In a
1-l. three-necked flask fitted with a
thermometer, a
dropping funnel, and a
mechanical stirrer are placed
65 g. (0.3 mole) of 1-methyl-1-(1-naphthyl)-2-thiourea (p. 609) (Note
1) and
400 ml. of glacial acetic acid (Note
2). To the mechanically stirred suspension, maintained at 18–20° by cooling in a
water bath, is added dropwise over a 30-minute period
48 g. (0.3 mole) of bromine in
50 ml. of glacial acetic acid. The light-yellow addition product is stirred an additional 15 minutes at 18–20°. After the thermometer is replaced by an outlet tube (Note
3), the mixture is heated in a water bath maintained at 80–85° (Note
4) for 3 hours.
Hydrogen bromide is evolved copiously with the formation of a white, crystalline
hydrobromide. When the mixture is cool, it is filtered and the precipitate is washed on the filter with
50 ml. of acetone and then with two 250- to
300-ml. portions of dry ether. One hundred milliliters of concentrated
ammonia is then added with stirring to the salt suspended in 700 ml. of warm water (60–65°). The imine base first separates as an oil (Note
5); after the mixture has been stirred and warmed on the
steam bath for 10 minutes, it is extracted with
500 ml. of chloroform and filtered by suction through a
Norit filter pad 5–6 mm. thick. The bottom layer is separated, washed with 350 ml. of water, and dried by stirring with
40 g. of potassium carbonate. The residual oil, after removal of the solvent on the steam bath under reduced pressure, is poured into an
evaporating dish and stirred while cooling. The brownish-colored crystals (Note
6) are dried at 80–85°; they melt at 97–99° and weigh
58–62 g. (
90–97%). If desired, the
naphthothiazoline (Note
7) can be crystallized from
petroleum ether (
60–90°) using 75 ml. per gram of product. The recovery of the almost colorless (cream) crystals is
85%. The recrystallized
thiazoline melts at
97–99°.