Oxidation of Aldehydes
Unlike ketones, aldehydes can be easily oxidised to carboxylic acids using a variety of oxidising agents. These reagents can be chromic acid, chromium trioxide, permanaganate or silver oxide.
Silver ions selectively oxidise —CHO group. This forms the basis of Tollen’s test. It involves the addition of a mixture of aqueous silver nitrate and aqueous ammonia which is known as Tollen’s reagent to the carbonyl compound.
Tollen’s reagent contains [Ag(NH3)2]+ complex ion. If an aldehyde is present, it gets oxidised to the carboxylic acid whereas the Ag+ ions are reduced to form silver metal which gets deposited on the walls of the test tube and this gives a mirror like shining appearance.
Aldehydes are also oxidised by Fehling solution, which contain Cu2+ (cupric) ions complexed with tartarate ions as the oxidant. These Cu2+ ions are reduced by the aldehydes in alkaline medium to give a brick red precipitate of cuprous oxide.
