A2-Level Advanced Organic Chemistry
Aldehydes can be oxidised to carboxylic acids. It is not possible to oxidise a ketone.
The ability to further oxidise an alcohol and not a ketone can be used to help distinguish between aldehydes and ketones.
Aldehydes can be identified by using Tollens’ reagent (silver precipitate formed) or Fehling’s solution (red precipitate formed).
Aldehydes and ketones can both be identified by 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.
Testing for an Aldehyde
It is easy to distinguish between an aldehyde and a ketone using Tollens’ reagent or Fehiling’s solution. In both cases, the aldehyde can be oxidised and cause a metal ion to be reduced. This reduction of a metal ion is accompanied by a colour change in solution (silver precipitate in the case of Tollens’ reagent and a 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, and at A-level it is 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. 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.