3-Aminothiophene (1.08 g, 10.9 mmol) was dissolved in ethanoic anhydride (20 mL) and the reaction was stirred for 3 hours at room temperature. After the reaction was completed, water (50 mL) was added and stirring was continued for 30 minutes. Subsequently, the pH was adjusted to >9 by slowly adding 4 M aqueous sodium hydroxide (150 mL). the reaction mixture was extracted with dichloromethane (3 x 100 mL). The organic phases were combined, washed with brine (30 mL), dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was crystallized by dichloromethane/petroleum ether mixed solvent to afford N-(3-thienyl)acetamide (994 mg, 65% yield) as a white crystalline solid. The structure of the product was confirmed by 1H-NMR (500 MHz, CDCl3) and 13C-NMR (125 MHz, CDCl3): 1H-NMR δ 2.17 (s, 3H, CH3), 7.00 (dd, J = 5.0, 1.3 Hz, 1H, Ar-H), 7.23 (dd, J = 5.0, 3.2 Hz, 1H, Ar-H) 7.45 (br, 1H, N-H), 7.55 (dd, J = 3.2, 1.3 Hz, 1H, Ar-H); 13C-NMR δ 18.64 (CH3), 105.03, 115.64, 119.26, 130.27 (Arom-C), 162.11 (C=O).
