2,4-Dinitrophenylhydrazine

Encyclopedia : 2 : 24 : 24D : 2,4-Dinitrophenylhydrazine

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.

The above is the reaction mechanism for any carbonyl compound. The H⁺ ion in stage two comes from the acidified solution. (Chemistry in Context 4th Edition, 2000, Graham Hill and John Holman)

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.

Dinitrophenylhydrazine is also known to have a strongly offensive odor.

External links

• [MSDS for 2,4-Dinitrophenylhydrazine]

• For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].

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