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The first reaction step is nucleophilic addition of the base (for instance the hydroxide anion) to the carbonyl carbon of the 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.

The mechanism of the Cannizzarro reaction

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 an alkoxide anion (RCH2O-) and a carboxylic acid, which is rapidly deprotonated to form the 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 enolate ion undergo the Cannizaro reaction. The aldehyde cannot have an enolizable proton. Under the basic conditions that facilitate the reaction, aldehydes that can form an enolate instead undergo aldol condensation. Examples of aldehydes that can undergo a Cannizzaro reaction include formaldehyde and aromatic aldehydes such as benzaldehyde    !!!!!

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