Quick Notes Identifying Carbonyls

  • Aldehydes can be identified by using Tollens’ reagent (silver precipitate formed) or Fehling’s solution (red precipitate formed).
    • For both tests, the aldehyde reduces a metal ion (Ag+ in Tollens’; Cu2+ in Fehling’s).
  • Aldehydes and ketones can both be identified using Brady’s reagent, producing an orange precipitate.
  • A methyl carbonyl group can be identified using a (warm) mixture containing iodine and sodium hydroxide, forming a yellow solid.

Full Notes Identifying Carbonyls


Testing for an Aldehyde

It’s easy to distinguish between separate samples of an aldehyde and a ketone using Tollens’ reagent or Fehiling’s solution. In both cases, the aldehyde can be oxidised, causing a metal ion to be reduced. This reduction of a metal ion is accompanied by a colour change in solution (silver precipitate for Tollens’ reagent; brick red precipitate for Fehiling’s solution).

Tollens' Reagent Fehlings Solution aldehyde oxidation carboxylic acid

Tollens’ reagent and Fehling’s solution both form carboxylate ions in the reaction, although these are easily converted into carboxylic acids by protonation. At A-level it’s often written as the formation of a carboxylic acid from aldehyde.

Aldehyde with Tollens' Reagent silver mirror Aldehyde with Fehling's Solution copper oxide

Testing for a Carbonyl Group

As beneficial as the above two tests are, they do not actually confirm the presence of a ketone, only an aldehyde. Ketones do not react with Tollens’ reagent or Fehling’s solution. This means the tests cannot be used to identify a carbonyl group, only an aldehyde.

Brady's reagent carbonyl group orange precipitate ethanal-2,4-dinitrophenylhydrazone

Brady’s reagent, a solution of 2,4-dinitrophenylhydrazine in dilute acid, reacts with both aldehydes and ketones to give a positive result (orange-yellow crystals).

Testing for Methyl Carbonyl

methyl group carbonyl group methyl-carbonyl

If a methyl-carbonyl group is heated with a solution containing iodine and sodium hydroxide (aqueous), yellow crystals of triiodomethane are formed.

methyl carbonyl test tri iodomethane test yellow crystals