Empirical parameters of solvents

Many approaches have been used to correlate solvent effects. The approach used most often is based on the electrostatic theory, the theoretical development of which has been described in detail by Amis [114]. The reaction rate is correlated with some bulk parameter of the solvent, such as the dielectric constant or its various algebraic functions. The search for empirical parameters of solvent polarity and their applications in multiparameter equations has recently been intensified, and this approach is described in the book by Reich-ardt [115] and more recently in the chapter on medium effects in Connor s text on chemical kinetics [110]. 

FA of data matrices containing 35 physicochemical constants and empirical parameters of solvent polarity (c/ Chapter 7) for 85 solvents has been carried out by Svoboda et al. [140]. An orthogonal set of four parameters was extracted from these data, which could be correlated to solvent polarity as expressed by the Kirkwood function (fir — l)/(2fir + 1), to solvent polarizability as expressed by the refractive index function (rfi — + ), as well as to the solvent Lewis acidity and basicity. Thus,... 

Another problem that has been tackled by multivariate statistical methods is the characterization of the solvation capability of organic solvents based on empirical parameters of solvent polarity (see Chapter 7). Since such empirical parameters of solvent polarity are derived from carefully selected, strongly solvent-dependent reference processes, they are molecular-microscopic parameters. The polarity of solvents thus defined cannot be described by macroscopic, bulk solvent characteristics such as relative permittivities, refractive indices, etc., or functions thereof. For the quantitative correlation of solvent-dependent processes with solvent polarities, a large variety of empirical parameters of solvent polarity have been introduced (see Chapter 7). While some solvent polarity parameters are defined to describe an individual, more specific solute/solvent interaetion, others do not separate specific solute/solvent interactions and are referred to as general solvent polarity scales. Consequently, single- and multi-parameter correlation equations have been developed for the description of all kinds of solvent effects, and the question arises as to how many empirical parameters are really necessary for the correlation analysis of solvent-dependent processes such as chemical equilibria, reaction rates, or absorption spectra. 

A quantitative description of the influence of the solvent on the position of chemical equilibria by means of physical or empirical parameters of solvent polarity is only possible in favourable and simple cases due to the complexity of intermolecular solute/solvent interactions. However, much progress has recently been made in theoretical calculations of solvation enthalpies of solutes that can participate as reaction partners in chemical equilibria see the end of Section 2.3 and references [355-364] to Chapter 2. If the solvation enthalpies of all participants in a chemical equilibrium reaction carried out in solvents of different polarity are known, then the solvent influence on this equilibrium can be quantifled. A compilation of about a hundred examples of the application of continuum solvation models to acid/base, tautomeric, conformational, and other equilibria can be found in reference [231]. 

A linear correlation has been found between the solvent-dependent AG° values and the empirical parameter of solvent polarity, t(30) (see Section 7.4). Thus, the host/guest binding strength increases steadily on going from nonpolar solvents to water, thus shifting the complexation equilibrium more and more to the right-hand side with increasing solvent polarity. 

In conclusion, it can be said that the electrostatic theory of solvent effects is a most useful tool for explaining and predicting many reaction patterns in solution. However, in spite of some improvements, it still does not take into account a whole series of other solute/solvent interactions such as the mutual polarization of ions or dipoles, the specific solvation etc., and the fact that the microscopic relative permittivity around the reactants may be different to the macroscopic relative permittivity of the bulk solvent. The deviations between observations and theory, and the fact that the relative permittivity cannot be considered as the only parameter responsible for the changes in reaction rates in solution, has led to the creation of different semiempirical correlation equations, which correlate the kinetic parameters to empirical parameters of solvent polarity (see Chapter 7). 

Empirical Parameters of Solvent Polarity from Equilibrium Measurements... 

Since reaction rates can be strongly affected by solvent polarity cf. Chapter 5), the introduction of solvent scales using suitable solvent-sensitive chemical reactions was obvious [33, 34]. One of the most ambitious attempts to correlate reaction rates with empirical parameters of solvent polarity has been that of Winstein and his co-workers [35-37]. They found that the SnI solvolysis of 2-chloro-2-methylpropane (t-butyl chloride, t-BuCl) is strongly accelerated by polar, especially protic solvents cf. Eq. (5-13) in Section 5.3.1. Grunwald and Winstein [35] defined a solvent ionizing power parameter Y using Eq. (7-13),... 

Table 7-3. Empirical parameters of solvent polarity, t(30) [cf. Eq. (7-27)] and normalized Ej values [cf. Eq. (7-29)], derived from the long-wavelength UV/Vis charge-transfer absorption band of the negatively solvatochromic pyridinium IV-phenolate betaine dyes (44) and (45), measured at 25 °C and 1013 hPa, for a selection of 288 solvents, taken from reference [293]. ...

An analysis of the t(30) values, using multivariate statistical methods, has been carried out by Chastrette et al. [193]. According to this analysis, the x(30) values of non-HBD solvents are measures of the dipolarity and polarizability as well as the cohesion of the solvents. Another analysis of x(30) values in terms of functions of the dielectric constant sf and refractive index ( d) of forty non-HBD solvents has been given by Bekarek et al he emphasizes the predominant influence of the f(fir) term on the iix(30) parameter of those solvents [194]. For further correlations of the x(30) values with other empirical parameters of solvent polarity, see Section 7.6. 

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