2,4-Dinitrophenylhydrazine, 2,4-DNPH, is the reagent every A-level, IB and AP chemistry student uses for the carbonyl test and then has to explain in the exam: what it detects, why it gives a precipitate, why the precipitate does not appear with an ester, and how the same reaction identifies the exact compound. This page covers the reagent, the equation, the mechanism, the results table, the follow-up tests and the safety points, in the order a practical write-up or a six-mark question asks for them.
The reagent
| Name | 2,4-Dinitrophenylhydrazine (2,4-DNPH); in solution, Brady's reagent |
| Formula | C6H3(NO2)2NHNH2, C6H6N4O4 |
| Molar mass | 198.14 g/mol |
| Appearance | Red to orange crystalline solid; supplied wetted with water because the dry solid is shock-sensitive |
| Brady's reagent | 2,4-DNPH dissolved in methanol (or ethanol) with concentrated sulfuric acid; orange solution |
| Structure | A benzene ring carrying two nitro groups (positions 2 and 4) and a hydrazine group –NH–NH2 at position 1 |
The business end is the terminal –NH2 of the hydrazine group, which is a nucleophile. The two nitro groups are electron-withdrawing; they make the product hydrazones deeply coloured and poorly soluble, which is why the test gives a precipitate rather than a colour change in solution.
What it tests for, and the result
Add two or three drops of the unknown to about 1 cm3 of Brady's reagent and shake.
| Compound class | Contains C=O? | Result with 2,4-DNPH |
|---|---|---|
| Aldehyde (e.g. ethanal, benzaldehyde) | Yes | Yellow to orange precipitate |
| Ketone (e.g. propanone, cyclohexanone) | Yes | Yellow to orange precipitate |
| Carboxylic acid | Yes | No precipitate |
| Ester | Yes | No precipitate |
| Amide, acyl chloride | Yes | No precipitate (acyl chlorides hydrolyse first) |
| Alcohol, ether, alkene, alkane | No | No precipitate |
The colour shifts towards orange-red with conjugation: aliphatic carbonyls give yellow, aromatic ones (benzaldehyde, acetophenone) give orange to red. Report the colour, but the mark is for precipitate.
The equation
A condensation: the two molecules join and a molecule of water is lost.
R2C=O + H2N–NH–C6H3(NO2)2 → R2C=N–NH–C6H3(NO2)2 + H2O
For propanone: (CH3)2C=O + 2,4-DNPH → (CH3)2C=N–NH–C6H3(NO2)2 + H2O. The product is propanone 2,4-dinitrophenylhydrazone, a yellow solid, melting point 126 °C.
The mechanism: nucleophilic addition-elimination
- Protonation of the carbonyl oxygen by the acid catalyst makes the carbonyl carbon more electrophilic.
- Nucleophilic addition: the lone pair on the terminal nitrogen of 2,4-DNPH attacks the carbonyl carbon; the C=O π bond breaks and the electrons move onto oxygen, giving a tetrahedral intermediate with an –OH and an –NH2+– on the same carbon.
- Proton transfer from nitrogen to oxygen gives a carbinolamine with an –OH2+ leaving group.
- Elimination of water: the nitrogen lone pair forms the C=N double bond as water leaves.
- Deprotonation of the nitrogen regenerates the catalyst and gives the hydrazone.
It is the same addition-elimination pattern as the reaction of a carbonyl with hydroxylamine (to give an oxime) or with a primary amine (to give an imine), which is why exam questions sometimes ask for the mechanism with "a nucleophile of the form H2N–X".
Why acids, esters and amides give nothing
All three have the carbonyl carbon bonded to an oxygen or nitrogen that carries a lone pair. That lone pair is delocalised into the C=O (the same resonance that makes acetamide a weak base), which lowers the partial positive charge on the carbonyl carbon and makes it a much poorer electrophile. The nucleophilic nitrogen of 2,4-DNPH does not attack it at a useful rate. The mark scheme phrase: the carbonyl carbon in an acid or ester is less electrophilic because of delocalisation of the lone pair on the adjacent O, so no addition occurs.
Aldehyde or ketone? The second test
2,4-DNPH says only "carbonyl". Take a fresh sample of the unknown and use a mild oxidising agent; aldehydes are oxidised to carboxylic acids, ketones are not.
| Reagent | With an aldehyde | With a ketone |
|---|---|---|
| Tollens' reagent (ammoniacal silver nitrate, warm) | Silver mirror on the tube: Ag+ reduced to Ag | No change |
| Fehling's or Benedict's solution (warm) | Brick-red precipitate of Cu2O from the blue Cu2+ | Stays blue |
| Acidified potassium dichromate(VI) (warm) | Orange to green | Stays orange |
Two tests, one conclusion: 2,4-DNPH positive and Tollens' positive is an aldehyde; 2,4-DNPH positive and Tollens' negative is a ketone.
Identifying the exact compound from the derivative
The precipitate is a pure, crystalline solid with a sharp melting point that is different for every carbonyl compound. Filter it off, recrystallise from ethanol, dry, and take the melting point; compare with the data book. This was the standard method of identifying an unknown aldehyde or ketone before spectroscopy, and it is still a practical in most A-level and IB courses.
| Carbonyl compound | Boiling point of compound | Melting point of its 2,4-DNPH derivative |
|---|---|---|
| Ethanal | 21 °C | 168 °C |
| Propanal | 49 °C | 155 °C |
| Propanone | 56 °C | 126 °C |
| Butanone | 80 °C | 116 °C |
| Benzaldehyde | 179 °C | 237 °C |
| Cyclohexanone | 156 °C | 162 °C |
Two liquids with close boiling points (propanal 49 °C, propanone 56 °C) give derivatives 29 °C apart, which is the point of the method.
Safety
The dry solid is a sensitive explosive; it is stored wetted with at least 30 % water, and a bottle that has dried out is not opened. Brady's reagent contains concentrated sulfuric acid and methanol: gloves, goggles, small volumes. 2,4-DNPH is toxic if swallowed and irritant to skin. In the write-up, the hazard to name is the acid and the flammable solvent, and the control measure is small volumes in a test tube.
The six-mark answer, in order
- Reagent: 2,4-dinitrophenylhydrazine (Brady's reagent).
- Positive result: orange precipitate; the compound is an aldehyde or ketone.
- Reaction type: condensation (nucleophilic addition-elimination); water is eliminated.
- Product: a 2,4-dinitrophenylhydrazone.
- Esters and acids: no precipitate, carbonyl carbon less electrophilic.
- To identify: purify the derivative, measure its melting point, compare with data; to distinguish aldehyde from ketone, Tollens' or Fehling's.
How we teach A-level and IB organic chemistry
One tutor, one student, the tests and mechanisms drawn by the student on a blank page while the tutor watches, then corrected against the board's mark scheme. Carbonyl chemistry takes three lessons: reactions, mechanisms, and the identification practical. The first 30-minute class is free: send the topic and the last test on WhatsApp or the contact page. The catalysis that makes this and most carbonyl reactions go is in acid-base catalysis; the subject page is online chemistry tutoring.
Questions parents ask
1What does 2,4-dinitrophenylhydrazine test for?
2What is Brady's reagent?
3Why does the test not work on carboxylic acids and esters?
4How do you tell whether the compound is an aldehyde or a ketone?
See how we teach this, 1-on-1 online →
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