Question

Aniline is converted into benzene by the following sequence of reactions: \[ \text{Aniline} \xrightarrow[\text{0--5}^\circ C]{NaNO_2/HCl A \xrightarrow{H_3PO_2} B \] The compounds \(A\) and \(B\) respectively are:}

• A. Benzene diazonium chloride, Benzene
• B. Nitrobenzene, Benzene
• C. Benzene diazonium chloride, Phenol
• D. Chlorobenzene, Benzene

Hint:

Remember: \[ ArN_2^+Cl^- \xrightarrow{H_3PO_2} ArH \] Hypophosphorous acid replaces diazonium group by hydrogen.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This problem represents a classic sequential organic synthesis involving primary aromatic amines. Aniline ($\text{C}_6\text{H}_5\text{NH}_2$) contains a strongly bound amino group that can be converted into a highly reactive intermediate known as a diazonium salt. This intermediate acts as a powerful synthetic launchpad because the diazonium group ($-\text{N}_2^+\text{Cl}^-$) is an excellent leaving group that can easily be replaced by various nucleophiles or reductive agents.
Step 2: Detailed Explanation:
Let's track each chemical transformation step-by-step: 1. First Step: Diazotization Reaction (Formation of A) When aniline reacts with nitrous acid (generated in situ by mixing sodium nitrite, $\text{NaNO}_2$, and hydrochloric acid, $\text{HCl}$) at ice-cold temperatures ($\text{0-5}^\circ\text{C}$), it undergoes a diazotization reaction. The amino group ($-\text{NH}_2$) converts into a diazonium group ($-\text{N}_2^+\text{Cl}^-$), yielding Benzene diazonium chloride ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$). Thus, compound A is Benzene diazonium chloride. $$ \text{C}_6\text{H}_5\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{\text{0-5}^\circ\text{C}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- (\text{A}) + \text{NaCl} + 2\text{H}_2\text{O} $$ 2. Second Step: Deamination / Reduction Reaction (Formation of B) When Benzene diazonium chloride ($\text{A}$) is treated with hypophosphorous acid ($\text{H}_3\text{PO}_2$, also called phosphinic acid) in the presence of water, a reduction reaction takes place. The hypophosphorous acid acts as a reducing agent, replacing the entire diazonium group with a hydrogen atom ($-\text{H}$) while oxidizing itself into phosphorous acid ($\text{H}_3\text{PO}_3$). This complete removal of the nitrogen group yields Benzene ($\text{C}_6\text{H}_5\text{H}$ or $\text{C}_6\text{H}_6$). Thus, compound B is Benzene. $$ \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- (\text{A}) + \text{H}_3\text{PO}_2 + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_6 (\text{B}) + \text{N}_2 \uparrow + \text{H}_3\text{PO}_3 + \text{HCl} $$ Therefore, compounds A and B are Benzene diazonium chloride and Benzene respectively. This matches option (A).
Step 3: Final Answer:
The compounds A and B respectively are Benzene diazonium chloride and Benzene.