Limitations of the Hughes-Ingold Rules

Even in those cases where the rate constants, for a reaction in various solvents, are not significantly different, the activation parameters may indicate a significant amount of interaction between solute and solvent, as shown for the unimolecular decomposition of di-tert-butyl peroxide in Table 5-14 [172, 227]. The rate of decomposition of the per- 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less, 

The final limitation of the pure electrostatic theory is its inability to predict solvent effects for reactions involving isopolar transition states. Since no creation, destruction, or distribution of charge occurs on passing from the reactants to the activated complex of these reactions, their rates are expected to be solvent-independent. However, the observed rate constants usually vary with solvent, although the variations rarely exceed one order of magnitude [cf. Section 5.3.3). These solvent effects may be explained in terms of cohesive forces of a solvent acting on a solute, usually measured by the cohesive pressure of the solvent [cf. Section 5.4.2). 

In spite of these limitations, the electrostatic Hughes-Ingold theory remains a good guide in predicting the solvent influence on chemical reactions, at least in a qualitative way. Exceptions can be safely assumed to involve strong specific solute-solvent interactions. 

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