Which of the following compounds will give silver mirror with ammoniacal silver nitrate?
A. Formic acid
B. Formaldehyde
C. Benzaldehyde
D. Acetone
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Tollens' reagent only needs a C–H on the carbonyl carbon, so hunt for an aldehyde-type H.

1. What Tollens' needs. Ammoniacal silver nitrate is a mild oxidising agent. It reduces Ag⁺ to metallic silver only when the compound can be oxidised easily, and that means a hydrogen sitting directly on the carbonyl carbon.

   RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag v + 4NH₃ + 2H₂O

   Compound	H on the carbonyl carbon?	Silver mirror?
   Formic acid, HCOOH	Yes, it carries an -CHO unit	Yes
   Formaldehyde, HCHO	Yes, an aldehyde	Yes
   Benzaldehyde, C₆H₅CHO	Yes, an aldehyde	Yes
   Acetone, CH₃COCH₃	No, a ketone	No

2. The odd ones. Formic acid is the only carboxylic acid that reduces Tollens' reagent, and it is oxidised to CO₂ and water. Benzaldehyde fails Fehling's test but passes Tollens' test. Acetone is a ketone and has no such hydrogen.
3. So. A, B and C give the silver mirror and D does not, so the answer is (c).

The idea to keep. For Tollens' test, look for H attached to C=O. All aldehydes pass, including aromatic ones, and formic acid passes too. Ketones fail. Do not mix this up with Fehling's test, which aromatic aldehydes fail.

Answer from the NTA final answer key. Solution by PrepWiser.