General procedure for the synthesis of 5-bromo-2-chloronitrobenzene from 3-nitro-4-chloroaniline: 4-chloro-3-nitroaniline (17.2 g, 0.1 mol) was dissolved in 260 mL of 48% hydrobromic acid (HBr) at 0 °C. Subsequently, an aqueous solution of sodium nitrite (NaNO2, 13.8 g, 0.2 mol) was added slowly and dropwise. The reaction mixture was stirred continuously for 1 h at 0 °C. After that, cuprous bromide (CuBr, 24 g, 0.17 mol) was added in batches and stirring was continued for 1 hour at 0°C. Upon completion of the reaction, water was added and the mixture was gradually warmed to room temperature and extracted with ethyl acetate (EtOAc). The crude product was purified by silica gel column chromatography to give the final 5-bromo-2-chloronitrobenzene (13 g, 55% yield). The structure of the product was confirmed by 1H NMR (CDCl3, 400 MHz): δ 8.03-8.02 (m, 1H), 7.66-7.63 (m, 1H), 7.45-7.42 (m, 1H).
