Charge-site effects on the radical reactivity of distonic lons

The perturbation on the intrinsic reactivity of a radical moiety by the presence of a nearby charged moiety was probed by comparing the reactivity of analogous positively and negatively charged distonic ions. The possible contributions of various factors (collisional encounter probability, ion-molecule solvation effects, reaction exothermicity, polar effects) to the overall perturbation are discussed. The N-(3-dehydrophenyl)-pyridinium and 3-dehydrobenzoate ions were chosen as distonic ion models of the phenyl radical for this study. The significant differences in their reaction rates are examined and the origins of these rate differences are explored. Observations that nucleophilic radicals react more rapidly with electron-deficient reagents and electrophilic radicals react more rapidly with electron-rich reagents can be made slightly more quantitative by comparing certain thermochemical values of each distonic ion (radical moiety IE and EA) with those of each neutral reagent. The smaller the relevant IE-EA energy difference, the lower the transition state energy and the faster the reaction. The overwhelming control of the transition state energy by the charge site of the distonic phenyl radical analogues described here emerges as an important caveat to their use as models for phenyl radical reactivity.