Tollens Test

Tollens Test, also known as the silver mirror test, is a chemical test that is used to detect the presence of aldehydes in a given organic compound. The test is based on a redox reaction between the aldehyde and Tollens’ reagent, which results in the formation of a silver mirror. Tollens’ reagent is widely used in organic chemistry for the qualitative analysis of organic compounds, especially in distinguishing aldehydes from other carbonyl compounds. The reagent used for the test is called Tollens Reagent which is an Ammoniacal Silver Nitrate Solution.

In this article, we will learn what is Tollens test, Tollens’ reagent, its reaction, its mechanism, and its applications.

What is Tollens’ Test?

Tollens test, also known as the silver mirror test, is a chemical test used to detect the presence of aldehydes in a given alpha-hydroxyorganic compound. When an aldehyde is present, it will react with Tollens’ reagent to produce a silver mirror on the inside of a test tube. This test is often used in qualitative organic chemistry to distinguish between aldehydes and ketones because ketones do not give a positive Tollens test (except alpha-hydroxy ketone).

Tollens Reagent

Tollens’ reagent is often referred to as “Tollens’ reagent” or “Ammoniacal Silver Nitrate solution.” It is named after Bernhard Tollens, the German chemist who developed the reagent in the late 19th century. The reagent is used for the qualitative detection of aldehydes. Its formula is ([Ag(NH3)₂]⁺

Preparation of Tollens Reagent

Tollens’ Reagent is prepared by mixing silver nitrate (AgNO₃) with aqueous ammonia (NH₃). In order to prepare Tollens’ reagent, Sodium hydroxide is added to a solution of silver nitrate dropwise until a light brown precipitate is obtained. To this, concentrated ammonia solution is added dropwise until the brown precipitate of Ag₂O dissolves completely. The amount of ammonia should be sufficient enough to dissolve the precipitate completely until a clear solution is obtained. The complex obtained at the end is (Ag(NH₃)₂)⁺ in which silver is in +1 oxidation state, i.e. Ag⁺ acts as the main component of the Tollens’ reagent. NaOH that was initially used is reformed at the end and helps in maintaining the needed pH balance.

Step 1:

2AgNO₃ + NaOH → Ag₂O↓(Brown ppt.) + 2NaNO₃ + H₂O

Step 2:

Ag₂O + 4NH₃ + 2NaNO₃ + H₂O → 2 [(Ag(NH₃)₂]NO₃ (Tollen’s reagent) + 2NaO

The silver ions (Ag⁺) in the reagent are complexed with ammonia to form a soluble, colorless complex ion, [Ag(NH3)₂]⁺

Reaction of Tollens Test

The Tollens test, also called as the Silver Mirror test, is a test which is used to detect the presence of aldehydes in a given organic compound. This test is based on the reaction between Tollens’ reagent and aldehydes, which results in the formation of a Silver Mirror on the inner surface of a test tube. This test is widely used in qualitative organic chemistry to identify and characterize aldehydes in various chemical reactions. Tollens’ reagent should be prepared and used with caution, as it contains ammonia and silver ions, which can be hazardous.

The reactions taking place in the Tollens test is explained as:

Reaction with Aldehyde: When Tollens’ reagent is added to a solution containing an aldehyde (Example: formaldehyde or glucose), the aldehyde undergoes oxidation. The aldehyde molecule is oxidized to form a carboxylic acid.

Reduction of Silver Ions (Ag⁺): The silver ions in Tollens’ reagent are reduced to metallic silver (zero oxidation state). The aldehyde acts as a reducing agent in this reaction. The reduction of silver ions to metallic silver is a characteristic feature of the Tollens test.

Formation of Silver Mirror: As metallic silver is deposited on the inside the teat tube. This is the confirmation of a positive result for the presence of aldehydes in the given sample. The formation of the silver mirror is an indication that an aldehyde group is present.

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.

Mechanism of Tollens Test

When an aldehyde is introduced to Tollens’ reagent, it acts as a reducing agent. Aldehydes have a hydrogen atom on the carbonyl group (C=O) that can be oxidized. The aldehyde is oxidized by the silver ions in the reagent as follows:

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH^– → RCOO^– + 2Ag + 4NH₃ + 2H₂O

In this reaction, the aldehyde (RCHO) is oxidized to a carboxylate ion (RCOO^–) while silver ions (Ag⁺) are reduced to metallic silver (Ag). The reduced silver ions forms a solid precipitate on the inner surface of the test tube forming a silver mirror.

Thus the important steps in the Tollens test are the Oxidation of the Aldehyde and the Reduction of Silver ions.

Reaction of Tollens Reagent with Aldehyde

When an aldehyde is introduced to Tollens’ reagent, it acts as a reducing agent. Aldehydes have a hydrogen atom on the carbonyl group (C=O) that can be oxidized. The aldehyde reacts with the [Ag(NH3)2]⁺ complex as follows, where R represents the organic group:

RCHO + 2[Ag(NH₃)₂]⁺ + 4OH^– → RCOO^– + 2Ag + 6NH₃ + 2H2O

In this reaction, the aldehyde is oxidized to a carboxylate ion (RCOO^–) while silver ions (Ag⁺) are reduced to metallic silver (Ag).

So, Tollens test is given by compounds having aldehydic group (aldehydes ,alpha-hydroxy ketones and formic acid-its -COOH behaves like a aldehydic group, reducing sugars/aldoses). It gives a precipitate of Shiny Silver mirror (where the silver salt is reduced to silver metal and the aldehyde is oxidized to silver salt of carboxylic acid.

Uses of Tollens Test

The Tollens test is mainly use in the following cases:

• Qualitative Analysis: The primary use of the Tollens test is to do qualitative identification of aldehydes in organic compounds. When an aldehyde is present, it will react with Tollens’ reagent to form a silver mirror on the inner surface of the test tube, confirming the presence of aldehydic group.
• Distinguishing Aldehydes from Ketones: Another major applications of the Tollens test is to differentiate between aldehydes and ketones. Aldehydes give a positive Tollens test by forming a silver mirror, whereas ketones (except alpha hydroxy ketone) do not react with Tollens’ reagent and, therefore, do not produce a silver mirror. This helps in differentiating between these two functional groups.
• Food Industry: Tollens’ reagent is also used in the food industry to detect reducing sugars, such as glucose and fructose, in various food products. This is important in quality control, especially for products like honey and fruit juices, as reducing sugars can show the presence of contaminants or spoilage.

Limitations of Tollens’ Test

• There are some sugars which can be isomerized under alkaline circumstances, certain carbohydrates without an aldehyde group may give positive Tollens’ test.
• Silver nitrate is a hazardous chemical so when it comes in contact with skin or clothing, the color of the same will disappear. So it is recommended to wear gloves and also ammonia fumes must not be inhaled.

Sample Questions on Tollens Test

Question 1: Can Tollens’ Test be performed in a beaker or conical flask?

Answer:

No. Tollens’ Test always be carried out in the test tube because the Brown precipitate of Metallic Silver is better identified in a test tube and not in a conical flask or beaker.

Question 2: How does Tollens’ reagent react with Formaldehyde?

Answer:

Tollens reagent when added to formaldehyde gets oxidized to corresponding carboxylic acids (Formic acid). Then formic aid is further oxidizes to metallic silver and produces CO₂, and H₂O.

Question 3: What are the functional groups that give Tollens Test?

Answer:

The functional group that gives a positive result in the Tollens test is the aldehyde group, which is represented as -CHO. Other functional groups that can potentially give positive results in the Tollens test are compounds that can be tautomeric with the aldehyde form, such as enols or certain tautomeric ketones. Alpha Hydroxy Ketone gives positive Tollens Test.

Question 4: Which reagent/test is used to differentiate between benzaldehyde and acetophenone?

Answer:

Tollens reagent/test is used to differentiate between benzaldehyde and acetophenone as benzaldehyde gives Tollens’ test but acetophenone does not as benzaldehyde contain Aldehyde functional group where acetophenone don’t.

Question 5: Does Tollens Test have any limitations?

Answer:

The most common limitations of Tollens Test are:

• There are some sugars which can be isomerized under alkaline circumstances, certain carbohydrates without an aldehyde group may give positive Tollens’ test.
• Silver nitrate is a hazardous chemical so when it comes in contact with skin or clothing, the color of the same will disappear. So it is recommended to wear gloves and also ammonia fumes must not be inhaled.

FAQs on Tollens Reagent

What is Tollens Test for Aldehyde?

Tollens test is given by compounds having aldehydic group (aldehydes ,alpha-hydroxy ketones and formic acid-its -COOH behaves like a aldehydic group, reducing sugars/ aldoses). It gives a precipitate of Shinny Silver mirror (where the silver salt is reduced to silver metal and the aldehyde is oxidised to silver salt of carboxylic acid.

RCHO + 2[Ag(NH₃)₂]⁺ + 4OH- → RCOO^– + 2Ag + 6NH₃ + 2H₂O

Do Ketones gives Tollens Test?

Ketones do not show Tollens test as it can not be Oxidized by Tollens reagent like aldehydes. Ketones require a strong Oxidizing agent to oxidize but Tollens’ reagent is mild and also due to the absence of Hydrogen atom because of that it do not give tollens test as this Hydrogen get reduced in case of aldehyde but in ketone it is absent. Thus, Ketone don’t give Tollens test.

What Tollens Test can do?

Tollens’ test, which is also known as Silver-Mirror Test, is a qualitative test which is used to distinguish between an Aldehyde and a Ketone. It distinguishes in a way that aldehydes are readily Oxidized whereas Ketones are not.

What Color shows the Confirmation of Positive Tollens Test?

The formation of a Dark grey precipitate or Silver Mirror on inner sides of the test tube which indicates a positive result, which means that the given sample contains Aldehydic group.

What is the Alpha Hydroxy Ketone?

A molecule which has adjacent Ketone and Alcohol groups is known as Alpha-Hydroxy Ketone.

Can Alpha-Hydroxy Ketones give Tollens Test?

Alpha-Hydroxy Ketones gives positive Tollens’ test since α-Hydroxy Ketones can Tautomerize to aldehydes, and then the aldehyde gives the Tollens test. An α-Hydroxy Ketone that cannot Tautomerize to a aldehyde won’t give a positive Tollens.

Why Methanoic Acid (HCOOH) gives a Positive Tollens Test?

Formic acid contain Aldehydic-H along with the -COOH group, hence being an acid it can give Tollens test.

Reaction of Formic acid with Tollens’ reagent:

HCOOH + 2[Ag(NH₃)₂]⁺ + 2OH^– → 2Ag↓ + CO₂ + 4NH₃ + 2H₂O

Why Ketone do not show Tollens Test?

Ketones do not show Tollens test as it can not be Oxidized by Tollens reagent like aldehydes. Ketones require a strong Oxidizing agent to oxidize but Tollens’ reagent is mild. Ketones do not contain hydrogen atom bonded with the Carbonyl group (c=o). This Hydrogen atom gets reduced to give the corresponding acid that is present in aldehyde. Thus ketone do not give Tollens test.

Can alcohols give positive Tollens’ Test?

Alcohol does not contain –CHO group in its structure it contains -OH group so it not possible for alcohol to give Tollens test.

What is Formula of Tollens Reagent?

The formula of Tollens Reagent is 2 [(Ag(NH₃)₂]NO₃

What is Tollens Test Reaction?

Tollens Test Reaction is given as RCHO + 2[Ag(NH₃)₂]⁺ + 4OH- → RCOO^– + 2Ag + 6NH₃ + 2H₂O