The reagents used to carry out the chlorination of benzene are chlorine (Cl2) and a Lewis acid catalyst such as aluminum chloride (AlCl3). The reaction is as follows:
C6H6 + Cl2 → C6H5Cl + HCl
The Lewis acid catalyst is necessary to polarize the Cl2 molecule, making it a better electrophile and allowing it to attack the benzene ring. Without the catalyst, the reaction would not proceed at a reasonable rate.