Absolute rate constants for hydrocarbon autoxidation. 27. On the question of a reversible rate controlling propagation reaction

Abstract

Autoxidations of cumene, 3-methylpentane, cyclopentane, and cyclohexane have been performed to high conversion in the presence of (CH3)3COOD from 0.1 to 1 atm of oxygen and it has been found that very low yields of C6H5C(CH3)2D, C6H13D, c-C5H9D, and c-C6H11D are produced. Furthermore, autoxidation of C6H5C(CH3)2D in the presence of (CH3)3COOH does not give detectable concentrations of C6H5C(CH3)2H and relative reactivities of cumene and 3-methylpentane to the tert-butylperoxy radical in solution do not depend on the pressure of oxygen above the liquid from 1 to 100 atm. These results have been interpreted in term of a slow reaction of the alkyl radical with alkyl hydroperoxide outside the solvent cage in competition with a fast reaction of the alkyl radical with oxygen. A rate constant for the former reaction has been estimated from the relative yields of 5-hexene and methylcyclopentane produced by decomposition of 5-hexenylformyl peroxide in an inert solvent containing tert-butyl hydroperoxide.The relative reactivity of the tertiary hydrogens in cumene and 3-methylpentane to the tert-butylperoxy radical in the gas phase is about 0.8 times the value in solution. This relative reactivity difference between the two phases is probably due to the involvement of tert-butoxy radicals in rate controlling propagation reactions in the gas phase rather than to significant reversal of the H-atom transfer reaction between the peroxy radical and the substrate inside the solvent cage.