Electrophilic substitution reaction
Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta-directing group.
Aldehydes which do not have an a-hydrogen atom undergo self oxidation and reduction (i.e., disproportionation) reaction on treatment with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol and another is oxidised to carboxylic acid salt.
Aldehydes containing a-hydrogen atoms when heated with concentrated alkali give brown resinous mass.