Question

The reagent ‘R’ in the given sequence of chemical reaction is:
graphics[width=0.5\linewidth]{Screenshot 2026-04-20 175228.png}

• A. CH\(_3\)CH\(_2\)OH
• B. HI
• C. CuCN/KCN
• D. H\(_2\)O
• E.

Hint:

If the reagent had been \(\text{H}_2\text{O}\), the product would have been a phenol. If it had been \(\text{HI}\), the product would have been an aryl iodide.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The given reaction sequence outlines the removal of an amino group (\(-\text{NH}_2\)) from an aromatic ring.
This process is called deamination. It is a two-step procedure involving the formation of a diazonium salt intermediate followed by its reduction.
Step 2: Key Formula or Approach:
The general approach for deamination involves two key reactions:
1. Diazotization: \(\text{Ar-NH}_2 + \text{NaNO}_2 + \text{HCl} \xrightarrow{0-5^\circ\text{C}} \text{Ar-N}_2^+\text{Cl}^-\)
2. Reduction (replacement by H): \(\text{Ar-N}_2^+\text{Cl}^- \xrightarrow{\text{Reducing Agent (R)}} \text{Ar-H} + \text{N}_2 \uparrow + \text{HCl}\)
Step 3: Detailed Explanation:
The first step shows 2,4,6-tribromoaniline reacting with \(\text{NaNO}_2\) and \(\text{HCl}\) at cold temperatures. This successfully forms the diazonium salt, 2,4,6-tribromobenzenediazonium chloride.
In the second step, the diazonium group (\(-\text{N}_2^+\text{Cl}^-\)) needs to be replaced by a hydrogen atom to yield 1,3,5-tribromobenzene.
This is a reduction reaction. The mild reducing agents typically employed for this specific transformation are either hypophosphorous acid (\(\text{H}_3\text{PO}_2\)) in the presence of water, or ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)).
When ethanol is used, it acts as a hydride donor, reducing the diazonium salt while it gets oxidized to acetaldehyde (\(\text{CH}_3\text{CHO}\)).
The reaction is:
\(\text{Ar-N}_2^+\text{Cl}^- + \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{Ar-H} + \text{N}_2 + \text{HCl} + \text{CH}_3\text{CHO}\)
Evaluating the options, \(\text{CH}_3\text{CH}_2\text{OH}\) is the correct mild reducing agent required for this conversion.
Step 4: Final Answer:
The reagent 'R' is ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)).