Which of the following gives positive Tollens test ?
(A) Phenylethanal
(B) Acetophenone
(C) Formic acid
(D) Benzene carbaldehyde
Choose the correct answer from the options given below :
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (C) and (D) only
Tollens’ reagent (ammoniacal silver nitrate) is a mild oxidising agent. It oxidises aldehydes and gives a silver mirror; ketones are not oxidised by it:
\( \displaystyle \ce{RCHO + 2[Ag(NH3)2]+ + 3OH^- -> RCOO^- + 2Ag v + 2H2O + 4NH3} \)
(A) Phenylethanal — Correct. \( \ce{C6H5CH2CHO} \) is an aldehyde, so it gives the silver mirror.
(B) Acetophenone — Incorrect. \( \ce{C6H5COCH3} \) is a ketone; it is not oxidised by Tollens’ reagent.
(C) Formic acid — Correct. HCOOH contains an aldehyde-like H–C=O group, so it is easily oxidised (to \( \ce{CO2} \) and water) and reduces Tollens’ reagent; it is the only carboxylic acid that does.
(D) Benzene carbaldehyde — Correct. Benzaldehyde, \( \ce{C6H5CHO} \), is an aldehyde and gives the silver mirror (aromatic aldehydes do not give Fehling’s test, but they do give Tollens’ test).
NCERT (2026-27 edition):
Oxidation
