Arrhenius parameters of elementary reactions involved in the oxidation of neopentane

Abstract

The reactions of neopentyl radicals in an oxidising environment have been studied by adding neopentane to slowly reacting mixtures of H₂+ O₂ over the temperature range 380–520 °C. Over a wide range of mixture composition, the only detectable initial products at these temperatures are 3,3-dimethyloxetan (DMO), acetone, i-butene, methane, and formaldehyde. A relatively simple mechanism involving the formation of neopentylhydroperoxide (QOOH) radicals gives a quantitative interpretation of the product yields. Although the major source of i-butene is the C—C homolysis of neopentyl radicals, a significant proportion is formed in reaction (6)(CH₃)₂C(CH₂OOH)CH₂→(CH₃)₂CCH₂+ HCHO + OH. (6)

From measurements of the product ratios ([acetone]+[DMO])/[i-butene] and [acetone]/[DMO] at each temperature used, Arrhenius parameters have been determined for a number of the elementary steps. The recommended value of k₃= 1.20 × 10¹³ exp (– 120 kJ mol^–1/RT) S^–1 for the 1,5p H-atom transfer in neopentylperoxy radicals is compared with previously determined parameters for the 1,4p H-atom transfer in ethylperoxy radicals. With the Arrhenius expressions for these two transfers as a basis, thermochemical calculations are used to obtain a self-consistent set of Arrhenius parameters from values of rate constants at 480 °C for primary, secondary and tertiary H-atom transfers in alkylperoxy radicals involving ring sizes in the transition state varying from 4 to 8 (CH₃)₃CCH₂O₂→(CH₃)₂C(CH₂OOH)CH₂. (3)