Thioglycolic Acid → 5-Fluoro-1-benzothiophene-2-carboxylic acid

In a glass pressure bottle, 2,5-difluorobenzaldehyde (11 ml, 0.1 mol) was added to a solution of 2-mercaptoacetic acid (9 ml) in potassium hydroxide (16.5 g) and water (155 ml). The bottle was then sealed and heated in a hot oil bath to 125±5° C. for 1.25 hours. The bottle was then removed from the hot oil bath and cooled. A pale yellow precipitate was obtained in the bottle. At ambient temperature, the bottle was opened and sufficient water was added to dissolve the precipitate. The resultant aqueous material was washed with ether and then acidified with hydrochloric acid to produce a pale yellow solid. The pale yellow solid was collected on a filter, washed with water and dried, to produce 15.5 g of 5-fluoro-2-benzo-[b]thiophenecarboxylic acid (80.1% yield, mp>300° C.).