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).