Question

The presence of unsaturation in organic compounds can be tested with:
a.) Schiff’s reagent
b.) Tollen’s reagent
c.) Fehling’s reagent
d.) Baeyer’s reagent

Answer 1

Hint: These are those carbon compounds that contain at least one double or triple bond between the carbon atoms are called Unsaturated carbon compounds. The reagent used for testing unsaturation is purple colored.
Complete answer:
Schiff’s reagent:- At the point when the dilute solution of p-rosaniline hydrochloride, pink in color, went through sulfur dioxide gas forms a colorless solution known as Schiff’s reagent. This reestablishes its color by the reducing nature of aldehyde while ketones give no reaction.

Tollen’s reagent:- It is a freshly prepared ammoniacal silver nitrate solution. Aldehydes react with tollens' reagent and a bright silver mirror is formed because of the development of silver metal. The aldehydes will get oxidized to carboxylate anion. This reaction occurs in basic mediums. Ketones won't give silver mirror tests.
The following reaction can take place;
${ RCHO+2[Ag({ NH }_{ 3 } }{ ) }_{ 2 }{ ] }^{ + }{ +{ 3OH }^{ - } }{ \rightarrow RCOO }^{ - }{ +2Ag+2{ H }_{ 2 } }{ O+4NH }_{ 3 }$
 So, we can say that Tollen’s reagent is used to test the presence of aldehyde.

Fehling’s reagent:- It is a mixture of equal volumes of Fehling A and Fehling B solution.
Fehling A: It is an alkaline solution of copper sulfate ${ CuSO }_{ 4 }$
Fehling B: It is a solution of aqueous potassium sodium tartrate and sodium hydroxide (a strong alkali).
The following reaction can take place;
${ RCHO+2Cu }^{ 2+ }{ +5OH }^{ - }{ \rightarrow RCOO }^{ - }{ +Cu }_{ 2 }{ O+3H }_{ 2 }{ O }$
When we add this solution to aliphatic aldehydes, the latter is reduced to give a red precipitate of cuprous oxide.
So, we can say that Fehling’s reagent is used to test the presence of an aldehyde.

Baeyer’s reagent:- It is a cold dilute alkaline solution of potassium permanganate { KMnO }_{ 4 }. It is used in organic chemistry as a qualitative test for the presence of unsaturation like double and triple bonds. It forms a purple-colored solution and the solution discharges its color in the presence of unsaturation.

Hence, the correct option is D.
Note: The possibility to make a mistake is that ${ KMnO }_{ 4 }$ is purple in color but the discoloration of Baeyer’s reagent shows the unsaturation of organic compounds.