Tollens Test - Formula, Preparation, Uses, FAQs

Tollens Test or Tollens Reagent Test
The Tollens reagent test was discovered by Bernhard Christian Gottfried Tollens. Tollens test is used to distinguish between the compounds as they are aldehyde or ketones. The silver mirror test is the other name of the tollens test. In the Tollens test one of the main components is tollens reagent.

Tollens Reagent

Tollens reagent is used to determine the aldehyde functional group and an aromatic aldehyde functional group and alpha hydroxy ketone group in any given substance.

Tollens reagent is named after Bernhard Tollens, who coined the term Tollen’s reagent. Bernhard Tollens is a famous Chemist from Germany.

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Tollens’ Reagent formula

Tollen reagent formula is Ag(NH3)2OH

Q – What other term can we use in place of the tollens test?

• The Tollens test is also known as silver mirror test

• Q- What is the composition of Tollens reagent?

• The tollens reagent is a mixture above-mentioned silver nitrate and ammonia.

• Silver nitrate = AgNO3

• Ammonia = Nh3

Q – Tollens test mechanism?

• As we know tollens reagent is an alkaline solution of ammoniacal silver nitrate, tollens reagent i..e.. ammoniacal silver nitrate is used for the test of aldehyde or alpha diketones. Silver ions play an important role in this as among the first silver nitrate oxidises aldehydes group to carboxylic group and the silver ions being reduced to metallic silver and due to this conversion, we say this compound is true for the over mirror test or not true for silformirror test.

• Tollens reaction of Tollens reagent reaction

• 2Ag⁺ (aq) + 2OH^- (aq) Aga 2 O(s) + H2O

• Ag₂O (s) + the 4 NH₃ (aq)a + H₂O(l) ? 2 [Ag(NH₃)₂]+ (aq) are2 OH^- (aq)

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Tollens Reagent Preparation

Since Tollens reagent has a short life span, it has to be prepared in the laboratory with precautions:

• So to form tollens reagent we have to follow these given steps : -

• First of all we have to take an aqueous silver nitrate solution and then we have to introduce a few drops of dilute NaOH ( we can not use concentrated NaOH in place of dilute NaOH ) in the aqueous solution of silver nitrate . Then the reaction occurs in which water ( from aqueous silver nitrate ) will act as ligand and the aqueous solution of silver nitrate also contains silver aqua complexes. And then the hydroxide ions ( from water ) convert silver aqua complexes to become silver oxide. And due to formation of this silver oxide precipitation will occur in the solution i..e... silver oxide precipitate in the form of brown solid from their solutions.

• Reaction of above discussed event will be 2AgNO₃ + 2NaOH → Ag₂O ( brown ppt ) + 2NaNO₃ + H₂O

• Silver oxide = Ag2O ( molecular symbol )

• Then we have to dissolve the precipitated brown solid of silver oxide in aqueous ammonia. Then the new solution will be formed after reaction containing [Ag(NH₃)₂]⁺ complex. The complex i..e.. formed is the primary component of tollens reagents.

• And the reaction of above mentioned step i..e.. formation of primary component of tollens reagent [Ag(NH₃)₂]⁺ complex will be:-

• 2 Ag⁺ (aq) + 2 OH^- (aq) Ag₂O(s) + H₂O(l)

• Ag₂O ( brown ppt ) + 4NH₃ + 2NaNO₃ + H₂O → 2[Ag(NH₃)₂]NO₃ + 2NaOH

Tollens Test :-

Q - What happens when we mix aldehyde with the tollens reagent we recently prepared in the laboratory ?

• The first thing that will happen is the oxidation of aldehyde i..e.. aldehyde is oxidized by tollens reagent to form a carboxylic acid.

• In this process silver ions that are present in tollens reagent will be reduced and form metallic silver.

• Basically any test i..e.. any laboratory test is done in a glass test tube . And due to the presence of glass test tubes when silver ions are reduced to metallic silver then formation of silver mirror will happen on the test tube.

• And then we can easily find out why tollens test is also known as silver mirror test i..e.. due to the formation of silver mirror on the test tube ( because of reduction of silver ions to metallic silver ) .

Q - what tollens reagent? describe one usefulness of this reagent ?

• Tollens reagent is the solution of silver nitrate and aqueous ammonia .

Tollens reagent complex is [Ag(NH₃)₂]⁺

Its Uses:-

We use tollens reagent to determine either of these functional groups i.e.. aldehyde , aromatic aldehyde or alpha hydroxy ketone is present.

Benzaldehyde tollens test :-

Benzaldehyde i.e... benzene + aldehyde will give positive tollens test due to presence of aldehyde group to it as its aldehyde portion will be reduced by silver aqua complex and forms mirror i..e.. silver mirror on the test tube and hence proves it positive test.

Q – What is the name of tollens reagent?

• Basically tollens reagent is ammoniacal silver nitrate as to be more accurate we would define tollens reagent as an alkaline solution of ammoniacal silver nitrate i..e.. used in distinguishing functional groups like aldehyde and ketones.

  As We all know that main working principle for tollens test is that aldehyde is oxidised to carboxylic acid and tollens reagent is reduced ( silver ions is reduced to metallic silver ) and thus gives us symbol in the form of silver mirror about the presence of aldehyde group in our given compound or the compound we uses in tollens test.

Q – what is an ammoniacal silver nitrate formula ?

• As we know that ammoniacal silver nitrate is tollens reagent means the reagent used in tollens reagent is ammoniacal silver nitrate itself.

• And formula of ammoniacal silver nitrate is Ag(NH₃)₂OH)

• In ammoniacal silver nitrate formula the ratio of Ag : N : H : O is 1 : 2 : 7 : 1

• Nature Ammoniacal silver nitrate is alkaline .

Questions like; which of the following reduces tollen's reagent , we have to note one thing that the compound which will reduce tollens reagent will also give positive tollens test or silver mirror test.

Q – which compounds will reduce tollens reagent?

Answer will be that all aldehyde and terminal alpha hydroxy ketone will reduce tollens reagent i..e.. ammoniacal silver nitrate and also these compounds ( all aldehyde and terminal alpha hydroxy ketone ) will give positive silver mirror test and tollens test.

Q – Do ketone reduce tollens reagent ?

• And the answer will be no as ketones do not reduce tollens reagent and give tollens test or silver mirror test .

• But there is an exception in case of alpha diketones i...e... compound consists of two ketone functional groups which are situated at adjacent positions to each other.

• In general ketones neither reduce tollens reagent nor reduce fehling's reagent that means ketone does not give tollens and fehlings tests.

Also check-

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NCERT Chemistry Notes:

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