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Concept
Tollens' reagent
Résumé
Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver nitrate, ammonium hydroxide and some sodium hydroxide (to maintain a basic pH of the reagent solution). It was named after its discoverer, the German chemist Bernhard Tollens. A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.
This reagent is not commercially available due to its short shelf life, so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute sodium hydroxide are added to some aqueous 0.1 M silver nitrate. The OH- ions convert the silver aquo complex form into silver(I) oxide, Ag2O, which precipitates from the solution as a brown solid:
2AgNO3 + 2NaOH -> Ag2O(s) + 2NaNO3 + H2O
In the next step, sufficient aqueous ammonia is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH3)2]+ complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:
Ag2O(s) + 4NH3 + 2NaOH + H2O -> 2[Ag(NH3)2]OH + 2NaOH
Alternatively, aqueous ammonia can be added directly to silver nitrate solution. At first, ammonia will induce formation of solid silver oxide, but with additional ammonia, this solid precipitate dissolves to give a clear solution of diamminesilver(I) coordination complex, [Ag(NH3)2]+. Filtering the reagent before use helps to prevent false-positive results.
Once the presence of a carbonyl group has been identified using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish ketone vs aldehyde. Tollens' reagent gives a negative test for most ketones, with alpha-hydroxy ketones being one exception.
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