aldol

An aldol is a beta-hydroxy carbonyl compound formed by the aldol reaction between an enolate of an aldehyde or ketone and another carbonyl compound. The products are typically beta-hydroxy aldehydes or beta-hydroxy ketones. When dehydration occurs, the reaction is termed an aldol condensation, yielding an α,β-unsaturated carbonyl compound.

Mechanism and scope: Under base- or acid-catalyzed conditions, a nucleophilic enolate attacks the electrophilic carbonyl carbon of a second molecule, forming a new carbon–carbon bond and an alkoxide. Protonation gives the aldol; subsequent dehydration produces an enone or enal. The reaction can proceed with identical or different carbonyl partners (self- or cross-aldol). Control of enolate formation and electrophile reactivity allows selective outcomes, and many variants explore stereochemical control, including enantioselective catalysis.

Conditions and variations: Base catalysts such as hydroxide, alkoxide, or amines, and sometimes Lewis acids, promote the reaction. Solvent choice and temperature influence whether dehydration occurs. Enantioselective versions use organocatalysts or metal-based catalysts to bias stereochemistry and improve selectivity for complex targets.

Applications: The aldol reaction is a fundamental carbon–carbon bond-forming step in organic synthesis, widely used to construct polyketides, carbohydrates, and various natural products. It is central to strategies such as the Robinson annulation for ring formation and is employed in the synthesis of fragrances, flavor compounds, and other fine chemicals.

History: The reaction has been known since the 19th century; the name derives from the aldehyde and alcohol functionalities in the aldol product.