Question

Which one of the following compounds will give orange precipitate when treated with 2,4-dinitrophenyl hydrazine ? 

• A. A
• B. B
• C. C
• D. D

Hint:

2,4-DNP gives a positive test only with aldehydes and ketones. Esters, acids, amides and anhydrides do not respond to this test.

Answer 1

The Correct Option is A

2,4-Dinitrophenylhydrazine (2,4-DNP), also called Brady’s reagent, is used to detect the presence of aldehydes and ketones.
These compounds react with 2,4-DNP to form brightly coloured 2,4-dinitrophenylhydrazones (orange or yellow precipitate).
Analysis of given compounds:
Acetophenone: It is a ketone and contains a reactive carbonyl (>C=O) group. Hence, it gives an orange precipitate with 2,4-DNP.
Ethyl benzoate: It is an ester. Due to resonance stabilisation of the carbonyl group, esters do not undergo nucleophilic addition with 2,4-DNP.
Ethyl salicylate: Also an ester, so it does not give the 2,4-DNP test.
Salicylic acid: A carboxylic acid. Carboxylic acids do not react with 2,4-DNP.
Hence, only acetophenone gives a positive 2,4-DNP test. \[ \boxed{\text{Acetophenone}} \]