18.3Tests for Aldehydes and Ketones

18.3    Tests for Aldehydes and Ketones

Test for carbonyl group using 2,4-dinitrophenylhydrazine(2,4-DNPH)

  • 2,4-dinitrophenylhydrazine or 2,4-DNPH can be used to detect the presence of carbonyl group, C=O. The structure of 2,4-DNPH is shown below:

  • This test is usually carried out using Brady’s reagent, that is, a solution of the 2,4-dinitrophenylhydrazine in methanol and sulfuric acid

  • When a little aldehyde or ketone is added to the Brady’s reagent, an orange-yellow precipitate is formed.


  • A condensation reaction occurs when a carbonyl compound is added to 2,4-DNPH. During this reaction, a water molecule is The final compound is seen as orange-yellow precipitate.


Test for aldehydes using Tollens’ reagent(silver mirror test)


  • Tollens’ reagent contains diamminesilver(I) ions, [Ag(NH3)2]. Aldehydes will reduce the diamminesilver(I) ions to metallic silver, aldehyde itself is oxidised to a salt of carboxylic acid


  • Since ketones will not be oxidised, it will not reduce it to metallic silver

  • Therefore, when a few drops of aldehyde is added to the freshly prepared Tollens’ reagent, and warmed in a water bath for a few minutes, a grey precipitate or a silver mirror is observed only if aldehyde is present


  • Take ethanal as an example, the equation is:



  • The details on the preparation of Tollens’ reagent are as shown below


Test for aldehydes using Fehling’s solution

  • Fehling’s solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide. It is a blue solution.

  • Only aldehydes will reduce the complexed copper(II) ion to copper(I) Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.


  • When a few drops of the aldehyde is added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes, a red precipitate is observed only if aldehydes present


  • Take ethanal as an example, the equation is: