2,4-Dinitrophenylhydrazine

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Structure of Brady's reagent

2,4-Dinitrophenylhydrazine (or Brady's reagent) is the chemical compound C₆H₃(NO₂)₂NHNH₂.

This otherwise specialized aromatic compound is widely used to detect if an organic compound contains a ketone or aldehyde functional group. A positive test is signaled by a yellow or red precipitate (known as a dinitrophenylhydrazone):

RR'C=O + C₆H₃(NO₂)₂NHNH₂ → C₆H₃(NO₂)₂NHNCRR' + H₂O

Crystals of different hydrazones have characteristic melting and boiling points, hence 2,4-dinitrophenylhydrazine can be used to distinguish between various compounds containing carbonyl groups. The method was particularly important because melting point determinations require only low-cost instrumentation. This application in analytical chemistry was developed by Brady and Elsmie.^[1]

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 mechanism for the reaction between 2,4-dinitrophenylhydrazine and an aldehyde or ketone is shown:

Adapted from Chemistry in Context, 4th Edition, 2000, Graham Hill and John Holman

Dinitrophenylhydrazine does not react with other carbonyl-containing functional groups such as carboxylic acids, amides, and esters. 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 relatively sensitive to shock and friction; it is a shock explosive so care must be taken with its use.

1 Synthesis and early use
2 References
3 See also
4 External links

[edit] Synthesis and early use

"2,4-DNP" is prepared by the reaction of hydrazine with 2,4-dinitrochlorobenzene.^[2]

[edit] References

^ Brady, O. L.; Elsmie, G. V. 'The use of 2:4-dinitrophenylhydrazine as a reagent for aldehydes and ketones' Analyst 51, 77-78, (1926). DOI: 10.1039/AN9265100077; Brady, O. L. J. Chem. Sec. 1931, 757.
^ Allen, C. F. H. "2,4-Dinitrophenylhydrazine" Organic Syntheses, Coll. Vol. 2, p.228 (1943).