Thermal Decomposition of Ions. 1. Pyrolysis of Protonated Ethers. Activation Energies and A Factors

Article abstract of DOI:10.1021/j150666a043

Unimolecular rate constants (k_d) have been measured at the high-pressure limit for the thermal decomposition of protonated dimethoxyethane (glyme) and bis(2-methoxyethyl)ether (diglyme).In both cases the decomposition involves loss of CH3OH and formation of an oxycarbonium ion.The variation of k_d with temperature gives log A=11.1 and E_a=21.1 kcal*mol^-1 for the pyrolysis of (glyme)H⁺ and log A=13.7 and E_a=30.7 for (diglyme)H⁺.To the best of our knowledge these are first Arrhenius parameters ever determined for the decomposition of cations in the vapor phase under equilibrium conditions at the high-pressure limit.The data and thermochemistry are consistent with a transition state in which electron shifts result in the disruption of an internal hydrogen bond.