The reason for the unusual acidity of the α – hydrogens in aldehydes are as follows:
The carbonyl group is strongly electron withdrawing and when an aldehyde loss a proton the anion enolate ion is produced is stabilized by delocalization.
Two resonance structures A and B can be written for delocalization of ion. Both structures contribute to the hybrid, Although structure A is favoured by the strength of its carbon-oxygen bond relative to the weaker carbon-carbon bond of B. Structure B makes a greater contribution to the hybrid because oxygen, being electronegative is better able to accommodate the negative charge. We can depict the enolate hybrid in the following way.
When this resonance stabilized enolate accepts a proton it can do so in either of two ways:
(1) It can accept the proton at the carbon to form the original carbonyl compound which is called the keto form or it may accept the proton at oxygen to form an enol.
(2) The enolate is the conjugate base of both the enol and keto forms. Both of these reactions are reversible.