Polar effects control hydrogen-abstraction reactions of charged, substituted phenyl radicals

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 similar to Cl < CN (most reactive); ortho, H < CF3 similar to Cl similar to 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 polarity of the transition state, without an associated increase in reaction exothermicity. The increase in the electron affinity (AEA) of the radical caused by a given substituent provides a sensitive probe for the substituent's barrier-lowering effect (in the few cases studied in detail, the barrier is lowered by about 10% of Delta EA(v)). Another way to lower the barrier involves lowering the ionization energy of the substrate. Indeed, all the radicals follow the reactivity trend of thiophenol > 4-fluorothiophenol > pentafluorothiophenol. This trend reflects the decreasing ionization energies of the three substrates rather than the decreasing reaction exothermicities or increasing homolytic bond-dissociation energies (4-fluorothiophenol > thiophenol > pentafluorothiophenol). Apparently, the polar control overrides the enthalpic control in this case. The results reported for radicals with different distances between the radical site and the charged group suggest that similar substituent effects are expected for neutral phenyl radicals, and that the hydrogen abstraction ability of heteroaromatic radicals is likely to be tunable by pH.