10.7 LIQUID SULFUR DIOXIDE
In addition to the protonic solvents already discussed, chemistry has been carried out in liquid sulfur
dioxide for many years. Although the SO 2 molecule has a signifi cant dipole moment, the liquid is a
good solvent for many covalent substances. The molecule is polarizable because of the π electrons, so
the London forces between SO 2 and solute molecules lead to solubility. In accord with the hard-soft
interaction principle, compounds such as OSCl 2 , OPCl 3 , and PCl 3 are very soluble. Although aliphatic
hydrocarbons are not soluble in liquid SO 2 , aromatic hydrocarbons are appreciably soluble in the liquid.
This difference in solubility provides a way to separate the two types of hydrocarbons by means of
solvent extraction. Ionic compounds are almost insoluble unless they consist of very large ions which
contribute to polarizability and low lattice energies. Physical properties for liquid sulfur dioxide are
shown in Table 10.5 . As will be described later, sulfur dioxide is a useful solvent for carrying out reactions
utilizing superacids such as the HOSO 2 F/SbF 5 mixture.