Tollens' reagent is a mild oxidising agent used to distinguish aldehydes from ketones. It is also called ammoniacal silver nitrate, because it contains the complex ion [Ag(NH3)2]+, the diamminesilver(I) ion.Preparing Tollens' reagent (it must be made fresh):
1. Add a few drops of sodium hydroxide solution to aqueous silver nitrate. A brown precipitate of silver(I) oxide forms.
2. Add dilute aqueous ammonia drop by drop until the precipitate just dissolves, giving a colourless solution.The test: add a few drops of the compound to the Tollens' reagent in a clean test tube and warm it in a hot water bath. A water bath is used rather than a naked flame because many carbonyl compounds are flammable.Results:
• Aldehyde: a silver mirror forms on the inside of the tube (or a grey-black precipitate of silver).
• Ketone: no change; the solution stays colourless.What happens: the aldehyde is oxidised, and the silver(I) ions are reduced to silver metal:
Ag+ + e− → Ag
Because the reagent is alkaline, the aldehyde is oxidised to the carboxylate ion (the anion of the carboxylic acid) rather than the free acid.Example: ethanal warmed with Tollens' reagent is oxidised to ethanoate ions, and a silver mirror forms. Propanone, a ketone, gives no silver mirror.The test works because aldehydes, with an H atom on the carbonyl carbon, are easily oxidised, whereas ketones are not. It does not tell different aldehydes apart.
1. Add a few drops of sodium hydroxide solution to aqueous silver nitrate. A brown precipitate of silver(I) oxide forms.
2. Add dilute aqueous ammonia drop by drop until the precipitate just dissolves, giving a colourless solution.The test: add a few drops of the compound to the Tollens' reagent in a clean test tube and warm it in a hot water bath. A water bath is used rather than a naked flame because many carbonyl compounds are flammable.Results:
• Aldehyde: a silver mirror forms on the inside of the tube (or a grey-black precipitate of silver).
• Ketone: no change; the solution stays colourless.What happens: the aldehyde is oxidised, and the silver(I) ions are reduced to silver metal:
Because the reagent is alkaline, the aldehyde is oxidised to the carboxylate ion (the anion of the carboxylic acid) rather than the free acid.Example: ethanal warmed with Tollens' reagent is oxidised to ethanoate ions, and a silver mirror forms. Propanone, a ketone, gives no silver mirror.The test works because aldehydes, with an H atom on the carbonyl carbon, are easily oxidised, whereas ketones are not. It does not tell different aldehydes apart.