15.6:
Stereochemical Effects of Enolization
Carbonyl compounds bearing hydrogen at the chiral α carbon, when treated with acids or bases, undergo racemization via the formation of an achiral enol.
In the acidic medium, protonation of the C=O oxygen and subsequent deprotonation of the α-carbon atom results in an enol.
In the basic medium, deprotonation of the α-carbon atom leads to the formation of a resonance-stabilized achiral enolate intermediate.
Subsequently, protonation of the enolate oxygen generates the enol.
In both cases, as the enol formed is achiral, reprotonation can occur from either side of the plane of the double bond with equal probability.
This leads to the formation of a racemic mixture—one with retained and the other with inverted stereochemistry.
In case the carbonyl compound bears an additional chiral carbon, enolization changes the stereochemistry of the α carbon to generate a more stable isomer.
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.