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