A Levels Chemistry (9701)•9701/13/M/J/21
Explanation
2,4-DNPH detects aldehydes and ketones via carbonyl groups Steps:
- Identify functional groups in each skeletal formula: R shows C=O (carbonyl), S shows OH (alcohol), T shows C=C (alkene).
- Recall that 2,4-dinitrophenylhydrazine (2,4-DNPH) reacts with carbonyls in aldehydes and ketones to form orange hydrazones.
- Test R: carbonyl present, so positive reaction.
- Test S and T: no carbonyls, so no reaction.
Why A is correct:
- R contains a carbonyl group, which forms a hydrazone with 2,4-DNPH per the reagent's specific reaction mechanism for C=O bonds.
Why the others are wrong:
- B includes S, an alcohol without a carbonyl, yielding no hydrazone.
- C includes T, an alkene without a carbonyl, yielding no hydrazone.
- D includes S and T, both lacking carbonyls, yielding no hydrazones.
Final answer: A
Topic: Carbonyl compounds
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