The Cannizzaro Reaction

When a non-enolizable aldehyde is heated in strong aqueous base, a redox transformation known as the Cannizzaro reaction takes place. Two examples are shown in the following diagram. In the first, formaldehyde disproportionates into methanol and formic acid (sodium salt). In the second, a benzaldehyde derivative is similarly converted into an equimolar mixture of the corresponding benzyl alcohol and benzoic acid derivatives. A hydride transfer mechanism analogous to that of the MVP reaction is drawn in the shaded box. If the Cannizzaro reaction is run in D₂O with NaOD as a base, no C-D incorporation is observed. Thus the new carbon-bonded hydrogen in the alcohol cannot have come from the solvent.
It is important to remember that the the Cannizzaro reaction is restricted to non-enolizable aldehydes. The strong base used for this reaction would initiate aldol and other reactions that take place via enolate anions. A useful crossed Cannizzaro reaction employs an excess of formaldehyde to reduce aryl aldehyde substrates to 1 º-alcohols. The success of this procedure may be attributed to the high concentration of the hydrate, H₂C(OH)₂, in aqueous solutions of formaldehyde, making it the only significant hydride donor in the system.

An intramolecular Cannizzaro reaction, sometimes termed a Cannizzaro rearrangement will be displayed above by clicking on the diagram. A variant of the Cannizzaro reaction, known as the Tischenko reaction is also shown. In this reaction the alcohol and acid products combine to form an ester.