Hydrochloric Acid → 7-Methylisatin

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In a 5 l flask is placed chloral hydrate (90 g, 0.54 mole) and water (1200 ml). To this solution is added sodium sulfate decahydrate (1300 g, 10.9 mole) followed by a solution of o-methylaniline (54 g, 0.5 mole) in water (300 ml) containing concentrated hydrochloric acid (43 ml, 0.5 mole). Then a solution of hydroxylamine hydrochloride (110 g, 1.58 mole) in water (50 ml) is added. The reaction mixture is heated to reflux over a 90 minute period and refluxed 30 minutes. The reaction is cooled in ice and the resulting crystalline ortho-isonitrosoacetotoluidide is isolated by filtration and air dried. The orthoisonitrosoacetotoluidide is dissolved in portions in concentrated sulfuric acid (325 ml) that is preheated to 50° C. with vigorous stirring. The reaction temperature is maintained under 75° C. When the addition is complete, the mixture is heated at 80° C. for 30 minutes, cooled, and poured unto ice (3 kg). The 7-methylisatin is isolated by filtration. The 7-methylisatin is purified by dissolution in dilute sodium hydroxide. The basic solution is treated with 4N hydrochloric acid until a slight amount of precipitation is evident. The mixture is filtered and the filtrate is acidified. The pure 7-methylisatin is recovered by filtration and dried in vacuo at 80° C. and 0.1 mm pressure, mp 114°-145° C. (A. Wahl, et al., Ann. Chim. 5 314 (1926).

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