Note: If a chiral carbon has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with enol serving as the intermediate, this process is called racemization.

→ this racemization is a two step process: firstly, deprotonation to form a planar enolate. secondly, protonation from either 'face' (aka side) to re-form a recemized carbonyl compound.

Overview ↓: Enol–keto tautomerism proceeds by either acid or base catalysis.

Figure ↑: In the acid-catalyzed process, the enol form is protonated at the double-bonded carbon away from the hydroxy-bearing neighbor. Moreover, the resulting cation is resonance-stabilized by the attached hydroxy group, and inspection of the corresponding resonance form reveals it to be simply the protonated carbonyl compound. Deprotonation then gives the product.

Figure ↑: Base simply removes the proton from the enol oxygen, reversing the initial protonation. Subsequent (and slower) C-protonation furnishes the thermodynamically more stable keto form.

General Note ↑: Both the acid- and base-catalyzed enol–keto interconversions occur rapidly in solution whenever there are traces of the required catalysts. Remember that although the keto form (usually) predominates, the enol-to-keto conversion is reversible and the mechanisms by which the keto form equilibrates with its enol counterpart are the exact reverse of the preceding two schemes.

Note ↑: The pKa values of aldehyde and ketone -hydrogens range from 16 to 21, much lower than the pKa values of ethene (44) or ethyne (25), but comparable with those of alcohols (15–18). Strong bases can therefore remove an -hydrogen. The anions that result are known as enolate ions or simply enolates.

We know that acid strength is enhanced by stabilization of the conjugate base. In the enolate ion, the inductive effect of the positively polarized carbonyl carbon strongly stabilizes the negative charge at the -position. Aldehydes are stronger acids than ketones because their carbonyl carbon bears a larger partial positive charge. Further strong stabilization is provided by delocalization of charge onto the electronegative oxygen, as described by the resonance forms just pictured. The effect of delocalization is also reflected in the electrostatic potential map of the acetone enolate shown down below (on an attenuated scale), which exhibits negative charge (red) on the -carbon as well as on the oxygen. An example of quantitative enolate formation is the deprotonation of cyclohexanone (pK_a = 19) by lithium diisopropylamide (LDA, pK_a of amine = 36).