Topic: Carbonyls and Optical Isomerism

Reading Beyond A-Level

The laboratory reduction of pyruvate using an achiral reducing agent like sodium borohydride (NaBH₄) yields a racemic mixture of the two enantiomers of lactic acid. In stark contrast, the biochemical reduction in the human body, catalysed by the enzyme lactate dehydrogenase, produces only a single enantiomer, (S)-(+)-lactic acid.

Critically analyse these two transformations. Your explanation should focus on how the planar geometry of the pyruvate carbonyl group allows for non-selective attack by an achiral nucleophile (e.g., :H⁻ from NaBH₄), leading to a 50/50 product mixture. Furthermore, investigate how the chiral three-dimensional active site of an enzyme creates a stereospecific environment that forces the reaction to proceed down a single pathway, resulting in an enantiomerically pure product.