15.40 : Aldéhydes non énolisables en acides et alcools : la réaction de Cannizzaro

The Cannizzaro reaction is a redox process producing an alcohol and a carboxylic acid from two aldehyde molecules in the presence of a base.

The reaction is specific with nonenolizable aliphatic or aromatic aldehydes that contain no α hydrogens.

Aldehydes bearing α hydrogens, under basic conditions, often undergo deprotonation to give the enolate ion, followed by an aldol reaction.

Nonenolizable aldehydes endure the Cannizaro reaction pathway wherein the anion of the base attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate.

The base further deprotonates the intermediate resulting in an unstable dianionic species.

The strong electron-donating ability of O^- facilitates the transfer of the aldehydic hydrogen to another aldehyde molecule.

This intermolecular hydride transfer generates an alkoxide and a carboxylate ion.

The more basic alkoxide ion is protonated by water to produce an alcohol, while the less basic carboxylate ion requires an acid workup to generate the carboxylic acid.

The Cannizzaro reaction is a disproportionation reaction where one aldehyde molecule is oxidized while the other is reduced.