Q.What happens when -

(a) Formic acid gives a silver mirror with Tollens' reagent (it is oxidised, releasing Ag and CO2) because it has a –CHO group. (b) Aniline + CHCl3 + alcoholic KOH → phenyl isocyanide (carbylamine test) with a very offensive smell.

(a) Formic acid with ammoniacal silver nitrate (Tollens' reagent):

• Ammoniacal silver nitrate is Tollens' reagent, [Ag(NH3)2]⁺, an oxidising reagent that responds to an aldehyde (–CHO) group.
• Formic acid, HCOOH, is unique among carboxylic acids: its structure H–COOH contains both a –COOH and an aldehyde-like –CHO group. Hence it acts as a reducing agent.
• It reduces the silver ammonia ion to metallic silver, which deposits as a bright silver mirror on the walls of the tube, while formic acid is oxidised to carbon dioxide and water: HCOOH + 2[Ag(NH3)2]OH → 2Ag↓ (silver mirror) + CO2 + H2O + 4NH3 + H2O (essentially: HCOOH + 2Ag⁺ → 2Ag + CO2 + 2H⁺)
• Observation: a shining silver mirror forms — showing formic acid has reducing (aldehydic) character. (Acetic acid and higher acids do not give this test.)

(b) Aniline with chloroform and NaOH (or KOH) — the carbylamine reaction:

• Aniline is a primary amine (C6H5NH2). When a primary amine is warmed with chloroform (CHCl3) and alcoholic potassium (or sodium) hydroxide, it forms an isocyanide (carbylamine) with an extremely unpleasant, offensive smell: …