A grouping within a molecular entity that may be considered a focus of @[email protected] At least one of these must be present in every @[email protected] (though the presence of stereogenic units does not conversely require the corresponding chemical species to be @[email protected]). Three basic types are recognized for molecular entities involving atoms having not more than four substituents:
  1. A grouping of atoms consisting of a @[email protected] and distinguishable @[email protected], such that the interchange of any two of the substituents leads to a stereoisomer. An @[email protected] atom (@[email protected]) is the traditional example of this stereogenic unit.
  2. A chain of four non-coplanar atoms (or rigid groups) in a @[email protected] @[email protected], such that an imaginary or real (restricted) rotation (with a change of sign of the @[email protected]) about the central bond leads to a stereoisomer.
  3. A grouping of atoms consisting of a double bond with substituents which give rise to @[email protected]
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2219 [Terms] [Paper]