Question

An organic compound answers Tollen’s test. What is the functional group in the compound?

Answer 1

Hint: Tollens test is used for compounds containing aldehydes and ketones as a functional group, aldehydes responds positively to this test while ketone does not. If a compound responds positively to Tollens test then the functional group of the compound will be aldehyde

Complete step by step answer:
Tollens’ test, which is also called a silver-mirror test, is a laboratory test used to differentiate between an aldehyde and a ketone and is named after German chemist Bernhard Tollen. The aldehyde oxides readily whereas ketone doesn’t. Tollens’ reagent is a colourless basic aqueous solution having silver ions attached to ammonia in it \[{\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + }\]. Tollens Reagent has a comparatively short shelf-life that is why this reagent is prepared directly in the laboratory and is not available commercially abundantly. It is prepared using a two-step procedure.

Step 1: Few drops of dilute \[NaOH\] is introduced into an aqueous solution of silver nitrate. This solution contains silver aquo complexes whereas water acts as a ligand. The hydroxide ions from water converts these aquo complexes into silver oxides which precipitates as a brown solid.
\[2AgN{O_3} + 2NaOH \to A{g_2}O\left( {{\text{brown ppt}}} \right) + 2NaN{O_3} + {H_2}O\]

Step 2: The brown precipitate of silver oxide generated above is now mixed with aqueous ammonia. This leads to the formation of \[{\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + }\] complex which is primary component of Tollens Reagent.
\[A{g_2}O\left( {{\text{brown ppt}}} \right) + 4N{H_3} + 2NaN{O_3} + {H_2}O \to 2\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]N{O_3} + 2NaOH\]
This reagent oxidizes an aldehyde into the carboxylic acid. The reaction is accompanied by the reduction of metallic metallic silver, which turns the glass tube into a mirror.



Note:
Tollens Reagent refers to the chemical reagent which can also be used in the detection of an aromatic aldehyde functional group, or an alpha hydroxy ketone functional group.