Application of Non-Steady-State Kinetics to Resolve the Kinetics of Proton-Transfer Reactions between Methylarene Radical Cations and Pyridine Bases

Abstract

Apparent deuterium kinetic isotope effects (KIEapp) of four different methylarene radical cation−pyridine base reactions in dichloromethane (0.2 M tetrabutylammonium hexafluorophosphate) were observed to increase toward a constant value with increasing extent of reaction. The reactions were studied by derivative cyclic voltammetry (DCV), and rate constants were assigned by comparing the experimental with the theoretical DCV data. The kinetic results rule out a simple second-order proton-transfer reaction and implicate a mechanism in which a complex is first formed that then undergoes proton transfer, followed by separation of the products. That KIEapp are extent of reaction-dependent is observed before steady-state is reached. The concurrent analysis of kinetic data for the reactions of both ArCH3•+ and ArCD3•+ with bases under non-steady-state conditions facilitates the resolution of the apparent rate constant [kapp = kfkp/(kb + kp)] into the microscopic rate constants (kf, kb, and kp) for the individual...