Benzoinkondensation

Benzoinkondensation, known in English as the benzoin condensation, is an organic reaction that couples two molecules of an aryl aldehyde to form a 2-hydroxy ketone, most commonly benzoin (2-hydroxy-1,2-diphenylethanone). The classic example uses benzaldehyde to give benzoin, Ar-CH(OH)-C(O)Ar. The transformation is typically catalyzed by nucleophilic catalysts such as cyanide ion or thiamine (vitamin B1). Mechanistically, cyanide adds to one aldehyde to form a cyanohydrin intermediate, which then attacks a second aldehyde molecule; after proton transfer and catalyst turnover, benzoin is formed and the cyanide catalyst is regenerated. Thiamine-catalyzed variants operate via thiamine diphosphate to generate an acyl anion equivalent, enabling the same carbon–carbon bond formation.

Cross-condensation between different aldehydes is possible but often yields mixtures; reaction conditions, substrate electronics, and catalyst choice influence selectivity toward homocoupled versus cross-coupled products. The reaction is a classic example of carbon–carbon bond formation and has been studied extensively in organic synthesis.

Product applications and variations: benzoin is a versatile synthetic intermediate and can be oxidized to benzil (the dibenzil derivative). Modern advances include organocatalytic and asymmetric versions using N-heterocyclic carbenes (NHCs) that provide enantioenriched products under mild conditions. In practice, cyanide-based catalysts are effective but pose toxicity concerns, while safer catalysts and greener solvents have been explored in contemporary protocols.