Question

A compound ‘A’ with molecular formula C₄H₈O gives positive 2,4-DNP test. It gives yellow precipitate of compound ‘B’ on treatment with sodium hydrogen sulphite. Compound ‘A’ does not react with Tollen’s or Fehling’s reagent; on oxidation with KMnO₄, it forms carboxylic acid ‘C’. Elucidate the structures of ‘A’, ‘B’ and ‘C’. Also give their IUPAC names.

Hint:

Use functional group tests like DNP and oxidation behavior to identify aldehydes vs ketones.

Answer 1

Positive 2,4-DNP test and no reaction with Tollen’s or Fehling’s test indicate ‘A’ is a ketone. C₄H₈O ketone → likely structure: Butan-2-one (CH₃COCH₂CH₃)

‘B’ is the sodium bisulfite addition product of the ketone.

On oxidation, ketones give carboxylic acids at the more oxidizable position, forming: CH₃COOH (ethanoic acid) and CO₂. But since one carboxylic acid is obtained, structure of ‘C’ is likely CH₃COOH (ethanoic acid).

Structures:

‘A’ = Butan-2-one (CH₃COCH₂CH₃)

‘B’ = Bisulfite adduct of butan-2-one

‘C’ = Ethanoic acid (CH₃COOH)