How are the Cahn-Ingold-Prelog rules applied to chiral molecules without a stereogenic carbon center? i.e biphenyls and allenes
Structures with axial chirality (e.g. an allene or a 2,2′,6,6′-tetrasubstituted biphenyl) with two pairs of substituents (ab and cd) are regarded as elongated tetrahedra and viewed along the chiral axis. The substituents (a, b, c, and d) are arranged in the elongated tetrahedral system.

In each pair (ab as well as cd), one higher-ranking substituent (a > b and c > d) is chosen using the sequence rule.

If the path going from a to b to c (while looking toward d) is clockwise, the chirality is described by the symbol Ra. The symbol is Sa if this path is anticlockwise.