Polar Effects in Hydrogen Atom Transfer Reactions from a Proton-Coupled Electron Transfer (PCET) Perspective: Abstractions from Toluenes

Rate constants for hydrogen atom transfer (HAT) reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals are reanalyzed here using the free energies of related proton transfer (PT) and electron transfer (ET) reactions, calculated from an extensive set of compiled or estimated pK(a) and E degrees values. The Eyring activation energies Delta G(HAT)double dagger do not correlate with the relatively constant Delta G degrees(HAT), but do correlate close-to-linearly with Delta G degrees(PT) and Delta G degrees(ET). The slopes of correlations are similar for the three radicals except that the Bu-t(center dot) barriers shift in the opposite direction from the oxyl radical barriers -a clear example of the qualitative polar effect in HAT reactions. When cast quantitatively in free energy terms (Delta G(HAT)double dagger vs Delta G degrees(PT/ET)), this effect is very small, only 5-10% of the typical Bell-Evans-Polanyi (BEP) effect of changing Delta G degrees(HAT). This analysis also highlights connections between polar effects and the concepts of asynchronous or imbalanced HAT reactions in which the PT and ET components of Delta G degrees(HAT) contribute differently to the barrier. Finally, these observations are discussed in light of the traditional explanations of polar effects and the potential for a rubric that could predict the extent to which contra-thermodynamic selectivity may be achieved in HAT reactions.

Automated summary

The study reanalyzed the rate constants for HAT reactions of substituted toluenes with tert-butyl, tert-butoxy, and tert-butylperoxyl radicals, and found a qualitative polar effect in the reactions.