B.
2,4,6-Tribromobenzoic acid. A
5-l. three-necked flask, equipped with a
mechanical stirrer and
thermometer and surrounded by an
ice-salt bath, is charged with a precooled mixture of
1.02 l. of concentrated sulfuric acid and 500 ml. of water. Cooling during the reaction period is assisted by the ice-salt bath, but is effected chiefly by periodic additions of large amounts of crushed Dry Ice directly to the reaction mixture. The temperature of the stirred mixture is lowered to −5° to −10° and
37 g. (0.54 mole) of sodium nitrite is added in small portions over a period of about 15 minutes. Cold
50% hypophosphorous acid (193 ml., 1.86 moles) (Note
3) is then added over a period of 10–15 minutes, the temperature still being maintained below −5°. A solution of the bromination product in
1.85 l. of glacial acetic acid is then added to the stirred diazotizing solution from a
dropping funnel. During the 1–1.25 hours required to complete this addition the temperature is held between −10° and −15° (Note
4). Stirring of the slurry is continued for approximately 2 hours longer, and during this period the temperature is allowed to rise gradually to +5°. The loosely stoppered flask is kept in a
refrigerator for 36 hours (Note
5), during which time
nitrogen and some
oxides of nitrogen are evolved
(Caution!). Most of the
acetic acid is then removed by volatilization in a current of steam, 3 l. of distillate being collected. During the steam distillation the product starts to precipitate from the clear solution, and
nitrogen dioxide and some
hydrogen sulfide are given off. The residual liquid is cooled, and the product is collected on a
sintered-glass funnel and washed with water. It is next suspended in 800 ml. of water, and approximately
9 g. of anhydrous sodium carbonate is added with shaking to bring it into solution. The yellow liquid is filtered to remove small quantities of insoluble impurities, and the acid is reprecipitated by acidification to pH 1–2 with
5% hydrochloric acid, while swirling vigorously. The cream-colored precipitate is separated by filtration, washed with water, and dried. The yield of
2,4,6-tribromobenzoic acid, sufficiently pure for preparative purposes, is
50–57.5 g. (
70–80%), m.p.
192.5–194.5° (Note
6).