stereocentrums

Stereocenters, also called stereogenic centers, are atoms at which exchanging two substituents gives a different stereoisomer. In most organic molecules this is a tetrahedral carbon bonded to four different groups, so swapping two substituents yields the enantiomer, a non-superimposable mirror image. Molecules with several stereocenters can exist as multiple stereoisomers, including enantiomeric pairs and diastereomers.

Stereocenters are not limited to carbon. Other atoms can be stereogenic if they bond to four different substituents or to a unique three-dimensional environment; examples include sulfur in sulfoxides and phosphorus in certain phosphoranes, as well as some silicon and metal centers in coordination complexes. Nitrogen can be stereogenic in constrained systems where inversion is blocked, but such cases are uncommon at room temperature.

To identify a stereocenter, check whether the atom has four different substituents. If two substituents are identical, it is not a stereocenter by the usual definition. Stereochemistry can also arise from restricted rotation (atropisomerism) or from axial or planar chirality, which do not involve a single central atom.

Prochirality and symmetry: a prochiral center becomes chiral upon substitution. A molecule with multiple stereocenters may be meso if it possesses an internal plane of symmetry, making it achiral despite having stereogenic sites.

Notation and counting: absolute configurations are designated R or S by CIP rules. For a molecule with n stereocenters, up to 2^n stereoisomers exist, though symmetry can reduce this number.

Examples and significance: lactic acid contains a single stereocenter; 2-butanol contains one. Stereocenters determine properties and biological activity, motivating enantioselective synthesis and chiral purification in chemistry and pharmacology.