Cannizzaro

The Cannizzaro reaction is a redox disproportionation of non-enolizable aldehydes in strong base, yielding an alcohol and a carboxylate salt. It applies to aldehydes that lack α-hydrogens and does not require any catalyst other than hydroxide.

In aqueous alkali, two molecules of an aldehyde react: one is reduced to the corresponding primary alcohol, and the other is oxidized to the carboxylate. The overall stoichiometry is 2 RCHO + OH− → RCOO− + RCH2OH. Reactions are typically carried out with concentrated sodium hydroxide in water, often at or near room temperature. The alcohol and carboxylate form salts that can be isolated after workup.

Mechanistically, hydroxide adds to the carbonyl of one aldehyde to form a tetrahedral intermediate, followed by a hydride transfer from this alkoxide to a second aldehyde molecule. This transfers reducing equivalents between aldehyde molecules, producing the alcohol and the carboxylate. Aldehydes bearing α-hydrogens generally do not undergo Cannizzaro under these conditions and may instead undergo aldol condensation or related chemistry.

Examples include benzaldehyde, which gives sodium benzoate and benzyl alcohol, and formaldehyde, which yields formate and methanol. The reaction can be extended to crossed Cannizzaro with two different aldehydes, but selectivity can be limited; typically the more easily oxidized aldehyde forms the carboxylate while the other is reduced to the corresponding alcohol.

History and naming: the reaction is named after the Italian chemist Stanislao Cannizzaro, who described it in 1853. It remains a classical demonstration of redox chemistry in organic synthesis.