Other Scales of Solvent Polarity

The scales of solvent polarity discussed above are based on rate processes (6) in which a polar transition state is formed from a covalent initial state. There are also several scales based on an electronic transition in which an electron is transferred from one species to another or from one end of a molecule to another [eqn (7) Reichardt, 1965 Reichardt and Dimroth, 1968 Kosower, 1968]. All these scales represent aspects of the microscopic behav- 

Since the parameters based on electronic transitions represent microscopic properties, these should be appropriate for comparison with rate processes. Unfortunately, experimental difficulties limit their usefulness for our purposes. The Z parameter (Kosower, 1958), based on the solvent dependence of charge transfer transitions in pyridinium iodides (e.g. [50]), is limited to solvents of Y 1 

By constructing betaines (e.g. [51], [52]) in which the solvent-sensitive absorption band is displaced to longer wavelengths, Dimroth et al. (1963) were able to obtain directly a solvent polarity scale (Ej), including more polar solvents. Unfortunately, acidic solvents cannot be studied because the oxygen atom of the indicator is protonated by these solvents. Thus in two of the solvents (formic acid and trifluoroacetic acid), independent measures of solvent polarity, which would have been particularly helpful in analysing the results of rate correlations (e.g. Fig. 12), are not available. 

A summary of the empirical scales of solvent polarity for the pure solvents frequently used for solvolytic studies is shown in Table 5. 

Because of the experimental difficulties discussed above, there is limited opportunity for direct comparison between the different scales. There is a satisfactory qualitative trend in all the scales (including dielectric constant) for water, methanol, ethanol, propan-2-01, and f-butyl alcohol. However, it has been noted that a graph of Z versus Ej for pure alcohols and water is markedly curved (Fig. 16) and that 2,2,3,3-tetrafluoropropan-l-ol does not fit the curve (Dimroth et al., 1963). The results for both Y and Z scales suggest that the ET parameter for water is surprisingly low. Consequently, 

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