https://doi.org/10.1351/goldbook.R05260
- The configurationof anystereogenic(asymmetric) centrewith respect to any other stereogenic centre contained within the same molecular entity. Unlikeabsolute configuration, relative configuration is reflection-invariant. Relative configuration, distinguishingdiastereoisomers, may be denoted by the configurational descriptors
, (or l) and , (or u) meaning, respectively, that the two centres have identical or opposite configurations. For molecules with more than twoasymmetriccentres the prefixrel- may be used in front of the name of oneenantiomerwhere R and S have been used. If any centres have knownabsolute configurationthen only and can be used for the relative configuration.See also:α (alpha), β (beta) (Defs. 1 and 3) - Two different molecules Xabcd and Xabce, may be said to have the same relative configurations if e takes the position of d in the tetrahedral arrangement of ligandsaround X (i.e. the pyramidal fragments Xabc are superposable). By the same token theenantiomerof Xabce may be said to have the opposite relative configuration to Xabcd. The terms may be applied tochiralmolecular entities with central atoms other than carbon but are limited to cases where the two related molecules differ in a single ligand.
Both definitions can be generalized to include
stereogenic units
other than asymmetric
centres.