Cannizzaro reaction

Cannizzaro reaction
Named after	Stanislao Cannizzaro
Reaction type	Organic redox reaction
Identifiers
Organic Chemistry Portal	cannizzaro-reaction
RSC ontology ID	RXNO:0000218

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position.

Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:

The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.

The reaction involves a nucleophilic acyl substitution on an aldehyde, with the leaving group concurrently attacking another aldehyde in the second step. First, hydroxide attacks a carbonyl. The resulting tetrahedral intermediate then collapses, re-forming the carbonyl and transferring hydride to attack another carbonyl. In the final step of the reaction, the acid and alkoxide ions formed exchange a proton. In the presence of a very high concentration of base, the aldehyde first forms a doubly charged anion from which a hydride ion is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ions. Subsequently, the alkoxide ion acquires a proton from the solvent.

Overall, the reaction follows third-order kinetics. It is second order in aldehyde and first order in base:

At very high base a second path (k') becomes important that is second order in base:

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