15.42: Nonenolizable Aldehydes to Acids and Alcohols: The Cannizzaro Reaction
The Cannizzaro reaction is a base-promoted redox reaction producing a primary alcohol and a carboxylic acid from two molecules of a nonenolizable aldehyde. The reaction commences when the anionic counterpart of the base attacks the carbonyl carbon, resulting in a tetrahedral alkoxide intermediate. The base then abstracts a proton from the intermediate to generate an unstable dianionic species. This intermediate enables the release of the aldehydic hydrogen as a hydride ion. An intermolecular hydride shift to another aldehyde molecule produces an alkoxide and a carboxylate ion. The more basic alkoxide undergoes protonation by the water molecule to form an alcohol, while the carboxylate is protonated via an acid work-up to yield a carboxylic acid. A disproportionation reaction involving two different aldehydes is called a Crossed-Cannizzaro reaction.