How are the Cahn-Ingold-Prelog rules applied to chiral molecules without a stereogenic carbon center? i.e biphenyls and allenes

Question

It's easy to apply CIP rules to chiral carbons but what about in chiral molecules without these carbon cneters like biphenyls, sulphoxides and allenes? Are they applied at all?

Answer 1

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 R_a. The symbol is S_a if this path is anticlockwise.