Solvents polarity parameters

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

There have been several other attempts to define solvent polarity parameters, among the more successful being those related to solvatochromic shifts the shift in wave-length/frequency of a band in the spectrum of a suitable absorbing species resulting from its interaction with the molecules of a series of different solvents. Particularly large shifts were observed with the zwitterion (51),... 

Notes [1] Empirical solvent polarity parameter (see Reference 32)... 

In the first DNMR studies of push-pull ethylenes, a strong effect of solvent polarity on the C=C barriers was noted. Thus Kende et al. (64) found AG = 18.0 kcal/mol for 46a in N,/V-dimethylformamide (dielectric constant e = 38) and 19.4 kcal/mol in Ph20 ( = 4). Similar observations have been made by many other workers, and they have been seen as a strong support for a zwitterionic transition state. Kessler et al. (140) observed reasonably linear correlations between AG for two ketene aminals and the solvent polarity parameter T (141) with variations in AG of ca. 2.5 kcal/mol over T values between 25 and 46. Similarly, Shvo et al. (78) found linear correlations between log km and the polarity parameter Z (141) for three compounds from Table 12. 

Table 2.5 Solvent polarity parameters Y from rates of solyvolysis of (CH3)3CCL (Reichardt 1988)...

The correlation between the Dimroth-Reichardt ET(30) and the Kosower Z solvent polarity parameters, Eq. (4.8), both in kcal mol 1... 

Oxygenation of 2-substituted adamantanes with methyl(trifluoromethyl)dioxirane showed a reaction constant, p = -2.31, consistent with a strongly electron-demanding transition state. Analysis of the effect of solvents on the rate yielded a positive regression coefficient with Dimroth-Reichardt Ej solvent polarity parameter. A mechanism involving an electrophilic O atom insertion has been postulated for the formation of alcohols and carbonyl compounds.201... 

The eluting strength of the solvent is inversely related to its polarity, as indicated in Table 2.3 where P is the solvent polarity parameter. A solvent with a low P is chosen initially, and quantities of a second solvent with a greater P are added until the desired separation is achieved. More details on the development of mobile phases can be found elsewhere.1,6,7... 

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... 

Then, Dimroth and Reichardt proposed a solvent polarity parameter, Ey(30), based on transition energy for the longest-wavelength solvatochromic absorption band of the pyridynium N-phenolate betaine dye, which is dye No. 30 in a table constructed by these authors. The x(30) values have been determined for more than 360 pure organic solvents and many binary solvent mixtures. 

The photophysical properties of a xanthene dye have been extensively studied. In order to explain a relationship between the non-radiative decay rate constant (k r) of the dye and a solvent polarity parameter ( t(30)), Quitevis et al. proposed a two-state model [36,37]. According to the model, kot is given by... 

Figure 5 Rate constants for the reaction of An2CH + BCU with 2-methyl-l-pen-tene as a function of the solvent polarity parameter EA30). (From Ref. 58.) log k = 0.0995 Et(30) - 4.175 (r = 0.986) ki values ( — 30° C) in parentheses.

The aforementioned macroscopic physical constants of solvents have usually been determined experimentally. However, various attempts have been made to calculate bulk properties of Hquids from pure theory. By means of quantum chemical methods, it is possible to calculate some thermodynamic properties e.g. molar heat capacities and viscosities) of simple molecular Hquids without specific solvent/solvent interactions [207]. A quantitative structure-property relationship treatment of normal boiling points, using the so-called CODESS A technique i.e. comprehensive descriptors for structural and statistical analysis), leads to a four-parameter equation with physically significant molecular descriptors, allowing rather accurate predictions of the normal boiling points of structurally diverse organic liquids [208]. Based solely on the molecular structure of solvent molecules, a non-empirical solvent polarity index, called the first-order valence molecular connectivity index, has been proposed [137]. These purely calculated solvent polarity parameters correlate fairly well with some corresponding physical properties of the solvents [137]. 

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. 

Because of the complicated interactions between solvents and solutes, the prediction of solvent effects on reaction rates, and the correlation of these effects with intrinsic solvent properties, is very difficult. Nevertheless, many authors have tried to establish -empirieally or theoretically - correlations between rate constants or Gibbs energies of aetivation and characteristic solvent parameters such as relative permittivity, r, dipole moment, fi, refractive index, n, solubility parameter, 5, empirical solvent polarity parameters, etc., as schematically shown by Eq. (5-9). 

As an example of the use of SC-CO2 in an enzymatic reaction, the lipase-catalyzed esterification of oleic acid with racemic ( )-citronellol should be mentioned. At 31 °C and 8.4 MPa, the (—)-(5)-ester is formed enantioselectively in SC-CO2 with an optical purity of nearly 100% [924]. The reaction rate is enhanced by increasing pressure, i.e. by increasing the solvation capability or solvent polarity of SC-CO2. A linear correlation has been found between reaction rate and the solvatoehromie solvent polarity parameter 1(30) see Section 7.4 for the definition of t(30). 

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

Similar solvent effects on HFS constants have been observed for other aminyl-oxides such as diphenyl aminyloxide [207, 212], di-r-butyl aminyloxide [218, 385], r-butyl aminyloxide [213, 217], and 2,2,6,6-tetramethylpiperidyl-l-oxide [384, 387, 388]. The constants of di-t-butyl and two other aminyloxides have been proposed as an empirical solvent polarity parameter because a( " N) is easily measured in most solvents [218, 389] cf. Section 7.4. 

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. 

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. 

The solvent dependence of the n n transition energies of two meropoly-methine dyes was used by Brooker et al. [77] to establish the solvent polarity parameters /r and Xb ( / Table 7-2). is based on the positively solvatochromic merocyanine dye no. 1 in Table 6-1 of Section 6.2.1 (red shift with increasing solvent polarity), while Xb represents the transition energies of the negatively solvatochromic merocyanine dye no. 13 in Table 6-1. 

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