The photochemical reaction between benzene and chlorine gas to form chlorobenzene in the presence of sunlight is known as the photochlorination of benzene. The mechanism of this reaction involves several steps:1. Initiation: In the presence of sunlight UV light , chlorine gas Cl2 undergoes homolytic cleavage, resulting in the formation of two chlorine radicals Cl .Cl2 + h sunlight 2 Cl2. Propagation: The chlorine radical generated in the initiation step reacts with benzene, forming a benzene radical cation and a chloride ion. The benzene radical cation is highly reactive and short-lived.Benzene + Cl Benzene radical cation + Cl3. The benzene radical cation reacts with another molecule of chlorine gas, forming chlorobenzene and a new chlorine radical.Benzene radical cation + Cl2 Chlorobenzene + ClThe newly formed chlorine radical can then react with another benzene molecule, continuing the chain reaction.4. Termination: The reaction can be terminated when two radicals react with each other, forming a stable molecule. For example, two chlorine radicals can react to form chlorine gas, or a chlorine radical can react with a benzene radical cation to form a chlorobenzene radical.2 Cl Cl2Cl + Benzene radical cation Chlorobenzene radicalOverall, the photochemical reaction between benzene and chlorine gas in the presence of sunlight results in the formation of chlorobenzene. This reaction is an example of a radical substitution reaction, where a hydrogen atom in benzene is replaced by a chlorine atom.