2,4-Dinitrophenylhydrazine

Synthesis

It can be prepared by the reaction of hydrazine sulfate with 2,4-dinitrochlorobenzene:

:[[Image:Preparation of 2,4-DNPH.png|300px]]

DNP test

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:

:[[Image:NERYOZ.png|left|thumb|X-ray structure of DNP-derived hydrazone of [[benzophenone]]. Selected parameters: C=N, 128 pm; N-N, 1.38 pm, N-N-C(Ar), 119]]

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 Modern spectroscopic and spectrometric techniques have superseded these techniques.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

Dry DNPH is friction and shock sensitive. For this reason, it is supplied damp or ‘wetted’ when a school purchases it from a chemical supplier. If DNPH is stored improperly and left to dry out, it can become explosive.

References

References

1. Allen, C. F. H.. (1933). "2,4-Dinitrophenylhydrazine".
2. (1998). "Experimental Organic Chemistry: A Balanced Approach, Macroscale and Microscale". W.H. Freeman and Company.
3. Adapted from ''Chemistry in Context'', 4th Edition, 2000, Graham Hill and John Holman
4. (2006). "Benzophenone 2,4-dinitrophenylhydrazone". Acta Crystallographica Section E.
5. (1926). "The use of 2:4-dinitrophenylhydrazine as a reagent for aldehydes and ketones". [[Analyst (journal).
6. (7 November 2016). "What is 2,4-DNPH and Why Are Schools Carrying Out Controlled Explosions?".
7. (2 November 2016). "Bomb disposal squads detonate chemical stocks in British schools". The Guardian.