The Friedel-Crafts alkylation of benzene with n-propyl chloride in the presence of AlCl₃ gives mostly isopropylbenzene because of a rearrangement that occurs during the reaction. Here's a breakdown:

1. Formation of the Carbocation: AlCl₃ acts as a Lewis acid, accepting a pair of electrons from the chlorine atom in n-propyl chloride. This generates a propyl carbocation (CH₃CH₂CH₂⁺).

2. Carbocation Rearrangement: The primary propyl carbocation is highly unstable. It undergoes a hydride shift, where a hydrogen atom from the adjacent carbon migrates to the positively charged carbon. This forms a more stable secondary carbocation, the isopropyl carbocation (CH₃CH⁺CH₃).

3. Electrophilic Attack: The isopropyl carbocation, being more stable and reactive, attacks the electron-rich benzene ring, leading to the formation of isopropylbenzene.

Why isopropylbenzene is favored:

* Carbocation Stability: Secondary carbocations are more stable than primary carbocations due to the electron-donating effect of the alkyl groups. The isopropyl carbocation is more stable than the propyl carbocation, making it the preferred intermediate for the reaction.

* Steric Hindrance: The bulky isopropyl group encounters less steric hindrance when it attacks the benzene ring compared to the linear propyl group.

In summary: The Friedel-Crafts alkylation of benzene with n-propyl chloride results in the formation of isopropylbenzene due to the rearrangement of the primary propyl carbocation to the more stable isopropyl carbocation. This rearrangement is favored due to the increased stability of the secondary carbocation and the reduced steric hindrance during the electrophilic attack on the benzene ring.