A. In a 2-l. round-bottomed flask are placed 61 g. (0.5 mole) of benzoic acid and 300 ml. of concentrated sulfuric acid (sp. gr. 1.84). To this mixture is added 100 ml. of fuming nitric acid (sp. gr. 1.54) in portions of 2 or 3 ml. The temperature during the addition of the acid is kept between 70° and 90° by means of external cooling with cold water (Note 1), and the addition should be carried out in a good hood. The flask is covered with a watch glass and allowed to stand for an hour, or overnight, in the hood. The flask is then heated on a steam bath in the hood for 4 hours, during which time considerable amounts of brown fumes are evolved. The reaction mixture is then allowed to cool to room temperature (Note 2), whereupon yellow crystals separate from the solution. An additional 75 ml. of fuming nitric acid is added, and the mixture is heated on the steam bath for 3 hours and then in an oil bath at 135–145° for 3 hours (hood). Brown fumes are evolved continuously, especially during the heating in the oil bath. The color of the reaction mixture is light to reddish yellow.

The mixture is allowed to cool and is poured into 800 g. of ice and 800 ml. of water. After standing for 30 minutes the 3,5-dinitrobenzoic acid is filtered with suction and washed with water until free of sulfates. The crude product weighs 62–65 g. and melts at 200–202°. This product is recrystallized from 275 ml. of hot 50% ethanol. The purified 3,5-dinitrobenzoic acid weighs 57–61 g. (54–58%) and melts at 205–207° (Note 3).

B. Two hundred grams of benzoic acid (1.6 moles) is stirred into 1 l. of concentrated sulfuric acid contained in a 3-l. round-bottomed flask. The flask is surrounded by cold water, and the temperature is maintained below 45° while 80 ml. of fuming nitric acid is added. When the temperature of the reaction mixture has fallen to 30°, 240 ml. of fuming nitric acid is added, a beaker is inverted over the neck of the flask, and the mixture is allowed to stand for 6 weeks.

The flask is heated on a steam bath for 4 hours and then in an oil bath at 145° until all the crystals are dissolved. A mush of crystals separates when the mixture is cooled to room temperature. These are filtered on a Büchner funnel without any filtering medium. After the crystals are pressed dry, they are placed in 1.5 l. of cold water, washed thoroughly, filtered through paper, and dried in the air. The yield is 200–213 g. (60%), and the product melts at 205–207° (cor.) (Note 4) and (Note 5).