In a 500 mL three-necked round-bottomed flask equipped with a reflux condenser and a mechanical stirrer, 100 g (0.413 mol) of 3,4,5,6-tetrafluorophthaloyl difluoride (derived from the distillation isolate of Example 2), 43.8 g (0.413 mol) of anhydrous sodium carbonate and 200 g of xylene were added. The reaction mixture was heated to 130°C and kept for 1.5 hours under vigorous stirring. Upon completion of the reaction, the insoluble inorganic salt was removed by thermal filtration. Subsequently, the filtrate was distilled under reduced pressure to remove the solvent, resulting in 84.9 g of the white solid product 3,4,5,6-tetrafluorophthalic anhydride in 93.4% yield.
