Which of the following compounds will not give a red precipitate when heated with Fehling's solution?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Benzaldehyde
Fehling’s reagent (alkaline copper(II) tartrate) oxidises an aldehyde to the carboxylate ion and is itself reduced to a reddish brown precipitate of Cu2O:
\( \displaystyle \ce{RCHO + 2Cu^2+ + 5OH^- -> RCOO^- + Cu2O v + 3H2O} \)
Fehling’s solution is a weak oxidising agent, and aromatic aldehydes do not respond to this test.
Option (1) Methanal — Gives a red precipitate with Fehling’s solution. HCHO is an aliphatic aldehyde.
Option (2) Ethanal — Gives a red precipitate with Fehling’s solution. \( \ce{CH3CHO} \) is an aliphatic aldehyde.
Option (3) Cyclohexane carbaldehyde — Gives a red precipitate with Fehling’s solution. Its –CHO group is on a saturated ring carbon, so it behaves as an aliphatic aldehyde.
Option (4) Benzaldehyde — Does not give a red precipitate. It is an aromatic aldehyde (–CHO on a benzene ring), and aromatic aldehydes do not respond to Fehling’s test.
NCERT (2026-27 edition):
Fehling’s Test – “Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.”