The properties of organic liquids that are relevant to their use as solvating solvents

The more widely used solvent parameters that have been proposed for the description of the polarity and the hydrogen bond and electron-pair donation and acceptance properties of more than 180 solvents have been compared and correlated. These properties contribute to the exoergic solute-solvent interactions that are required for the solute to be soluble in the solvent in the first place and to solvent effects on spectra and reactions in the second place Three mutually independent quantities play roles in this respect these are measured by the solvatochromic parameters α (for HBD), β (for HBA), and φ* (for polarity/polarizability) of the solvent. The main endoergic contribution to solute-solvent interactions is the formation of a cavity in the solvent to accommodate the solute. This is a chemical property of the solvent, depending on the association of its molecules in the liquid state. It is measured by the cohesive energy density, δ_H², which is independent of the former three parameters when all the solvents are considered together. This measure of the stiffness or tightness of the solvent is related indirectly to its 'structuredness', which is measured by the entropy deficiency of the liquid solvent relative to the solvent in the ideal gas state, corrected for its compression. This quantity, however, has not been calculated so far for most solvents at room temperature. When only a small set of related solvents is considered, mutual correlations of the solvent parameters may arise, and this must be guarded against when causes for solvent effects are sought, since not all the four parameters need be operative for a given process The mutual orthogonality of the parameters employed must be tested and confirmed. It must also be stressed that certain physical properties of solvents, such as the relative permittivity (dielectric constant, ∈) may be very important where charged solute species are concerned. Low ∈ values lead to solute-solute interactions (ion pairing of unlike charged species) even in dilute solutions, but this effect is outside the scope of this paper. Also, certain chemical properties of solvents, such as hydrophobicity and miscibility or mutual solubility with water are not directly relevant to the solvation ability of solvents, although they play important roles in chromatography or liquid-liquid distribution. The solvating ability of solvents is described by the HBA and HBD abilities, the polarity, and tightness, without having to invoke their behaviour towards water. However, when interaction takes place with a very 'soft' solute, the softness of the solvent should also be taken into account as an additional solvent property.