stereogene

Stereogene is a term used in chemistry to refer to a stereogenic element within a molecule—an atom or a defined point whose substitution can lead to different stereoisomers. In practice, a stereogene is typically an atom, most often carbon, that is bonded to four different substituents, such that exchanging any two substituents produces a distinct stereoisomer. When a molecule contains one or more stereogenes, it may be chiral, meaning it has non-superimposable mirror images (enantiomers).

Commonly, the expression stereogene is used interchangeably with “stereogenic center” or “stereogenic atom,” though the more usual English term is stereogenic center. Substituents around a stereogene may be diverse, and their arrangement determines the molecule’s absolute configuration, often designated R or S by the Cahn–Ingold–Prelog rules. The presence of multiple stereogenes yields a number of possible stereoisomers up to 2^n, where n is the number of independent stereogenic elements, subject to symmetry that can reduce this number.

Stereogenes are central to stereochemistry, the study of three-dimensional arrangements in molecules and their chemical behavior. They underpin enantioselectivity in reactions and the activity of many biological molecules, such as pharmaceuticals, where different stereoisomers can have markedly different properties. Detecting and characterizing stereogenes involves methods like optical rotation, circular dichroism, and chiral chromatography, as well as structural determination by spectroscopy or crystallography.

Historically, the concept underlies the development of asymmetric synthesis and the broader field of chirality, with roots in 19th-century work on optical activity. In modern usage, stereogene is most often discussed in the context of stereogenic centers and their impact on molecular chirality. See also chiral center, enantiomer, diastereomer, and stereochemistry.