1. The @[email protected] of any @[email protected] @[email protected] with respect to any other stereogenic centre contained within the same molecular entity. Unlike @[email protected], relative configuration is reflection-invariant. Relative configuration, distinguishing @[email protected], may be denoted by the configurational descriptors \(R^{\text{*}}\), \(R^{\text{*}}\) (or l) and \(R^{\text{*}}\), \(S^{\text{*}}\) (or u) meaning, respectively, that the two centres have identical or opposite configurations. For molecules with more than two @[email protected] centres the prefix @[email protected]- may be used in front of the name of one @[email protected] where R and S have been used. If any centres have known @[email protected] then only \(R^{\text{*}}\) and \(S^{\text{*}}\) can be used for the relative configuration.
    See also:
    α (alpha), β (beta) (Defs. 1 and 3)
  2. 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 @[email protected] around X (i.e. the pyramidal fragments Xabc are superposable). By the same token the @[email protected] of Xabce may be said to have the opposite relative configuration to Xabcd. The terms may be applied to @[email protected] molecular 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 @[email protected] other than @[email protected] centres.
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2217 [Terms] [Paper]