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2,4-Dinitrophenylhydrazine (or Brady's reagent) is a chemical compound that can be used in a chemical test to detect whether an organic compound has a ketone or aldehyde functional group. If it does, a yellow or red precipitate is formed upon reaction with the carbonyl forming a hydrazone (C=N–NR2). It does not react with carboxylic acids, amides and esters. Crystals of different hydrazones have precise but different melting and boiling points, hence 2,4-dinitrophenylhydrazine is used to distinguish between various compounds containing carbonyl groups.
This reaction can be described as a condensation reaction, with two molecules joining together with loss of water. It is also called addition-elimination reaction : nucleophilic addition of the -NH2 group to the C=O carbonyl group, followed by the removal of a H2O molecule.
For carboxylic acids, amides and esters, there is resonance associated stability as a lone-pair of electrons interacts with the p-orbital of the carbonyl carbon resulting in increased delocalisation in the molecule. This stability would be lost by addition of a reagent to the carbonyl group. Hence, these compounds are more resistant to addition reactions.