Distillation with addition of linseed oil is old method. SOCl2 is not stable at atmospheric pressure boiling temperature so vacuum transfer is preferable after stirring with PPh3 at subboiling temperature.
Crude SOCl2 can be freed from sulfuryl chloride, sulfur monochloride and sulfur dichloride by refluxing it with sulfur and then fractionally distilling twice. [The SOCl2 is converted to SO2 and sulfur chlorides. The S2Cl2 (bp 135.6) is left in the residue, whereas SCl2 (bp 59) passes over in the forerun.] The usual purification is to distil it from quinoline (50g SOCl2 to 10g quinoline) to remove acid impurities, followed by distillation from boiled linseed oil (50g SOCl2 to 20g of oil). Precautions must be taken to exclude moisture.
Thionyl chloride is used extensively in organic syntheses and can be prepared by distillation of technical SOCl2 in the presence of diterpene (12g/250mL SOCl2), and avoiding overheating. Further purification is achieved by redistillation from linseed oil (1-2%) [Rigby Chem Ind (London) 1508 1969]. Gas chromatographically pure material is obtained by distillation from 10% (w/w) triphenyl phosphite [Friedman & Wetter J Chem Soc (A)36 1967, Larsen et al. J Am Chem Soc 108 6950 1986]. HARMFUL VAPOURS.
Source : Purification of Laboratory Chemicals, W.L.F. Armarego & C.L.L. Chai
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