2,4-Dinitrophenylhydrazine test, MSDS

2,4-Dinitrophenylhydrazine
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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 considered an 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:
Crystals of different hydrazones have characteristic melting and boiling points, allowing the identity of a substance to be determined in a method known as derivatization. In particular, the use of 2,4-dinitrophenylhydrazine was developed by Brady and Elsmie.[3] Modern spectroscopic and spectrometric techniques have since superseded these techniques.

2,4-DNP reacting with 2-heptanone

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.


Procedure

Add a solution of 1 or 2 drops or 30 mg of unknown in 2 mL of 95% ethanol to 3 mL of 2,4-dinitrophenylhydrazine reagent. Shake vigorously, and, if no precipitate forms immediately, allow the solution to stand for 15 minutes.
2,4-Dinitrophenylhydrazine reagent: Dissolve 3 g of 2,4-Dinitrophenylhydrazine in 15 mL of conc. sulfuric acid. This solution is then added with stirring to 20 mL of water and 70 mL of 95% ethanol. This solution is mixed thoroughly and filtered.

Positive Test

Formation of a precipitate is a positive test.

Complications
Some ketones give oils which will not solidify.
Some allylic alcohols are oxidized by the reagent to aldehydes and give a positive test.
Some alcohols, if not purified, may contain aldehyde or ketone impurities.


Safety data for 2,4-dinitrophenylhydrazineGeneral
Synonyms: 2,4-DNPH, (2,4-dinitrophenyl)hydrazine, 1-hydrazino-2,4-dinitrobenzene, DNPH, DNP-hydrazine
Use:
Molecular formula: C6H6N4O4
CAS No: 119-26-6
EINECS No: 204-309-3
Physical data
Appearance: red powder
Melting point: 200 - 202 C
Boiling point:
Vapour density:
Vapour pressure:
Density (g cm-3):
Flash point:
Explosion limits:
Autoignition temperature:
Water solubility:

Stability
Stable when wet, but explosive when dry. May be shock sensitive when dry. Highly flammable. Incompatible with strong oxidizing agents.

Toxicology
Harmful if swallowed.

Toxicity data

(The meaning of any toxicological abbreviations which appear in this section is given here.)
ORL-RAT LD50 500 mg kg-1
IPR-RAT LD50 450 mg kg-1
Risk phrases
(The meaning of any risk phrases which appear in this section is given here.)
R1 R11 R22.

Transport information

(The meaning of any UN hazard codes which appear in this section is given here.)
UN No 1325. Hazard class 4.1. Packing group II.
Personal protection
Safety glasses, adequate ventilation. Do not dry or heat this material.
Safety phrases
(The meaning of any safety phrases which appear in this section is given here.)