Synthesis
A. Synthesis of 2-fluoro-4-nitrophenol
To a stirred dichloromethane solution of 2-fluorophenol (32.3 g, 0.288 mol) was slowly added 90% nitric acid (1 g, 0.31 mol HNO3) in an ice-salt bath cooled to -10°C. The addition process was controlled to be completed in 1 hour. During the reaction, the temperature was maintained at about -5 °C. After the addition was completed, stirring was continued at 0°C for 1 hour. At the end of the reaction, the precipitate was collected by filtration and washed several times with cold dichloromethane. Analysis by GC and thin layer chromatography (silica gel, 7:3 hexane-acetone) showed that the product was mainly a single compound and that the by-products produced in the reaction were of low polarity and volatility. The solid was dissolved in ether, washed with water, dried (MgSO4) and the solvent evaporated. The resulting solid was recrystallized with methylcyclohexane to give 13 g of a pale yellow solid with a melting point of 119°-121°C. NMR (CDCl3) analysis confirmed the product as 2-fluoro-4-nitrophenol.
The dichloromethane mother liquor was treated, washed with water, dried (MgSO4) and the solvent evaporated. The solid obtained was ground with boiling hexane (3 x 150 ml) to effectively remove the least polar and highly volatile reaction by-products. The hexane solution was treated with activated charcoal, filtered and concentrated to about 300 ml and cooled to give 13.5 g (30% yield) of the by-product 2-fluoro-6-nitrophenol as a yellow solid with a melting point of 70°-86 °C.
References
[1] Patent: US4750931, 1988, A
[2] Patent: EP142328, 1991, B1
