Temperature Effects on Rates of Dehalogenation of Aromatic Anion Radicals

Article abstract of DOI:10.1021/j100273a038

The temperature dependence of the unimolecular dehalogenation of radical anions of nitrobenzyl halides and haloacetophenones was measured between -7 and 70 deg C.Activation parameters range from E_a=11.2-16.9 kcal/mol and log A=12.7-17.1.Both E_a and log A increase from p- to o-nitro radicals and from chloro to bromo radicals.Unfavorable steric effects that move the halogen atom out of the aromatic plane result in lowered A factors.In general, the variation of k₂₉₄ with structure depends in a complex way on the combination of E_a and log A factors, which suggests caution in the evaluation of rate constants at one temperature.The fast unimolecular dissociation of (p-NO2C6H4CH2Br)(1-). (k₂₉₄=4.6*10⁵ s^-1) allows measurement of the slower bimolecular electron transfer (C6H5NO2)(1-).+p-NO2C6H4CH2Br->(p-NO2C6H4CH2Br)(1-).+C6H5NO2 (k₂₉₄=1.9*10⁶ M^-1 s^-1).Both the activation energy and probability factor contribute to the slow rate, possibly due to a geometry change upon the reduction of ArNO2.Extending the temperature studies to supercooled solutions shows no discontinuity of the unimolecular rate constants near the phase transition temperatures.