Polar Effects Control Hydrogen-Abstraction Reactions of Charged, Substituted Phenyl Radicals

Abstract

The rate of hydrogen atom abstraction from tributyltin hydride, benzeneselenol, thiophenol, and tetrahydrofuran was measured in the gas phase for charged phenyl radicals with different neutral substituents at the meta- or ortho-position. A charged pyridinium substituent (meta or para) allowed the manipulation of the radicals in the Fourier transform ion cyclotron resonance mass spectrometer that was used to carry out the experiments. All the reaction rates were found to be similarly affected by substituents on the radical: meta, H < Br ∼ Cl < CN (most reactive); ortho, H < CF3 ∼ Cl ∼ F. The experimental observations parallel the transition-state energies calculated for hydrogen abstraction from methanol. However, the calculated reaction exothermicities do not correlate with the reactivity trends. Instead, a correlation exists between the reactivity and electron affinity of the radicals. We conclude that the electron-withdrawing substituents studied here lower the reaction barrier by increasing the polari...