Hydrogen Abstraction by Alkoxyl Radicals: Computational Studies of Thermodynamic and Polarity Effects on Reactivities and Selectivities

摘要

Density functional theory calculations (omega B97X-D) are reported for the reactions of methoxy, tert-butoxy, trichloroethoxy, and trifluoroethoxy radicals with a series of 26 C-H bonds in different environments characteristic of a variety of hydrocarbons and substituted derivatives. The variations in activation barriers are analyzed with modified Evans-Polanyi treatments to account for polarity and unsaturation effects. The treatments by Roberts and Steel and by Mayer have inspired the development of a simple treatment involving the thermodynamics of reactions, the difference between the reactant radical and product radical electronegativities, and the absence or presence of a- unsaturation. The three- parameter equation (Delta H-double dagger = 0.52 Delta H-rxn(1 - d) - 0.35 Delta chi(2)(AB) + 10.0, where d = 0.44 when there is a-unsaturation to the reacting C-H bond), correlates well with quantum mechanically computed barriers and shows the quantitative importance of the thermodynamics of reactions (dictated by the reactant and the product bond dissociation energies) and polar effects.

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