prochirale

Prochirale, often written prochiral in English, describes a molecule that is not chiral but contains a site that can become stereogenic by a single chemical modification. Concretely, a center is prochiral if two of its substituents are identical; replacing one of those identical substituents by a different group would give a chiral center with four different substituents.

A common manifestation is a methylene group (CH2) adjacent to a heteroatom or unsaturated system, where the two hydrogens are enantiotopic. For example, in ethanol (CH3-CH2-OH), the CH2 group next to the hydroxyl is prochiral; replacing one hydrogen with X gives CH3-CH(X)-OH, a chiral secondary alcohol.

Another example is the methylene group alpha to a carbonyl in molecules such as propanal (CH3-CH2-CHO); substituting one hydrogen by X yields CH3-CH(X)-CHO, a chiral center at the former CH2 carbon.

In alkenes, the two faces of a planar double bond are prochiral; addition of a reagent to one face can generate a chiral product. This principle underlies many enantioselective transformations and is exploited by enzymes and chiral catalysts to produce enantiomerically enriched compounds.

Significance: the concept of prochirality helps explain why achiral substrates can give chiral products and informs strategies for stereoselective synthesis and biocatalysis.