2,4-Dinitrophenyl hydrazine Test for Carbonyl Groups Detection

The 2,4-Dinitrophenylhydrazine (2,4-DNPH) test, also known as Brady’s test, is a classical qualitative test used in organic chemistry to detect the presence of carbonyl functional groups in organic compounds, specifically aldehydes and ketones.
The reagent, 2,4-DNPH, reacts with the carbonyl group (C=O) to form a solid, colored 2,4-dinitrophenylhydrazone derivative.

The appearance of a yellow, orange, or red precipitate confirms the presence of a carbonyl compound.

What is 2,4-Dinitrophenylhydrazine?

2,4-Dinitrophenylhydrazine is an aromatic hydrazine derivative containing two electron-withdrawing nitro groups attached to a benzene ring. It is usually used in an acidic alcoholic medium, which enhances its reactivity toward carbonyl compounds. The presence of nitro groups increases the stability and color intensity of the reaction product.

Principle of the 2,4-DNP Test

The test works on a condensation reaction between the carbonyl group of an aldehyde or ketone and 2,4-DNP. In an acidic medium, the carbonyl carbon becomes more reactive and undergoes nucleophilic attack by the hydrazine group.

2,4‑DNPH reacts with aldehydes and ketones to form solid 2,4‑dinitrophenylhydrazone derivatives, producing yellow/orange/red precipitates.

General Reaction:

R‑CO‑R’ + H₂NNHC₆H₃(NO₂)₂ → R‑C=N‑NH‑C₆H₃(NO₂)₂ + H₂O

This diagram showing the reaction of acetone with 2,4 dinitrophenyl hydrazine forming a yellow hydrazone precipitate in the carbonyl compound test.

🧪Procedure

Add 1–2 mL of 2,4‑DNPH reagent into a test tube.
Add 2–3 drops of the unknown sample.
Shake or warm gently.
Formation of a precipitate indicates a positive test.

The color depends on the structure of the aldehyde or ketone.
Aliphatic carbonyls → yellow precipitate
Aromatic or conjugated carbonyls → orange to red precipitate

This diagram illustrates the 2,4 dinitrophenyl hydrazine test where carbonyl compounds react to form yellow precipitate for isolated carbonyl groups and orange precipitate for conjugated carbonyl systems

Observation and Result

A positive 2,4-DNP test is confirmed by the formation of a yellow, orange, or red precipitate. The intensity of the color depends on the nature of the carbonyl compound. Aliphatic aldehydes and ketones usually give a yellow precipitate, while aromatic carbonyl compounds form deeper orange or red precipitates due to greater conjugation.

If no precipitate is formed, the test is considered negative, indicating the absence of aldehyde or ketone groups.

Compound Type	2,4-DNP Test Result
Formaldehyde	Yellow ppt
Acetone	Yellow ppt
Benzaldehyde	Orange ppt
Acetophenone	Orange-red ppt
Crotonaldehyde (unsaturated)	Deep red ppt
Alcohols / Acids	No reaction

Why Aldehydes and Ketones Respond to 2,4-DNP

Aldehydes and ketones readily give the 2,4-DNP test because they contain a polar and reactive carbonyl group. The carbonyl carbon is electron-deficient and easily attacked by nucleophiles such as the hydrazine group. This reactivity leads to the formation of a stable hydrazone derivative, which separates out as a precipitate.

Compounds That Do Not Give the Test

Alcohols, carboxylic acids, esters, and ethers do not respond to the 2,4-DNP test because they lack a free carbonyl group capable of undergoing condensation. As a result, no precipitate is formed in these cases.

Role of 2,4-DNP in Identification of Carbonyl Compounds

The 2,4-DNP test is widely used not only for detection but also for identification of unknown aldehydes and ketones. The hydrazone derivatives formed are crystalline solids with sharp melting points. By comparing the melting point of the derivative with known values, specific carbonyl compounds can be identified in organic analysis

Importance and Applications

Detecting carbonyl functional groups in organic compounds
Differentiating carbonyl compounds vs. non-carbonyl compounds
Useful for preliminary functional group analysis in labs
Formation of hydrazones helps in purification and identification
Used in qualitative organic analysis and academic laboratories

Viva questions

What does the 2,4-DNP test detect?
Why does a precipitate form in this test?
What colors are observed for aldehydes and ketones?
Why do alcohols not give this test?
What is the principle behind hydrazone formation?
What is the reagent used in this test?
Why is the reagent kept acidic?
What is Brady’s reagent?

Multiple Choice Questions

MCQ 1

The 2,4‑DNPH test is used for detecting which group?

A. Hydroxyl

B. Carbonyl

C. Carboxyl

D. Amide

MCQ 2

2. A positive DNPH test gives:

A. Blue solution

B. Yellow/orange/red precipitate

C. White fumes

D. No change

MCQ 3

3. Which of the following gives a positive DNPH test?

A. Methanol

B. Ethanoic acid

C. Propanone

D. Methylamine

MCQ 4

4. DNPH reacts with carbonyl compounds to form:

A. Esters

B.Hydrazones

C. Alcohols

D. Amines

MCQ 5

5. DNPH test cannot distinguish between:

A. Aldehydes

B. Ketones

C. Aldehydes & ketones

D. Alcohols

MCQ 6

6. Which compound will not give 2,4-DNP test?

A. Acetone

B. Ethanal

C. Ethanol

D. Benzaldehyde

FAQ’s

It indicates the presence of an aldehyde or ketone.

The acidic medium increases the reactivity of the carbonyl group, allowing the condensation reaction to occur easily.

Aromatic aldehydes have greater conjugation, which intensifies the color of the hydrazone formed.

Orange to orange-red precipitate.

Brady’s reagent

References

Brady, O. L. (1931). A simple qualitative test for aldehydes and ketones. Journal of the Chemical Society.
Furniss, B. S., et al. Vogel’s Textbook of Practical Organic Chemistry.
Reusch, W. (n.d.). Carbonyl compound identification. MCC Organic Chemistry.