Toluene

The tollen test is used to evaluate whether an aldehyde or ketone is present in a given unknown solution. 

This test, commonly known as the Silver-Mirror Test, is named after German scientist Bernhard Tollens. Compounds with the functional group >C=O are known as carbonyl compounds. Aldehydes (R-CHO) or Ketones can be used (R-CO-R).

Any of the alkyl groups can be represented by R.

Tollen’s Test: What Is It Good For?

When an unknown organic compound is given for identification, a systematic study of the substance to detect the functional group involved becomes a critical stage in the entire identification process. 

The application of Tollen’s Reagent Test in a reaction is explained in the following points: The existence of a carbonyl group in the molecule is confirmed by an appropriate positive reaction with 2,4-DNP (2,4-Dinitrophenylhydrazine) or Brady’s reagent. 

However, it is unclear whether the carbonyl group is an aldehydic or a ketonic group. A Tollens’ reagent test is used to discriminate between the two. 

Mild oxidizing agents can easily convert aldehydes to their respective carboxylic acids. Ketones, unlike aldehydes, cannot be converted by mild oxidizing agents to any carboxylic acids. 

Strong oxidizing substances are required for the oxidation of ketones. Tollens’ reagent can be used as a mild oxidizer. 

Tollens’ reagent is a mild oxidizing reagent that reduces aldehyde groups to their respective acids, resulting in the reduction of Silver from its +1 oxidation state (Ag+) to its elemental form (Ag). 

Except for alpha diketones (two ketone functionalities on neighboring atoms) and alpha-hydroxy ketones (keto and alcohol functional groups on adjacent atoms), which can tautomerize into aldehydes via keto-enol tautomerism in an alkaline medium), ketones do not decrease Tollens’ reagent. 

The following is the reaction that occurs during the Tollens’ Test:

(Carboxylic acid) (Tollens’ reagent) (Tollens’ reagent) (Elemental Silver)

What is Tollens’ Reagent, and how does it work?

Tollens’ reagent is a chemical reagent that is used in the Tollens test to determine the presence of aldehyde functional groups as well as some alpha-hydroxy ketones in an unknown solution.

 It was first discovered by a German chemist Bernhard Tollens. It’s a colourless aqueous solution made up of a silver ammonia complex suspended in ammonia.

 The presence of sodium hydroxide in the solution keeps the pH of the solution basic, i.e. (pH>7).

 (Ag(NH3)2)NO3 is Tollens’ reagent formula. Due to its limited shelf life, it is synthesized fresh in the laboratory at the time of the reaction. 

Preparation of Tollens’ Reagent To make Tollens’ reagent, add sodium hydroxide to a silver nitrate solution drop by drop until a light brown precipitate forms. 

Drop by drop, concentrated ammonia solution is added to this until the brown Ag2O precipitate is fully dissolved. 

The ammonia concentration should be high enough to completely dissolve the precipitate until a clear solution is obtained. The final combination is (Ag(NH3)2)+, with silver in the +1 oxidation state, implying that Ag+ is the major component of the Tollens’ reagent.

 The NaOH that was utilized in the beginning is regenerated at the conclusion, which aids in maintaining the required pH balance. 

 (Conc. Ammonia solution) (Tollen To obtain the freshly manufactured reagent, many times a single-step procedure is used instead of a two-step one.

 To do so, a little amount of aqueous ammonia (2ml) is directly added to a solution of silver nitrate (2ml) in a test tube, and then a sufficient amount of ammonia solution is poured dropwise to dissolve the precipitate created, resulting in a clear solution.

 When freshly made Tollens’ reagent is introduced to the solution, it produces a gray-black precipitate or a silver mirror. 

The Basic Methodology 

As follows: A little amount of the provided substance (50mg) is dissolved in an alcohol that is free of aldehydes (2ml-acting as a neutral solvent). 

Freshly made Tollens’ reagent (1ml) is added to this solution, which is then warmed in a hot water bath. 

The presence of an aldehyde is confirmed if a gray-black precipitate forms or a silver mirror appears on the test tube walls. 

The silver ion is reduced and oxidized to its respective acid by going from +1 oxidation state to 0 oxidation state or elemental state during the reaction. 

As a result, the reaction in question is a redox reaction. Because the reaction takes place in an alkaline media, carboxylic acid is not produced directly; instead, carboxylate ions are produced. 

Silver is precipitated out in its elemental form, resulting in the silver mirror on the test tube’s edges or walls.

 If the Tollens’ reagent isn’t a clear solution, a gray-black precipitate with the silver mirror is often formed, which is also a positive test for the aldehydes. 

Carbohydrates are Polyhydroxy Aldoses or Ketoses in their most basic form. Several carbohydrates include a free aldehyde group, and these sugars can easily reduce Tollens’ reagent, Fehling’s reagent, or Benedict’s solution, and hence are referred to as reducing sugars. 

Some sugars or carbohydrates that are low in sugar or carbohydrates can help.

Conclusion

Tollens’ reagent is a chemical reagent that is used in the Tollens test to determine the presence of aldehyde functional groups as well as some alpha-hydroxy ketones in an unknown solution.

 It was first discovered by a German chemist Bernhard Tollens. It’s a colorless aqueous solution made up of a silver ammonia complex suspended in ammonia.

 The presence of sodium hydroxide in the solution keeps the pH of the solution basic, i.e. (pH>7).