Introduction:
When benzene reacts with 1-chloropropane in the presence of anhydrous aluminium chloride, it undergoes Friedel-Crafts alkylation reaction resulting in the formation of iso-propyl benzene instead of n-propyl benzene. This is due to the steric hindrance caused by the branched structure of the isopropyl group.

Friedel-Crafts alkylation reaction:
The Friedel-Crafts alkylation reaction is a type of electrophilic aromatic substitution reaction. In this reaction, a substituent is introduced into an aromatic ring using a Lewis acid catalyst such as anhydrous aluminium chloride. Benzene is a highly stable aromatic compound and does not undergo addition reactions easily. However, with the help of a catalyst, it can undergo substitution reactions.

Mechanism of the reaction:
The mechanism of Friedel-Crafts alkylation involves the formation of a carbocation intermediate. The carbocation is formed by the reaction of the Lewis acid catalyst with the electrophile. In this case, the electrophile is 1-chloropropane. The carbocation intermediate is then attacked by the aromatic ring, resulting in the substitution of the chloro group with the alkyl group.

Formation of iso-propyl benzene:
When benzene reacts with 1-chloropropane, two possible isomers of propyl benzene can be formed. These are n-propyl benzene and iso-propyl benzene. However, the bulky isopropyl group experiences steric hindrance and prefers to attach to the carbon atom adjacent to the carbon atom that is bonded to the benzene ring. This results in the formation of iso-propyl benzene instead of n-propyl benzene.

Conclusion:
In conclusion, the formation of iso-propyl benzene instead of n-propyl benzene on treating benzene with 1-chloropropane is due to the steric hindrance caused by the branched structure of the isopropyl group. This is a result of the Friedel-Crafts alkylation reaction, which involves the substitution of a hydrogen atom in the benzene ring with the alkyl group.