Animated Solution for Chemistry - Organic Chemistry: In the chemical reaction, compounds A and B respectively are

The problem asks us to subject this aniline to a specific sequence of reagents. The first step involves treating aniline with sodium nitrite (${\text{NaNO}}_2$) and hydrochloric acid ($\text{HCl}$) at a chilly temperature of $278\text{K}$ (which is $5^{\circ }\text{C}$).

Whenever you see a primary aromatic amine reacting with ${\text{NaNO}}_2$ and $\text{HCl}$ at ice-cold temperatures ($0-5^{\circ }\text{C}$), your mind should immediately jump to one of the most famous reactions in organic chemistry: Diazotization.

Diazotization In the diazotization process, the sodium nitrite and hydrochloric acid react in situ to generate nitrous acid (${\text{HNO}}_2$). This nitrous acid then attacks the amino group ($-{\text{NH}}_2$) of the aniline. Through a series of protonations and water eliminations, the amino group is transformed into a diazonium group ($-{\text{N}}_2^{+}$).

Because we are using $\text{HCl}$, the counter ion is chloride (${\text{Cl}}^{-}$). Thus, the product formed, Compound A, is Benzene diazonium chloride (${\text{C}}_6{\text{H}}_5{\text{N}}_2^{+}{\text{Cl}}^{-}$). This compound is highly reactive and serves as a versatile synthetic intermediate, acting as a gateway to synthesize a myriad of substituted benzene derivatives.

The Sandmeyer Reaction Now, let's move to the second step. Compound A is treated with cuprous cyanide ($\text{CuCN}$) and heated ($\Delta$). This is a classic example of a nucleophilic aromatic substitution ($S_N$) reaction, specifically known as the Sandmeyer reaction.

The diazonium group ($-{\text{N}}_2^{+}$) is an exceptionally good leaving group because it leaves as nitrogen gas (${\text{N}}_2$), a highly stable and neutral molecule. The departure of nitrogen gas provides a massive thermodynamic driving force for the reaction. As the nitrogen leaves, the cyanide nucleophile ($-\text{CN}$) from the cuprous cyanide takes its place on the benzene ring.

This substitution yields Compound B, which is Benzonitrile (${\text{C}}_6{\text{H}}_5\text{CN}$). Therefore, our intermediate A is benzene diazonium chloride, and our final product B is benzonitrile. This perfectly matches option (b).