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.

Additional recommended knowledge

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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.