Tollens' Reagent

Tollens' reagent is a mild oxidising agent used to distinguish aldehydes from ketones. It is made from silver nitrate solution and aqueous ammonia, and contains the complex ion [Ag(NH₃)₂]⁺. It should be freshly prepared.The test: add Tollens' reagent to the sample in a clean test tube and warm it in a hot water bath. A water bath is used rather than a flame because organic compounds are flammable.• Aldehyde: a silver mirror forms on the inside of the tube (or a black-grey precipitate of silver).
• Ketone: no change; the solution stays colourless.What happens: the aldehyde is oxidised to a carboxylic acid (in the alkaline reagent it forms the carboxylate ion). At the same time the silver(I) ions are reduced to silver metal:Ag⁺ + e⁻ → Ag
RCHO + [O] → RCOOHKetones give no silver because they are not oxidised by mild oxidising agents.Worked example: two unlabelled bottles contain propanal and propanone (both C₃H₆O). Warming a sample of each with Tollens' reagent, the one that gives a silver mirror is propanal; the one that stays colourless is propanone. The silver mirror comes from Ag⁺ gaining an electron, so the aldehyde has acted as a reducing agent.