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

Solution

Chemistry ma’am · 3 steps

A C=O alone is not enough: 2,4-DNP only reacts with the C=O of an aldehyde or a ketone.

1. What 2,4-DNP detects. 2,4-Dinitrophenylhydrazine (Brady's reagent) adds to a carbonyl carbon and then loses water. The product is a 2,4-dinitrophenylhydrazone, a yellow, orange or red precipitate. This is the test for aldehydes and ketones.
2. Acids and esters fail. In -COOH and -COOR, a lone pair on the attached oxygen is delocalised into the C=O. This makes the carbonyl carbon much less electrophilic, so these groups do not form a hydrazone with 2,4-DNP.

   Option	Compound	Carbonyl type	Orange precipitate?
   (a)	Ethyl salicylate	ester (plus a phenol -OH)	No
   (b)	Ethyl benzoate, C₆H₅COOC₂H₅	ester	No
   (c)	Salicylic acid	carboxylic acid (plus a phenol -OH)	No
   (d)	Acetophenone, C₆H₅COCH₃	ketone	Yes

   C₆H₅COCH₃ + H₂NNHC₆H₃(NO₂)₂ → C₆H₅C(CH₃)=NNHC₆H₃(NO₂)₂ v + H₂O

3. So. Only acetophenone has a true ketone group. It forms the orange 2,4-dinitrophenylhydrazone, so the answer is (d).

The idea to keep. 2,4-DNP is the test for aldehydes and ketones only. If the C=O carbon also carries an -OH, -OR or -NH₂, resonance weakens it and the test is negative.

Answer from the NTA final answer key. Solution by PrepWiser.