Two hundred forty cubic centimeters (5.7 moles) of c.p. fuming nitric acid (sp. gr. 1.50) is added to 1.4 l. of concentrated c.p. sulfuric acid (sp. gr. 1.84) in a 3-l. beaker, and the mixture is cooled in an ice bath. As soon as the temperature of the mixed acids reaches 12°, 200 g. (1.36 moles) of commercial phthalimide is stirred in as rapidly as possible while the temperature of the nitrating mixture is kept between 10° and 15°. The reaction mixture is allowed to warm to room temperature in the ice bath as the ice melts, and left overnight.

The clear, pale yellow solution is poured slowly with vigorous stirring onto 4.5 kg. of cracked ice; the temperature of this mixture must not rise above 20°. The crude nitration product is filtered through cloth on a 20-cm. Büchner funnel, using suction, and the mass is pressed as dry as possible. The cake is removed and stirred vigorously with 2 l. of ice water. The solid is filtered; the cake is removed and stirred again with 2 l. of ice water. This washing is repeated four times. The crude product, after drying in the air, melts at 185–190° and weighs 165–174 g. (63–66 per cent of the theoretical amount). This material is purified by crystallization from 3 to 3.2 l. of 95 per cent ethyl alcohol. This furnishes 136–140 g. (52–53 per cent of the theoretical amount) of 4-nitrophthalimide melting at 198°.