Universal polarity scale

The large number of empirical polarity scales derived from solvatochromic sensors raises the question of the degree of correlation among them. In principle, there is no reason to expect a correlation between any pair of these empirical scales. The solvatochromic behavior of a given compound reflects a sum of specific and nonspecific solute-solvent interactions that vary from probe to probe. Good correlations are to be expected only between scales based on solvatochromic compounds that present a similar response to a range of solvents. As a result of this, the concept of solvent polarity is elusive and the claims of a universal polarity scale based on solvatochromic probes cannot be maintained. Different solvents may assume different polarity values, according to the nature of the scale employed to define them. 

Drago developed a universal polarity scale (the S scale) from more than three hundred spectral data (electronic transitions, F and N chemical shifts and RSE coupling constants) for 30 solutes in 31 non-protic solvents from cyclohexane to propylene carbonate. He used the equation... 

As already established for combinations of cations with n-nucleo-philes [33,160,219], the situation is less complicated for the reactions of carbocations with 7r-systems. Solvent polarity plays only a minor role (Section III.D.3) and, for many 7r-systems, the relative reactivity has been found to be electrophile-independent (Fig. 10, Section III.D.4.b). Also for these systems, the construction of a universal nucleophilicity scale is not unproblematic, however. Remember Fig. 11, which shows that An2CH + reacts 3.4 times faster with allyltrimethylsilane than with 2-methyl-2-bu-... 

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]. 

Whrai a radical ar atom attacks a polar O—or N—bond, the reactant Y forms a hydrogen bond of the type O—H...Y or N— H...Y in polar solvents. The hydrogen bond shields the reactant and slows down the reactimi regardless of vdiat kind of radical, polar or nonpolar, attacks it A universal kinetic scale was pressed for the estimation of the effectiveness of such shielding. 

It has been very fortunate that the S scale and the universal primary reference, tetram-ethylsilane (TMS), were widely accepted rather early in 29Si NMR history. (For other suggested references, their relative merits and conversion factors, see earlier reviews9.) Not only does TMS have a relatively short relaxation time 7 3 14, but it also has 12 protons that can be utilized for polarization transfer. Thus TMS can be used as a reference at low... 

At best, this approach provides a quantitative index to solvent polarity, from which absolute or relative values of rate or equilibrium constants for many reactions, as well as absorption maxima in various solvents, can be derived. Since they reflect the complete picture of all the intermolecular forces acting in solution, these empirical parameters constitute a more comprehensive measure of the polarity of a solvent than any other single physical constant. In applying these solvent polarity parameters, however, it is tacitly assumed that the contribution of intermolecular forces in the interaction between the solvent and the standard substrate is the same as in the interaction between the solvent and the substrate of interest. This is obviously true only for closely related solvent-sensitive processes. Therefore, an empirical solvent scale based on a particular reference process is not expected to be universal and useful for all kinds of reactions and absorptions. Any comparison of the effect of solvent on a process of interest with a solvent polarity parameter is, in fact, a comparison with a reference process. 

In contrast, newer techniques such as FID and evaporative light-scattering detection (ELSD, Universal Detector ) are more sensitive and have been used for the analysis of many lipids, including the polar and neutral lipid classes [6]. For preparative scale separations, if the UV absorbance is more than what the detector can handle, an evaporative light-scattering detector is used [B2]. 

Figure 4 Giddings elutrophic series originally presented in a Science article in 1968 is replotted on the left, while a solvent strength scale based on the solvatochromic dye, Nile Red, is presented on the right. Note that pure carbon dioxide is much less polar on the Nile Red scale than in Giddings series. Significant concentrations of polar modifier are required (like 15% methanol) in the Nile Red scale to equal the estimated value in Giddings series. It is now nearly universally accepted that the Giddings estimate was much too optimistic.

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