stereogenic

Stereogenic is an adjective used in chemistry to describe a molecule, atom, or arrangement that can give rise to stereoisomerism. A stereogenic element is any atom or group whose replacement or rearrangement leads to a different stereoisomer. The most familiar example is a stereogenic center, typically a tetrahedral carbon bonded to four different substituents, which can produce non-superimposable mirror images known as enantiomers.

Beyond stereogenic centers, other elements can create stereoisomerism. Stereogenic axes arise when restricted rotation around a bond generates axial chirality, as seen in certain biaryl compounds. Stereogenic planes occur in planar-chiral systems where reflection across a plane yields distinct stereoisomers. Helicity in structures like helicenes can also generate stereogenic elements. In total, a molecule may be stereogenic through one or more of these elements.

Not all stereogenic-containing molecules are chiral in the absolute sense. Symmetry can produce meso forms where internal planes of symmetry render the molecule achiral despite the presence of stereogenic centers. When a molecule is stereogenic, it can have different stereoisomers, typically categorized as enantiomers (mirror images) or diastereomers (not mirror images).

Descriptors such as R/S are used for chiral centers according to CIP rules, while axial and planar chirality use corresponding conventions. The term “stereogenic” therefore signals the potential for stereoisomerism, even if the actual number of stereoisomers is limited by symmetry or other constraints.