Prochirality

Prochirality describes achiral molecules that can be converted into chiral products by changing only one substituent. For example, the reduction of 2-butanone with sodium borohydride produces an equimolar mixture of (R)-2-butanol and (S)-2-butanol enantiomers. Prochiral molecules have trigonal carbon centers where the incoming group's attachment point determines the product's stereochemistry.

Prochiral molecule faces are named using the Cahn–Ingold–Prelog system. Priorities are assigned to substituents at the trigonal carbon based on atomic numbers. If the one-two-three sequence is clockwise, the face is labeled 're'; if counterclockwise, it is labeled 'si'. Each face is unique and determines which enantiomer forms when a group adds to that face.

Without chiral reagents, the hydride group in 2-butanone reduction is equally likely to attach from either the 're' or 'si' face, producing equal amounts of both (R)-2-butanol and (S)-2-butanol. This equimolar mixture of enantiomers is called a racemic mixture. Chiral catalysts or enzymes can favor one enantiomer, making the reaction enantioselective.

The 're' and 'si' designations describe the stereochemical orientation of prochiral molecule faces. Using the Cahn–Ingold–Prelog priority system, a 're' face has a clockwise one-two-three sequence, while an 'si' face has a counterclockwise sequence. In 2-butanone, hydride addition from the 're' face yields (S)-2-butanol, while addition from the 'si' face yields (R)-2-butanol.

Chiral catalysts or enzymes preferentially direct the incoming group to one face of the prochiral molecule, favoring formation of one enantiomer over the other. This makes the reaction enantioselective. Without chiral catalysts, both faces are equally accessible, producing a racemic mixture. Enzymes in biological systems are particularly effective at achieving high enantioselectivity.

These terms classify substituents on prochiral carbons. Homotopic substituents are identical and indistinguishable in any environment. Enantiotopic substituents are related by a mirror plane and become distinguishable only in a chiral environment. Diastereotopic substituents are not mirror images and are distinguishable in any environment. In 2,6-dimethylcyclohexanone, different hydrogen groups exemplify each classification.

The Cahn–Ingold–Prelog system assigns priorities to substituents at the trigonal carbon based on atomic numbers at the first point of difference. These priorities determine whether each molecular face is labeled 're' or 'si' based on whether the one-two-three sequence runs clockwise or counterclockwise. This systematic naming allows chemists to predict which enantiomer forms when a group adds to a specific face.