If two identical substituents are attached to an sp3-hybridized atom, the descriptors pro-R and pro-S are used to distinguish between the two. Replacing the pro-R substituent results 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 chirality center when a substituent is added to the re or si face of the molecule.
- John McMurry. Organic Chemistry (6ed ed.). Brooks/Cole. pp. 301–303.