this reaction is given by only those aldehydes & ketones which don't have acidic H

in this formaldehye behaves as an acid derivative rather than aldehyde......this reaction is an example of disproportion recation

aldehydes which do not contain a-hydrogen undergoes disproportionation when heated with NaOH.............

in this reaction one molecule of aldehyde get oxidised and the other is reduced..........

hiiii

it is the reaction in which aldehydes that do not have a hydrogen

attached to the carbon adjacent to the carbonyl group, upon encountering

strong alkali, readily form an alcohol and an acid salt.

 mechanism

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.

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  !!!!!