CROSSED CANNIZARO REACTION:

When a mixture of two different aldehyde (alpha hydrogen less)  like formaldehyde and a non enolizable aldehyde (benzaldehyde) is treated with a strong base, the later is preferentially reduced to alcohol while formaldehyde is oxidized to formic acid. This variant is known as crossed Cannizzaro reaction.

Illustrative example: Benzyl alcohol and formic acid are obtained when a mixture of benzaldehyde and formaldehyde is treated with alkali.

Important note:

(1) In cross cannizaro reaction if one of the reactant is formaldehyde, then oxidation of formaldehyde take place , and reduction of another aldehyde take place .The reason may be: the initial nucleophillic addition of hydroxide anion is faster on formaldehyde as there are no electron donating groups on it.

(2) The preferential oxidation of formaldehyde in crossed Cannizzaro reactions may be utilized in the quantitative reduction of some aldehydes.