What Is Tollens Reagent Principle Reaction Equation and Uses
A tollen test is performed to determine the presence of an aldehyde or a ketone in a given unknown solution. This test is named after the German chemist Bernhard Tollens and is also known as the Silver-Mirror Test. Carbonyl compounds are the compounds that contain >C=O as their functional group. These can be either:
Aldehydes (R-CHO) or
Ketones (R-CO-R).
Where R stands for any of the alkyl groups.
What is the Purpose of Tollen's Test?
Whenever an unknown organic compound is given for identification, systematic analysis of the compound in order to detect the functional group involved becomes a very crucial step in the overall process of identification. The following points explain the use of Tollen's Reagent Test in a reaction:
A suitable positive reaction with 2,4-DNP (2,4-Dinitrophenylhydrazine) or Brady's reagent confirms the presence of a carbonyl group in the compound. But it doesn't state if the carbonyl group is an aldehydic group or a ketonic group. In order to differentiate between the two, a Tollens' reagent test is done.
Aldehydes can easily be oxidized to their respective carboxylic acids by using mild oxidizing agents. Unlike aldehydes, ketones cannot be oxidized to any carboxylic acids by mild oxidizing agents. For the oxidation of ketones, strong oxidizing agents are required. Tollens' reagent can act as a mild oxidizing reagent.
Tollens' reagent is a mild oxidizing reagent that oxidizes aldehyde groups to their respective acid and it gets reduced, the reduction from +1 oxidation state of Silver (Ag+) to elemental form of Silver (Ag). Ketones do not reduce Tollens' reagent (except alpha diketones (i.e. two ketone functionalities on the adjacent 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).
The reaction that takes place during the Tollens' Test is as follows:
RCHO + 2(AG(NH3)2OH) + 2NaOH → RCOONH4 + 3NH3 + 2Ag (s) + H2O
(Aldehyde) (Tollens’ reagent) (Carboxylic acid) (Elemental Silver)
What is Tollens' Reagent?
Tollens' reagent, which was initially discovered by a German chemist Bernhard Tollens is the chemical reagent that is used in the Tollen's test to determine the presence of aldehyde functional groups along with some alpha-hydroxy ketones in an unknown solution. It is a colourless, aqueous solution consisting of a silver ammonia complex in an ammonia solution. Sodium hydroxide present in the solution maintains a basic pH, i.e. (pH>7) of the solution.
Tollens' reagent formula is:
(Ag(NH3)2)NO3
Because of its short shelf life, it's freshly prepared in the laboratory during the time of reaction.
Tollens' Reagent Preparation
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.
2AgNO3 + NaOH → Ag2O + 2NaNO3 + H2O
(Silver Nitrate) (Sodium Hydroxide) (Brown ppt.) (Soluble sodium nitrate)
Ag2O + 4NH3 + 2NaNO3 + H2O → 2 (Ag(NH3)2)NO3 +2NaOH
(Brown ppt.) (Conc. Ammonia solution) (Tollen's reagent)
Many times, instead of following a two-step process, a single-step process is taken in order to obtain the freshly prepared reagent. For this, firstly, a small amount of aqueous ammonia (2ml) is directly added to a solution of silver nitrate (2ml) taken in a test tube and then in order to dissolve the precipitate formed, a sufficient amount of ammonia solution is poured dropwise in order to obtain a clear solution.
Tollens' Test
Aldehyde gives a grey-black precipitate or a silver mirror when freshly prepared Tollens' reagent is added to the solution.
The Basic Procedure Followed as:
The small amount (50mg) of the given compound is dissolved in an aldehyde free alcohol (2ml-acting as a neutral solvent). To this solution, freshly prepared Tollens' reagent (1ml) is added and the solution is warmed in a hot water bath. If a grey-black precipitate is formed or a silver mirror is seen on the walls of the test tube, it confirms the presence of an aldehyde.
In the reaction, the silver ion goes from +1 oxidation state to 0 oxidation state or elemental state, thereby getting reduced and oxidizing the aldehyde to its respective acid. Therefore, the reaction involved is a redox reaction.
Since the reaction occurs in an alkaline medium therefore carboxylic acid isn't obtained directly; instead, carboxylate ion is obtained. Silver is precipitated out in its elemental form, forming the silver mirror on the edges or on the walls of the test tube. Many times, if the Tollens' reagent used isn't a clear solution, then a grey-black precipitate is obtained along with the silver mirror, which is also a positive test for the aldehydes.
Carbohydrates are basically Polyhydroxy Aldoses or Ketoses. There are several carbohydrates that have a free aldehyde group and such sugars easily reduce Tollens' reagent, Fehling's reagent or Benedict's solution and are therefore called reducing sugars. Some reducing sugars or carbohydrates do not have an aldehydic group, but they also give Tollens' test as positive because of isomerization in the alkaline medium and such sugars are also categorized as reducing sugars. Therefore tollens' reagent is used in the identification and differentiation of carbohydrates/sugars on the basis of their ability to reduce Tollens' reagent/Benedict's solution or Fehling's solution.
Example of Tollens’ Test
Glucose is a reducing sugar as it has a free aldehyde group. Fructose does not have any aldehydic group still. It can isomerise into glucose and mannose (by keto-enol tautomerism because of the presence of alpha-hydroxy ketone) in the alkaline medium therefore giving a positive test with Tollens' reagent and therefore acts as a reducing sugar. However, Sucrose, a disaccharide, does not react with Tollen's reagent, thereby acting as a non-reducing sugar because of the lack of a free carbonyl group.
Tollens' reagent has a mildly basic pH because of the presence of NaOH; however, it isn't corrosive in nature. So, basic laboratory safety measures are enough while handling those chemicals.
Tollens' reagent is a mild oxidizing agent that can selectively oxidize aldehyde to their respective acid and themselves getting reduced to elemental silver precipitate forming a silver mirror coating on the walls of the container/test tubes. And this mild oxidizing property of Tollens' reagent can be used selectively in the laboratory or in the chemical industry for various purposes such as during systematic analysis of the organic compound etc.
FAQs on Tollens Reagent and the Silver Mirror Test in Chemistry
1. What is Tollens reagent?
Tollens reagent is an ammoniacal silver nitrate solution containing the complex ion [Ag(NH3)2]+ used to test for aldehydes. It is prepared by adding aqueous ammonia to silver nitrate solution until the brown precipitate dissolves. In organic chemistry, it is commonly called the silver mirror reagent because it produces a shiny silver coating when it reacts with aldehydes.
2. What is the formula of Tollens reagent?
The active species in Tollens reagent is the complex ion [Ag(NH3)2]+. It is formed when silver nitrate reacts with aqueous ammonia, producing a diamminesilver(I) complex in alkaline solution. This complex ion acts as a mild oxidizing agent in aldehyde tests.
3. What is the Tollens test used for?
The Tollens test is used to distinguish aldehydes from ketones by oxidizing aldehydes to carboxylate ions while reducing silver ions to metallic silver. In this reaction:
- Aldehyde is oxidized to a carboxylate ion.
- Ag+ is reduced to Ag(s), forming a silver mirror.
Ketones generally do not react, so no silver mirror forms.
4. Why does Tollens reagent give a silver mirror?
Tollens reagent gives a silver mirror because silver(I) ions are reduced to metallic silver during the oxidation of an aldehyde. A simplified balanced reaction is:
R–CHO(aq) + 2[Ag(NH3)2]+(aq) + 3OH-(aq) → R–COO-(aq) + 2Ag(s) + 4NH3(aq) + 2H2O(l)
The deposited Ag(s) coats the inner surface of the test tube, forming a shiny mirror.
5. How do you prepare Tollens reagent in the laboratory?
Tollens reagent is prepared by adding aqueous ammonia to silver nitrate solution until the initial precipitate dissolves to form [Ag(NH3)2]+. The steps are:
- Add a few drops of NaOH(aq) to AgNO3(aq) to form brown Ag2O(s).
- Add dilute NH3(aq) dropwise.
- The precipitate dissolves, forming a clear solution of the diamminesilver(I) complex.
It must be freshly prepared because it can form explosive silver compounds on standing.
6. What is the difference between Tollens reagent and Fehling’s solution?
The main difference is that Tollens reagent forms a silver mirror, while Fehling’s solution forms a brick-red precipitate of Cu2O when reacting with aldehydes. Key differences:
- Tollens reagent contains [Ag(NH3)2]+.
- Fehling’s solution contains alkaline Cu2+ complexed with tartrate ions.
- Tollens gives a silver mirror; Fehling’s gives red Cu2O(s).
Both are used to detect reducing sugars and aldehydes in qualitative analysis.
7. Do ketones react with Tollens reagent?
Most ketones do not react with Tollens reagent because they are not easily oxidized under mild conditions. Since ketones lack a hydrogen atom attached to the carbonyl carbon, they do not reduce Ag+ to Ag(s). An important exception is α-hydroxy ketones, which can give a positive result.
8. How does Tollens reagent react with aldehydes?
Tollens reagent oxidizes aldehydes to carboxylate ions while being reduced to metallic silver. The general reaction is:
R–CHO + 2[Ag(NH3)2]+ + 3OH- → R–COO- + 2Ag(s) + 4NH3 + 2H2O
- Aldehyde undergoes oxidation.
- Silver(I) ions undergo reduction.
This is a classic example of a redox reaction in organic chemistry.
9. Can Tollens reagent detect reducing sugars?
Yes, Tollens reagent detects reducing sugars because they contain a free aldehyde group or can form one in solution. For example:
- Glucose gives a positive silver mirror test.
- Sucrose does not react because it is a non-reducing sugar.
This property is widely used in carbohydrate chemistry to identify reducing sugars.
10. Why must Tollens reagent be freshly prepared?
Tollens reagent must be freshly prepared because it can form explosive silver compounds such as silver nitride (Ag3N) on standing. When left to dry, these compounds may detonate upon disturbance. Therefore:
- Prepare it just before use.
- Do not store the solution.
- Dispose of it safely after the experiment.
This safety precaution is essential in laboratory practice.
