereocenters at room temperature unless constraints prevent inversion.

Stereocenters are central to fields such as organic synthesis, medicinal chemistry, and pharmacology because enantiomers can differ in taste, odor, reactivity, and biological activity.