The photochemical reaction of benzene and chlorine is a substitution reaction, specifically a chlorination reaction. The mechanism involves the following steps:1. Initiation: Ultraviolet UV light causes the homolytic cleavage of the chlorine molecule Cl2 into two chlorine radicals Cl .Cl2 + UV light 2 Cl2. Propagation: The chlorine radical reacts with the benzene molecule, forming a benzene radical and hydrochloric acid HCl .Benzene + Cl Benzene radical + HCl3. Termination: The benzene radical reacts with another chlorine radical, forming chlorobenzene.Benzene radical + Cl ChlorobenzeneThe photochemical reaction of benzene and bromine is similar to that of benzene and chlorine, but with some differences in the mechanism:1. Initiation: UV light causes the homolytic cleavage of the bromine molecule Br2 into two bromine radicals Br .Br2 + UV light 2 Br2. Propagation: The bromine radical reacts with the benzene molecule, forming a benzene radical and hydrobromic acid HBr .Benzene + Br Benzene radical + HBr3. Termination: The benzene radical reacts with another bromine radical, forming bromobenzene.Benzene radical + Br BromobenzeneThe main difference between the two mechanisms is the type of halogen radical involved chlorine vs. bromine and the corresponding halogenated benzene and hydrogen halide products. Additionally, the reaction rate of bromination is generally slower than chlorination due to the lower reactivity of bromine radicals compared to chlorine radicals.