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The pyrolysis of 4,5-dicyano-l,3-dithiol-2-one was carried out in a vertical quartz tube, 2 cm in diameter and 25 cm in length, and packed with quartz tubes of 0.5 cm diameter, cut into pieces 0.5 ern long. The tube temperature was controlled by a thermocouple contained in a well in the center of the tube and connected to a "Pyro Vane" temperature control unit. The lower end of the tube was connected to a 200-ml round-bottomed flask containing a mixture of 75 g of 4,5-dicyano-1,3-dithiol-2-one and 75 g of sand (Fisher Scientific Co. silica, SiO2). The upper end of the pyrolysis tube was connected to two inverted traps in series, the first (1) containing a magnetic stirring bar and cooled with Dry Iceacetone, the second (2) cooled with liquid nitrogen. The whole apparatus was shielded. With the tube temperature at 800°, the whole system was evacuated to ca. 0.2 mm, using an oil pump. The outlet of the pump vented into a hood. The flask was then immersed in an air bath, the temperature of which was raised to ca. 135 C during 1-2 hr. During this time, the pressure increased to ca. 3-5 mm. When the pressure had decreased again to ca. 2 mm, the bath temperature was raised slowly to 160 C and kept there until the pressure decreased to below 1 mm. The whole pyrolysis took about 6-8 hr. Nitrogen was admitted to the system and the stirred contents of trap 1 were flash-distilled under vacuum into another trap, cooled with Dry Ice-acetone. During this operation, trap 1 was immersed in hot water and the pressure was regulated to prevent excessively fast distillation which resulted in freezing of the dicyanoacetylene in trap 1.
The yield of dicyanoacetylene was 20-26 g (59-76%). The product so obtained usually contained less than 0.1% of carbon disulfide and if the precursor was dried azeotropically, ca. 0.5-1% of benzene. The carbon disulfide can be removed by distillation through an efficient column; dicyanoacetylene boils at 76 C (atmospheric pressure) Dicyanoacetylene was analyzed by gas chromatography, using a Dow-Corning 550 silicone oil column for the separation of dicyanoacetylerie and benzene and a DowCorning XF-1150 silicone oil column for detection of carbon disulfide. The contents of trap 2 were allowed to evaporate in the hood. Before reuse, the sulfur and dark deposits were removed by passing air through the shielded pyrolysis tube at 600-800°; the sulfur ignited. Pure oxygen had been used in place of air for a number of times, but an explosion occurred in one instance.
© Collected Ruslan Anatolievich Kiper, burewestnik@mail.ru