R–S notation

R–S notation assigns absolute configurations to stereogenic centers by ranking attached groups under Cahn–Ingold–Prelog rules and tracking their spatial order.

Cahn–Ingold–Prelog priority rules: Rules for ranking atoms or groups by atomic number, isotope mass, and successive points of difference. These rules establish the priority order used to assign each center’s configuration.
Stereogenic center: An atom at which exchanging two attached groups produces a stereoisomer. R–S notation is assigned at stereogenic centers, though not every such center is tetrahedral.
IUPAC nomenclature: The systematic naming framework developed by the International Union of Pure and Applied Chemistry. R–S descriptors specify stereochemistry within systematic names of compounds.
D–L notation: A configurational naming system that relates molecular structures to glyceraldehyde as a reference. D and L labels use a different reference system and do not generally determine R or S.
Tetrahedral stereochemistry: The spatial arrangement of four different substituents around a central atom. The common tetrahedral case turns the ranked substituents into an R or S designation.
Chirality: The property of an object that makes it non-superimposable on its mirror image. Many R–S assignments describe centers that contribute to molecular chirality.
Thalidomide: A drug associated with severe birth defects when taken during pregnancy and used in treating some conditions. Its stereochemistry illustrates why configurations and interconversion must be treated carefully in drug discussions.
E–Z notation: A system for specifying the arrangement of higher-priority substituents across a double bond. E and Z describe double-bond geometry rather than configuration at a stereogenic center.
Fischer projection: A two-dimensional convention for depicting stereochemistry, with horizontal bonds directed toward the viewer and vertical bonds away. Its fixed bond directions determine how a Fischer drawing must be oriented before assigning R or S.
Enantiomer: One of a pair of stereoisomers that are nonsuperimposable mirror images. Enantiomers generally have opposite configurations at corresponding stereogenic centers.

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