The preparation of 2,4-dichlorobenzoic acid from benzoic acid and chlorine gas involves a stepwise electrophilic aromatic substitution EAS reaction mechanism. The chlorination reaction occurs in two steps, with the first chlorination at the ortho or para position and the second chlorination at the other ortho position. Here's the stepwise mechanism:1. Activation of chlorine: Chlorine gas Cl2 reacts with a Lewis acid catalyst, such as aluminum chloride AlCl3 , to form a highly reactive electrophile, the chloronium ion Cl+ . This step is essential for the electrophilic aromatic substitution to occur. Cl2 + AlCl3 Cl+ + AlCl4-2. First chlorination: The electrophile Cl+ attacks the benzoic acid at the ortho or para position relative to the carboxyl group, forming a sigma complex. This is because the carboxyl group is a deactivating and meta-directing group, but the ortho and para positions are sterically hindered due to the presence of the carboxyl group. C6H5COOH + Cl+ C6H4 Cl COOH ortho or para 3. Aromaticity restoration: A proton is removed from the sigma complex by the AlCl4- anion, restoring the aromaticity of the ring and forming the first chlorinated product, either ortho-chlorobenzoic acid or para-chlorobenzoic acid. C6H4 Cl COOH + AlCl4- C6H4 Cl COO- + HAlCl44. Second chlorination: The first chlorinated product reacts with another equivalent of Cl+ to form a new sigma complex, with the second chlorination occurring at the other ortho position relative to the carboxyl group. C6H4 Cl COOH + Cl+ C6H3 Cl2 COOH 2,4-dichlorobenzoic acid 5. Aromaticity restoration: A proton is removed from the sigma complex by the AlCl4- anion, restoring the aromaticity of the ring and forming the final product, 2,4-dichlorobenzoic acid. C6H3 Cl2 COOH + AlCl4- C6H3 Cl2 COO- + HAlCl4In summary, the chlorination reactions occur in steps 2 and 4, with the first chlorination at the ortho or para position and the second chlorination at the other ortho position.