In a
500-ml. three-necked round-bottomed flask, fitted with a
stirrer,
thermometer, and
250-ml. dropping funnel, are placed 19 ml. of water and
27.8 ml. (0.5 mole) of concentrated sulfuric acid (sp. gr. 1.84), and the whole is warmed to 70°. From the dropping funnel is added, dropwise, over a 30-minute period,
53.5 g. (0.5 mole) of o-toluidine (Note
1). The temperature is allowed to reach about 85° in order to prevent the separation of solid before all the amine has been added. When addition is complete, the temperature is raised to 100–105° for 5 minutes. The flask is then placed on a
steam bath, the stirrer and dropping funnel are removed, and the flask is evacuated by means of a
water pump to about 15 mm. (Note
2); after 21 hours all the water has been removed. The flask is then immersed in an
oil bath, and the internal temperature is raised to 180° over a period of 2 hours and kept at 180–195° (bath temperature about 190–205°) for 7 hours while the same diminished pressure is maintained (Note
3). The crude product is dissolved in 1.3 l. of boiling water, and
99 g. of barium hydroxide octahydrate is added with stirring; the solution is now alkaline to litmus. After the addition of
3 g. of Norit, the solution is heated for 1.5 hours on a steam bath, and
17.5 ml. of concentrated sulfuric acid (sp. gr. 1.84) diluted with 20 ml. of water is added with good stirring. The solution is heated to 96° and filtered through a
large, preheated Büchner funnel; the filter cake is rinsed with 100 ml. of boiling water (Note
4).
The filtrate is transferred to a
3-l. round-bottomed flask and concentrated under a pressure of about 50 mm. to a volume of 200 ml. (Note
5). After the mixture has been chilled overnight in a
refrigerator, the crystalline acid (
68–74 g.) is filtered by suction and dried (Note
6). An additional small amount (
5–8 g.) can be obtained by evaporating the filtrate to dryness. This residue is allowed to remain in contact with
75 ml. of methanol for about an hour with occasional shaking. The solid is filtered, dried, and weighed. The total yield of
o-toluidinesulfonic acid is
74–79 g. (
79–83%) (Note
7) and (Note
8).