The photochemical reaction of benzene with chlorine is a substitution reaction known as chlorination. It involves the replacement of one of the hydrogen atoms in the benzene ring with a chlorine atom. The reaction proceeds via a free radical mechanism and requires ultraviolet UV light to initiate the process. Here's a step-by-step description of the mechanism:1. Initiation: The reaction starts with the homolytic cleavage of the chlorine molecule Cl2 into two chlorine radicals Cl when exposed to UV light. This process is called photodissociation.UV lightCl2 2 Cl2. Propagation: The chlorine radical generated in the initiation step reacts with the benzene molecule to form a benzene radical cation and a hydrochloric acid HCl molecule. The benzene radical cation is a highly reactive intermediate species.Benzene + Cl Benzene radical cation + HClNext, the benzene radical cation reacts with another chlorine molecule, resulting in the substitution of one hydrogen atom with a chlorine atom, forming chlorobenzene and a new chlorine radical.Benzene radical cation + Cl2 Chlorobenzene + ClThe newly formed chlorine radical can then react with another benzene molecule, propagating the reaction.3. Termination: The reaction can be terminated when two free radicals react with each other, forming a stable molecule. For example, two chlorine radicals can combine to form a chlorine molecule, or a benzene radical cation can react with a chlorine radical to form chlorobenzene.Cl + Cl Cl2Benzene radical cation + Cl ChlorobenzeneOverall, the photochemical chlorination of benzene results in the formation of chlorobenzene and hydrochloric acid:Benzene + Cl2 Chlorobenzene + HClIt's important to note that this reaction is not very selective, and multiple chlorination can occur, leading to the formation of di-, tri-, and polychlorinated benzenes as side products.