GENERAL METHOD: Thiourea dioxide (20 mmol) was added in batches to a mixture of water (15 mL) and ethanol (5 mL) containing 4-chloro-2-nitroaniline (5 mmol) and sodium hydroxide (20 mmol) at 70 °C. The reaction mixture was continuously stirred at the same temperature and the progress of the reaction was monitored by thin layer chromatography (TLC) until the reaction was completed. Upon completion of the reaction, the mixture was cooled to room temperature, followed by slow addition of 10% sodium hydroxide solution to adjust the pH to 9-10. The precipitated solid was collected by filtration and washed thoroughly with deionized water to afford the crude product of 5-chlorobenzimidazole. The crude product was further purified by aqueous recrystallization to obtain the target compounds in white crystalline form. The physical properties and spectral data of compounds 2a-h are described below.
