enolisaatio

Enolization, also spelled enolisation, is the chemical process by which a carbonyl compound, typically an aldehyde or ketone, interconverts with its enol or enolate tautomer. In the enol form, a carbon–carbon double bond is adjacent to a hydroxyl group; in the enolate form the oxygen bears a negative charge or is coordinated to a metal. The equilibrium between these forms is influenced by structure and conditions, and the enol or enolate form is often present only in small amounts but can play a crucial role in reactivity.

Mechanistically, enolization can occur via acid- or base-catalyzed pathways. In base-catalyzed enolization, a base abstracts an alpha hydrogen to give an enolate, which can be protonated to yield the enol or react further as a nucleophile. In acid-catalyzed enolization, protonation of the carbonyl oxygen increases the acidity of the neighboring alpha hydrogen, allowing deprotonation to form the enol. The reverse tautomerization regenerates the carbonyl compound. Factors such as conjugation, intramolecular hydrogen bonding, and metal chelation can stabilize the enol or enolate, shifting the equilibrium toward the enol form in certain systems like enones or β-dicarbonyl compounds.

Enolization is central to many organic reactions. Enolates are versatile nucleophiles used in aldol condensations, Claisen condensations, Michael additions, and various alkylations and acylations. In biochemistry, enolization features in metabolic processes, for example the formation of phosphoenolpyruvate from 2-phosphoglycerate by the enzyme enolase. Overall, enolization describes a fundamental tautomeric mechanism that influences reactivity, selectivity, and the outcome of numerous synthetic and biological transformations.