15.6 : Stereochemical Effects of Enolization

The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.

Under an acidic medium, an oxygen atom of the carbonyl group is protonated with simultaneous removal of the α-hydrogen, giving an enol.

Alternatively, in the basic medium, the removal of α-hydrogen generates a resonance-stabilized enolate. Next, the protonation of the enolate oxygen gives an enol. It is to be noted that deprotonation of a carbonyl compound to form enol is favored when the C(α)–H(α) and C=O bonds are perpendicular to each other, allowing the overlap of σ orbital and π orbital.

Since the enol alkene has planar geometry, the incoming electrophilic proton can attack the molecule either from the top or the bottom of the plane with the same probability, resulting in a racemic mixture of enantiomers. In this racemic mixture, the stereochemistry of one of the enantiomers is retained while it is inverted for the other.