Question

Benzene diazonium chloride on reaction with phenol in weakly basic medium gives :

• A. Benzene
• B. Anisole
• C. p-hydroxyazobenzene
• D. Benzophenone

Hint:

In weakly basic medium the diazonium ion couples at the para position of phenoxide to give the azo dye p-hydroxyazobenzene.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Benzene diazonium chloride, \( \mathrm{C_6H_5N_2^+Cl^-} \), is a weak electrophile.
It attacks electron-rich rings such as phenol. This is called a coupling reaction.

Step 2: Role of the weakly basic medium:
In weakly basic medium (pH 9-10) phenol turns into the phenoxide ion.
The phenoxide ring is much richer in electrons than phenol. So it reacts readily with the diazonium ion.

Step 3: Position of attack:
The diazonium ion attacks the para position of phenol, because it is free and less crowded.
The new group is \( \mathrm{{-}N{=}N{-}} \), an azo group.
\[ \mathrm{C_6H_5N_2^+Cl^- + C_6H_5OH \xrightarrow{\text{weakly basic}} p\text{-}HO{-}C_6H_4{-}N{=}N{-}C_6H_5 + HCl} \]

Step 4: Why the other options are wrong.
Benzene forms only when the diazonium group is reduced by \( \mathrm{H_3PO_2} \) or ethanol.
Anisole is a methyl ether of phenol and does not form here.
Benzophenone has a C=O group between two rings, and no such group appears.

Final Answer:
The product is p-hydroxyazobenzene, an orange-red dye, option (C).\[ \boxed{\text{p-hydroxyazobenzene}} \]