A
1-l. three-necked flask is equipped with a
Hershberg stirrer operating in a ground-glass bearing (Note
1), a
250-ml. pressure-equalizing dropping funnel, and a
coil condenser. The exit from the condenser is connected to a
train consisting of a trap (of at least 50-ml. capacity below the bottom of the inlet tube) cooled in ice, a
drying tube (about 6 in. long by 1 in. I.D.) filled with indicating Drierite and calcium chloride, an efficient trap of at least 150-ml. capacity cooled in Dry Ice-acetone to −70° or below, and a drying tube containing Drierite. A mixture of
95% ethanol (400 ml.), water (80 ml.), and
300 g. (4.6 g. atoms) of zinc dust is placed in the reaction flask. The
addition funnel is charged with
260 g. (2.34 moles) of 2,3-dichloropropene (Note
2), the reaction mixture is stirred and heated to reflux, and the
2,3-dichloropropene is added dropwise at such a rate that reflux is maintained without external heating (2–3 hours). After the addition is complete, heating is resumed for 1 hour. The ice-cooled trap is warmed to about 25°, and the residual
allene is purged from the reaction flask with a very slow stream of
nitrogen.
The trap cooled in Dry Ice-acetone contains about
105 g. of crude product which, when distilled through a
column packed with glass helices (Note
3), yields about
75 g. (
80%) of
allene (Note
4). No external heat is needed during the distillation. The distillation flask is allowed to warm to room temperature, the distillation beginning at a liquid temperature of −34° and virtually stopping at about 10°. The distilled product contains no detectable
ethanol, water,
2,3-dichloropropene, or
methylacetylene as determined by gas-liquid chromatography (Note
5).