General procedure for the synthesis of 4-chloro-5-fluoro-2-nitrophenol from 4-chloro-3-fluorophenol: 4-chloro-3-fluorophenol (2.38 g, 16.2 mmol) was dissolved in dichloroethane (DCE, 32 mL), followed by addition of tetrabutylammonium bromide (0.524 g, 1.62 mmol). A 7% nitric acid solution was prepared by diluting 70% nitric acid (HNO3, 2.1 mL, 32 mmol) with water (18.9 mL). This nitric acid solution was slowly added to the reaction mixture and the reaction was stirred at room temperature for 4 h. The progress of the reaction was monitored by thin layer chromatography (TLC) until the reaction was completed. After completion of the reaction, the reaction mixture was poured into water and extracted with dichloromethane (DCM, 3 × 30 mL). The organic layers were combined, dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure. The obtained residue was adsorbed on silica gel and purified by fast column chromatography to afford the target product 4-chloro-5-fluoro-2-nitrophenol (2.29 g, 74% yield). The structure of the product was confirmed by 1H NMR (400 MHz, DMSO-d6): δ 11.76 (br.s, 1H), 8.18 (d, J = 8.1 Hz, 1H), 7.10 (d, J = 10.6 Hz, 1H).
