Q.12 The major product for the following reaction is

Introduction

Friedel–Crafts alkylation reactions are commonly tested in organic chemistry, especially questions involving carbocation rearrangements. In this problem, benzene reacts with isobutyl chloride in the presence of AlCl₃. To find the major product, it is essential to analyze carbocation stability and possible rearrangements.

Key Concept

• Friedel–Crafts alkylation proceeds via carbocation formation

• More stable carbocation forms preferentially

• Rearrangements (hydride or alkyl shift) are common

• Product depends on carbocation stability, not the original alkyl halide structure

Step-by-Step Reaction Explanation

Step 1: Formation of Carbocation

Isobutyl chloride reacts with AlCl₃ to form an isobutyl carbocation, which is a primary carbocation and hence unstable.

Step 2: Carbocation Rearrangement

The unstable primary carbocation undergoes a hydride shift to form a more stable tertiary carbocation (tert-butyl carbocation).

(CH3)2CHCH2+  →  (CH3)3C+(CH₃)₂CHCH₂^+ \;\rightarrow\; (CH₃)₃C^+(CH3​)2​CHCH2+​→(CH3​)3​C+

Step 3: Electrophilic Substitution

The tert-butyl carbocation, being highly stable, attacks the benzene ring, resulting in tert-butylbenzene as the major product.

✅ Correct Answer

Option (C): tert-Butylbenzene

Explanation of All Options

Option (A)

• Shows two substituents on benzene.

• Friedel–Crafts alkylation introduces only one alkyl group.

❌ Incorrect

Option (B)

• Represents isobutylbenzene.

• Does not consider carbocation rearrangement.

❌ Incorrect

Option (C)

• Represents tert-butylbenzene.

• Formed after hydride shift to a tertiary carbocation.

• Most stable and favored product.

✅ Correct

Option (D)

• Contains chlorine substitution, which does not occur in Friedel–Crafts alkylation.

• AlCl₃ removes Cl⁻ completely.

❌ Incorrect

Summary Table

Final Conclusion

In Friedel–Crafts alkylation reactions, the most stable carbocation determines the major product. Since isobutyl carbocation rearranges to a tert-butyl carbocation, the final product formed is:

tert-Butylbenzene (Option C)​