The general procedure for the synthesis of 2,5-difluorobenzenesulfonyl chloride from 2,5-difluoroaniline was as follows: 12.9 g (99.9 mmol) of 2,5-difluoroaniline was rapidly added to a mixture prepared from 33 mL of concentrated hydrochloric acid and 10 mL of acetic acid in ethanol cooled to just below -10°C. The solution of 2,5-difluorobenzenesulfonyl chloride in ethanol was then added dropwise. Subsequently, a solution of 7.4 g (107 mmol) of sodium nitrite dissolved in 15 mL of water was added dropwise. After the dropwise addition, stirring was continued for 45 minutes at a temperature range of -10 to -25°C. Maintaining the temperature of the reaction solution at about -25 °C, a mixture consisting of 100 mL of acetic acid saturated with sulfur dioxide and 2.5 g (25.3 mmol) cuprous chloride was added in batches, a process that required cooling to 10 °C in an ice bath. A large amount of nitrogen generation was observed during the reaction. The reaction mixture was brought back to room temperature and kept for 2 h. Subsequently, it was poured into 450 mL of ice-water mixture and extracted with ether. The ether extracts were combined, washed with saturated aqueous sodium bicarbonate, dried over sodium sulfate and concentrated. The resulting oily residue was distilled under reduced pressure to give 14.3 g (87% yield) of a liquid product consisting mainly of 2,5-difluorobenzenesulfonyl chloride with a boiling point of 65-70 °C (0.5 mmHg), which could be used in subsequent steps without further purification.
