Cannizzaro Reaction: Mechanism
The disproportionation of an aldehyde into a carboxylic acid and the corresponding alcohol in the Cannizaro reaction is base-catalyzed. Taking benzaldehyde as an example, the first step involves addition of a hydroxyl ion to the benzaldehyde.
In the second step, a hydride ion is transfered from the carboxylate ion to another molecule of benzaldehyde. Normally, hydride ions are bad candidates for leaving groups. However, since the aldehyde represents a Lewis acid, this transfer is facilitated . Presumably, hydride transfer to the carbonyl carbon and proton transfer to the carbonyl oxygen occur simultaneously because no free hydride ion is observed during the reaction.
The Cannizaro reaction is a redox reaction. In the following example, a molecule of benzaldehyde is oxidized to benzoic acid and another is being reduced to benzyl alcohol.