Question

A reagent used to test for unsaturation of alkene is
A. Conc $H_{2}SO_{4}$
B. Ammonical $Cu_{2}Cl_{2}$
C. Ammonical $AgNO_{3}$
D. Solution of $Br_{2}$ in $CCl_{4}$

Answer 1

Hint: Saturated hydrocarbons are those hydrocarbons that are made up of hydrogen and carbon and have a single bond between the two adjacent carbons. Alkanes is an example of saturated hydrocarbons. Unsaturated hydrocarbons are those hydrocarbons that have a double or a triple bond present between the two adjacent carbons. Unsaturated hydrocarbons are classified into alkenes, alkynes and aromatic compounds based on what type of bond is present. Alkenes have a double bond present between the two adjacent carbons, alkynes have a triple bond whereas aromatic compounds are the ring-shaped hydrocarbons that contain delocalised pi electrons.

Complete Step by Step Answer:
The test for unsaturation is used to identify the presence of a pi bond or a functional group. Alkaline potassium permanganate ($KMnO_{4}$) test (Baeyer's test) and bromine test are the two methods for detecting the unsaturation.

Baeyer's test:- When alkaline potassium permanganate is added to an unsaturated hydrocarbon, the pink colour of potassium permanganate ($KMnO_{4}$) disappears. This disappearance of the pink colour of $KMnO_{4}$ indicates the presence of unsaturation. The following reaction takes place while unsaturation:-
$2KMnO_{4}{+} H_{2}O\longrightarrow 2KOH{+} 2MnO_{2}{+}3[O]$
$-C=C-{+}H_{2}O\rightarrow -C(OH)-C(OH)-{+}KOH$

Bromine test:- When we add bromine to any unsaturated hydrocarbon, the brown colour of the bromine disappears. This indicates the unsaturation. The following reaction takes place:-
$2KMnO_{4}{+} H_{2}O\longrightarrow 2KOH{+} 2MnO_{2}{+}3[O]$
$-C=C-{+}Br_{2}\rightarrow -C(Br)-C(Br)-$

While testing for unsaturation of alkene the given organic compound is first added to the 1 ml of carbon tetrachloride in a test tube and then the $Br_{2}$ is added dropwise to the test tube while constant shaking. Add $Br_{2}$ constantly to the test tube till the brown colour completely disappears.
Thus, the correct answer is option D:- Solution of $Br_{2}$ in $CCl_{4}$.

Note: To confirm the presence of unsaturation, bring the rod dipped with ammonium hydroxide. If the white fumes are absent, that confirms that the organic compound is an unsaturated compound (alkene or alkyne).