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
ligands around X (i.e. the pyramidal fragments Xabc are superposable). By the same token
the
enantiomer of Xabce may be said to have the opposite relative configuration to Xabcd. The terms
may be applied to
chiral molecular entities with central atoms other than carbon but are limited to cases
where the two related molecules differ in a single ligand.
,
(or l) and
,
(or u)
meaning, respectively, that the two centres have identical or opposite configurations.
For molecules with more than two asymmetric centres the prefix rel- may be used in front of the name of one enantiomer where
R
and
S have been used. If any centres have known absolute configuration then only
and
can be used for the relative configuration.
