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:
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