Step 1: Understanding the Concept:
An azo coupling reaction is a organic chemical transformation belonging to the class of electrophilic aromatic substitution (EAS) reactions. In this reaction, an aromatic diazonium ion ($\text{Ar-N}_2^+$) acts as a relatively weak electrophile. Because the diazonium ion carries a weak positive charge, it can attack another benzene ring only if that target ring is heavily enriched with electron density by highly activating groups.
Step 2: Detailed Explanation:
Let's analyze the activation state of the aromatic rings in each given option:
- (A) Aniline ($\text{C}_6\text{H}_5\text{NH}_2$): Contains an amino group ($-\text{NH}_2$). The nitrogen atom has a lone pair that it donates into the ring via a strong positive resonance effect ($+\text{M}$), making the ring highly activated and reactive toward azo coupling.
- (B) Phenol ($\text{C}_6\text{H}_5\text{OH}$): Contains a hydroxyl group ($-\text{OH}$). The oxygen atom donates its lone pairs through resonance ($+\text{M}$), creating an activated ring that couples easily with diazonium salts to form brightly colored azo dyes.
- (C) Anisole ($\text{C}_6\text{H}_5\text{OCH}_3$): Contains a methoxy group ($-\text{OCH}_3$). Like phenol, the oxygen lone pairs activate the aromatic ring via resonance ($+\text{M}$), allowing it to undergo electrophilic azo attacks.
- (D) Nitrobenzene ($\text{C}_6\text{H}_5\text{NO}_2$): Contains a nitro group ($-\text{NO}_2$). The nitro group is a powerful electron-withdrawing group due to both negative resonance ($-\text{M}$) and negative inductive ($-\text{I}$) effects. It pulls electron density away from the benzene ring, severely deactivating it.
Because the benzene ring in nitrobenzene is extremely electron-poor, the weak diazonium electrophile cannot attack it. Therefore, nitrobenzene will not undergo an azo coupling reaction. This matches option (D).
Step 3: Final Answer:
The compound that will not undergo an azo coupling reaction is Nitrobenzene.