2,4-Dinitrophenylhydrazine

2,4-Dinitrophenylhydrazine (or Brady's reagent) is the chemical compound C₆H₃(NO₂)₂NHNH₂. Dinitrophenylhydrazine is relatively sensitive to shock and friction; it is a shock explosive so care must be taken with its use. It is a red to orange solid, usually supplied wet to reduce its explosive hazard. It is a substituted hydrazine, and is often used to qualitatively test for carbonyl groups. The hydrazone derivatives can also be used as evidence toward the identity of the original compound.

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Synthesis

2,4-Dinitrophenylhydrazine is commercially available usually as a wet powder. It can be prepared by the reaction of hydrazine with 2,4-dinitrochlorobenzene.^[1]

Brady's test

2,4-Dinitrophenylhydrazine can be used to qualitatively detect the carbonyl functionality of 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

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:^[2]

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.^[3]

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.

See also

• Tollens' reagent

References

1. ^ Allen, C. F. H. (1943). "2,4-Dinitrophenylhydrazine". Org. Synth.; Coll. Vol. 2: 228. 
2. ^ Adapted from Chemistry in Context, 4th Edition, 2000, Graham Hill and John Holman
3. ^ Brady, O. L.; Elsmie, G. V. (1926). "The use of 2:4-dinitrophenylhydrazine as a reagent for aldehydes and ketones". Analyst 51: 77-78. doi:10.1039/AN9265100077.