Question Details

Which of the following compounds will not give azo coupling reaction with benzene diazonium chloride?

Options

A

Nitrobenzene

B

Aniline

C

o-Toluidine

D

Phenol

Show Answer

Correct Answer :

Option A

Nitrobenzene

Solution :

The correct option is Nitrobenzene.

Azo coupling is an electrophilic aromatic substitution reaction where a diazonium ion acts as the electrophile. The benzene diazonium cation is represented as:

C 6 H 5 N 2 +

Due to the delocalization of the positive charge over the nitrogen atoms, the diazonium ion is a relatively weak electrophile.

Because the diazonium ion is a weak electrophile, it requires a highly activated aromatic ring to react. Aromatic rings are activated by strong electron-donating groups that increase electron density on the ring through resonance.
Examples of such activating groups include the hydroxyl group:

O H

present in phenol, and the amino group:

N H 2

present in aniline and o-toluidine. Therefore, phenol, aniline, and o-toluidine readily undergo azo coupling reactions.

In contrast, nitrobenzene contains a nitro group:

N O 2

The nitro group is a strongly electron-withdrawing group due to both resonance and inductive effects. It deactivates the benzene ring by drastically reducing its electron density. As a result, the deactivated ring of nitrobenzene is not nucleophilic enough to attack the weak benzene diazonium electrophile, and no azo coupling reaction takes place.

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