This MedLibrary.org supplementary page on Tollens' reagent is provided directly from the open source Wikipedia as a service to our readers. Please see the note below on authorship of this content, as well as the Wikipedia usage guidelines. To search for other content from our encyclopedia supplement, please use the form below:
Related Sponsors
![Ball-and-stick model of the diamminesilver(I) cation, [Ag(NH3)2]+](Image%3ADiamminesilver(I)-3D-balls.png)
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.
When adding the aldehyde or ketone to Tollens' reagent, put the 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 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 silver carbide 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.
Laboratory Preparation
- To aqueous silver nitrate, a drop of dilute sodium hydroxide is added, forming a brown precipitate of silver oxide.
- Concentrated ammonia is then added until the precipitate just dissolves.
- This is ammoniacal silver nitrate, [Ag(NH3)2NO3 (aq).
The reaction is:
- 2 AgNO3 + 2 NaOH → Ag2O + 2 NaNO3 + H2O
- Ag2O + 2 NH3 + NaNO3 +H2O → Ag(NH3)2NO3 (aq) + 2NaOH
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 silver azide.
External links
Wikipedia content modification information:
- This page was last modified on 2 October 2008, at 00:48.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Tollens' reagent".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.