Application of non-steady-state kinetics to resolve the kinetics of proton-transfer reactions between methylarene radical cations and pyridine bases

Apparent deuterium kinetic isotope effects (KIEapp) of four different methy larene radical cation-pyridine base reactions in dichloromethane (0.2 M tet rabutylammonium hexafluorophosphate) were observed to increase toward a con stant 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 resu lts rule out a simple second-order proton-transfer reaction and implicate a mechanism in which a complex is first formed that then undergoes proton tr ansfer, followed by separation of the products. That KIEapp are extent of r eaction-dependent is observed before steady-state is reached. The concurren t analysis of kinetic data for the reactions of both ArCH3.+ and ArCD3.+ wi th bases under non-steady-state conditions facilitates the resolution of th e apparent rate constant [k(app) = k(f)k(p)/(k(b) + k(p))] into the microsc opic rate constants (k(f), k(b), and k(p)) for the individual steps. The KI Eapp observed during proton-transfer reactions need not be the real kinetic isotope effects (KIEreal). Having access to the microscopic rate constants for the steps in which the proton is transferred allows KIEreal to be eval uated and compared with the corresponding KIEapp. The present study shows t hat the KIEreal are much greater than the KIEapp derived in the usual way f rom the rate of the overall reaction.