Introduction to CH-acidic Compounds
As clearly depicted by the resonance formulas, an enolate anion has two nucleophilic positions, namely the α carbon and the carbonyl oxygen. Enolate anions are ambident, that is, they possess two reactive (nucleophilic) centers. Even if the α carbon were to be more nucleophilic, as is particularly the case in reactions with soft Lewis-acid electrophiles, protonation may occur at the α carbon, as well as at the carbonyl oxygen, since a proton is an extremely hard Lewis acid. Consequently, protonation of an enolate may yield two different products, the enol (enolic form of a carbonyl compound), or the carbonyl compound (keto form).
- The keto form and enolic form are in equilibrium. The establishment of equilibrium may be catalyzed by both acids and bases. Through suitable means, such as through fractional crystallization or careful distillation in the absence of any acid and any base, the keto and the enolic form may be separated from each other. The keto and enolic form of a carbonyl compound are constitutional isomers.
The separation must be carried out in the absence of all acids and bases, as the equilibrium reaction would otherwise proceed too rapidly. As a result, the separated, pure keto and enolic form would immediately be "contaminated" at least to some degree by the other form again. The glass of a flask is also capable of catalyzing keto-enol tautomerism. It must be noted that in contrast to an enol, the enolate is not in equlibrium with a keto form! Rather, the various structural formulas of an enolate, in which the negative charge is located at various positions, are actually merely resonance formulas of one and the same compound!
Equilibrium position in keto-enol tautomerism
The position of the keto-enol equilibrium is influenced by the temperature and the solvent (if present). The keto form usually exceeds the enolic form to a considerable degree. Acetone, for instance, contains only 1.5*10-4% of the enolic form.
In addition, phenol's enolate is much more stable than non-aromatic enolates are, as its negative charge is stabilized through resonance with the aromatic system. As a result, phenol is considerably more acidic than other enols or alcohols. The value of phenol amounts to 9.95.