Polarity scales, of solvents

Recent attempts to unify the polarity scales of solvents (for non-specific interactions) are of great interest in rationalizing the medium effects166. Generally, the spectroscopic properties of appropriate substances are used to check the solvating ability of solvents. 4-Nitroaniline is a useful indicator for estimating solvent polarity because it is an electron acceptor molecule which presents incomplete complexation with weak donor solvents167. 

The P scale of solvent polarity is based on a combination of gas-liquid partition coefficients reported by Rohrschneider [43]. 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

The .p(30) scale of solvent polarity is sensitive to both solvent dipolarity and polarizability, as well as the solvent hydrogen-bond donor ability. 

Although c is an e irical paraseter, it can be correlated to sore general solvent strength scales, such as the Kavlet-Taft constants [251] and Reichardt s E,(30) scale of solvent polarity [252]. 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

Kamlet MJ, Abboud J-L, Taft R (1977) The solvatochromic comparison method. 6. The it scale of solvent polarities. JACS 99 6027-6038... 

Empirical scales of solvent polarity based on solvatochromic shifts... 

Quantitative determination of solvent polarity is difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property, due to the complexity of solute-solvent interactions, and for this reason empirical scales of solvent polarity based on chemical... 

Figure 1.12 Reichardt s betaine dye in its zwitterionic ground state (a) and first excited state (b). The ground state has a larger dipole moment (15 D) than the excited state (6 D). Measurement of the energy of the transition between these two states (n - n ) is the basis for the Ej scale 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-... 

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

Figure 16. Comparison of Z and Ej scales of solvent polarity (data from Dimroth et aL, 1963).

Because of the different effects of electrophilic solvation of the various negative charges (i.e. Cl- for Y, "OTs for Tots I" for Z, 0 for Ej), direct comparisons between the various scales should be done cautiously. A wide variety of correlations giving clear indications of trends, has been reported by Reichardt and Dimroth (1968), but the significance of a recent general survey of scales of solvent polarity is doubtful, because of the many parameters used in the correlations (Fowler et al., 1971). The multi-parameter approach has also been adopted and reviewed by Koppel and Palm (1972). 

Inevitably this specific electrophilic solvation must be included in the Y, Z and T scales of solvent ionizing power. Ideally some correction for the different extents of solvation of the anions Cl-, OTs-, I-) should be made, and then the various scales of solvent polarity may be more comparable. 

Since then, solvent-dependent absorptions of a great variety of compounds have been studied as potential reference processes for establishing empirical scales of solvent polarity. Most approaches include negatively or positively solvatochromic dyes because they are experimentally easy to handle. The use of solvatochromic dyes as solvent polarity indicators has been reviewed [1, 2, 293-296, 316], see, in particular, reference [293] only a few of these dyes can be mentioned in this Section. 

The solvent, temperature, pressure, ionophore, and substituent effects on the UV/ Vis spectra of the pyridinium A -phenolate betaine dyes indicate the extreme sensitivity of this class of compounds to small changes in the environment. Their behaviour may be eompared to that of the Princess and the Pea in one of H. C. Andersen s fairy-tales [76] . Their utility for setting up linear Gibbs energy relationships is demonstrated by the fact that the same betaine dye can be used for establishing kinetic and spectroscopic scales of substituents cf. Figs. 7-1 and 7-2) as well as a spectroscopic scale of solvent polarity cf. Table 7-3). 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

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. 

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