Solvent properties and parameters

The macroscopic dielectric constant of a solvent (Sr) is the most popular parameter to characterize the polarity of the medium and controls the ionic dissociation of salts. In better terminology, Sy is the relative permittivity and amounts to siso, with So being the permittivity of the vacuum. Values of Sj range from about 2 (apolar, hydrocarbons) to high values such as 180 for secondary amides. Electrostatic interactions (ion-ion, ion-dipole. 

It is noted that the polarity of water falls off at high temperatures and pressures as anticipated since the H-bond network breaks down. For example, synthesis of benzimidazoles at 358°C and 20.0 MPa was found to be successful, - at least in part due to the improved solubility of the organic reactants. 

Solvent polarity is one of the most common solvent characteristics that have been used for correlations of rate constants with the nature of the reaction medium. Since relative permittivities and dipole moments did not give satisfactory results, there was a strong need for a microscopic parameter and many attempts have been made to develop empirical solvent parameters that are based on a physical parameter which is sensitive to the solvent polarity. Solvatochromic dyes have been the most successful and particularly Reichardt s t(30) solvent polarity parameter. The latter is based on the transition energy of the longest wavelength solvatochromic absorption band of the betaine dye pyridinium-N-phenoxide (in fact, this is dye number 30 in the first publication ). Its value is given by 

The solubility of the dye in water is limited, but the t(30) value (63.1) can be determined and again demonstrates the high polarity of water. In Table 2.1 the normalized values are listed  

Herein water (E = 1) and tetramethylsilane (TMS E = 0) are employed as reference solvents. 

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Organic liquids only occupy a certain percentage of space. The volume of filled space by a solvent is defined as its packing coefficient (PC), and is another bulk solvent property and parameter. It is a ratio of the sum of the van der Waals volumes for a solvent (Ryv) to the given volume of space (k). 

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