A grouping within a molecular entity that may be considered a focus of stereoisomerism
. At least one of these must be present in every enantiomer
(though the presence of stereogenic units does not conversely require the corresponding
chemical species to be chiral
). Three basic types are recognized for molecular entities involving atoms having
not more than four substituents:
A grouping of atoms consisting of a central atom and distinguishable ligands, such that the interchange of any two of the substituents leads to a stereoisomer.
An asymmetric atom (chirality centre) is the traditional example of this stereogenic unit.
A chain of four non-coplanar atoms (or rigid groups) in a stableconformation, such that an imaginary or real (restricted) rotation (with a change of sign of the
torsion angle) about the central bond leads to a stereoisomer.
A grouping of atoms consisting of a double bond with substituents which give rise
to cis-trans isomerism.
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2219
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook