The reaction between benzene and chlorine in the presence of a catalyst is called electrophilic aromatic substitution, specifically chlorination of benzene. The catalyst used in this reaction is usually either aluminum chloride AlCl3 or ferric chloride FeCl3 . The mechanism of this reaction involves the following steps:1. Generation of the electrophile:The catalyst AlCl3 or FeCl3 reacts with chlorine Cl2 to form a complex, which generates the electrophile, the chloronium ion Cl+ . This process is as follows:Cl2 + AlCl3 [AlCl4] + Cl+2. Formation of the sigma complex arenium ion :The electrophile Cl+ attacks the benzene ring, which is rich in electron density due to the delocalized electrons. This attack leads to the formation of a sigma complex, also known as an arenium ion. During this process, one of the carbon atoms in the benzene ring forms a bond with the chlorine atom, and the delocalization of the electrons is disrupted, resulting in a positive charge on the carbon atom.3. Deprotonation and regeneration of the aromatic system:A base, usually the [AlCl4] ion, abstracts a proton from the positively charged carbon in the sigma complex. This step restores the aromaticity of the benzene ring, and a molecule of HCl is formed as a byproduct. The catalyst is also regenerated in this step.Overall, the reaction can be summarized as:C6H6 + Cl2 C6H5Cl + HClIn this reaction, the intermediate formed is the sigma complex arenium ion , which has a positive charge on one of the carbon atoms in the benzene ring. The reaction proceeds through electrophilic aromatic substitution, and the catalyst AlCl3 or FeCl3 plays a crucial role in generating the electrophile and facilitating the reaction.