Gas-Phase Proton Transfer from Toluenes to Benzyl Anions

Article abstract of DOI:10.1021/ja00199a002

Gas-phase proton-transfer kinetics of PhCH2(1-) + ArCH3 -> ArCH2(1-) + PhCH3 were studied for reactions having ΔH⁰_rxn = 0 to -20 kcal/mol.These reactions are very slow in the absence of thermodynamic driving force; their reaction efficiencies range from 0.004 to 0.7.RRKM theory was applied to obtain energy differences between the proton-transfer transition state and the loose orbiting transition state from reaction efficiencies.Marcus theory provides a general model for a rate-equilibrium relationship with a constant intrinsic energy barrier of 7 kcal/mol forthe degenerate proton transfer from toluene to benzyl anion.The barrier is inferred from an RRKM fit to the energy difference of -5 kcal/mol between the proton-transfer transition state and the energy of the reactants and an estimated -12 kcal/mol for the energy of the collision complex relative to the reactants.In the reaction involving 3-nitrotoluene, electron transfer, which is some 11-12 kcal/mol less favorable than proton transfer, dominates almost exclusively.