Which of the following compound will not undergo Azo coupling reaction?

The correct option is Nitrobenzene.

An azo coupling reaction is an electrophilic aromatic substitution reaction. In this reaction, an electron-rich aromatic ring (the substrate) reacts with a diazonium salt (the electrophile). For the reaction to occur efficiently, the substrate must possess strong electron-donating groups that increase the electron density of the benzene ring, making it highly nucleophilic.

Let us analyze the given compounds:
1. Aniline: Contains an amino group (-NH₂) which is a strong activating (electron-donating) group via resonance (+M effect). Thus, it readily undergoes azo coupling.
2. Phenol: Contains a hydroxyl group (-OH) which is also a strong activating (electron-donating) group via resonance (+M effect). Thus, it undergoes azo coupling easily.
3. Anisole: Contains a methoxy group (-OCH₃) which is an activating group (+M effect), enabling it to participate in azo coupling.
4. Nitrobenzene: Contains a nitro group (-NO₂) which is a very strong electron-withdrawing group due to both resonance (-M effect) and inductive effect (-I effect). This group deactivates the benzene ring by drastically reducing its electron density, making it extremely poor as a nucleophile. Consequently, nitrobenzene is not reactive enough to undergo electrophilic attack by a weakly electrophilic diazonium ion.

Therefore, nitrobenzene will not undergo the azo coupling reaction.