aldolkondensation

Aldol condensation is a carbon–carbon bond–forming reaction in organic chemistry in which an enolate or enolizable carbonyl compound adds to the carbonyl of another aldehyde or ketone, yielding a β-hydroxy carbonyl compound (an aldol). Under heating or with acid or base catalysts, this aldol product commonly undergoes dehydration to form an α,β-unsaturated carbonyl compound.

Mechanism and scope

In base-catalyzed variants, a deprotonated enolate from one carbonyl compound attacks the carbonyl carbon of a second molecule, producing a new C–C bond and an alkoxide that is subsequently protonated to give the aldol. Proton transfer and dehydration then generate the enone or enal. Acid-catalyzed versions follow a different pathway but achieve the same overall result. The reaction can occur in self-condensation (one molecule acts as donor and acceptor), or as a cross-aldol when two different carbonyl compounds are used. Intramolecular aldol condensations can form cyclic β-hydroxy carbonyls and related ring systems.

Variants and selectivity

Control of reactivity is important in cross-aldol reactions to minimize mixtures. Enantioselective and diastereoselective versions exist, often using organocatalysts (for example, chiral proline derivatives) or metal-based catalysts to induce stereocontrol at the new β-position. The dehydration step to the α,β-unsaturated carbonyl is favored under heat or with dehydrating conditions and often drives the reaction forward.

Applications

Aldol condensations are widely used in organic synthesis to build complex molecules, including intermediates for natural products, fragrances, and pharmaceuticals. They also underpin certain polymerization strategies that yield polyβ-hydroxy carbonyls and related materials.