III. Determining Radical Philicity

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Since radical philicity appears to have an important role in radical reactivity, it is valuable to be able to determine easily whether a particular radical is electrophilic or nucleophilic. A procedure that accomplishes this task would be especially useful if it could be implemented simply by inspecting the structure of the radical in question. Fortunately, two such procedures exist. One is based on atom electronegativity¹⁶ and the other on cation and anion stability.¹⁷ More sophisticated techniques for determining radical philicity also are known. One of these, principal component analysis,^18,19 extracts information about nucleophilicity and electrophilicity of radicals from experimentally determined rate constants. A second, based on ab initio molecular orbital calculations, determines radical philicity from the extent and direction of charge transfer between a radical and an alkene at the transition state for an addition reaction.^3,20–23 These different approaches to determining radical philicity are described in the next four sections.