A
1-l. three-necked round-bottomed flask is fitted with a
rubber slip-sleeve-sealed stirrer, a
dropping funnel with the tip extended to reach almost to the bottom of the flask (Note
1), and an efficient condenser leading to a
gas absorption trap.
2 o-Xylene (106 g., 1 mole) (Note
2) is placed in the flask, which is heated with an
oil bath and illuminated with a
sun lamp (Note
3) placed 1–5 cm. from the upper portion of the flask. When the temperature of the
o-xylene reaches 125°, the dropwise addition of
352 g. (2.2 moles) of bromine is commenced with stirring. The rate of addition is regulated so that all the
bromine is introduced in 1.5 hours. The mixture is stirred at 125° under illumination for an additional 30 minutes. It is then allowed to cool to 60° and poured into
100 ml. of boiling 60–68° petroleum ether contained in a
beaker, the transfer being assisted with small amounts of warm solvent. As the homogeneous solution cools slowly to room temperature it is stirred frequently to prevent caking of the brown crystalline product that separates. After the mixture is cool and the bulk of the dibromide has crystallized, the beaker is placed in a
refrigerator for 12 hours (Note
4). The product is then separated by suction filtration, washed twice with
25-ml. portions of cold petroleum ether, and then pressed on the filter until nearly dry. Final drying is effected in a
vacuum desiccator containing solid
potassium hydroxide. The brown crystalline product amounts to
123–140 g. (
48–53% yield), the melting point ranging between
89° and
94° (Note
5).