15.1: Reactivity of Enols
Enols are a class of compounds where a hydroxyl group is attached to a carbon–carbon double bond, which implies that it is a vinyl alcohol. A carbonyl compound with an α hydrogen undergoes keto–enol tautomerism and remains in equilibrium with its tautomer, the enol form. Usually, the keto tautomer is present in a higher concentration than the enol tautomer due to the higher bond energy of C=O compared to C=C. Moreover, the direction of the keto–enol equilibrium is governed by factors like conjugation, intramolecular hydrogen bonding, and aromatic energy. Recall that tautomers are constitutional isomers with distinct atomic arrangements, while resonance forms are different representations of one molecule. This tautomerization is reversibly catalyzed by both acids and bases involving protonation and deprotonation steps. While protonation precedes deprotonation in presence of an acid leading to enols, the reverse order is followed in base-catalyzed enolization that yields enolates.
Enols are electron-rich and hence nucleophilic in nature, like other compounds containing carbon–carbon double bonds. Due to the strong electron-donating resonance effect of the hydroxyl group, a second resonance structure of the enol can be drawn where the negative charge is on the α carbon. Consequently, this carbon atom is especially nucleophilic and reacts with electrophiles (E+) to form a new C–E bond. Loss of a proton in the next step leads to a neutral product. The net result amounts to the substitution of hydrogen by an electrophile on the α carbon. Therefore, carbonyl compounds are halogenated at the α position by halogens, such as bromine, in both acidic and basic solutions. This results in the formation of different products under various reaction conditions.
When a carbonyl compound with at least one α hydrogen is dissolved in D2O, to which DCl or NaOD are added, the α hydrogens are gradually replaced by deuterium (D) atoms via enol formation. If this carbonyl compound is chiral owing to a stereogenic center at the α carbon, its chirality is destroyed by rapid interconversion between keto and enol forms through a planar, achiral enol intermediate. The spontaneous racemization prevents the synthesis of chiral ꞵ-keto esters whose only stereogenic center lies in between the two keto groups.
