Carbonyl compounds are the compounds which contain >C=O as their functional group. These can be either:
Aldehydes (R-CHO) or
Where R stands for any alkyl group.
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.
A positive suitable 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 acts as a mild oxidizing reagent.
Tollens’ reagent being a mild oxidizing reagent oxidizes aldehydic group to its 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).
Tollens’ reagent which was initially discovered by a German chemist Bernhard Tollens and so is the name of the reagent, consists of silver ammonia complex in 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 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.
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, sufficient amount of ammonia solution is poured dropwise in order to obtain a clear solution.
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 itself getting reduced and oxidizes the aldehyde to its respective acid. Therefore, the reaction involved is a redox reaction.
Figure 1. Tollens’ Test 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 which 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: Glucose is a reducing sugar as it has a free aldehydic 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 free carbonyl group.
Tollens’ reagent has a mild 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 which 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 chemical industry for various purposes such as during systematic analysis of the organic compound etc.