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Ball-and-stick model of the diamminesilver(I) cation, [Ag(NH3)2]+

Tollens' reagent is usually ammoniacal silver nitrate, but can also be other compounds, as long as there is an aqueous diamminesilver(I) complex. It was named after Bernhard Tollens.

Contents

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. It should be noted that the actual oxidation product is the carboxylate ion, which on acidification gives the corresponding carboxylic acid.

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. Importantly, there is a special case in which Tollens' Reagent will give a positive for a ketone. If the ketone is an alpha-hydroxy ketone, then the Tollens' reagent will react.

When adding the aldehyde or ketone to Tollens' reagent, the test tube is put 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 easily. 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 the metal acetylide is formed in this case.

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

It also was used in the Victorian times to make mirrors.

In Anatomic pathology, ammoniacal silver nitrate is used in the Fontana-Masson Stain, which is a silver stain technique used to detect melanin, argentaffin and lipofuscin in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.[1]

Use of Tollens' reagent to detect aldehyde

Laboratory Preparation

  1. To aqueous silver nitrate, a drop of dilute sodium hydroxide is added, forming a brown precipitate of silver oxide.
  2. Concentrated ammonia is then added until the precipitate just dissolves.
  3. This is ammoniacal silver nitrate, [Ag(NH3)2]NO3 (aq).[2] The reagent should then be filtered.

The reaction is:

2 AgNO3 (aq) + 2 NaOH (aq) → Ag2O (s) + 2 NaNO3 (aq) + H2O (l)
Ag2O (s) + 4 NH3 (aq) + 2 NaNO3 (aq) +H2O (l) → 2 Ag(NH3)2NO3 (aq) + 2 NaOH (aq)

Safety

The reagent should be freshly prepared and stored refrigerated in a dark glass container. It has an approximate shelf-life of 24 hours when stored in this way.[3] 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 silver azide.

External links

References

  1. ^ Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF Accessed 4 February, 2009
  2. ^ Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF Accessed 4 February, 2009
  3. ^ Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF Accessed 4 February, 2009
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