A
250-mL, three-necked, round-bottomed flask containing a
2.5-cm magnetic stirring bar is equipped with a
Dewar-type reflux condenser containing ice, a
dropping funnel, and a
gas inlet tube. A
calcium chloride drying tube is attached to the condenser and the apparatus is flushed with dry deoxygenated
nitrogen (Note
1). The gas inlet tube is then replaced by a
thermometer, and a suspension of
5.0 g of aluminum trichloride in
50 mL of benzene is introduced into the flask (Note
2). A solution of
100 g (1.85 mol) of 2-butyne (Note
3) and (Note
4) in
50 mL of cold dry benzene is added, with vigorous stirring, through the dropping funnel, over a period of 1 hr. During the addition, the temperature of the reaction mixture is kept between 30 and 40°C through the use of a
water bath. Stirring is continued for 5 hr at 30–40°C after the addition has been completed. The catalyst is then decomposed by pouring the mixture onto 50 g of crushed ice in a
500-mL separatory funnel, whereupon the dark brown color turns pale yellow. When the ice has melted completely, the organic layer is separated, washed with two 25-mL portions of cold water, dried over anhydrous
potassium carbonate, and filtered.
Benzene and unreacted
butyne (Note
5) are removed in a
rotary evaporator using a water bath at 40°C and a water aspirator vacuum. The residual liquid is distilled through a
short-path distillation head under reduced pressure using a capillary. The yield is
38–50 g (
38–50%) of
hexamethyl Dewar benzene, bp
43°C/10 mm, mp
7–8°C,
nD20 1.4480 (Note
6) and (Note
7).