Question

Which of the following compounds will not undergo an Azo coupling reaction?

• A. Aniline
• B. Phenol
• C. Anisole
• D. Nitrobenzene

Hint:

Azo coupling occurs easily with activated aromatic compounds such as phenols and aromatic amines. Strong electron withdrawing groups like \(-NO_2\) prevent azo coupling.

Answer 1

The Correct Option is D

Concept: Azo coupling reaction is an important electrophilic aromatic substitution reaction involving diazonium salts. In this reaction: \[ ArN_2^+ \] acts as a weak electrophile. Therefore, azo coupling occurs only with highly activated aromatic rings containing strong electron donating groups. Important points:
• Electron donating groups activate the ring.
• Electron withdrawing groups deactivate the ring.
• Activated rings undergo azo coupling easily. Examples of activating groups: \[ -OH,\ -NH_2,\ -OR \] Examples of deactivating groups: \[ -NO_2,\ -COOH,\ -CN \]

Step 1: Analyzing Aniline \((C_6H_5NH_2)\).
Aniline contains: \[ -NH_2 \] group. The lone pair on nitrogen participates in resonance and increases electron density in the benzene ring. Thus:
• Ring becomes highly activated.
• Electrophilic substitution becomes easier. Hence, Aniline undergoes azo coupling reaction.

Step 2: Analyzing Phenol \((C_6H_5OH)\).
Phenol contains: \[ -OH \] group. Oxygen donates electron density through resonance effect. Therefore:
• Ring becomes activated.
• Phenol reacts readily with diazonium salts. Hence, Phenol undergoes azo coupling reaction.

Step 3: Analyzing Anisole \((C_6H_5OCH_3)\).
Anisole contains: \[ -OCH_3 \] group. Methoxy group donates electrons through resonance. Thus:
• Aromatic ring becomes electron rich.
• Electrophilic substitution occurs easily. Hence, Anisole also undergoes azo coupling reaction.

Step 4: Analyzing Nitrobenzene \((C_6H_5NO_2)\).
Nitro group: \[ -NO_2 \] is a strong electron withdrawing group. It withdraws electron density by:
• \(-I\) effect
• \(-M\) effect As a result:
• Benzene ring becomes strongly deactivated.
• Weak diazonium electrophile cannot attack the ring. Therefore, Nitrobenzene does not undergo azo coupling reaction.

Step 5: Write the final answer.
Hence: \[ \boxed{\text{Nitrobenzene}} \] Therefore, the correct option is: \[ \boxed{(D)} \]