In a
500-mL, three-necked flask, equipped with a
thermometer,
stirrer, and a
reflux condenser with drying tube, are placed
12.6 g (0.1 mol) of 1-octyn-3-ol,
154 mL of dioxane,
7.24 g (0.0504 mol) of cuprous bromide,
7.4 g of paraformaldehyde, and
18.54 g (0.183 mol) of diisopropylamine (Note
1). The resulting mixture is gently refluxed and stirred for 2 hr and then cooled to room temperature and filtered through a
Celite plug. The dark-brown filtrate is concentrated under vacuum (Rotavapor) to a gummy residue and then diluted with 50 mL of water followed by
100 mL of ether and acidified with
6 N hydrochloric acid to pH 2. The ether–water layers are decanted from any residue, the
ether layer is separated, and the aqueous solution is extracted with
ether (5 × 50 mL). The
ether extracts are combined and washed with small portions of water until pH 6.5 is reached. The organic layer is then washed with saturated
sodium chloride solution and dried over anhydrous
MgSO4. After removal of
ether by distillation through a
20-cm Vigreux column (water aspirator vacuum) while heating on a
water bath, ≤40°C, the residual liquid is fractionated under reduced pressure through a
10-cm Vigreux column. The main fraction is collected at
41–42.5°C(0.15 mm) to give
8.65 g of pure allene (Note
2), with additional fractions of a less pure material.