The preparation of 2,4-dichlorobenzoic acid from benzoic acid and chlorine gas can be achieved through electrophilic aromatic substitution. The reaction mechanism involves the formation of a chloronium ion, which acts as an electrophile and attacks the aromatic ring of benzoic acid.Reaction stoichiometry:C7H6O2 benzoic acid + 2 Cl2 chlorine gas C7H4Cl2O2 2,4-dichlorobenzoic acid Reaction mechanism:1. Formation of the chloronium ion: Cl2 + FeCl3 catalyst Cl+ chloronium ion + FeCl4-2. Electrophilic aromatic substitution: Benzoic acid reacts with the chloronium ion, forming a sigma complex.3. Rearrangement and deprotonation: The sigma complex loses a proton, and the aromaticity is restored, forming 2-chlorobenzoic acid.4. The 2-chlorobenzoic acid undergoes a second electrophilic aromatic substitution with another chloronium ion, following the same mechanism, to form the final product, 2,4-dichlorobenzoic acid.Safe and effective reaction conditions:1. Use a Lewis acid catalyst, such as ferric chloride FeCl3 , to facilitate the formation of the chloronium ion.2. Perform the reaction in a suitable solvent, such as dichloromethane DCM or chloroform, under anhydrous conditions to avoid side reactions with water.3. Carry out the reaction at low temperatures 0-5C to minimize side reactions and control the selectivity of the reaction.4. Monitor the reaction progress using thin-layer chromatography TLC or other analytical techniques to avoid over-chlorination.5. After the reaction is complete, quench the reaction mixture with water, extract the product with an organic solvent, and purify it using column chromatography or recrystallization.Remember to follow safety precautions, such as working in a well-ventilated area or a fume hood, wearing appropriate personal protective equipment PPE , and handling chemicals with care.