Aldehydes and Ketones
10.0 Cannizzaro Reaction
10.0 Cannizzaro Reaction
Mechanistically, Cannizzaro reaction is a characteristic reaction of aromatic aldehydes and of aliphatic aldehydes without $\alpha $-hydrogen. In the presence of a strong base, such aldehydes undergo self-oxidation-reduction yielding salt of carboxylic acid and an alcohol.
When the Cannizzaro reaction is performed in ${D_2}O$, the alcohol formed has no carbon-bound deuterium thereby establishing that the reaction sequence involves a direct transfer of hydrogen from one molecule of aldehyde to the other. An aldehyde molecule produces an anion by interaction with a hydroxide ion which transfers a hydride ion to the carbonyl carbon of the second aldehyde molecule.
Aldehydes that possess $\alpha $-hydrogens do not undergo this reaction, because base-catalysed aldol condensation becomes the dominant process.
The Cannizzaro reaction between two dissimilar aldehyde moelcuels (crossed Cannizzaro reaction) may yield all possible products. However, when one of the aldehydes is formaldehyde the formate ion and the alcohol corresponding to the other aldehyde are formed exclusively. The exclusive formation of the formate ion is due to the high reactivity of formaldehyde which preferentially adds a hydroxide ion before losing one of its two hydrogens as hydride ion to the less reactive aldehyde.
