Tollens’ Test: Tollens’ Reagent, Principle of Tollens’ Test, Examples, Purpose, Uses and Limitations of Tollens’ Test, Practice Problems and Frequently Asked Questions

Tollens’ tests are usually performed with compounds that have an aldehyde group (aldehydes, alpha hydroxy ketones, formic acid). It gives greyish-white silver stains when we perform the test (reduces silver salts to silver metals and oxidizes aldehydes to silver salts of carboxylic acids)

The Tollens’ Reagent is named after Bernhard Tollens’, A German chemist who discovered this reagent and its uses. Tollens’ reagent is a solution of silver nitrate (AgNo₃) and Aq. Ammonia 

(NH₄OH).

Table of content

• Tollens’ Reagent
• Principle of Tollens’ Test
• Tollens’ Reagent Preparation
• Tollens’ Test
• Purpose of Tollen’s Test
• Uses of Tollens’ Test
• Limitations of Tollen’s Test
• Practice Problems on Tollen's Reagent 
• FAQ's on Tollen's Reagent 

Tollens’ Reagent

Tollens’ Reagent([Ag(NH₃)₂]⁺) refers to the chemical reagent which is used in the detection of an aldehyde functional group, an aromatic aldehyde functional group, or an alpha hydroxy ketone functional group in a given test substance.

Principle of Tollens’ Test

• The Tollens’ reagent is the solution of alkaline silver nitrate ((AgNo₃)) combined with aqueous ammonia solution (NH₄OH), leading to the development of a complex.
• Silver nitrate in water produces a silver-aqua complex, in which water acts as a ligand.
• Hydroxide ions convert the aqua complexes into silver oxides (Ag₂O).
• The [Ag(NH₃)₂]⁺ complex is formed when silver oxide develops a brown precipitate, which would be subsequently dissolved by aqueous ammonia.
• The complex subsequently forms a carboxylic acid by oxidizing the aldehyde group in a given compound.
• The silver ions in the reagent will be converted to metallic silver.
• A silver mirror forms on the bottom side of a test tube as silver ions are reduced to metallic silver.
• A terminal hydroxy ketone produces a positive Tollens test because this Tollen's reagent will convert the ketone into an aldehyde.

Tollens’ Reagent Preparation

The reagent is not available in the commercial market, as Tollens’ Reagent has a rather limited shelf life. This reagent is therefore commonly produced in the laboratory directly. The following steps describe one such technique for preparing Tollens’ Reagent.

Step 1:

A few drops of diluted NaOH are added to an aqueous silver nitrate solution. Then silver complexes will now be converted into silver oxides by hydroxide ions. From this solution, (Ag₂O) silver oxide precipitates as a brown solid. The following is an equation for expressing the reaction.

Step 2:

Step 1 produced a brown silver oxide precipitate, which is now dissolved in aqueous ammonia. The Ag(NH₃)₂]⁺  complex is formed in the solution produced by the addition of aqueous ammonia. Tollens’ Reagent has this complex as its main component.

Tollens’ Test

When an aldehyde is introduced to the Tollens’ reagent, two things occur:

The aldehyde is oxidized by the Tollens’ reagent and forms a carboxylate salt. This reaction can be written as follows:

The silver ions present in the Tollens’ reagent are reduced into metallic silver. 

Examples of Tollens’ Test

Glucose has a free aldehyde group and is a reducing sugar (Reducing sugars are carbohydrates that can act as a reducing agent due to the presence of a free aldehydic group). Fructose does not yet have an aldehyde group. Still, it can function as a reducing sugar because it can isomerize to glucose and mannose in alkaline media (due to keto-enol tautomerism in the presence of alpha-hydroxy ketone) and gives a positive test with Tollens' reagent. However, since the disaccharide sucrose does not react with the Tollens’ reagent, it functions as a non-reducing sugar(Non-reducing sugars are carbohydrates that cannot act as a reducing agent due to the absence of a free aldehydic group) in the absence of a free carbonyl group. 

The Tollens' reagent has a slightly basic pH due to the presence of NaOH. However, it is not corrosive. Therefore, basic laboratory safety measures are sufficient when handling these chemicals. 

Tollens' reagent is a mild oxidant capable of selectively oxidizing aldehydes to their respective acids, which itself is reduced to a silver precipitate of the element, producing a silver mirror coating on the walls of the container/test tube. To do in addition, this mild oxidizing property of Tollens’ reagent can be used for a variety of purposes, especially in the laboratory and chemical industry in the systematic analysis of organic compounds, etc

Purpose of Tollens’ Test

• When an unknown compound is presented for identification, systematic examination of the substance to discover the functional group included then this test becomes a critical stage in the entire identification process.
• Mild oxidizing agents may easily convert aldehydes to their corresponding carboxylic acids. Ketones, unlike aldehydes, cannot be converted by mild oxidizing agents into any carboxylic acids. Strong oxidizing substances are necessary for the oxidation of ketones. Tollens' reagent has the ability to operate as a moderate oxidiser.

Uses of Tollens’ Test

• In chemical laboratories, Tollens’ test is used to differentiate aldehydes from ketones in qualitative organic analysis.
• This test is mainly intended to distinguish between reducing and non-reducing sugars.

Limitations of Tollens’ Test

• Because such sugars are isomerised under alkaline circumstances, certain carbohydrates without an aldehyde group may give positive Tollens’ test.
• When skin or clothing comes into touch with silver nitrate, the color of the same will disappear. It is recommended that you wear gloves. Ammonia fumes must not be inhaled.
• To dissolve the silver coating on the flask (or test tube), we need to use strong HNO₃.

Practice Problems on Tollen's Test

Q 1. Which reagent is used to differentiate benzaldehyde and acetophenone?

(A) Ag(NH₃)₂]⁺                                                     (B) NaOH/I₂

(C) Ag(NH₃)₂]²⁺                                                   (D) Both (A) and (B)

Answer: (D)

Benzaldehyde gives Tollens’ test but acetophenone does not. On the other hand acetophenone (ph - COCH₃ gives iodoform test due to the presence of  - COCH₃ but benzaldehyde does not.

Q 2. Which of the following will give the Tollens’ test?

(A) Fructose                                                               (B) acetophenone

(C) Sucrose                                                                (D) None of these

Answer: (A)

Even though fructose contains ketone group still it will give positive Tollens’ test due to keto-enol tautomerism of alpha hydroxy ketone. But acetophenone does not give this test because it does not contain any alpha hydroxy group and sucrose being a disaccharide does not react with Tollens reagent.

Q 3. The compound that gives positive Tollens’ test is 

(A) Acetone(CH₃COCH₃)                                                            (B) Acetaldehyde(CH₃CHO)

(C) Acetic acid (CH₃COOH)                                                         (D) Diethyl ether(C₂H₅OC₂H₅)

Answer: (B)

Since acetone is ketone it won’t give a positive Tollens’ test. Among acids, only formic acid will give positive Tollens’ test so acetic acid will not give this test and ethers won’t give positive Tollens’ test. Acetaldehyde comes under aldehydes so it will give a positive Tollens’ test. 

Q 4. Which of the following reagent reacts with both acetaldehyde and acetone?

(A) Schiff’s reagent                                                                 (B) Tollens’ reagent

(C) Fehling's reagent                                                               (D) Grignard reagent

Answer: (D)

Both acetaldehyde and acetone react with Grignard reagent giving alcohols as the products. Acetaldehyde being aldehyde will give a positive test for Tollens’,Fehling’s and Schiff's reagents.

FAQ's on Tollen's Reagent

Q 1. Why do ketones not give the Tollens’ test?

Answer: The main importance of Tollens’ reagent is used to distinguish aldehydes and ketones. The reaction of aldehydes with Tollens’ reagent is an oxidation reaction which converts aldehydes to carboxylic acid. Tollens’ reagent is a mild oxidizing agent which can oxidize aldehydes but not ketones. Because aldehydes have the aldehydic-H which can be oxidized under mild oxidizing agents like Tollens reagent but ketones do not have the aldehydic-H.

Q 2. Why do alpha hydroxy ketones give positive Tollens’ test?

Answer: Generally, alpha-hydroxy ketones can give a positive Tollens’ test because α-Hydroxyketones have the ability to tautomerize to aldehydes and aldehydes give a Tollens’ test. And also some non-terminal alpha hydroxy ketones will give positive Tollens’ test like benzoin.

Q 3. Why methanoic acid (HCOOH) gives a positive Tollens’ test?

Answer: Formic acid has the aldehydic-H along with the -COOH group, hence being an acid it can give Tollens test. 

Hence, it gives both Tollens’ and Fehling's test positive.

 Reaction of formic acid with Tollens’ reagent:

Q 4. What is tautomerism?

Answer: Tautomerization is a phenomenon in which a single compound tends to be present in two or more interconvertible structures with different relative positions of nuclei, which are generally hydrogen. The two structures are called tautomers, and these types of isomer compounds usually differ only in the number of electrons and protons. They also exist in dynamic equilibrium. 

When a reaction occurs between these compounds, generally protons move. Tautomerism is also known as desmotropism.

Q 5. Does Alpha Hydroxy Ketone give Fehling’s Test? 

Answer: Alpha-hydroxy ketones are positive in Fehling's solution. This is because α-hydroxyketones are converted to aldehydes by tautomerism. Aldehydes are positive in Fehling's solution, while ketones are not positive in Fehling's solution.

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