Which reagent distinguishes an aldehyde from a ketone by forming a silver mirror on the inner surface of the test tube?
Answer and explanation
Correct answer: C. Tollens' reagent (ammoniacal silver nitrate)
- Step 1: Tollens' reagent contains the diamminesilver(I) complex ion, [Ag(NH3)2]+.
- Step 2: Aldehydes are readily oxidized to carboxylate ions, reducing Ag+ to metallic silver which coats the glass wall.
- Step 3: Ketones lack an oxidizable hydrogen atom on the carbonyl carbon and do not react with Tollens' reagent.
Answer
Tollens' reagent (ammoniacal silver nitrate).
Why not the others: 'Benedict's solution' forms a brick-red Cu2O precipitate rather than a silver mirror; 'Bromine water' is used to test for unsaturation (C=C double bonds); 'Acidified potassium manganate(VII)' decolors from purple to colorless but produces no mirror.
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