Purification of Thionyl Chloride

This substance is ordinarily purified by treatment with quinoline and linseed oil, a procedure that gives poor yields and difficult-to-handle residues. Pratt modified the method by using a lower aliphatic ketone in place of quinoline and sulfur in place of the linseed oil. The latter procedure has been modified herein as follows:

900 mL of crude technical thionyl chloride was refluxed for 4.5 h with 25g of flowers of sulfur and distilled rapidly through a 30-bulb Snyder column attached to an all-glass setup. A 94% yield of colored product was taken overhead. The distillate was fractionated through the 30-bulb column, about 12 h being needed to remove the colored forerun. The colorless portion was taken overhead in about two hours and distilled over no range with a thermometer that was graduated in degrees. The yield in the second step was 88% making an 82% overall yield. The product was colorless when viewed crossways in a liter graduate and very slightly yellow when viewed from the top. The still went to dryness and the residue was yellow with some black material reminiscent of organic matter. No attempts to improve the above procedure were made. The sulfur probably aids in changing the sulfuryl chloride to sulfur dioxide and sulfur chlorides. The sulfur monochloride, bp 135.6°C, presumably is left behind in the first distillation and sulfur dichloride, bp 69°C, is the forerun of the fractionation which gives the colorless thionyl chloride, bp 78.8°C. It is probably impractical to try removing both sulfur chlorides in one distillation because of the equilibrium between sulfur monochloride on the one hand and sulfur dichloride and sulfur on the other.

[1] JACS 68, 1380-1381 (1946)