Proton catalyzed hydride transfer from alkanes to methylated benzyl cations : I. Kinetics

Hydride transfer from isoalkanes to methylated benzyl cations in strong proton acids and proton exchange between isoalkanes and strong proton acids have comparable velocities. The rate of proton exchange between isobutane and several strong proton acids has been studied by PMR line broadening and the following parameters were obtained: (with HSO3F) ¿H¿ = 5.5 kcal/mole., ¿S¿ = -35 cal/mole.°K; (with CF3SO3H) ¿H¿ = 4.3 kcal/mole., ¿S¿ = -43 cal/mole.°K. Using 2-2H-isobutane and tritiated HSO3F, labelled hexamethylbenzene was prepared via hydride transfer to pentamethylbenzyl cations at various temperatures. The analyses for deuterium and tritium were combined with the known rates of proton exchange to get an estimate for the rate of hydride transfer and the corresponding activation parameters: ¿H¿ = 1 kcal/mole., ¿S¿ = -46 cal/mole.°K.