prochiral

Prochiral is a term used in stereochemistry to describe an achiral molecule or a specific atom within a molecule that could become a chiral center by a single chemical modification that distinguishes two otherwise equivalent substituents. The concept is central to understanding how enantioselective reactions can generate optical activity from symmetry.

Typically, prochirality arises at a carbon atom bearing two identical substituents (often hydrogens) and two other different groups. The two identical substituents are enantiotopic; replacing one of them with a new group Z yields a stereogenic center at that carbon, producing two enantiomers if Z is different from the existing substituents. An example is a molecule of the form R-CH2-Y, where the methylene hydrogens are enantiotopic. If one hydrogen is replaced by Z, the product R-CH(Z)-Y has four different substituents and is chiral. The starting material is achiral due to symmetry.

Prochiral substrates are central to enantioselective synthesis, where a reaction discriminates between enantiotopic positions to give enantioenriched products. In this context, terms like pro-R and pro-S describe the two possible positions or faces that could become the stereogenic center upon reaction. Prochirality applies to various sites in molecules, including carbon atoms adjacent to carbonyls, alkenes, or other motifs, especially when a chiral catalyst or enzyme directs the outcome.