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2,4-Dinitrophenylhydrazine



2,4-Dinitrophenylhydrazine
IUPAC name (2,4-dinitrophenyl)hydrazine
Other names 2,4-DNPH; 2,4-DNP; Brady's reagent
Identifiers
CAS number 119-26-6
SMILES NNc1ccc(cc1[N+]([O-])=O)[N+]([O-])=O
Properties
Molecular formula C6H6N4O4
Molar mass 198.14
Appearance Red or orange powder
Melting point

198 - 202 °C dec.

Solubility in water Slight
Hazards
Main hazards Flammable, possible carcinogen
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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

Contents

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

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.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "2,4-Dinitrophenylhydrazine". A list of authors is available in Wikipedia.
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