Step 2: Synthesis of 4-bromo-3-fluorobenzonitrile
Sodium sulfite (0.97 g, 14.117 mmol) was added to sulfuric acid (6 mL) in batches at 5 °C, followed by stirring at room temperature for 30 min. The reaction mixture was cooled to 0 °C, acetic acid (9.8 mL) was added slowly and stirring was continued for 5 min. After that, 4-amino-3-fluorobenzonitrile (1.6 g, 11.764 mmol) was added in small portions. The mixture was stirred at room temperature for 1 hour. Subsequently, a solution of copper (I) bromide (2.5 g, 17.647 mmol) was added to the aqueous solution. Hydrobromic acid (6 mL) was added slowly at 10 °C. The resulting mixture was stirred overnight at room temperature and the progress of the reaction was monitored by thin layer chromatography (TLC). Upon completion of the reaction, water was added and extracted with ether. The organic layer was dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (100-200 mesh) using a hexane solution containing 2% ethyl acetate as eluent, resulting in 4-bromo-3-fluorobenzonitrile (0.8 g, 34.7% yield) as a white solid. Mass spectrum (MS): 201.1 [M + 1].
