zum Directory-modus

Introduction to the Reactions of Enols and Enolates

Racemization of Carbonyl Compounds

If aldehydes or ketones with an α hydrogen atom are chiral because of an asymmetric α carbon, racemization occurs relatively rapidly when these are treated with an acid or base. Why is this? Aldehydes and ketones with an α hydrogen atom are in equilibrium with their corresponding enols or enolates due to keto-enol tautomerism. The α carbon in enols and enolates is sp2-hybridized. Thus, its substituents are arranged trigonal planar. As a result, the stereochemical information of the sp3-hybridized carbonyl compound's α carbon is lost. Therefore, reconversion of the enol to the carbonyl compound does not proceed stereoselectively. Thus, racemization occurs.

Fig.1
Loss of stereochemical information (racemization) through enolization under basic conditions.
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Fig.2
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Fig.5
Loss of stereochemical information (racemization) through enolization under acidic conditions.
Hint
It must be kept in mind that racemization in the α position of carbonyl compounds (and other CH-acidic compounds) is possible when an enantioselective synthesis is constructed.

Exercise

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