Reactions of cation radicals of EE systems. 8. Effects of initial cation radical concentration on the rate-determining step in the half-regeneration mechanism

The reactions of the cation radical derived from 9,10-diphenylanthracene with the nucleophiles pyridine and 4-cyanopyridine in acetonitrile have been studied by three kinetic techniques: single potential step spectroelectrochemistry, open circuit relaxation spectroelectrochemistry, and stopped-flow kinetic spectrophotometry. The half-regeneration mechanism is shown to account for the observed reaction dynamics for both reaction systems. Under conditions of high initial cation radical concentration, the experimental rate law is found to be first order in both cation radical and nucleophile concentration and independent of the concentration of the neutral precursor, while at low initial concentration of cation radical, second-order dependence on cation radical concentration is found with a first-order dependence on nucleophile concentration and an absence of precursor concentration dependence. In the former case adduct formation from the initial reaction of nucleophile and cation radical is rate determining. A shift in kinetic control to the monoelectronic oxidation of reversibly formed adduct by free cation radical is found under the latter reaction conditions.