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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 C=O (carbonyl) group. (N.B. It only identifies carbonyl groups in aldehydes or ketones.) If it does, a yellow or red precipitate is formed. This precipitate is a hydrazone, containing the functional group C=N–N. 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.
However, 2,4-dinitrophenylhydrazine only reacts with ketones and aldehydes to yield the yellow/orange precipitate. It does not react with carboxylic acids, amides and esters.
This reaction can be described as a condensation reaction, as two large molecules are joining together, producing a smaller molecule at the joining point. 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 C 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.
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