The general procedure for the synthesis of 4,5-dichlorothiophene-2-carboxylic acid from methyl 4,5-dichlorothiophene-2-carboxylate was as follows: methyl 4,5-dichlorothiophene-2-carboxylate (190 g, 0.9 mol) was dissolved in tetrahydrofuran (THF, 1200 mL). Subsequently, 4 M of aqueous lithium hydroxide (LiOH) (250 mL) and diatomaceous earth were added to this solution and mixed well. The reaction mixture was stirred at room temperature for 30 minutes. Upon completion of the reaction, THF was removed by distillation under reduced pressure.The remaining aqueous phase was acidified with 1 N hydrochloric acid and subsequently extracted three times with ethyl acetate. The organic phases were combined, dried over anhydrous magnesium sulfate (MgSO4) and decolorized by activated carbon. After filtration, the white solid product 4,5-dichlorothiophene-2-carboxylic acid (170 g, 96% yield) was obtained. The melting point of the product was 196-197 °C (literature value: 194-195 °C). IR spectrum (KBr): 1689 cm-1 (C=O stretching vibration).1H NMR (DMSO-d6): δ 13.48 (broad peak, 1H, COOH), 7.68 (single peak, 1H, H-3).13C NMR (DMSO-d6): δ 161.5, 132.5, 132.3, 130.1, 124.6.Elemental analysis: C5H2Cl2O2S Calculated values: c, 30.48; h, 1.02. measured values: c, 30.28; h, 1.15. mass spectrum (ESI): m/z 196.0 [M-H]-.
