Solvent polarity linear solvation energy

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

A linear solvation energy relationship (LSER) study of tautomerism in aromatic Schiff bases and related azo compounds indicates that the aminoenone tautomer is always the more polar, and is specifically favoured by proton donor solvents (binding to the second lone pair of the carbonyl). Effects of aromatization and benzo fusion are also discussed.26... 

The Kamlet-Taft u polarity/polarizability scale is based on a linear solvation energy relationship between the n it transition energy of the solute and the solvent polarity ( 1). The Onsager reaction field theory (11) is applicable to this type of relationship for nonpolar solvents, and successful correlations have previously been demonstrated using conventional liquid solvents ( 7 ). The Onsager theory attempts to describe the interactions between a polar solute molecule and the polarizable solvent in the cybotatic region. The theory predicts that the stabilization of the solute should be proportional to the polarizability of the solvent, which can be estimated from the index of refraction. Since carbon dioxide is a nonpolar fluid it would be expected that a linear relationship... 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

Electric polarization, dipole moments and other related physical quantities, such as multipole moments and polarizabilities, constitute another group of both local and molecular descriptors, which can be defined either in terms of classical physics or quantum mechanics. They encode information about the charge distribution in molecules [Bbttcher et al, 1973]. They are particularly important in modelling solvation properties of compounds which depend on solute/solvent interactions and in fact are frequently used to represent the -> dipolarity/polarizability term in - linear solvation energy relationships. Moreover, they can be used to model the polar interactions which contribute to the determination of the -> lipophilicity of compounds. 

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. 

Kamlet, M.J. and Taft, R.W. (1979b). Linear Solvation Energy Relationships. Part 1. Solvent Polarity-Polarizability Effects on Infrared Spectra. J.Chem.Soc.Perkin Trans.2,337-341. 

The rates of the 5 2 reaction between phenacyl bromide and 2-mercaptobenzothia-zole in 17 protic and aprotic solvents have been measured. The effect on the rates is assessed in terms of the electrophilicity, the hydrogen-bond donor ability, the specific polarizability, and a non-specific polarity of the solvent. The relative infiuence of each factor is given and a linear solvation energy equation is proposed. 

With this in mind, the Welton group reasoned that the same approach could be taken to the study of the effects of ionic liquids on the rates of reactions. They used the Kamlet-Taft polarity scales to develop Linear Solvation Energy Relationships (LSERs) to describe the effects of solvents on the reaction kinetics of various 8 2 nucleophilic substitution reactions f Schemes 10.1-10.4). 8 2 reactions occur in a concerted step in which the nucleophile replaces the nucleofuge or leaving group as it dissociates from the subsmate. Their reaction profiles have the form of that in Figure 10.1. 

The three scales devised by Kamlet, Abboud and Taft have been used many times to formulate relationships between reaction rate constants and solvent polarity. These are known as linear solvation energy relationships (LSERs). The rate of amide formation for example, the most common single reaction in medicinal chemistry, is inversely proportional to jS for entropic reasons (Figure 3.4). Limonene and its derivative p-cymene were thus justified as excellent options for a renewable amidation solvent, not only in terms of performance but also because they are produced from a renewable feedstock. Other solvents are less suitable according to their solvatochromic polarity parameters (Table 3.3). As hydrocarbons, some aquatic toxicity concerns surround the use of limonene and p-cymene, but ideally these would be minimised with recycling. 

MJ Kamlet, TN Hall, J Boykin, RW Taft. Linear solvation energy relationships. 6. Additions to and correlations with the Pi-Star scale of solvent polarities. J Org Chem 44 2599, 1979. 

It was claimed [126] that infrared scales could provide the basicity descriptor for multiparameter linear solvation energy relationships, which relate the variation of a solvent-dependent property to the polarizability, polarity, acidity and basicity of the solvent. This implies that infrared scales are related to thermodynamic scales. The Gibbs energy and enthalpy of the reaction 4.28 have been measured for 41 (42 for AG) diverse Lewis bases B [96] in CCI4 at 298 K. 

Grate JW, MegiU P-A, Hilvert D. Analysis of solvent effects on the decarboxylation of benzisoxazole-3-carboxylate ions using linear solvation energy relationships - relevance to catalysis in an antibody-binding site. J zlm Chem Soc. 1993 115 8577—8584. Kamlet MJ, Abboud JL, Taft RW. The solvatochromic comparison method. 6. The.pi. scale of solvent polarities. J Am Chem Soc. 1977 99 6027-6038. 

Polar solvents have no effect on the rate constant of the reaction R02 + RH [56], This means that the solvation energies of the peroxyl radical R02 and TS R02 HR are very close. A different situation was observed for the reaction of cumylperoxyl radical with benzyl alcohol (see Table 7.10). The rate constant of this reaction is twice in polar dimethylsulfoxide (s = 33.6) than that in cumene (a 2.25). It was observed that the very important property of the solvent is basicity (B), that is, affinity to proton. A linear correlation... 

The basic requirement for the development of a more generally applicable solvent concept is the need to try to separate the various factors responsible for the solvating power of a solvent. It is important to find criteria for the solvents character that can be correlated not only to salt solubility and apparent conductivity but also to the impact of the solvents on the thermodynamics and kinetics of the electrochemical reactions. There are several approaches to defining a typical solvent property that can represent its polarity and be correlated to the thermodynamics and kinetics of reactions conducted in its solutions (i.e., a linear free-energy relationship). A comprehensive review of such approaches by Reichardt [12] divides them into three categories ... 

It would appear from these observations that the solvation capability might be better characterized using a linear Gibbs energy relationship approach than functions of relative permittivity. There are now numerous examples known, for which the correlation between the rates of different reactions and the solvation capability of the solvent can be satisfactorily described with the help of semiempirical parameters of solvent polarity [cf. Chapter 7). 

---