Solvent polarity scale

One of the earliest solvent polarity scales is Person s D scale. This scale is based on the endojexo ratio of the Diels-Alder reaction between cyclopentadiene and methyl acrylate (Figure 3.5-2, O = logio endo/exo). This reaction has been conducted in a number of ionic liquids, giving values in the 0.46-0.83 range [26]. 

Correlations among solvent polarity scales, dielectric constant and dipole moment, and a means to reliable predictions of polarity scale values from current data. T. R. Griffiths and D. C. Pugh. Coord. Chem. Rev., 1979, 29,129-211 (130). 

Buncel E. and Rajagopal S. (1990) Solvato-chromism and Solvent Polarity Scales, Acc. Chem. Res. 23, 226—231. 

The original solvent polarity scale, known as the E (30) scale, was defined simply as the energy of the longest wavelength adsorption band for the dye, measured in kcalmol-1. This scale has now been revised and normalized because of the introduction of SI units, and Ej is defined in Equation 1.4. 

A complicated problem is that of solvent substitution for organic reaction optimization. The solvent is important in organic synthesis, and several solvent polarity scales have been developed to attempt to quantify this, specifically work by Reichardt (Rei-chardt, 1988). 

For reviews of solvent polarity scales, see Abraham Grellier Abboud Doherty Taft Can. J. Chem. 1988,66. 2673-2686 Kamlet Abboud Taft Prog. Phys. Org. Chem. 1981,13,485-630 Shorter Correlation Analysis of Organic Reactivity Wiley New York, 1982, pp. 127-172 Reichardt, Ref. 386 Reichardt Dimroth, Ref. 386 Abraham Prog. Phys. Org. Chem. 1974, II, I -87 Koppel Palm, in Chapman Shorter Advances in Linear Free Energy Relationships, Plenum New York, 1972, pp. 203 280 Ref. 384. See also Chastrette Carrclto Tetrahedron 1962,38, 1615 Chastrette Rajzmann Chanon Purcell J. Am. Chem. Soc. 1965,107, 1. 

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

By constructing betaines (e.g. [51], [52]) in which the solvent-sensitive absorption band is displaced to longer wavelengths, Dimroth et al. (1963) were able to obtain directly a solvent polarity scale (Ej), including more polar solvents. Unfortunately, acidic solvents cannot be studied because the oxygen atom of the indicator is protonated by these solvents. Thus in two of the solvents (formic acid and trifluoroacetic acid), independent measures of solvent polarity, which would have been particularly helpful in analysing the results of rate correlations (e.g. Fig. 12), are not available. 

Solvent polarity parameters — use solvatochromic dyes (dyes whose electronic transitions are strongly dependent on the nature of the solvent) as indicators of solvent polarity. A comprehensive solvent polarity scale was first proposed by Kosower who defined the polarity parameter, Z, as the molar transition energy, Ej, for the charge transfer band of 1-ethyl-(methoxycarbonyl)pyridynium iodide in a given solvent as... 

Further well-known molecular liquids that have recently found renewed interest as solvents for many applications are water and perfluorohydrocarbons. These occupy the two extreme positions on empirical solvent polarity scales, with water being the most polar solvent and perfluorohydrocarbons being among the least polar of solvents cf. Chapter 7). 

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. 

By a quantitative structure-property relationship (QSPR) analysis of a total of 45 different empirical solvent scales and 350 solvents, the direct calculation of predicted values of solvent parameters for any scale and for any previously unmeasured solvent was possible using the CODESS A program [ie. comprehensive descriptors for structural and statistical analysis) developed by Katritzky et al. [244]. The QSPR models for each of the solvent scales were constructed using only theoretical descriptors, derived solely from the molecular solvent structure. This QSPR study enabled classification of the various solvent polarity scales and ultimately allowed a unified PCA treatment of these scales. This PCA treatment, carried out with 40 solvent scales as variables (each having 40 data points for 40 solvents as objects), allowed a rational classification and grouping... 

Consideration of the dipolarity of the two activated complexes can explain the observed trend. If the reactants are pictured as lying in roughly parallel planes, the dipole moments for the exo orientation are seen to be nearly opposite in direction, whereas for the endo orientation they are parallel. Therefore, the net dipole moment for the endo transition state is greater than that for the exo. Thus, the solvation of the endo activated complex will be more pronounced as the polarity of the solvent increases. This leads to a lowering of the activation enthalpy and preferential formation of the endo adduct. The logarithm of the endojexo product ratio in various solvents has been used to define an empirical solvent polarity scale [124] [cf. Section 7.3). Analogous solvent-dependent endolexo product ratios have been obtained in [4 -1- 2]cycloadditions of cyclopentadiene to other acrylic acid derivatives [560]. Theoretical calculations on exoj endo structures for activated complexes of [4 + 2]cycloadditions have shown that the observed endo preference in polar solvents is due to the influence of the medium, and that secondary orbital interactions are not involved [808]. The solvent has the decisive influence on the exo/endo selectivity. 

The visible n n absorption band is shifted hypsochromically by A1 = 53 nm on changing the medium from n-hexane to water. Based on the negative solvatochrom-ism of this aminyloxide radical, a spectroscopic solvent polarity scale, called the scale of Eewis acidity has been proposed [336]. Because of its pronounced negative sol-vatochromism, 4-nitropyridine-l-oxide has been recommended as an empirical indicator of the HBD acidity of solvents [330] cf. Section 7.4. The solvatochromic range of this A-oxide, measured in 48 different solvents, amounts to Av = +2840 cm (A1 = —31 nm) for the solvent change n-hexane water. 

Z values cover a range from 94.6 (water) to about 60 kcal/mol (z-octane) and were originally measured for 21 pure solvents and 35 binary solvent mixtures [5, 56], as well as some electrolytes [57] and surfactant solutions [58]. Various authors have since gradually extended this to include 45 pure solvents. Z values for a further 41 pure solvents have been determined by Griffiths and Pugh [172], who also compiled all available Z values and their relationships with other solvent polarity scales. A selection of Z values together with some other spectroscopic solvent polarity parameters is given in Table 7-2. 

Further solvent polarity scales based on UV/Vis absorption as well as fluorescence spectra have been proposed by Brooker et al. [77], Dahne et al. [78], de Mayo et al. [217], Dubois et al. [79], Mukerjee et al. [218] and Wrona et al. [219], Walter et al. [220], Walther [81] and Lees et al. [82], Zelinskii et al. [80], Winnik et al. [222], Kamlet and Taft [84, 84a, 224, 226]. Buncel et al. [333], and Catalan et al. [296, 334-337]. In addition to these scales, a great variety of further positively and negatively solvato-chromic dyes have been recommended as solvent polarity indicators. A review describes about 60 organic and inorganic compounds, the solvatochromism of which is sufficiently large for their potential application as empirical solvent polarity probes [293]. 

Mukerjee et al. [218] and Wrona et al. [219] have used the highly solvatochromic 71 transition energy of the stable 2,2,6,6-tetramethylpiperidine-l-oxide radical (TMPNO) for the development of a solvent polarity scale. So-called values, as empirical measures of solvent Lewis acidity, have been determined for 53 pure organic solvents and some binary solvent/water mixtures [219]. 

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]. 

At present, the situation is not quite as bad as in the correlation analysis of substituent effects, where even more substituent parameters than common substituents seem to be known. It has been suggested that new solvent polarity scales should only be introduced into the literature if they exhibit significant advantages over existing solvent scales [235]. 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

Of particular interest is the analysis of other, experimentally determined empirical solvent polarity scales by means of the theoretically derived TLSER equation (7-67). For example, correlation of the solvatochromic Ej scale [see Eq. (7-29) in Section 7.4] with the TLSER descriptors of Eq. (7-67) leads to Eq. (7-68) for n = 29 solvents, with r = 0.937 and S = 0.075 [351] ... 

Evidence has been provided that adsorption of several (t] -arene)tricarbonylchromium(O) complexes on to silica gel can perturb the electronic structure of these complexes and the quantum efficiencies with which they undergo photodecomposition. The steric constraint imposed by the support also seems to influence the photoreactivity of these complexes. M(CO)4(bipym) (where M = Cr or W, bipym = 2,2 -bipyrimidyl) and (W(CO)4]2(bipym) both have a lowest MLCT state, and on irradiation CO is lost. Wavelength-dependent quantum yields have been obtained for this transformation and at 366 nm = 2.4x 10 quanta/min for Cr-bipym, 2.5 x 10 quanta/min for W-bipym, and 1.1 x 10 quanta/min for W-bipym-W." Correlations have been described between the MLCT absorption energies of a series of M(CO)4 diimine complexes and Reichardt s solvent parameters (Et), and Kamlet s and Taft s solvent polarity scale Paramagnetic complexes... 

Several solvent polarity scales were proposed to quantify the polar effects of solvents on physical properties and reactivity parameters in solution, such as rate of sol-volyses, energy of electronic transitions, and solvent-induced shifts in IR or NMR... 

Y polarity scale. A solvent polarity scale proposed by Grunwald and Winstein [Grunwald and Winstein, 1948] based on solvolytic rate ko of r-butyl chloride in 80 % aqueous ethanol at 25 °C. The Y polarity value for a given solvent is calculated by ... 

Et polarity scale. A solvent polarity scale proposed by Dimroth, Reichardt and coworkers [Dimroth et al., 1963 Reichardt, 1965] based on the solvatochromic band shifts of the 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide and its trimethyl derivative. This scale is one of the most comprehensive for the number of considered solvents and is widely used. 

A wide variety of correlations among solvent polarity scales were studied [Reichardt and Dimroth, 1968] however, because of the different reference compounds used to define them, direct comparison should be done with caution [Bentley and von Schleyer, 1977]. 

Katritzky, A.R., Mu, L. and Karelson, M. (1997b). QSPR Treatment of the Unified Nonspecific Solvent Polarity Scale. J.Chem.Inf.Comput.ScL, 37,756-761. 

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