Tollens' reagent

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Tollens' reagent is usually ammoniacal silver nitrate, but can also be other things, as long as there is an aqueous diamminesilver(I) complex.

Uses

The diamminesilver(I) complex is an oxidizing agent, which is itself reduced to silver metal, which in a clean glass reaction vessel forms a "silver mirror". This feature is used as a test for aldehydes, which are oxidized to carboxylic acids.

Once it has been identified that there is a carbonyl group on the organic molecule using 2,4-dinitrophenylhydrazine (also known as Brady's reagent or 2,4-DNPH), Tollens' reagent can be used to ascertain whether the compound is a ketone or an aldehyde.

When adding the aldehyde or ketone to Tollens' reagent, put test tube in a warm water bath. If the reactant under test is an aldehyde, Tollens' test results in a silver mirror. If the reactant is a ketone, it will not react because a ketone cannot be oxidized. A ketone has no available hydrogen atom on the carbonyl carbon that can be oxidized, unlike an aldehyde which has this hydrogen atom.

Tollens' reagent is also a test for alkynes with a triple bond in the 1-position. A yellow precipitate of silver carbide is formed in this case.

Tollens' reagent also gives a positive test with formic acid (methanoic acid), as does Fehling's reagent.

Laboratory Preparation

• To aqueous silver nitrate, add a drop of dilute sodium hydroxide. A brown precipitate of silver oxide will form.
• Add ammonia until the precipitate just dissolves.
• This is ammoniacal silver nitrate, [Ag(NH₃)₂]NO₃ (aq).

Safety

The reagent should be freshly prepared and never stored for more than a couple of hours. After the test has been performed, the resulting mixture should be acidified with dilute acid before disposal. These precautions are to prevent the formation of the highly explosive fulminating silver, which is mostly silver nitride, AgN₃ or silver azide.