Question

Which from following reaction results in azo coupling?

• A. \(\text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{HNO}_2} \dots\)
• B. \(\text{C}_6\text{H}_5\text{N}_2\text{Cl} + \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{OH}^-} \dots\)
• C. \(\text{C}_6\text{H}_5\text{N}_2\text{Cl} \xrightarrow{\text{HBr}_4} \dots\)
• D. \(\text{C}_6\text{H}_5\text{N}_2\text{Cl} \xrightarrow{\frac{\text{Cu (Powder)}}{\text{HCl}}} \dots\)

Hint:

Azo coupling usually means: \[ \text{diazonium salt} + \text{phenol / aniline} \rightarrow \text{azo dye} \]

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Azo coupling is a reaction between a diazonium salt and a highly reactive aromatic ring (like phenol or aniline) to form an azo dye (\(\text{Ar-N=N-Ar'}\)).
Step 2: Key Formula or Approach:
Identify the combination of a diazonium salt with a phenol/amine in a suitable pH environment.
Step 3: Detailed Explanation:
- (A) represents diazotization (formation of diazonium salt).
- (C) is the first step of Balz-Schiemann reaction.
- (D) is the Gattermann reaction (substitution of \(\text{N}_2\text{Cl}\) with halogen).
- (B) involves benzene diazonium chloride (\(\text{C}_6\text{H}_5\text{N}_2\text{Cl}\)) reacting with phenol (\(\text{C}_6\text{H}_5\text{OH}\)) in alkaline medium (\(\text{OH}^-\)). This forms p-hydroxyazobenzene, an orange-colored azo dye.
\[ \text{C}_6\text{H}_5\text{N}_2\text{Cl} + \text{C}_6\text{H}_5\text{OH} \xrightarrow{\text{OH}^-} \text{C}_6\text{H}_5\text{-N=N-C}_6\text{H}_4\text{OH} + \text{HCl} \] Step 4: Final Answer:
Reaction (B) results in azo coupling.