Liquid sulfur dioxide is immiscible with saturated hydrocarbon liquids and will form a separate lower phase when combined with them. Thus, if a saturated hydrocarbon solvent is used as a chaser to drive off the reaction solvent from a first reaction, sulfur dioxide can be added to extract the reaction mixture content into a lower phase provided the desired material is soluble in sulfur doxide. The viscosity of a hydrocarbon fraction that is sufficiently high boiling to work as a ‘chaser’ may mix only sluggishly with liquid sulfur dioxide at -10 C. If it proves necessary to decrease the viscosity of such a hydrocarbon solution it can be mixed with a lower molecular weight saturated hydrocarbon.
Because at 1.46 g/ml the density of liquid sulfur dioxide is almost twice that of a typical hydrocarbon and since saturated hydrocarbons have only a limited solubility in liquid sulfur dioxide, they will form a separate liquid phase from which the liquid sulfur dioxide lower layer can be cut away.
Therefore, liquid sulfur dioxide can be used to transfer a solute from a higher-boiling hydrocarbon solvent mixture to a lower-boiling solvent such as ethyl acetate by first extracting the solute into a liquid sulfur dioxide phase and then displacing the sulfur dioxide with the second solvent (such as ethyl acetate).
There is a shortage of usable solvents that have a higher density than water since halogenated solvents have fallen under a regulatory cloud. Liquid SO2 is such a dense solvent.
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