In a four-necked flask equipped with a stirrer, a thermometer, a cooling tube and a dropping funnel under a nitrogen atmosphere, 11.8 g of magnesium and 250 mL of tetrahydrofuran were added, followed by the addition of 2.05 g of ethyl bromide, which was allowed to react under heated reflux for 15 minutes. Then, 50 g of was added dropwise while maintaining reflux.
A mixture of 2-chloro-3-methylthiophene and 4.1 g of ethyl bromide was then added dropwise while maintaining reflux and allowed to react under heat for 30 minutes. Then, 4.11 g of ethyl bromide was added and allowed to react under heated reflux for another 1 hour.
Carbon dioxide was introduced into the resulting reaction solution at 25 to 35C and allowed to react at room temperature for 2 hours. Water was added to the resulting reaction solution, and then concentrated hydrochloric acid was added to adjust the reaction solution to a pH of 2 or less. Then, the aqueous layer was removed by partitioning, water was added to the resulting organic layer, and then the solvent was removed by distillation, thereby obtaining a slurry of 3-methyl-2-thiophenecarboxylic acid. The resulting slurry was filtered and dried, thereby obtaining 50.5 g of 3-methylthiophene-2-carboxylic acid.
