Prochirality

Prochirality refers to the concept of two or more achiral molecules reacting to give rise to chiral products.

For example, consider the reduction of 2-butanone to 2-butanol. Here, the achiral molecules 2-butanone and sodium borohydride react to generate an equimolar mixture of the chiral enantiomers (R)-2-butanol and (S)-2-butanol.

Achiral molecules that can be converted to chiral products by changing only one substituent are referred to as prochiral.

In trigonal prochiral molecules, the chiral configuration of the product depends on whether the incoming group adds to the sp²-hybridized center from the top face or the bottom face of the molecule.

As such, each face of the butanone molecule is unique and should have its own distinct name, which can be assigned with the Cahn–Ingold–Prelog system.

Similar to the naming of chiral molecules, priorities are assigned to the substituents at the trigonal carbon atom based on their atomic numbers at the first point of difference. The faces of the molecule are then labeled depending on whether the one-two-three sequence is clockwise or counterclockwise.

If the sequence is clockwise, the face is labeled as “re”. If the sequence is counterclockwise, the face is labeled as “si”.

In the case of 2-butanone, if the hydride group adds from the “re” face of 2-butanone, (S)-2-butanol is obtained. In contrast, addition of the hydride ion from the “si” face generates (R)-2-butanol.

In the absence of chiral reagents, the hydride group is equally likely to attach to the molecule from either face, and a racemic mixture of the product is obtained.

However, chiral catalysts or enzymes can be used to favor the formation of one enantiomer, which makes the reaction ‘enantioselective’ for that enantiomer.