To a mixture of 2，5-Dichloroaniline (1.02 g, 7.84 mmol), anhydrous sodium sulfate (8.94 g, 62.71 mmol), hydroxylamine hydrochloride (2.24 g, 31.35 mmol) and 1M hydrochloric acid (8.0 mL) in water (60 mL) is added chloral hydrate (1.58 g, 9.41 mmol) at room temperature. The resulting mixture is warmed to 55 °C and stirred for 6 hours. While the mixture is cooling to room temperature, solid precipitate is formed and collected by filtration, washed with water and dried under vacuum to yield the hydroxyliminoacetanilide intermediate, which is added in small portions to concentrated sulfuric acid (5.0 mL) that is preheated to 55 °C. The temperature of the reaction mixture is maintained at below 58 °C throughout the addition. After the addition is completed, the dark-colored mixture is heated to 80 °C for 10 minutes before cooling down to room temperature. The mixture is then poured into crushed ice, swirled, and left to stand for 30 minutes. 4,7-Dichloroisatin formed is collected by filtration, washed with water and dried under vacuum.
