Tollen Test

Tollens Test is a great way to tell the difference between aldehydes and ketones. The silver mirror test is another name for this qualitative lab test.

Compounds with an aldehydic group are commonly used in the Tollens test (aldehydes,alpha-hydroxy ketones and formic acid-its -COOH behaves like an aldehydic group). 

It produces a white silver ppt (where the silver salt is converted to silver metal and the aldehyde is oxidised to carboxylic acid silver salt).

Tollens Reagent is a tollens reagent that is used to

The chemical reagent Tollens Reagent is used to detect an aldehyde functional group, an aromatic aldehyde functional group, or an alpha hydroxy ketone functional group in a test sample.

Bernhard Tollens, a German scientist, is credited with discovering the Tollens Reagent and its applications. Tollens reagent is a silver nitrate (AgNO3) and ammonia solution (NH3).

Tollens Alpha Hydroxy Ketone Test

Alpha hydroxy ketones can cause a positive Tollens’ test because they can tautomerize to aldehydes, and the aldehyde causes the Tollens’ test to be positive. A -hydroxy ketone that cannot tautomerize to an aldehyde, such as benzoin, will not produce a positive Tollens’ test.

Tollens Alpha Hydroxy Ketone Test

Preparation of Tollens Reagent

Because Tollens Reagent has such a short shelf life, it is not commercially available. As a result, this reagent is frequently made in the laboratory. The following is an example of a Tollens Reagent preparation method:

Step 1: Add a few drops of dilute NaOH to an aqueous silver nitrate solution. Silver aquo complexes with water as a ligand are found in aqueous silver nitrate solutions. These silver aquo complexes are being converted to silver oxides by the hydroxide ions. From this solution, silver oxide (provided by Ag2O) precipitates as a brown solid.

2AgNO₃ + 2NaOH → Ag₂O (brown ppt) + 2NaNO₃ + H₂O

Step 2: The brown silver oxide precipitate from step 1 is now dissolved in aqueous ammonia. The [Ag(NH₃ )2]+ complex is present in the solution generated by the addition of aqueous ammonia. Tollens Reagent is made up primarily of this complex.

Ag₂O (brown ppt) + 4NH₃ + 2NaNO₃  + H₂O → 2[Ag(NH₃ )₂]NO₃  + 2NaOH

Test of Tollens

When you add an aldehyde to the Tollens reagent, two things happen:

The Tollens reagent oxidises the aldehyde, forming a carboxylic acid.

The Tollens reagent contains silver ions, which are converted to metallic silver. In most cases, the Tollens Test is performed in clean glass test tubes. Because the silver ions are reduced to metallic silver, a silver mirror forms on the test tube. In the illustration below, a silver mirror is shown.

Tollens Test Positive

Because of the creation of this film of shiny silver on the test tube, the Tollens test is also known as the Silver Mirror test.

Conclusion

Tollens Reagent is a chemical reagent that is most commonly used to assess the functional groups of aldehydes and alpha-hydroxy ketones in a sample test material. However, the Tollens reagent can be used selectively in the laboratory or in the chemical industry for a variety of reasons).