B.
3,3-Dimethoxycyclopropene. A
500-ml., three-necked, round-bottomed flask is equipped with a magnetic stirrer, a
gas-inlet tube, a
thermometer, and an
acetone–dry-ice condenser topped with a
drying tube containing sodium hydroxide pellets. An acetone–dry-ice bath is placed under the flask, and
ammonia is condensed into the flask from a commercial cylinder. When
350–400 ml. of ammonia has condensed, the inlet tube is replaced by a stopper, and a small piece
(0.5 g.) of potassium metal is added to the
ammonia. The cooling bath is removed, and
ca. 0.05 g. of anhydrous iron(III) chloride is added. When the
ammonia reaches reflux temperature, the blue color of the dissolved
potassium turns to gray, and the remainder of the
potassium (11.7 g., 0.300-g.-atom total) is added in 0.5-g. pieces at such a rate that a gentle relux is maintained. The stopper is then replaced by an
addition funnel containing a solution of
1-bromo-3-chloro-2,2-dimethoxypropane (21.7 g., 0.100 mole) in
50 ml. of anhydrous diethyl ether, which is added to the gray
potassium amide–
ammonia suspension over a period of 15 minutes, during which time the mixture is maintained at −50° to −60° with the cooling bath (Note
3). After 3 hours at this temperature, solid
ammonium chloride (10.8 g., 0.20 mole) is added with stirring.
Ammonia is allowed to evaporate by removing the cooling bath, and during the course of the evaporation it is replaced with
350 ml. of anhydrous ether. When the reaction temperature reaches
ca. 0°, the resulting brown solution is filtered from inorganic salts and placed in a
500-ml., round-bottomed flask (Note
4). The ethereal solution is then subjected to a vacuum (50–80 mm.) applied through a
carbon tetrachloride–dry-ice condenser (
ca. −25°), while the flask is immersed in an
ice bath. After 4–5 hours, when the quantity of residue seems to remain constant, the dry-ice condenser is replaced with a distilling head. The pressure is decreased to 1–2 mm., the receiver is maintained at −78° with a cooling bath, and distillation yields
4.0–6.5 g. (
40–65%) of
3,3-dimethoxycyclopropene as a clear liquid (Note
5). This material has been purified further,
2 but it can be used directly in the next step. If it is stored, it should be kept below 0°.