The compound which will not respond to Fehling's test is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Benzaldehyde
On heating, an aliphatic aldehyde reduces the \( \ce{Cu^{2+}} \) of Fehling’s solution to a reddish brown precipitate of copper(I) oxide:
\( \displaystyle \ce{R-CHO + 2Cu^2+ + 5OH^- -> R-COO^- + Cu2O v + 3H2O} \)
Option (1) Glucose — Responds. Glucose has a free –CHO group in its open-chain form; it is a reducing sugar and is oxidised to gluconate.
Option (2) Ethanal — Responds. An aliphatic aldehyde: \( \ce{CH3CHO} \) is oxidised to acetate and gives \( \ce{Cu2O} \).
Option (3) Benzaldehyde — Does not respond (aromatic aldehyde). \( \ce{C6H5CHO} \) gives no red precipitate.
Option (4) Formaldehyde — Responds. The simplest aliphatic aldehyde, \( \ce{HCHO} \), also reduces Fehling’s solution.
NCERT (2026-27 edition):
Fehling’s Test
