Solvent dipolarity/polarizability

Another different 7t -scale which indicates solvent dipolarity/polarizability and which is a measure of the ability of the solvent to stabilize a charge or a dipole by virtue of the dielectric effect, has been proposed for numerous Lewis bases including sulphoxides89. 

Taft, Table 4.23 [507,508,514,515]. The T value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide - 1, which measures the ability of a solvent to... 

The same authors also introduced a r scale of solvent dipolarity/polarizability [84a]. This n scale is so named because it is derived from solvent effects on the n n electronic transitions of a selection of seven positively solvatochromic nitroaromatics of the type D-C6H4-A, where D and A stand for electron-donor e.g. NMe2) and electron-acceptor e.g. NO2) groups, respectively 4-nitroanisole, A,A-diethyl-3-nitroaniline, 4-methoxy-/ -nitrostyrene, 1-ethyl-4-nitrobenzene, A-methyl-2-nitro-p-toluidine, N,N-diethyl-4-nitroaniline, and 4-(dimethylamino)benzophenone. Given a solvatochromic indicator compound, the n value for a solvent S was defined according to Eq. (7-32) ... 

Interesting solvent scales based on NMR measurements have been proposed by Taft et al. [90] and by Gutmann, Mayer et al [91]. A solvent polarity parameter, designated as P, has been defined by Taft et al [90] as the F chemical shift (in ppm) of 4-fluoro-nitrosobenzene in a given solvent, relative to the same quantity in the reference solvent cyclohexane cf. Table 6-6 and the discussion in Section 6.5.1). These parameters define a scale ranging from P = 0.0 in cyclohexane to P = 2.7 in sulfolane, and can easily be measured in a wide variety of solvents. The P values appear to be related to the ability of the solvents to form specific 1 1 complexes with the nitroso group of the standard compound. A compilation of P values can be found in reference [92], In addition, chemical shifts of (trifiuoromethyl)benzene and phenylsulfur pentafiuoride have been used by Taft et al. to study nonspecific dipolar interactions with HBD solvents and utilized to define n values of solvent dipolarity/polarizability for protic solvents [249]. 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

The situation regarding the analysis of tert-butyl chloride solvolyses in terms of a LFER is clearly confused, a general acceptance of the beliefs that, at most, only a minor contribution arises from the term governed by solvent nucleophilicity and that the dominant contribution is from a combination of solvent dipolarity-polarizability, and that electrophilic solvation exists. 

The solvatochromic parameters are derived from spectroscopic and other measurements specifically designed to measure only a single interaction. In addition, the values are averages of the results from several solutes for each parameter and somewhat independent of solute identity. The most comprehensive solvatochromic treatment of solvent selectivity are the Tti, ai and Pi parameters of Kamlet and Taft, Table 4.15 [568-570, 578]. The rti value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide = 1. The i scale of hydrogen-bond acidity measures the... 

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