Facile proton transfer reactions of N,N-dimethylaniline cation radicals

Article abstract of DOI:10.1021/ja00023a026

Ring-substituted N,N-dimethylaniline (DMA) cation radicals undergo rapid proton transfer reactions with acetate ion or pyridine in acetonitrile. The difference in pK_a of the parent DMA and DMA* was estimated to equal 27 using electrode potentials in a thermochemical cycle. Second-order rate constants for the reaction with acetate ion at 298 K ranged from 3.2 × 10⁶ for the p-methoxy-substituted cation radical to 3 × 10⁹ M^-1 s^-1 for the corresponding p-nitro derivative. Rate constants for the reactions of the cation radicals with pyridine were observed to be as much as 10⁶ lower than with acetate ion. Activation parameters for the two series of reactions differed markedly. Enthalpies of activation (ΔH*) on the order of 20 kcal/mol and large positive activation entropies (ΔS*), 35-53 eu, were characteristic of the acetate ion reactions, while for pyridine, ΔH* ranged from 2 to 13 kcal/mol and ΔS* values were large and negative, -15 to -29 eu. Large k_H/k_D values for reactions of ArN(CD₃)₂*⁺ implicate proton transfer as the rate-determining step in both reaction series. The differences in activation parameters for the two sets of reactions were attributed to differences in the position of the preequilibrium between base and cation radicals. A large equilibrium constant is expected for the reversible ion combination between acetate ion and the cation radicals.