2,4-DNPH Test

2,4-dinitrophenylhydrazine (2,4-DNPH, also called Brady's reagent) is used to test for the carbonyl group, C=O, in aldehydes and ketones. It is usually used as a solution in methanol and sulfuric acid.Add a few drops of the compound to the 2,4-DNPH solution. A positive result is a yellow or orange precipitate of a 2,4-dinitrophenylhydrazone. Both aldehydes and ketones give a positive result, so this test shows that a carbonyl group is present but not which type. Fehling's, Benedict's or Tollens' reagent is then used to decide.Carboxylic acids and esters also contain C=O, but they do not give a precipitate, because their C=O group is less reactive.The reaction is a condensation (addition followed by elimination): the NH₂ group of 2,4-DNPH adds across the C=O and a molecule of water is lost, forming a C=N bond.R₂C=O + H₂NNHC₆H₃(NO₂)₂ → R₂C=NNHC₆H₃(NO₂)₂ + H₂OIdentifying the compound: the precipitate is filtered off, purified by recrystallisation and dried. Its melting temperature is measured and compared with a table of data. Each aldehyde and ketone gives a derivative with a different melting temperature.Worked example: compound X gives an orange precipitate with 2,4-DNPH and no change with Fehling's solution, so X is a ketone. The recrystallised derivative melts at 126 °C, which matches the data book value for the propanone derivative, so X is propanone.