Which of the following will give a positive 2,4-DNP test?
(A) CH3CHO
(B) CH3COCH3
(C) CH3COOH
(D) C6H5CHO
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B) and (D) only
The 2,4-DNP test is a test for the carbonyl group of aldehydes and ketones. 2,4-Dinitrophenylhydrazine adds to the C=O group and loses water, giving a yellow, orange or red precipitate of a 2,4-dinitrophenylhydrazone (Ar = 2,4-dinitrophenyl):
\( \displaystyle \ce{R2C=O + H2N-NH-Ar -> R2C=N-NH-Ar + H2O} \)
The reaction needs an electrophilic carbonyl carbon. In a carboxylic acid, the lone pairs of the –OH oxygen are delocalised onto the C=O group by resonance, so the carbonyl carbon is much less electrophilic and the acid does not give this test.
(A) CH3CHO — Correct. Ethanal is an aldehyde, so it gives the hydrazone precipitate.
(B) CH3COCH3 — Correct. Propanone is a ketone and also responds.
(C) CH3COOH — Incorrect. Ethanoic acid is a carboxylic acid; its C=O group does not show the typical reactions of aldehydes and ketones.
(D) C6H5CHO — Correct. Benzaldehyde is an aromatic aldehyde and responds like other aldehydes.
NCERT (2026-27 edition):
Chemical Reactions – …
