Polarity measures, empirical

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

Some empirical polarity / polarizability descriptors which were proposed to measure the ability of the compound to influence a neighbouring charge or dipole by virtue of dielectric interactions are reported below. 

This term is a measure of the exoergic balance (i.e. release of energy) of solute-solvent and solute-solute dipolarity / polarizability interactions. This term, denoted by n, describes the ability of the compound to stabilize a neighbouring charge or dipole by virtue of nonspecific dielectric interactions and is in general given by -> electric polarization descriptors such as -> dipole moment or other empirical - polarity / polarizability descriptors [Abraham et al, 1988]. Other specific polarity parameters empirically derived for linear solvation energy relationships are reported below. 

The empirical treatment of solvent-solute interactions by means of the jt parameters was re-examined 15 years after their introduction16. It was concluded that AAAAdimethyl-4-nitroaniline (7), not used in the original introduction of the jt scale, is superior to N,N-diethyl-4-nitroaniline (5) employed originally, since the former is not appreciably affected by differences in band shapes in different solvents as the latter is, due to vibrational structure. An extensive table of v(7) in 229 solvents and the gas phase was provided (and found well correlated linearly with v(2)). New jt values were obtained for several solvents that differed somewhat from the previously established values4,7. The wavenumbers (in cm-1) of 7 were shown to depend on the polarizability (measured by the refractive index n) and the polarity (measured by the relative permittivity e) of the solvent for 66 non-aromatic solvents as follows (equation 10) ... 

The term solvatochromism was introduced in 1922 by Hantzshlater [45] to explain the influence of the solvent on electronic absorption spectra. In 1951 Brooker et al. [46] suggested that the solvatochromic effect induced by a solvent on certain probes that have a strong absorption in the UV/Vis could be used as a visual indicator of the solvent polarity. One of the main reasons that spectroscopic measurements are such a popular method to obtain empirical polarity parameters is the simplicity of the experiment Electronic absorption spectroscopy is a readily available tool that requires little speciahst skill to operate. Hence, several of these scales have been used with ILs. 

In addition, empirical polarity parameters derived from kinetic measurements have been supplemented by parameters determined from spectral data. 

Remember that Donor Numbers (DN) are used as an empirical semiquantitative measure of the nucleophilic properties of solvents and are particularly used when discussing the influence of solvent polarity in reactions involving cations. They range from dichloromethane (DN —0.0 kcal moT, reference solvent) to HMPA... 

Equation (15) incorporates three empirically measured parameters the exchange current density, io (given here in terms of its value on the electrode, /o,e), and the anodic and cathodic transfer coefficients, a A and ac, respectively. These parameters are obtained from polarization measurements. Often, but not always, a -b c = It should be noted that while the Butler-Volmer equation correlates well many electrode reactions, there are numerous others, particularly when carried in the presence of plating additives, which do not follow it. 

Table 4.12. 7(30), an Empirical Measure of Polarity, Compared with Dielectric Constant... 

In Section 8.4 we will encounter many empirical measures of solvent polarity. These are empirical in the sense that they are model dependent that is, they are defined in terms of a particular standard reaction or process. Thus, these empirical measures play a role in the study of solvent effects exactly analogous to that of the substituent constants in Chapter 7.)... 

Some of these model-dependent quantities were formulated as measures of a particular phenomenon, such as electron-pair donor ability but many of them have been proposed as empirical measures of solvent polarity, with the goal, or hope, that they may embody a useful blend of solvent properties that quantitatively accounts for the solvent effect on reactivity. This section describes many, although not all, of these empirical measures. Reichardt has reviewed this subject. 

Z values are obtained from Eq. (8-76) for solvents having Z in the approximate range 63-86. In more polar solvents the CT band is obscured by the pyridinium ion ring absorption, and in nonpolar solvents l-ethyl-4-carbomethoxy-pyridinium iodide is insoluble. By using the more soluble pyridine-1-oxide as a secondary standard and obtaining an empirical equation between Z and the transition energy for pyridine-1-oxide, it is possible to measure the Z values of nonpolar solvents. The value for water must be estimated indirectly from correlations with other quantities. Table 8-15 gives Z values for numerous solvents. 

Bamford, Jenkins and coworkers131157 concluded that many of the limitations of the Q-e scheme stemmed from its empirical nature and proposed a new scheme containing a radical reactivity term, based on experimentally measured values of the rate constant for abstraction of benzylic hydrogen from toluene (Ay i), a polar term (the Hammett o value) and two constants a and J which are specific for a given monomer or substrate (eq. 57) 146... 

The rate constants in organic reaction in a solvent generally reflect the solvent effect. Various empirical measures of the solvent effect have been proposed and correlated with the reaction rate constant [5]. Of these, some measures have a linear relation to the solubility parameter of the solvent. The logarithms of kj and k2/ki were plotted against the solubility parameter of toluene, NMP and DMSO[6] in Fig. 2. As shown in Fig.2, the plots satisfied the linear relationship. The solvent polarity is increased by the increase of solubility parameter of the solvent. It may be assumed that increase of unstability and solvation of Ci due to the increase of solvent polarity make the dissociation reaction of Ci and the reaction between Ci and COisuch as SNi by solvation[7] easier, respectively, and then, k2/ki and ks increases as increasing the solubility parameter as shown in Fig. 2. 

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 solvatochromic phenolbetaine Reichardt s Dye (RD) allows to calculate a single parameter that indicates the overall polarity of the polymer. It is obtained by dissolving the dye in the polymer and measuring the absorbance maximum. The molar transition energy (Ex(30)) of RD is an empirical parameter to scale solvent polarity and is obtained by calculating18 Et(30) - hcvmaxNA OT 2.859vmaX R >. 

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... 

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