CAUTION: This reaction should be carried out in a well-ventilated hood. <br/>To a flask equipped with a mechanical stirrer and an air condenser topped with a tube leading to a gas trap is added 74.0 gm (0.5 mole) of phthalic anhydride, 162 gm (0.638 mole) of iodine, and 300 ml of 60% fuming sulfuric acid (1.84 moles). The flask is gently heated to 45-50°C, at which point the reaction commences. If the reaction becomes too vigorous it may be necessary to use a ice bath to lower the temperature to 40-50°C. The reaction mixture is eventually (4 hr) heated up to 65°C until all visible reaction has ceased. The reaction mixture is cooled to 10-20°C and an additional 81.0 gm (0.318 mole) of iodine is added and the reaction again slowly heated up to 65°C (1hr) and again when the reaction ceases it is cooled. Another 27.0 gm (0.107 mole) of iodine is added and the reaction is again heated up to 65°C (1 hr). The flask is heated with an oil bath to a bath temperature of 175-180°C, at which point the sulfur trioxide and iodine fumes evolve. After about 2 hr, when the gaseous evolution ceases, the flask is cooled to about 60°C and the mxiture then poured into a beaker of water. The contents are allowed to stand overnight at room temperature, filtered, washed with two 50-ml portions of cone, sulfuric acid and then with three 100 ml portions of water. The light yellow crystalline product is put into a beaker containing 1 liter of water and 10 gm of sodium bisulfite in order to remove the last traces of free iodine. The aqueous solution is decanted, the product washed five times with liter of water, washed twice with 100 ml of acetone and then dried at 60°C to afford 260-268 gm (80-82%), m.p. 327-328°C.
