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°.