2,4-DNP test detects aldehydes and ketones via yellow-orange hydrazone precipitate

Steps:

• 2,4-Dinitrophenylhydrazine reacts with the carbonyl (C=O) group in aldehydes and ketones to form insoluble hydrazones.
• The hydrazone precipitates as yellow-orange, distinguishing carbonyls from other functional groups.
• Evaluate options: only those with aldehyde or ketone will test positive.
• Select D, as it contains a carbonyl compound.

Why D is correct:

• Aldehydes and ketones react with 2,4-DNP to yield 2,4-dinitrophenylhydrazone, a yellow-orange precipitate, per the standard qualitative test for C=O groups.

Why the others are wrong:

• A: Alkane lacks carbonyl; no reaction occurs.
• B: Alcohol has OH group; does not form hydrazone precipitate.
• C: Carboxylic acid's C=O is unreactive under test conditions; no yellow-orange ppt.

Final answer: D