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

The rate of hydrogen atom abstraction from tributyltin hydride, benzenesele nol, thiophenol, and tetrahydrofuran was measured in the gas phase for char ged phenyl radicals with different neutral substituents at the meta- or ort ho-position. A charged pyridinium substituent (meta or para) allowed the ma nipulation of the radicals in the Fourier transform ion cyclotron resonance mass spectrometer that was used to carry out the experiments. All the reac tion rates were found to be similarly affected by substituents on the radic al: 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 reactivit y trends. Instead, a correlation exists between the reactivity and electron affinity of the radicals. We conclude that the electron-withdrawing substi tuents studied here lower the reaction barrier by increasing the polarity o f the transition state, without an associated increase in reaction exotherm icity. The increase in the electron affinity (AEA) of the radical caused by a given substituent provides a sensitive probe for the substituent's barri er-lowering effect (in the few cases studied in detail, the barrier is lowe red 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 > pentafluor othiophenol. This trend reflects the decreasing ionization energies of the three substrates rather than the decreasing reaction exothermicities or inc reasing homolytic bond-dissociation energies (4-fluorothiophenol > thiophen ol > pentafluorothiophenol). Apparently, the polar control overrides the en thalpic control in this case. The results reported for radicals with differ ent distances between the radical site and the charged group suggest that s imilar substituent effects are expected for neutral phenyl radicals, and th at the hydrogen abstraction ability of heteroaromatic radicals is likely to be tunable by pH.