Fluorine Substitution Enhances the Reactivity of Substituted Phenyl Radicals toward Organic Hydrogen Atom Donors

Abstract

Phenyl radicals with different positively charged, chemically inert substituents in the para position were generated by collision-activated dissociation of para-substituted iodobenzene ions in the gas phase inside a Fourier-transform ion cyclotron resonance mass spectrometer. The reactivity of the radicals was examined toward organic hydrogen atom donors. The findings parallel those made by others for neutral radicals in solution. For example, the efficiency of hydrogen atom abstraction increases in the expected order phenol < thiophenol < benzeneselenol. Inspite of great exothermicity, hydrogen atom abstraction by the charged phenyl radicals occurs at a small fraction of the gas-phase collision rate, suggesting the presence of a significant barrier on the reaction coordinate. Fluorine substitution on the phenyl ring was found to drastically enhance the reaction rate for all the substrates studied. For example, hydrogen abstraction from thiophenol by the (2,3,5,6-tetrafluoro-4-dehydrophenyl)trimethylphosp...