A prochiral molecule is an achiral molecule containing at least one pair of enantiotopic ligands.
1 is achiral, and Ha and Hb in 1 are enantiotopic. Thus, 1 is prochiral. In a prochiral molecule, an atom bearing a pair of enantiotopic ligands is called a prochiral center or prochirality center. If a prochirality center is a carbon atom, it can also be called a prochiral carbon.
see also chiral molecule, chiral center