Rate constants for the reactions of H atoms and OH radicals with ethers at 753 K

Abstract

A relative rate method has been used to determine kinetic data for the reactions of H and OH radicals with five ethers by adding trace amounts of each ether individually to mixtures of H₂ + O₂ at 753 K. Sensitivity analysis confirmed that reaction of the ethers with O and HO₂ radicals was of minor importance and that reliable values of the rate constants for H and OH could be obtained, as has been established with alkanes in previous studies. At 753 K, for the OH radical, the values are 5.7 × 10⁹(dimethyl ether), 8.9 × 10⁹(methyl ethyl ether), 1.23 × 10¹⁰(diethyl ether), 8.9 × 10⁹(methyl t-butyl ether) and 1.29 × 10¹⁰(ethyl t-butyl ether) dm³ mol⁻¹ s⁻¹, and for the H atom 1.11 × 10⁹(dimethyl ether), 2.0 × 10⁹(methyl ethyl ether), 2.95 × 10⁹(diethyl ether), 1.06 × 10⁹(methyl t-butyl ether) and 2.1 × 10⁹(ethyl t-butyl ether) dm³ mol⁻¹ s⁻¹. The uncertainties are discussed in the paper, and are in the region of ca. ± 15%. The values have been assessed in the context of other kinetic information in the literature, most of which has been obtained at low temperatures, and expressions have been derived for the rate constants for use over the range 250–1250 K. Further, a preliminary value of k = 3.5 × 10⁵ dm³ mol⁻¹ s⁻¹ with an uncertainty of about 20% has been determined at 753 K for HO₂ + diethyl ether from measurements of the initial yields of ethene and i-butene from tetramethylbutane + diethyl ether + O₂ mixtures. Full analytical studies were carried out for methyl t-butyl ether + H₂ + O₂ and ethyl t-butyl ether + H₂ + O₂ mixtures at 753 K. Low yields (ca. 1–2%) of 4,4-dimethyl-1,3-dioxalane and 2,4,4-trimethyl-1,3-dioxalane are formed as initial products from methyl t-butyl ether and ethyl t-butyl ether, respectively. Although structurally possible, no evidence was found for the formation of conjugate alkene in the initial products of methyl ethyl ether, diethyl ether and ethyl t-butyl ether, in marked contrast to the high yields found from structurally-related alkanes under identical conditions. The explanation lies in the very high rate constants for C–O homolysis in the ether radicals which are a factor of 10² higher than for C–C homolysis in alkyl radicals at 753 K.