Solvent effects on chemical equilibria

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

Among many examples of the solvent effects on chemical equilibria and reactions, the solvent effect on tautomerization has been one of the most extensively studied. Experi-... 

Ben-Naim, A. 1975. Solute and solvent effects on chemical-equilibria. Journal of Chemical Physics. 63, 2064. 

We now inquire into the nature of solvent effects on chemical equilibria, taking noncovalent molecular complex formation as an example. Suppose species S (substrate) and L (ligand) interact in solution to form complex C, K, being the complex binding constant. 

We now consider a type of analysis in which the data (which may consist of solvent properties or of solvent effects on rates, equilibria, and spectra) again are expressed as a linear combination of products as in Eq. (8-81), but now the statistical treatment yields estimates of both a, and jc,. This method is called principal component analysis or factor analysis. A key difference between multiple linear regression analysis and principal component analysis (in the chemical setting) is that regression analysis adopts chemical models a priori, whereas in factor analysis the chemical significance of the factors emerges (if desired) as a result of the analysis. We will not explore the statistical procedure, but will cite some results. We have already encountered examples in Section 8.2 on the classification of solvents and in the present section in the form of the Swain et al. treatment leading to Eq. (8-74). 

Thermodynamic Equilibrium, Kinetics, Activation Barriers, and Reaction Mechanisms for Chemical Reactions in Karst Terrains (White, 1997) Solvent Effects On Isomerization Equilibria—an Energetic Analysis in the Framework of Density Functional Theory (Lelj and Adamo, 1995)... 

Contemporary computer-assisted molecular simulation methods and modern computer technology has contributed to the actual numerical calculation of solvent effects on chemical reactions and molecular equilibria. Classical statistical mechanics and quantum mechanics are basic pillars on which practical approaches are based. On top of these, numerical methods borrowed from different fields of physics and engineering and computer graphics techniques have been integrated into computer programs running in graphics workstations and modem supercomputers (Zhao et al., 2000). 

We can use the transfer activity coefficients to predict solvent effects on chemical reactions and equilibria [22]. Some examples are shown below. 

The use of the Gutmann41 donor and acceptor numbers for describing solvent effects on rates, equilibria and other physicochemical properties has met with some success in organic chemistry. 62 63 However, because the donor and acceptor numbers of mixtures of solvents can not be inferred from the values of the pure solvents but must be determined experimentally, and also because the relationships describing the effects of solvent on chemical reactions were found to apply to non-associated solvents of medium to high dielectric constant, there has been very little attempt to introduce this approach into inorganic systems where the commonly used solvents are protic, i.e. associated. However, one such reaction that has been studied was63 equation (34) ... 

Thus, whenever a chemist wishes to carry out a chemical reaction he not only has to take into consideration the right reaction partners, the proper reaction vessels, and the appropriate reaction temperature. One of the most important features for the success of the planned reaction is the selection of a suitable solvent. Since solvent effects on chemical reactivity have been known for more than a century, most chemists are now familiar with the fact that solvents may have a strong influence on reaction rates and equilibria. Today, there are about three hundred common solvents available, nothing to say of the infinite number of solvent mixtures. Hence the chemist needs, in addition to his intuition, some general rules and guiding-principles for this often difficult choice. 

So far, we have treated the stationary-state quantum mechanics of an isolated molecule. The molecular properties so calculated are appropriate for gas-phase molecules not at high pressure. However, most of chemistry and biochemistry occurs in solution, and the solvent can have a major effect on the position of chemical equilibrium and on reaction rates. (For a survey of solvent effects on rates, equilibria, IR, UV, and NMR spectra, see C. Reichardt, Solvents and Solvent Ejfects in Organic Chemistry, VCH, 1988.) We now examine solvent effects on molecular and thermodynamic properties. 

Since then, the generality and importance of solvent effects on chemical reactivity and physical properties of species in dilute solutions has been widely acknowledged. Solvent-solute interactions for reactants and for products account for observed shifts in chemical equilibria those involving reactants and transition states determine changes in the rates of elementary processes. Shifts of the absorption and/or fluorescence maxima originate in differential solvent-solute interactions of the ground and electronically excited states of a dissolved species. The perturbations induced by the solvents are reflected by concurrent variations of such physical properties of the solute as ir, nmr, and epr spectra and partial molar properties. 

Chapter 12 gives an extensive coverage on the thermodynamics of chemical reactions, which emphasizes the importance of the real mbcture behavior on the description of reaction equilibria and the enthalpies of reaction as well as solvent effects on chemical equilibrium conversion. 

The main principles and concepts of the effect of solvent polarity on chemical reactions and equilibria are outlined in the following sections. However, this is a vast subject area beyond the scope of this work and the interested reader will find a detailed discussion elsewhere [1],... 

As described above, the role of ion solvation is crucial in the dissolution of electrolytes. Ion solvation also has significant effects on chemical reactions and equilibria. Ion-solvent interactions that may participate in ion solvation are shown in Table 2.3 [8],... 

Solvent Effects on the Position of Homogeneous Chemical Equilibria... 

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