Solvent scale

Each of the binding forces that hold solvent molecules together plays a role in determining the bulk properties of the solvent. By bulk properties, we are not referring to the microscopic interactions between the individual solvent molecules, but instead to the properties that the solvent displays as a whole. For example, boiling points and melting points, the solubilizing behavior to solutes, surface tension, and refractive index are all bulk solution properties. 

Solvents can be classified as protic or aprotic, and as polar or nonpolar. A protic solvent has a hydrogen atom attached to a heteroatom, such as O, N, or S, and can form hydrogen bonds with a solute molecule as well as with other solvent molecules. An aprotic solvent lacks a hydrogen on a heteroatom, and therefore cannot act as a donor. 

Creating a definition of a polar solvent is a more difficult task. Phenomenologically, a polar solvent can be described as a solvent that can solubilize salts or molecules with large 

The Effectof Solvent on Spectra. I. A New Empirical Measure of Solvent Polarity Z-Values. /. Am. Chem. Soc., 80,3253 (1958). Reichardt, C. (1988). Solvents and Sohent Effects in Organic Chemistry 2nd ed., VCH, Weinheim. Kamlet, M. J., Abboud, J.-L. M., Abraham, M. H., and Taft, R. W. LinearSolvation Energy Relationship. 23. A Comprehensive Collection of Solvatochromic Parameters, TM, K, and 2, and Some Method for Simplifying the Generalized Solvato-chromatic Equation. /. Org. Chem., 48,2877 (1983). 

Througjiout this chapter the e parameter will be used in various equations that describe binding forces (such as Eq. 3.1, below). Mathematically, it is defined as the ratio of the permittivity of the medium (e ) to the permittivity of a vacuum ( o)- Hence, e = s lso- Therefore, it is a dimensionless parameter, which is often referred to as the relative permittivity (also known as the dielectric constant). 

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Attempts to evaluate the effective solute hydrogen bond acidity and basicity40 produce quantitative scales of solute hydrogen-bonding, which may be of interest for application also to biochemical processes41. The indicator pair 4-nitroanilinc/4-nitro-/V,/V-dimethylaminoaniline are used to state the equivalence42 of solute and solvent scales of hydrogen bond basicity. 

The layer thickness or brush height h in a good solvent scales linear with the degree of polymerization N, as well as with the grafting density [Pg.400]

Of the existing solvent scales, the Kamlet and Taft n scale ( D appears appropriate for use with supercritical fluid solvents. The basis of the Kamlet and Taft scale is the effect of solvent polarity and polarizability on the energy of the n - it electronic transition of a solute probe molecule. Solvent molecules in proximity to a solute (the cybotatic region) differentially affect the electronic energy levels of solutes (the solvatochromic effect)... 

Since its introduction the Kamlet-Taft it scale has been applied to a large number of diverse liquid solvents ( 5). Several studies have shown that the it solvent scale is consistent with other popular measures of solvent strength (6J. It has also been shown that there are good correlations between the empirical measurements of the Kamlet and Taft scale and several theoretical models of solvent behavior (J). Use of the solutes chosen by Kamlet and... 

Persson, Sandstrom, and Goggin have proposed an empirical solvent scale, called the Ds scale, ranking the donor strength of 64 EPD solvents towards a soft acceptor... 

Finally, an attempt was made to establish a measure of the electron-donating and electron-accepting power of organic solvents by means of infrared [72, 73] and F1 NMR measurements [73], Further empirical Lewis acid and base parameters will be discussed in Chapters 7.2... 7.5. A thorough and critical compilation of empirical solvent scales, including Lewis acidity and basicity scales, has recently been made for non-FlBD solvents [342],... 

Finally, the Z>s and Z>h solvent scale of Persson et al. should be mentioned [241]. The empirical Z>s scale, ranking the EPD solvent s donor strength towards a soft acceptor, is based on the shift of the Raman IR absorption for the symmetrical stretching vibration of the Elg—Br bond of soft mercury(II) bromide in the solvent of interest relative to that in the gas phase = Av(Hg—Br)/cm = [v(Hg—Br)gas —... 

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... 

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 to this early empirical solvent scale, one of the more recent, introduced by Eliel and Hofer in 1973 [25] and based on the solvent-dependent conformational equilibrium of 2-z-propyl-5-methoxy-l,3-dioxane, should be mentioned cf. Table 4-9 in Section 4.4.3). In general, polar solvents shift this conformational equilibrium towards the more dipolar axial cis isomer. The authors proposed calling the standard molar Gibbs energy changes associated with this equilibrium, he D scale D for... 

Another approach to a new solvent scale was introduced by Gutmann in 1966 [26, 27]. Based on the fact that many chemical reactions are influenced primarily by coordi-... 

Since reaction rates can be strongly affected by solvent polarity cf. Chapter 5), the introduction of solvent scales using suitable solvent-sensitive chemical reactions was obvious [33, 34]. One of the most ambitious attempts to correlate reaction rates with empirical parameters of solvent polarity has been that of Winstein and his co-workers [35-37]. They found that the SnI solvolysis of 2-chloro-2-methylpropane (t-butyl chloride, t-BuCl) is strongly accelerated by polar, especially protic solvents cf. Eq. (5-13) in Section 5.3.1. Grunwald and Winstein [35] defined a solvent ionizing power parameter Y using Eq. (7-13),... 

The second-order rate eonstants of this quaternization reaction have been determined for seventy-eight solvents by Lassau and lungers [49, 50]. A seleetion from this relatively extensive solvent scale is given in Table 7-1. 

Spectroscopic parameters of solvent polarity have been derived from solvent-sensitive standard compounds absorbing radiation in spectral ranges corresponding to UV/Vis, IR, ESR, and NMR spectra cf. Chapter 6) [1-9], The first suggestion that solvato-chromic dyes should be used as indicators of solvent polarity was made by Brooker et al. [54] in 1951, but Kosower [5, 55] in 1958 was the first to set up a comprehensive solvent scale. 

Using the negative solvatochromism of the n ti absorption of iV,iV-(dimethyl)-thiobenzamide 5-oxide, an solvent scale has been proposed by Walter et al. [220]. This scale comprises of 36 solvents and three binary solvent/water mixtures and is thought to be particularly useful for characterizing protic solvents. 

Interesting solvent scales based on NMR measurements have been proposed by Taft et al. [90] and by Gutmann, Mayer et al [91]. A solvent polarity parameter, designated as P, has been defined by Taft et al [90] as the F chemical shift (in ppm) of 4-fluoro-nitrosobenzene in a given solvent, relative to the same quantity in the reference solvent cyclohexane cf. Table 6-6 and the discussion in Section 6.5.1). These parameters define a scale ranging from P = 0.0 in cyclohexane to P = 2.7 in sulfolane, and can easily be measured in a wide variety of solvents. The P values appear to be related to the ability of the solvents to form specific 1 1 complexes with the nitroso group of the standard compound. A compilation of P values can be found in reference [92], In addition, chemical shifts of (trifiuoromethyl)benzene and phenylsulfur pentafiuoride have been used by Taft et al. to study nonspecific dipolar interactions with HBD solvents and utilized to define n values of solvent dipolarity/polarizability for protic solvents [249]. 

A seleetion of SPP values is eolleeted in Table 7-5 for nearly the same set of solvents as given in Table 7-4. Whereas eyelohexane is often used as a nonpolar referenee solvent in other solvent seales, the SPP seale shows a eonsiderable gap between eyelohexane and the gas phase (0.557 units), whieh is nearly as wide as that between eyelohexane and polar hexafiuoro-2-propanol (0.457 units). Therefore, the ehoiee of eyelohexane as a reference solvent to define a solvent scale has been called into question [335]. For a comparison of the SPP scale with Kamlet and Taft s k scale, see reference [338]. 

From the previous Sections, we can conclude that there are many empirical solvent scales, the most comprehensive of which are the solvatochromic ones cf. for example Table 7-3. Unfortunately, too many solvent scales have been proposed during the last decades. Around 35 different solvent scales are known. Only about ten of them have found wider application in the correlation analysis of solvent effects, i.e. Y, Z, t(30), a,... 

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. 

Applying a quantitative structure-property relationship (QSPR) analysis of 45 different empirical solvent scales and 350 solvents, a direct calculation of predicted... 

A more detailed discussion of this Ej correlation and the TLSER correlations of other experimentally derived solvent scales can be found in reference [351]. Obviously,... 

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