Anhydrous
hydrogen fluoride (135 ml., approx. 7 moles) is liquefied by passing the gas through an
8-ft. spiral of
-in. I.D. copper tubing surrounded by an
isopropyl alcohol-dry ice bath (Note
2). The liquid is delivered to a
500-ml. polyethylene squeeze bottle (Note
3) containing a
magnetic stirring bar via a
polyethylene tube inserted through the screw cap of the bottle. The squeeze bottle is contained in a
2-l. beaker and is surrounded by powdered dry ice (Note
4). After the
hydrogen fluoride is collected, the cap and
delivery tube are removed, and
p-cymene (terpene-free, 67 g., 0.5 mole), precooled to −50° to −60°, just above the slush point, is added to the
hydrogen fluoride. The cap and polyethylene tube assembly is now attached to deliver
boron trifluoride. The delivery tube in this case dips below the surface of the
hydrogen fluoride, the bottom layer.
Boron trifluoride is bubbled through while the mixture is efficiently stirred with a magnetic stirrer (Note
5). A light orange color develops immediately, and the two layers become one in about 30 minutes (Note
6). Additional powdered dry ice must be added to the beaker during the
boron trifluoride addition. The volume of the complex increases about 25%. After homogeneity is effected (in 30 minutes), a somewhat slower stream of
boron trifluoride is added for an additional 30 minutes (Note
7). The delivery tube is replaced by a cap on the polyethylene bottle, the drying tube is removed, and the original side-arm tube is lowered to the bottom of the container. The cold reaction mixture is squirted, by squeezing the bottle, in a continuous small stream into a
4-l. beaker half-filled with cracked ice, vigorously hand-stirred. The bottle is rinsed, and the contents of the beaker are placed in a large
separatory funnel. The upper colorless layer is separated, and the aqueous phase is extracted 3 times with
50-ml. portions of hexane. The combined organic layers are washed with three 50-ml. portions of water and dried overnight with anhydrous
sodium sulfate under refrigeration (Note
8). The
hexane solution at this point contains
m-cymene with about 8% disproportionation impurities including
toluene. The solution is fractionated through a
1-ft. helices-packed column, with the
m-cymene at boiling point
173–176°,
50–54 g. (
75–80%) being collected (Note
9).