The chemical reaction in which the side chain results in the ring closure reaction is known as the intramolecular reaction.

For the ring closure to take place, the carbocation intermediate is generated by the loss of a leaving group.The molecule then rearranges itself to form a different compound with the formation of a new ring.

The sulphuric acid reacts with the given hydrocarbon chain which is attached to the benzene molecule. The hydrocarbon side chain consists of an alcohol group. The sulphuric acid results in the protonation of the alcohol group and releases a water molecule to form the carbocation.

Protonation of the alcohol group

The carbocation then rearranges itself to form the tertiary carbocation as shown in the following structure:

Formation of the tertiary carbocation

The electrophilic attack of the carbocation on the benzene ring takes place which results in the intramolecular nucleophilic attack to form the final product: