Synthesis
The general procedure for the synthesis of 4-bromo-5-chlorothiophene-2-carboxylic acid from 2-chlorothiophene-5-carboxylic acid was as follows: a solution of bromine (634 μL, 12.3 mmol) in acetic acid (2.5 mL) was added slowly and dropwise to an acetic acid solution (25 mL) containing 5-chloro-2-thiophenecarboxylic acid (2 g, 12.3 mmol) and ferric chloride (399 mg, 2.50 mmol). mL) at 25 °C. Subsequently, the reaction mixture was heated to reflux during which bromine (634 μL, 12.3 mmol) and ferric chloride (399 mg, 2.50 mmol) were added in portions. After the reaction lasted for 7 days, the reaction solution was poured into ice water, the precipitate precipitated was filtered and washed with cold water to finally obtain 4-bromo-5-chlorothiophene-2-carboxylic acid (3 g, quantitative yield) as a yellow powder. The product was analyzed by liquid chromatography-mass spectrometry (LC-MS) and showed m/z 242 ([M+H]+).
