The Cannizzaro reaction resembles the acyl exchange reactions among carboxylic acid derivatives, although it occurs with aldehyde. With aldehyes (or ketones) tetrahedral intermediate formation is not typically followed by the departure of an acyl type leaving group. Reactions of nucleophiles with aldehydes and ketones differ in this respect from reactions of carboxylic acids. Although sometimes the original carbonyl oxygen will depart (as in acetal or imine/enamine formation) the reaction of a nucleophile with an aldehydes most commonly results in a tetrahedral product) In the Cannizzaro reaction, a strong base reacts with an aldehyde having no ?-hydrogens. The hydroxyl adds to the aldehyde to produce a typical tetrahedral intermediate, except that this intermediate resolves itself with hydride (hydride!) departing as an ‘acyl type’ leaving group, forming a carboxylic acid. The freshly departed hydride then acts as a nucleophile upon another aldehyde, producing a new tetrahedral intermediate, which, after protonation, becomes an alcohol. One equivalent of aldehyde becomes carboxylic acid and the other leads to the alcohol form.
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