Question

What is Tollen’s reagent?

Answer 1

Hint: Tollen’s reagent is a classic reagent that is widely used for the detection of aliphatic and aromatic aldehydes. Since it is an oxidizing agent, it oxidizes an aldehyde to its corresponding carboxylic acid and itself gets reduced to silver metal.

Complete step by step solution:
> Tollen’s reagent consists of a solution of silver nitrate and ammonia; and is named after Bernhard Tollens, a German chemist.
> Method of preparation of Tollen’s reagent:
- Add a few drops of dilute sodium hydroxide (NaOH) solution to an aqueous solution of silver nitrate ($Ag{ NO }_{ 3 }$). NaOH reacts with the silver nitrate solution and converts it into silver oxide (${ Ag }_{ 2 }O$) which precipitates as a brown solid. The reaction is given below:
$\begin{matrix} 2Ag{ NO }_{ 3 }(aq) \\ Silver\quad nitrate \end{matrix}+\begin{matrix} 2NaOH(aq) \\ Sodium\quad hydroxide \end{matrix}\rightarrow \begin{matrix} { Ag }_{ 2 }O(s)\downarrow \\ Silver\quad oxide \end{matrix}+\begin{matrix} 2Na{ NO }_{ 3 } \\ Sodium\quad nitrate \end{matrix}+\begin{matrix} { H }_{ 2 }O(l) \\ Water \end{matrix}$
- This brown precipitate is then dissolved in an aqueous solution of ammonia which produces the Tollen’s reagent. The reaction is given below:
$\begin{matrix} { Ag }_{ 2 }O(s) \\ Silver\quad oxide \end{matrix}+\begin{matrix} 4N{ H }_{ 3 }(aq) \\ Ammonia \end{matrix}+\begin{matrix} 2Na{ NO }_{ 3 }(aq) \\ Sodium\quad nitrate \end{matrix}+\begin{matrix} { H }_{ 2 }O(l) \\ Water \end{matrix}\rightarrow \begin{matrix} 2\left[ Ag({ NH }_{ 3 }{ ) }_{ 2 } \right] { NO }_{ 3 }(aq) \\ Tollen's\quad reagent \end{matrix}+\begin{matrix} 2NaOH(aq) \\ Sodium\quad hydroxide \end{matrix}$
> Tollen’s reagent is used for the detection of aldehyde functional group in both aliphatic and aromatic aldehydes as well as for the detection of alpha hydroxy ketone functional groups. This test is called the Tollen’s test or silver mirror test since in this test the silver ions get reduced to metallic silver which appears as a shiny silver layer on the sides of the test tube. > The general reaction of Tollen’s reagent with an aldehyde is given below:
$\begin{matrix} RCHO \\ Aldehyde \end{matrix}+\begin{matrix} 2[Ag({ NH }_{ 3 }{ ) }_{ 2 }{ ] }^{ + }(aq) \\ Tollen's\quad reagent \end{matrix}+\begin{matrix} 2{ OH }^{ - }(aq) \\ Hydroxide\quad ions \end{matrix}\xrightarrow { \triangle } \begin{matrix} RCOOH \\ Carboxylic\quad acid \end{matrix}+\begin{matrix} 2Ag(s)\downarrow \\ Silver\quad mirror \end{matrix}+\begin{matrix} { H }_{ 2 }O(l) \\ Water \end{matrix}$
Hence Tollen’s reagent is an ammoniacal solution of silver nitrate ($[Ag({ NH }_{ 3 }{ ) }_{ 2 }{ ] }^{ + }(aq)$).

Note: Tollen’s test is an oxidation-reduction reduction. It is used to distinguish the carbonyl group present in aldehydes and ketones. Ideally ketones should not give a positive Tollens test since they are not good reducing agents. But alpha hydroxy ketones do give positive Tollens test since Tollen’s test is carried out in a basic medium which facilitates the tautomerization of alpha hydroxy ketones to alpha hydroxy aldehydes. But remember that only those alpha hydroxy ketones will tautomerize that have an alpha hydrogen atom. Therefore not all alpha hydroxy ketones give positive Tollens test.