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.
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