Sulfuric Acid → 4-Chloroisatin

Temperatur

90 °C

To 775 ml of concentrated sulfuric acid, preheated to 70° C., was added, with stirring, 136 g of isonitroso-3-chloroacetanilide at such a rate as to maintain the reaction medium at a temperature between 75° and 85° C. When all the solid had been added, the reaction mixture was heated at 90° C. for an additional 30 minutes. The reaction mixture was then cooled, and poured slowly onto ca 2 liters of ice, with stirring. Additional ice was added as necessary to maintain the temperature below room temperature. A red-orange precipitate formed which was recovered by filtration, washed with water and dried. The resultant solid was slurried in 2 liters of water, and then it was brought into solution by the addition of ca 700 ml of 3 N sodium hydroxide. The solution was filtered, and then pH was adjusted to 8 with concentrated hydrochloric acid. At this point, 120 ml of a mixture of 80 parts water and 20 parts concentrated hydrochloric acid was added. The solid which precipitated was recovered by filtration, washed with water and dried to give 50 g of crude 4-chloroisatin. The filtrate from which the 4-chloroisatin had been recovered was further acidified to pH 0 using concentrated hydrochloric acid, whereupon a further precipitate formed. It was recovered by filtration, washed with water and dried, to give 43 g of crude 6-chloroisatin.