A
500 ml., three-necked, round-bottomed flask is equipped with a
Trubore stirrer, a
pressure-compensating dropping funnel, and a
reflux condenser with drying tube. The flask is charged with
144 g. of triphenylphosphine (0.55 mole) (Note
1) and
125 ml. of acetonitrile (Note
2) and (Note
3). The solution is stirred and cooled in an
ice bath and
88 g. of bromine is added dropwise over a period of 20–30 minutes (Note
4). After the addition of the
bromine (Note
5) is complete, the ice bath is removed and
72 g. (0.50 mole) of β-naphthol (Note
6) in
100 ml. of acetonitrile (Note
7) is added in one portion and the reaction mixture is heated to 60–70° for at least 30 minutes (Note
8). The flask is now fitted for a simple distillation, stirring is discontinued, and the
acetonitrile is distilled (Note
9) under aspirator pressure until the
oil bath temperature reaches 110° (Note
10). After all the
acetonitrile has been removed, the condenser is replaced by a
short, large glass tube (Note
11) connected to a 500-ml. flask half-filled with water, and the oil bath is replaced by a
Wood's metal bath. The bath temperature is now raised to 200–220° and kept at this temperature until all the solid has melted (Note
12). The mixture is stirred and the bath temperature is raised to 340° (Note
13) and held at this temperature until evolution of
hydrogen bromide ceases (approximately 20–30 minutes). The Wood's metal bath is removed and the reaction mixture is cooled to approximately 100° and then poured into a
1-l. beaker and cooled to room temperature.
Pentane (300 ml.) (Note
14) is added and the solid is broken into a fine precipitate (Note
15). The solid is filtered by suction and washed thoroughly with two
300-ml. portions of pentane. The
pentane filtrates are combined, washed with
200 ml. of 20% sodium hydroxide, and dried over anhydrous
magnesium sulfate. The
pentane extract is then passed through a
25 mm. diameter column filled to 35 cm. in depth with alumina; distillation of the
pentane at reduced pressure gives
72–81 g. (
70–78%) (Note
16) of
2-bromonaphthalene, a white solid melting at
45–50° (reported:
2 55–56.4°) (Note
17), (Note
18).