Quantitative Theories of Solvent Effects on Reaction Rates

Quantitative Theories of Solvent Effects on Reaction Rates... 

Quantitative Theories of Solvent Effects on Reaction Rates 233 5.4.4 Reactions between Neutral Molecules and Ions... 

The first systematic investigation on the influence of solvent on reaction rates was reported by Menschutkin(l) as long ago as 1890. Quite soon after this study, chemists began to consider whether or not solvent influences on reaction rates were connected with the effect of solvents on the reactants (i.e. with initial-state effects). However, a careful and extensive investigation by Von Halban(2) in 1913 showed conclusively that for the reaction of trimethylamine with p-nitrobenzyl chloride, solvent effects on the reactants could not account quantitatively for the overall influence of solvent on the reaction rate constant. Little further progress was made on these lines until the advent of transition state theory, when it then became clear that in principle it was possible to dissect the influence of solvent on rate constants into initial-state and transition-state contributions(3-5). 

The present theories of the effects of solvents on the rates of polar (ionic) reactions do not permit a quantitative analysis of the above cited results. Fractions of AHp and ASp, due to solvation, apparently compensate each other, because the increase in the energy needed for desolvation is just compensated by equal contributions of the entropy (a more firmly bound or larger number of molecules are desol-vated). This compensation phenomenon is well known in organic chemistry. For instance, the difference between the activation enthalpies (AAH ) of the reaction of benzyl chloride with pyridine in DMF and CH3OH is equal to -5.3 kcal mole". ... 

In view of the simplifying assumptions which form the basis for TST, its success in many practical situations may come as a surprise. Bear in mind, however, that transition state theory accounts quantitatively for the most important factor affecting the rate—the activation energy. Dynamical theories which account for deviations from TST often deal with effects which are orders of magnitude smaller than that determined by the activation barrier. Environmental effects on the dynamics of chemical reactions in solution are therefore often masked by solvent effect on the activation free energy, IF (xso), with lF(x) given by Eq. (14.25). 

Many additional refinements have been made, primarily to take into account more aspects of the microscopic solvent structure, within the framework of diffiision models of bimolecular chemical reactions that encompass also many-body and dynamic effects, such as, for example, treatments based on kinetic theory [35]. One should keep in mind, however, that in many cases die practical value of these advanced theoretical models for a quantitative analysis or prediction of reaction rate data in solution may be limited. 

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