Stereozentrum

Stereozentrum, the German term for a stereogenic center, denotes an atom at which the interchange of two substituents would generate a different stereoisomer. The most familiar example is a tetrahedral carbon atom attached to four different substituents.

If all four substituents are different, the center is chiral and the molecule exists as non-superimposable mirror images (enantiomers). The two enantiomers have opposite configurations (R and S according to CIP rules) and typically rotate plane-polarized light in equal but opposite directions. When some substituents are identical or arranged in a way that internal symmetry cancels chirality, the center is not stereogenic, and the molecule may be achiral or possess meso forms.

Not all stereozenters are carbon. Other atoms such as nitrogen, sulfur, phosphorus, or silicon can be stereogenic under certain geometries, especially when inversion is blocked or at quaternary centers. For example, stereogenic nitrogen centers are uncommon because rapid inversion often racemizes the configuration, whereas quaternary ammonium or rigid, pyramidal systems can give stable stereoisomerism.

Determination and analysis of stereochemistry rely on priority rules (CIP) to assign R or S configurations, as well as experimental techniques such as NMR (coupling constants and NOE correlations), optical rotation measurements, and X-ray crystallography. Stereozentren tragen entscheidend zur Eigenschaftsvielfalt von Molekülen bei und beeinflussen Reaktivität, Diastereomerie und biologische Aktivität.