Question

For which group identification Tollens’ test is used. Give a chemical equation?

Answer 1

. Tollens’ test is used to differentiate between two different carbonyl compounds. Aldehydes gives silver mirror with Tollens’ test while ketones does not give. Tollens’ reagent is an oxidising agent and aldehydes more readily oxidized than ketones.

Complete answer:

Tollens’ test is also known as the silver mirror test. It is a qualitative test done in laboratories for identification of carbonyl groups of aldehydes. It is basically used to distinguish between an aldehyde (-RCHO group) and a ketone ($- C = O -$ group). This test is done using Tollens’ reagent. The main component of Tollens’ reagent is ${[Ag{(N{H_3})_2}]^ + }$. It is a mixture of silver nitrate ($AgN{O_3}$), ammonia ($N{H_3}$) and sodium hydroxide (NaOH).
All aldehydes, that is, aliphatic as well as aromatic aldehydes, give silver mirror with the Tollens’ reagent while ketones do not give. Thus we can also say that all aldehydes give positive Tollens’ test whereas ketones give negative Tollens’ test.
Once the presence of a carbonyl group is identified in an experiment, Tollens’ test is done to distinguish between aldehyde and ketone. The test is based on the fact that aldehydes more readily oxidize than ketones; this is because carbonyl carbon in aldehydes has attached hydrogen. The Tollens’ reagent or ${[Ag{(N{H_3})_2}]^ + }$ is an oxidising agent. Tollens’ reagent oxidizes the aldehyde to the corresponding acid and itself gets reduced to elemental silver. The chemical equation for this reaction is:
$2{[Ag{(N{H_3})_2}]^ + } + RCHO + {H_2}O \to 2Ag(s) + 4N{H_3} + RCOOH + 2{H^ + }$

Elemental silver, Ag (s) precipitates out onto the inner surface of the reaction vessel and thus aldehydes giving a characteristic “silver mirror”.

Note: It should be noted that there is one exception in ketones that give Tollens’ test and it is $\alpha$- hydroxy ketones. $\alpha$- hydroxy ketones besides aliphatic and aromatic aldehydes give silver mirrors with Tollens’ reagent. General $\alpha$- hydroxy ketone structure is: