What is "prochirality"

n. (context chemistry English) the property displayed by a prochiral molecule or other entity; protostereoisomerism

In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step. An achiral species which can be converted to a chiral in two steps is called proprochiral.

If two identical substituents are attached to a sp-hybridized atom, the descriptors pro-R and pro-S are used to distinguish between the two. Promoting the pro-R substituent to higher priority than the other identical substituent results in an R chirality center at the original sp-hybridized atom, and analogously for the pro-S substituent.

A trigonal planar sp-hybridized atom can be converted to a chiral center when a substituent is added to the re or si face of the molecule (pronounced "ray" and "sigh"). A face is labeled re if, when looking at that face, the substituents at the trigonal atom are arranged in decreasing Cahn-Ingold-Prelog priority order in a clockwise order, and si if the priorities decrease in counter-clockwise order; note that the designation of the resulting chiral center as S or R depends on the priority of the incoming group.