In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step.[1][2]
If two identical substituents are attached to a sp3-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 identical alternative would result in an R chirality center at the original sp3-hybridized atom, and vice versa.
A trigonal planar sp2-hybridized atom can be converted to a chiral center when a substituent is added to the re or si face of the molecule. These faces are labelled re if the substituents at the trigonal atom are disposed in decreasing Cahn-Ingold-Prelog priority order in a clockwise order, and si if the priorities decrease in counter-clockwise order; but the designation of the resulting chiral centre as s or r depends on the priority of the incoming group.