Why do we get isopropyl benzene on treating benzene with 1-chloropropane instead of n-propyl benzene?

This can be easily understood from the mechanism give below :

Thus according to above mechanism the formation of two type of carbocation is possible i.e 1^o and 2^o since the reaction take place through carbocation reaction intermedaite and we know that 2^o carbocation is more stable than 1^o . The expected major product  must be second one.