Which of the following compound will not undergo azo coupling reaction with benzene diazonium chloride? 

Nitrobenzene

The correct answer is option 4. Nitrobenzene.

Let us look into the chemistry behind azo coupling reactions and why certain compounds will or will not participate in these reactions with benzene diazonium chloride.

Azo coupling is a type of electrophilic aromatic substitution reaction where an aromatic diazonium salt reacts with an aromatic compound containing an electron-donating group. The product is an azo compound, characterized by the functional group \(-N=N-\).

Reactivity Towards Azo Coupling

The reactivity of an aromatic compound towards azo coupling depends on the presence and type of substituents on the aromatic ring:

Electron-donating groups (EDGs): These groups increase the electron density on the aromatic ring, especially at the ortho and para positions relative to the substituent. This makes the ring more nucleophilic and more reactive towards electrophiles, including the diazonium ion. Common EDGs include:

Amino group \((-NH_2)\)

Hydroxyl group \((-OH)\)

Methoxy group \((-OCH_3)\)

Electron-withdrawing groups (EWGs): These groups decrease the electron density on the aromatic ring, making it less nucleophilic and less reactive towards electrophiles. Common EWGs include:

Nitro group \((-NO_2)\)

Carboxyl group \((-COOH)\)

Carbonyl group \((-C=O)\)

Evaluating Each Compound

1. Aniline \((C_6H_5NH_2)\):

The amino group (-NH2) is a strong EDG. It activates the benzene ring, particularly at the ortho and para positions. Aniline readily undergoes azo coupling with benzene diazonium chloride.

2. Phenol \((C_6H_5OH)\):

The hydroxyl group \((-OH)\) is a strong EDG. It activates the benzene ring at the ortho and para positions. Phenol readily undergoes azo coupling with benzene diazonium chloride.

3. Anisole \((C_6H_5OCH_3)\):

The methoxy group \((-OCH_3)\) is a strong EDG. It activates the benzene ring at the ortho and para positions. Anisole readily undergoes azo coupling with benzene diazonium chloride.

4. Nitrobenzene \((C_6H_5NO_2)\):

The nitro group \((-NO_2)\) is a strong EWG. It deactivates the benzene ring by withdrawing electron density, especially at the ortho and para positions. This makes the ring less nucleophilic and less reactive towards electrophiles. Nitrobenzene does not readily undergo azo coupling with benzene diazonium chloride due to the decreased electron density and reactivity of the ring.

Detailed Explanation of Azo Coupling Reaction

In an azo coupling reaction, the diazonium ion \((ArN_2^+)\) acts as an electrophile. The aromatic compound with an EDG (the nucleophile) attacks the diazonium ion, typically at the para position (if available) or the ortho position due to the increased electron density provided by the EDG.

For example, in the case of aniline:

Formation of diazonium salt: Aniline is treated with nitrous acid \((HNO_2)\) at low temperature to form benzene diazonium chloride \((C_6H_5N_2^+Cl^-)\).

Coupling reaction: The benzene ring in aniline (which is electron-rich due to the -NH2 group) attacks the electrophilic diazonium ion.

Nitrobenzene and Azo Coupling: In contrast, nitrobenzene contains a nitro group \((-NO_2)\) that withdraws electron density from the benzene ring. This makes the ring less nucleophilic and less reactive towards the diazonium ion. The electron-withdrawing effect of the nitro group significantly reduces the reactivity of the benzene ring, preventing the coupling reaction from occurring effectively.

Conclusion

Therefore, among the given compounds, nitrobenzene is the one that will not undergo an azo coupling reaction with benzene diazonium chloride due to the presence of the electron-withdrawing nitro group, which deactivates the benzene ring and makes it less nucleophilic. The other compounds (aniline, phenol, and anisole) all contain electron-donating groups that activate the benzene ring, making them reactive towards azo coupling.