In a
5-l. round-bottomed flask fitted with an
efficient reflux condenser are placed
2750 ml. of 95% ethanol and
609 g. (8.0 moles) of thiourea. The mixture is brought to the reflux temperature on a
steam bath, and the refluxing solution is almost clear. The steam is turned off, and
751.5 g. (4.0 moles) of ethylene dibromide is added in one portion. Within 5 minutes a vigorous reaction (Note
1) ensues and
ethylene diisothiuronium bromide separates from solution. The exothermic reaction is allowed to continue to completion without further application of heat. The isothiuronium salt is collected by filtration and dried. The salt melts with decomposition at
225–227° (Note
2) and weighs
1104 g. (
81%).
Concentration of the filtrate to a volume of about 250 ml. and recrystallization from
95% ethanol of the crude isothiuronium salt which separates gives an additional
130 g. of material. The total yield of the salt is
1234 g. (
90%).
A mixture of
255 g. (0.75 mole) of ethylene diisothiuronium bromide and
640 g. (9.7 moles) of 85% potassium hydroxide in 1360 ml. of water is placed in a
5-l. round-bottomed three-necked flask and boiled under reflux for 5 hours.
Ammonia is evolved during the reflux period. The flask is then equipped with a
separatory funnel, a
gas-inlet tube, and a condenser set for steam distillation (Note
3).
Nitrogen is admitted through the inlet tube, and a cooled solution of
415 ml. of sulfuric acid in 760 ml. of water is added dropwise (Note
4). The addition is continued until the reaction mixture becomes acid to Congo red paper, and then a 20% excess of acid is added. Approximately 725–850 ml. of the acid solution is required. The heat of neutralization is sufficient to distil part of the dithiol. At the end of the addition of the acid, the passage of
nitrogen is discontinued and steam is admitted through the inlet tube. The steam distillation is continued until about 3 l. of distillate is collected. The oil is separated from the water in the distillate, which is then extracted with two
500-ml. portions of ether. The
ether solution and the oil are dried separately over
calcium chloride. After evaporation of the solvent, the residue is added to the oil and the crude product is fractionated through a
10-in. Vigreux column under reduced pressure in an atmosphere of
nitrogen. The
ethanedithiol boils at
63°/46 mm. (Note
5) and weighs
39–44 g. (
55–62%);
nD20 1.5589.