I. Introduction

A. The Evans-Polanyi Relation

For many radical reactions there is a simple relation between the energy of activation for the reaction and its enthalpy. This relation, which is referred to by several, similar names^1–3 (Evans-Po­lanyi being a common one), is given in eq 1. Equation 1 expresses in a quantitative fash­ion the notion that in a group of closely related reactions the enthalpy for a particular reac­tion should be related to its energy of activation; speci­fic­ally, energies of activation should decrease in a linear fashion as reactions become more exo­thermic.

Once the two constants in eq 1 have been deter­mined, it is pos­sible to predict the energy of activation for reaction of any member of the group from knowl­edge of the reaction enthalpy. The numerical value of the constant α rep­re­sents the fraction of the overall enthalpy change that exists at the trans­ition state. The value of α can be viewed as a measure of how far a reaction has pro­ceeded along the reaction coordinate when the transition state is reached. The later a transition state occurs in a reaction the closer α will be to unity.

B. Nucleophilic and Electrophilic Radicals

Although radicals are neutral species, they often exhibit behavior char­acteristic of either nucleophilic or electrophilic inter­me­diates.^4,5 This behavior facilitates certain types of reaction; for example, in the addition reactions shown in eq 2, the carbon-centered, cyclohexyl radical behaves as a nucleo­phile by adding more rapidly to compounds with more electron-deficient double bonds than to ones in which the double bonds are less electron-deficient.⁶ In contrast, the malonyl radical 1 can be viewed as electrophilic because it adds to electron-rich double bonds such as that in the D-glucal 2 (eq 3).⁷ A good beginning point for discussing radical philicity is examining some hydrogen-abstraction reactions.