In a 4-l. beaker is placed 210 g. (1.5 moles) of finely powdered m-nitroaniline (Note 1). A cold mixture of 450 cc. of water and 330 cc. of concentrated sulfuric acid is added with hand or mechanical stirring, and then about 800 g. of finely crushed ice. When a homogeneous mixture has resulted, a solution of 105 g. (1.52 moles) of sodium nitrite in 250 cc. of water is added rapidly over a period of eight to ten minutes at the bottom of the mixture through a separatory funnel (Note 2) until a permanent color is given to starch-iodide paper (about 25–30 cc. of nitrite solution remains unused). The temperature during diazotization should be maintained at 0–5°. Stirring is continued for five to ten minutes longer, and the solution allowed to settle for another five minutes. A heavy crystalline deposit of m-nitrobenzenediazonium sulfate settles at the bottom of the beaker, from which the supernatant liquid is decanted (Note 3).

While the diazotization is in progress, 1 l. of concentrated sulfuric acid is added to 750 cc. of water in a 5-l. round-bottomed flask and the mixture heated to boiling (160°) with a large ring burner. The liquor from the diazotization is then added from a separatory funnel at such a rate that the acid mixture boils very vigorously. About fifty minutes is required for this addition. The crystalline diazonium sulfate is then added in small portions at such a rate that the evolved nitrogen does not cause loss of material by excessive foaming. Boiling is continued for a few minutes longer, and the contents of the flask are poured into a large beaker (Note 4) set in running cold water, and vigorously stirred to obtain a homogeneous crystal magma.

When completely cold, the nitrophenol is filtered, thoroughly pressed out, drained with suction, and washed with several portions of iced water, 450 cc. in all being used. It is spread on large sheets of filter paper and dried in a warm room. As thus prepared it has a yellowish brown appearance with darker particles intermixed. The yield is 170–180 g. (81–86 per cent of the theoretical amount) (Note 5).

It may be purified by recrystallization from hot hydrochloric acid, as recommended in Org. Syn. 3, 89, but on account of the difficulty of filtration this procedure is satisfactory on a small scale only. It is preferably distilled in 200-g. lots from a 500-cc. Claisen flask under 10–15 mm. pressure, b.p. 160–165°/12 mm. It is important that no air inlet be used to prevent bumping, since under these conditions the nitrophenol partly sublimes and causes trouble by stopping up the connections. A filter or distilling flask is used as a receiver, and only a short but wide-bore (10-mm.) air condenser is necessary (Note 6). The product is pale yellow and melts at 95–96°. The yield from 200 g. of crude product is 180–185 g.