Question

When chlorobenzene is treated with $Cl_{2}$ in the presence of anhydrous $FeCl_{3}$ catalyst, it gives a mixture of 1,2-dichlorobenzene and 1,4-dichlorobenzene. This is an example of ________.

• A. Nucleophilic substitution reaction
• B. Electrophilic substitution reaction
• C. Free radical substitution reaction
• D. Nucleophilic addition reaction
• E. Electrophilic addition reaction

Hint:

Most benzene ring reactions involving catalysts like $FeCl_3$ or $AlCl_3$ are electrophilic substitutions.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
The question describes the chlorination of an aromatic ring (chlorobenzene). We need to classify this reaction based on the mechanism. Aromatic rings are characterized by their tendency to undergo substitution reactions that preserve the stable aromatic system, rather than addition reactions.
Step 2: Key Formula or Approach
The key is to identify the attacking species (reagent) and the nature of the transformation. - Reagents: \(Cl_2\) in the presence of a Lewis acid catalyst like \(FeCl_3\). This combination is characteristic of generating an electrophile. - Transformation: A hydrogen atom on the benzene ring is replaced by a chlorine atom. This is a substitution reaction.
Step 3: Detailed Explanation
1. Generation of the Electrophile:
The Lewis acid catalyst, \(FeCl_3\), interacts with the chlorine molecule, \(Cl_2\), to generate a strong electrophile, the chloronium ion (\(Cl^+\)) or a highly polarized complex. \[ Cl_2 + FeCl_3 \rightleftharpoons Cl^+[FeCl_4]^- \] An electrophile is an "electron-loving" species that seeks out electron-rich centers. 2. Attack on the Aromatic Ring:
The \(\pi\) electron cloud of the benzene ring is electron-rich and acts as a nucleophile. It attacks the electrophile (\(Cl^+\)). This is the characteristic step of an electrophilic attack on an aromatic ring. 3. Substitution Mechanism:
The attack temporarily disrupts the aromaticity of the ring, forming a resonance-stabilized carbocation intermediate (an arenium ion). To restore the highly stable aromatic system, the intermediate does not undergo addition. Instead, it loses a proton (H\(^+\)), which is a substitution process. The overall result is the replacement of an H atom with a Cl atom. 4. Directive Effect:
The existing chloro group (-Cl) on chlorobenzene is an ortho-, para-directing group for incoming electrophiles. This is why the products are the ortho (1,2-) and para (1,4-) isomers. This directive effect is a hallmark of electrophilic aromatic substitution. Conclusion:
Since a hydrogen atom on the aromatic ring is replaced by an electrophile (Cl\(^+\)), the reaction is an electrophilic substitution reaction.
Step 4: Final Answer
This reaction is an example of an electrophilic substitution reaction.