1-Methyl-1H-pyrazole (100 g, 1.18 mol) was slowly added dropwise to chlorosulfonic acid (325 mL, 4.84 mol) at 0 °C and under nitrogen protection, keeping stirring. The reaction mixture was then heated to reflux at 110 °C for 3 hours. Upon completion of the reaction, the mixture was cooled to room temperature and quenched by careful pouring into crushed ice with continuous stirring. The precipitated white solid product was collected by vacuum filtration and washed well with cold water. Finally, the product was dried under vacuum to afford 1-methyl-1H-pyrazole-4-sulfonyl chloride (72.2 g, 33% yield), which can be used in subsequent reactions without further purification.
