The preparation of 2,4-dichlorobenzoic acid from benzoic acid and chlorine gas can be achieved through electrophilic aromatic substitution. The balanced chemical equation for the reaction is:C6H5COOH + 2 Cl2 C6H3Cl2COOH + 2 HClHere's the step-by-step mechanism for the reaction:1. Catalyst activation: A Lewis acid catalyst, such as AlCl3 or FeCl3, is used to activate the chlorine gas. The catalyst reacts with Cl2 to form a complex, which is a stronger electrophile.Cl2 + AlCl3 [AlCl4] + Cl+2. Electrophilic attack: The activated chlorine electrophile Cl+ attacks the benzoic acid at the ortho or para positions, forming a sigma complex also known as an arenium ion . The ortho and para positions are preferred due to the activating effect of the electron-donating carboxyl group.C6H5COOH + Cl+ C6H4ClCOOH ortho or para 3. Deactivation of the catalyst: The sigma complex loses a proton H+ to the [AlCl4] ion, regenerating the AlCl3 catalyst and forming the monochlorinated product.C6H4ClCOOH sigma complex + [AlCl4] C6H4ClCOOH monochlorinated + AlCl3 + HCl4. The monochlorinated product undergoes a second electrophilic aromatic substitution with the same mechanism, forming the 2,4-dichlorobenzoic acid.C6H4ClCOOH + Cl2 + AlCl3 C6H3Cl2COOH + AlCl3 + HClThe role of the catalyst in this process is to activate the chlorine gas, making it a stronger electrophile that can react with the benzoic acid. The catalyst also helps to stabilize the intermediate sigma complex, allowing the reaction to proceed more easily. The catalyst is regenerated at the end of the reaction, so it is not consumed in the overall process.