DNPH is a reagent in instructional
analytical chemistry laboratories. '''Brady's reagent
or Borche's reagent''', is prepared by dissolving DNPH in a solution containing
methanol and some concentrated
sulfuric acid. This solution is used to
detect ketones and
aldehydes. A positive test is signalled by the formation of a yellow, orange or red
precipitate of the dinitrophenylhydrazone. Aromatic carbonyls give red precipitates whereas
aliphatic carbonyls give more yellow color. The reaction between DNPH and a generic ketone to form a
hydrazone is shown below: :RR'C=O + C6H3(NO2)2NHNH2 → C6H3(NO2)2NHN=CRR' + H2O This reaction is, overall, a
condensation reaction as two molecules joining together with loss of water. Mechanistically, it is an example of
addition-elimination reaction: nucleophilic addition of the -NH2 group to the C=O carbonyl group, followed by the elimination of a H2O molecule: :. Selected parameters: C=N, 128 pm; N-N, 1.38 pm, N-N-C(Ar), 119 is added to a solution of 2,4-DNPH and heated, an orange-red precipitate forms. DNP-derived hydrazones have characteristic melting points, facilitating identification of the carbonyl. In particular, the use of DNPH was developed by Brady and Elsmie. Modern spectroscopic and spectrometric techniques have superseded these techniques. DNPH does not react with other carbonyl-containing functional groups such as
carboxylic acids,
amides, and
esters, for which there is resonance-associated stability as a lone-pair of electrons interacts with the
p orbital of the carbonyl carbon resulting in increased delocalization 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. Also, with carboxylic acids, there is the effect of the compound acting as a base, leaving the resulting carboxylate negatively charged and hence no longer vulnerable to nucleophilic attack. ==Safety==