Which one of the following compounds gives a positive Fehling's Test?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Acetaldehyde
Fehling’s reagent is a mixture of Fehling solution A (aqueous \( \ce{CuSO4} \)) and Fehling solution B (alkaline sodium potassium tartrate). On warming with an aliphatic aldehyde, the aldehyde is oxidised to a carboxylate ion and Cu2+ is reduced to a red-brown precipitate of Cu2O:
\( \displaystyle \ce{R-CHO + 2Cu^2+ + 5OH^- -> R-COO^- + Cu2O v + 3H2O} \)
Acetaldehyde (ethanal, \( \ce{CH3CHO} \)) is an aliphatic aldehyde, so it gives a positive Fehling’s test.
Why the other options are incorrect
Option (1) Benzaldehyde — Incorrect. Aromatic aldehydes do not reduce Fehling’s solution, although they do give Tollens’ test.
Option (3) Butanone — Incorrect. A ketone; it has no hydrogen on the carbonyl carbon and is not oxidised by mild oxidants.
Option (4) t-Butanol — Incorrect. A tertiary alcohol; it has no carbonyl group and resists oxidation.
NCERT (2026-27 edition):
Fehling’s Test
