The ionizing power of solvents

The ability of a solvent to dissolve a substance highly depends on its ability to solvate this substance. If a substance can ionize, both the anion and cation must be solvated. Cations are usually smaller than anions (e.g., Ca(N03)2), and the solvation of cations is of primary importance. Solvation of a simple cation is, in fact, formation of a complex in which the solvent molecules are entering ligands. The positive sides of a dipole solvent molecule will be oriented towards anions and its negative side towards cations. 

Generally speaking, the dielectric constant (relative permittivity, e ) and the ability of a solvent to solvate ions are interrelated the higher the solvent dielectric constant, the higher is its solvation ability. There is unfortunately no quantitative correlation because the relation is very complex and various factors are involved.The following table illustrates this situation. Relative permittivity of 4 solvents and their dipole moments are compared and no correlation is observed. 

Water is often the best solvent because of its high relative permittivity and medium high dipole moment. Water has also an acidic hydrogen and a lone electron pair on oxygen, so it solvates well both acids and bases. On the other hand, NH3 is probably a better electron-pair donor than water, but its hydrogens are less acidic. In general, NH3, as solvent, ionizes the dissolved substance 

DMSO has a rather high relative permittivity, which supports ionization of both anions and cations besides, DMSO is quite a good Lewis base and can solvate cations efficiently. Yet, DMSO has no acidic hydrogen atom. 

The rate of a dissociative type reaction drastically depends on the solvent ionizing power, as shown by the classical kinetic studies of Winstein et Table 1.11.3 contains a series of solvents ordered according to their decreasing ionizing power, as determined by measurements of the rate of ionization of / -methoxyneophyl /j-toluenesulfonate  

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