General procedure for the synthesis of 4,5-dichloro-2-nitrophenol from 3,4-dichlorophenol: 3,4-dichlorophenol (30 g, 185 mmol) was dissolved in dichloromethane (DCM, 300 mL) at -15 °C, followed by the addition of sulfuric acid (24 g, 278 mmol). Nitric acid (19 g, 194 mmol) was slowly added dropwise at a temperature maintained at -15 °C to -5 °C for a controlled time of 20 min or more. After the dropwise addition, the reaction mixture was continued to be stirred at 0 °C for 1 hour. After completion of the reaction, the mixture was poured into ice water and extracted with ethyl acetate. The organic layers were combined and washed sequentially with water, saturated aqueous sodium bicarbonate (NaHCO3) and brine, and then dried over anhydrous sodium sulfate (Na2SO4). The dried organic phase was concentrated under reduced pressure, and the resulting residue was purified by rapid column chromatography on silica gel (eluent ratio of petroleum ether/ethyl acetate = 100:1) to give the target product 4,5-dichloro-2-nitrophenol (16 g, 42% yield) as a solid.
