Dry chlorine gas is admitted into a solution of 104 g. (1.00 mole) of cycloöctatetraene in 150 ml. of dry carbon tetrachloride contained in a tared, 500-ml., three-necked flask equipped with a gas-inlet tube, a low-temperature thermometer, and a calcium chloride drying tube. The reaction mixture is maintained between −28° and −30° throughout the addition, which is terminated after 71 g. (1.0 mole) of chlorine has been added. After the addition, which takes approximately 1 hour, the reaction mixture is allowed to warm to 0° and 50 g. of powdered sodium carbonate is added, and the contents are shaken gently for several minutes. This treatment removes any hydrochloric acid which may have been produced during the reaction. The mixture is then filtered directly into a 1-l., round-bottomed flask containing 135 g. (0.951 mole) of dimethyl acetylenedicarboxylate. A condenser is fitted to the flask and the solution is heated at gentle reflux for 3 hours (Note 1). The solvent is removed under reduced pressure (Note 2). The crude Diels-Alder adduct, which will slowly solidify on standing, is used directly in the next step.

The crude Diels-Alder adduct is transferred to a 500-ml., pressure-equalizing dropping funnel attached to a 1-l., three-necked, round-bottomed flask; the latter is immersed in an oil bath maintained at 200° and equipped with a distillation head, condenser, and receiving flask. The pressure inside the equipment is reduced to 20 mm. A magnetic stirrer in the pyrolysis flask is started, the Diels-Alder adduct (Note 3) is added slowly to the hot flask, and the pyrolysate collected in the receiving flask. The distillation temperature during the pyrolysis varies from 135° to 152°, depending on the rate of addition of the Diels-Alder adduct. After the addition is complete (about 1 hour), the pyrolysis is continued for a further 30 minutes or until very little material remains in the pyrolysis flask (Note 4). The crude pyrolysate is then redistilled at 12–15 mm., with all material boiling below 140° collected; this distillate consists mainly of a mixture of dichlorocyclobutene and 1,4-dichlorobutadiene, the residue being mainly dimethylphthalate.

A Final distillation at 55 mm. through a 36-in., platinum spinning band column yields 49–52 g. (40–43%) of pure cis-3,4-dichlorocyclobutene (b.p. 70–71°, 55 mm.); the forerun (b.p. 58–62°, 55 mm.) consists mainly of 1,4-dichlorobutadiene (Note 5).