Four compounds A, B, C and D were warmed separately with Tollens’ reagent. All the compounds except D formed a silver mirror on the inside of the test tube. Which of the following could be compound D?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Acetone
Tollens’ reagent (ammoniacal silver nitrate) is a mild oxidising agent. It oxidises an aldehyde to the carboxylate ion and is itself reduced to metallic silver, which forms the silver mirror:
\( \displaystyle \ce{RCHO + 2[Ag(NH3)2]^+ + 3OH^- -> RCOO^- + 2Ag v + 2H2O + 4NH3} \)
A ketone has no H on its carbonyl carbon and is oxidised only under vigorous conditions, so it gives no silver mirror. Acetone (\( \ce{CH3COCH3} \)) is a ketone, so it can be D.
Why the other options are incorrect
Option (1) Acetaldehyde — Incorrect. \( \ce{CH3CHO} \) is an aldehyde; it is oxidised to acetate and gives the silver mirror.
Option (2) Butyraldehyde — Incorrect. \( \ce{CH3CH2CH2CHO} \) is an aldehyde and gives the silver mirror.
Option (4) Benzaldehyde — Incorrect. \( \ce{C6H5CHO} \) is an aldehyde; aromatic aldehydes also reduce Tollens’ reagent (it is Fehling’s test that they fail).
NCERT (2026-27 edition):
8.4 Chemical Reactions – “Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures.”
“(i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal.”