Solvent effects on reaction rate

With reactions in solution, the solvent is generally present at a much higher concentration than the reactants and its concentration is approximately constant during the course of a reaction. If the solvent is also one of the reactants, as with water in hydrolyses or in the example of recombination of carbocations with water molecules we discussed previously, the order with which the solvent participates in the reaction rate cannot be obtained by kinetic studies in this solvent. To get this, we need kinetic studies with solvent mixtures or isotopically labelled molecules, spectroscopic studies, etc. 

In other systems, the solvent only acts as an inert medium where reaction occurs without affecting the mechanism. This appears to be the case of the decomposition of nitrogen pentoxide, in which the pre-exponential factor and the activation energy are very similar in the gas phase and in solvents of various types, as shown in Table 9.1. 

A more subtle effect of solvent is seen in the reactions first studied by Menschutkin [1], and which take his name. An example is the reaction 

The results shown in Table 9.2 indicate that the rate constants decrease by a factor of 140 times on going from acetonitrile to benzene. This solvent effect occurs without any change in mechanism, but what causes it Note, that there is also an effect in the same solvents on the free energy of reaction, AG°, which is seen as a decrease in the equilibrium 

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For analysing equilibrium solvent effects on reaction rates it is connnon to use the thennodynamic fomuilation of TST and to relate observed solvent-mduced changes in the rate coefficient to variations in Gibbs free-energy differences between solvated reactant and transition states with respect to some reference state. Starting from the simple one-dimensional expression for the TST rate coefficient of a unimolecular reaction a— r... 

E. S. Amis, "Solvent Effects on Reaction Rates and Mechanisms." Academic. New York, 1966. 

The effect of the medium (solvent) on chemical reactivity is a subject of great difficulty, one that can be studied at several levels of understanding. The literature of the field is large, and research interest continues to be bigb. In this chapter we can only summarize much that has been learned each topic can be pursued in detail by means of the citations to original work. Many authors have reviewed solvent effects on reaction rates. Section 8.1 introduces a few ideas that are treated more thoroughly in the rest of the chapter. 

Although the solvent effect on reaction rate could, in principle, be large, the limited availability of... 

Horne, R. A. (ed.) Water and aqueous solutions. New York Wiley-Interscience 1972. 31) Amis, E. S. Solvent effects on reaction rates and mechanisms. New York Academic Press 1966. 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

AMIS, E. s. Solvent Effects on Reaction Rates and Mechanisms (Academic Press, 1966). 

House, J. E. (2007). Principles of Chemical Kinetics (2nd ed.). Amsterdam, Netherlands Academic Press. Chapter 5 deals with solvent effects on reaction rates. 

Because the transition-state theory is essential for even a qualitative understanding of solvent effects on reaction rates, some important features of this theory are outlined below. 

In principle, when studying solvent effects on reaction rates, it is not sufficient to investigate only the change because, according to Eq. (5-5), this term is determined by both an enthalpy and an entropy term. There are four types of reaction rate control [41] ... 

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). 

Qualitative Theory of Solvent Effects on Reaction Rates... 

Qualitative Theory of Solvent Effects on Reaction Rates 165 Table 5-4. Predicted solvent effects on rates of nucleophilic substitution reactions [16, 44-46],... 

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