A mixed solution was prepared from 2,5-difluoro-3-nitrobenzoic acid and 3,6-difluoro-2-nitrobenzoic acid (2.48 g, 12.2 mmol, molar ratio 2:1) in carbon tetrachloride (60 mL). Mercury(II) oxide (red, 5.2 g, 24 mmol) was added to this solution and the reaction was carried out at room temperature. Subsequently, the reaction mixture was heated to reflux and reacted under light conditions. Bromine (3.8 g, 24 mmol) was slowly added dropwise to the reaction mixture over a period of 10 min. The reaction mixture was continued under reflux conditions for 6 hours. After completion of the reaction, it was cooled to room temperature and filtered to remove the solid insoluble material. The filtrate was washed sequentially with saturated sodium bicarbonate solution and water. After the solvent was removed by distillation under reduced pressure, the residue was purified by column chromatography (eluent: hexane/ethyl acetate, 8:1, v/v) to afford the target product 2,5-difluoro-3-bromonitrobenzene. The yield was 1.07 g in 38%. The product was characterized by 1H NMR (400 MHz, CDCl3): δ 7.76 (ddd, 1H), 7.64 (ddd, 1H).
