[Cl:1][C:2]1[C:10]2[CH:9]=[C:8]([C:11]([O:13]C)=[O:12])[S:7][C:6]=2[CH:5]=[CH:4][CH:3]=1.[OH-].[Na+].Cl>O>[Cl:1][C:2]1[C:10]2[CH:9]=[C:8]([C:11]([OH:13])=[O:12])[S:7][C:6]=2[CH:5]=[CH:4][CH:3]=1A mixture of methyl 4-chlorobenzo[b]thiophene-2-carboxylate (20 g), 5M aqueous sodium hydroxide solution (100 ml) and water (300 ml) was heated at 95° C. for 5 hours then poured into a mixture of concentrated hydrochloric acid (180 ml) and water (2000 ml) and stirred at ambient temperature for 2 hours. The resulting solid was collected by filtration, washed with water (3×500 ml), and dried in vacuo at 60° C. to give 4-chlorobenzo[b]thiophene-2-carboxylic acid (21.0 g) as an off-white solid, m.p. 246-248° C.
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