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Cannizzaro Reaction

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.

Hydroxyl addition

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.

Hydride transfer

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.

Redox process in the Cannizaro reaction
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