Stereocenter

A stereocenter is an atom in a molecule whose exchange of two attached groups produces a stereoisomer. It is often, but not always, a tetrahedral carbon bonded to four different groups.

Stereoisomer: A molecule that has the same connectivity as another molecule but a different spatial arrangement of its atoms. Exchanging two groups at a stereocenter produces a stereoisomer.
R–S notation: A system that labels a stereogenic center R or S according to substituent priorities and spatial order. It gives a compact description of the arrangement around many stereocenters.
Stereogenic center: A location in a molecule where exchanging two groups produces stereoisomers. It is often used interchangeably with stereocenter, but terminology can distinguish broader stereogenic sites.
Enantiomer: One of a pair of stereoisomers that are nonsuperposable mirror images of each other. A molecule with one stereocenter and no other stereogenic features commonly has an enantiomer.
Lactic acid: A three-carbon carboxylic acid with a hydroxyl group on its middle carbon; it exists as two enantiomers. Its central carbon is a straightforward example of a tetrahedral stereocenter.
Molecular connectivity: The pattern of which atoms in a molecule are bonded to one another. A stereocenter changes spatial arrangement without changing which atoms are connected.
Stereospecific reaction: A reaction in which the stereochemistry of the reactant determines the stereochemistry of the product. Such reactions can preserve or systematically change a stereocenter's configuration.
Chiral center: An atom whose arrangement makes it a local source of molecular chirality, commonly a tetrahedral atom with four different substituents. The term is often treated as synonymous, though stereocenter is defined by group exchange.
Diastereomer: A stereoisomer that is not the mirror image of another stereoisomer. Molecules with multiple stereocenters can differ at some, but not all, centers to form diastereomers.
Tartaric acid: A dicarboxylic acid with two stereogenic centers and three distinct stereoisomers, including a meso form. Its symmetry shows how two stereocenters can yield an achiral molecule.

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