Cannizzaro reaction

Reaction mechanism

The first reaction step is aldehyde. The resulting alkoxide is deprotonated to give a di-anion, known as the Cannizzaro intermediate. Formation of this intermediate requires a strongly basic environment.

Both intermediates can react further with aldehyde to transfer a hydride, "H"⁻. The hydridic character of the C-H is enhanced by the electron-donating character of the alpha oxygen anion. This hydride transfer simultaneously generates a hydroxyl anion and a carboxylate. Further evidence for the hydridic character of the Cannizzaro intermediate is provided by the formation of H₂ by its reaction with water.

Only aldehydes that cannot form an benzaldehyde.

Variations

A special condition is the crossed Cannizzaro reaction. This variation is more common these days because the original Cannizzaro reaction yields a mixture of alcohol and carboxylic acid. For example any aldehyde with no alpha hydrogens can be reduced when in the presence of atom economy is still low.

References

See also

• Meerwein-Ponndorf-Verley reduction
• Oppenauer oxidation
• Tishchenko reaction