Relative rate study of the addition of HO2 radicals to ethylene and to tetramethylethylene

Abstract

Studies of the competitive oxidation of ethylene and tetramethylethylene in the presence and absence of tetramethylbutane as a source of HO₂ radicals have been carried out between 400 and 500 °C over a range of mixture composition and total pressure using aged boric-acid-coated Pyrex vessels. From measurements of the initial relative yields of oxirane and tetramethyloxirane, values of log₁₀(A₇/A₅)=–0.77±0.09 and E₅–E₇= 36.5±0.9 kJ mol^–1 have been obtained: HO₂+ C₂H₄→ C₂H₄O + OH (5) [graphic omitted] Use of the published values of the Arrhenius parameters for reaction (5) gives log₁₀(A₇/dm³ mol^–1 s^–1)= 9.24±0.29 and E₇= 40.0±4.5 kJ mol^–1. No other values are available in the literature, but the present results are consistent with the very limited data available for the addition of HO₂ radicals to ethylene, propene and i-butene. Trends in the variation of the values of the rate constant and of the activation energy for HO₂ addition with structure of the alkene are very similar to those observed for the addition of CH₃O₂ and i-C₃H₇O₂ radicals. The relative rate of addition of i-C₃H₇O₂, CH₃O₂ and HO₂ radicals to any alkene at 130 °C is ca. 1:4:30.