Axial chirality

Axial chirality differs from central chirality (point chirality) in that axial chirality does not require a chiral center such as an asymmetric carbon atom, the most common form of chirality in organic compounds. Bonding to asymmetric carbon has the form Cabcd where a, b, c, and d must be distinct groups. Allenes have the form abC=C=Ccd and the groups need not all be distinct as long as groups in each pair are distinct: abC=C=Cab is sufficient for the compound to be chiral, as in penta-2,3-dienedioic acid. Similarly, chiral atropisomers of the form abC−Ccd may have some identical groups (abC−Cab), as in BINAP.

Nomenclature

The enantiomers of axially chiral compounds are usually given the stereochemical labels (R_a) and (S_a), sometimes abbreviated (R) and (S). The designations are based on the same Cahn–Ingold–Prelog priority rules used for tetrahedral stereocenters. The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority (1 and 2) than the far ones (3 and 4).