To a 1000 mL four-neck reaction flask (equipped with a mechanical stirrer, thermometer, dropping funnel, and reflux condenser) was added 136 g of p-toluic acid followed by 272 g of dichloroacetic acid as a solvent. Next, 0.2 g of sodium tungstate dihydrate was added as catalyst and 365 g of 30% hydrochloric acid was added. Stirring was initiated and the reaction mixture was heated to 50-60 °C. At this temperature, 136 g of 30% hydrogen peroxide was slowly added dropwise, and the dropwise process was controlled to be completed within 6 hours, during which the temperature was maintained at 50-60 °C. After the dropwise addition, the reaction was continued for 4 hours. The reaction mixture was analyzed by high performance liquid chromatography (HPLC) and the remaining content of p-toluic acid was confirmed to be 9.5%. After completion of the reaction, the reaction solution was distilled under reduced pressure. When no fraction evaporated, the temperature was lowered to 20-30 °C, 250 g of methanol was added, heated to reflux and then cooled to 10 °C again. The reaction mixture was filtered, the solid residue was collected and the filter cake was washed with methanol. The filter cake was dried to give a final product of 165.2 g of 3,5-dichloro-4-methylbenzoic acid in 80.6% yield and 98.5% HPLC purity.
