Among the following, the reaction that proceeds through an electrophilic substitution, is :

Options

A.
B.
C.
D.

Answer

To determine which reaction proceeds through an electrophilic substitution mechanism, we will analyze each of the given options step by step.

### Step 1: Analyze Option A
- **Reaction**: Benzene diazonium chloride treated with Cu2Cl2.
- **Mechanism**: The reaction involves the formation of an aryl cation (aryl carbocation) as an intermediate after the release of N2 gas. A nucleophile (Cl-) then attacks this cation.
- **Conclusion**: This reaction follows the mechanism of **nucleophilic substitution (AR SN1)**, not electrophilic substitution.

### Step 2: Analyze Option B
- **Reaction**: Benzene ring reacts with Cl2 in the presence of AlCl3.
- **Mechanism**: Here, Cl2 interacts with AlCl3 to form a Cl+ electrophile. This Cl+ then attacks the benzene ring, forming a sigma complex (an arenium ion) as an intermediate. A hydrogen atom is then abstracted, resulting in the formation of chlorobenzene and regenerating AlCl3.
- **Conclusion**: This reaction is a classic example of **electrophilic substitution**, where Cl+ replaces a hydrogen atom on the benzene ring.

### Step 3: Analyze Option C
- **Reaction**: UV light is used to initiate a reaction.
- **Mechanism**: In the presence of UV light, free radicals are generated, leading to a free radical addition reaction.
- **Conclusion**: This is not an electrophilic substitution reaction, but rather a free radical mechanism.

### Step 4: Analyze Option D
- **Reaction**: A compound is treated with HCl and heated.
- **Mechanism**: H+ from HCl attacks a lone pair of electrons on oxygen, leading to the formation of a carbocation. Water leaves as a good leaving group, resulting in a substitution product.
- **Conclusion**: This is a **nucleophilic substitution (SN2)** reaction, not an electrophilic substitution.

### Final Conclusion
After analyzing all options, the only reaction that proceeds through an electrophilic substitution mechanism is **Option B** (benzene with Cl2 in the presence of AlCl3).