Aldehydes that do not contain an α -hydrogen atom, when treated with conc. NaOH solution undergo disproportionation, i.e. self reduction oxidation. As a result, one molecule of the aldehyde is reduced to corresponding alcohol and the other is oxidised to corresponding carboxylic acid.This is known as Cannizzaro reaction.
The reaction of p-methyl benzaldehyde and NaOH is an example of Cannizzaro reaction..
