The general procedure for the synthesis of methyl 3-hydroxy-4-chlorobenzoate from methanol and 4-chloro-3-hydroxybenzoic acid was as follows: thionyl chloride (6 g) was added slowly and dropwise to a methanol (100 mL) solution of 4-chloro-3-hydroxybenzoic acid (4.4 g, 25.50 mmol). The reaction mixture was stirred at 70 °C for 2 hours. Upon completion of the reaction, the reaction solution was concentrated under reduced pressure to remove the solvent. The concentrated residue was diluted by adding hexane (50 mL) to the concentrated residue. The precipitated solid product was collected by filtration and dried in an oven under reduced pressure to afford methyl 3-hydroxy-4-chlorobenzoate as a light yellow solid (4 g, 84% yield). The product was analyzed by LC/MS (electrospray ionization, m/z): [M+H]+ 187.0. 1H NMR (300 MHz, DMSO-d6) δ 10.69 (s, 1H, OH), 7.55 (d, J=2.10 Hz, 1H, ArH), 7.49 (d, J=8.40 Hz, 1H, ArH), 7.36-7.40 ( m, 1H, ArH), 3.83-3.85 (d, J=8.40Hz, 3H, OCH3).
