The general procedure for the synthesis of 4-(chlorosulfonyl)benzoic acid from 4-methylphenylsulfonic acid was as follows: a solution of potassium permanganate (KMnO4, 1.6 g) in water (H2O, 13 mL) was added slowly and dropwise at 80 °C to a solution of 4-methylphenylsulfonic acid (1.7 g, 10 mmol) and potassium hydroxide (KOH, 0.68 g, 12 mmol) in water (H2O, a 15 mL) solution. Upon completion of the reaction, the mixture was poured into ethanol (EtOH, 20 mL), filtered and concentrated to give potassium 4-carboxybenzenesulfonate (1.2 g). This intermediate was used directly in the next reaction without further purification. At room temperature, chlorosulfonic acid (4 mL) was slowly added dropwise to a stirring solution of potassium 4-carboxybenzenesulfonate (1.2 g). The reaction mixture was continued to be stirred for 2 hours and then poured into crushed ice. The resulting solid product was collected by filtration and washed with water (H2O) to give 4-(chlorosulfonyl)benzoic acid (0.22 g) as a white solid in 20% yield. The melting point of the product was 230-232 °C (literature value: 228-232 °C); 1H-NMR (CDCl3, 400 MHz) data: δ 8.30 (d, J=8.0 Hz, 2H), 8.42 (d, J=8.0 Hz, 2H).
