Hydrochloric Acid → 4-Fluorosalicylic acid

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A solution of 2,4-difluorobenzoic acid (100 g, 0.63 mol) in 1,3-dimethyl-2-imidazolidinone (1,400 mL, 0.45 M) was treated with sodium hydroxide (88 g, 2.2 mol) and heated to 135° C. After stirring for 4 h, the solution was cooled 0° C., disolved in water (100 mL), and transferred to a 5 L Erlenmeyer flask and carefully treated with aq HCl (2,800 mL, 2 N). After filtering off the crude product, the precipitate was disolved in ethyl acetate, dried over sodium sulfate and decolorizing charcol, and filtered. The solution was concentrated under reduced pressure and recrystallized from ethyl acetate and heptane to give 4-fluorosalicylic acid (2 crops, 67 g, 68%) as off-white needles. mp: 188–189° C.; Rf 0.26 (20% methanol in dichloromethane).

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