Rate constants for the reactions of H atoms and OH radicals with ethers at753 K

A relative rate method has been used to determine kinetic data for the reac tions of H and OH radicals with five ethers by adding trace amounts of each ether individually to mixtures of H-2 + O-2 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 cou ld be obtained, as has been established with alkanes in previous studies. A t 753 K, for the OH radical, the values are 5.7 x 10(9)(dimethyl ether), 8. 9 x 10(9)(methyl ethyl ether), 1.23 x 10(10)(diethyl ether), 8.9 x 10(9)(me thyl t-butyl ether) and 1.29 x 10(10)(ethyl t-butyl ether) dm(3) mol(-1) s( -1), and for the H atom 1.11 x 10(9)(dimethyl ether), 2.0 x 10(9)(methyl et hyl ether), 2.95 x 10(9)(diethyl ether), 1.06 x 10(9)(methyl t-butyl ether) and 2.1 x 10(9)(ethyl t-butyl ether) dm(3) mol(-1) s(-1). The uncertaintie s are discussed in the paper, and are in the region of ca. +/-15%. The valu es have been assessed in the context of other kinetic information in the li terature, most of which has been obtained at low temperatures, and expressi ons have been derived for the rate constants for use over the range 250-125 0 K. Further, a preliminary value of k = 3.5 x 10(5) dm(3) mol(-1) s(-1) wi th an uncertainty of about 20% has been determined at 753 K for ether from measurements of the initial yields of ethene and i-butene from HO2 + diethy l tetramethylbutane]diethyl mixtures. Full analytical studies were carried out for methyl t-butyl ether and + H-2 + O-2 and ethyl t-butyl ether + H-2 + O-2 mixtures at 753 K. Low yields (ca. 1-2%) of 4,4-dimethyl-1,3-dioxalan e and 2,4,4-trimethyl-1,3-dioxalane are formed as initial products from met hyl t-butyl ether and ethyl t-butyl ether, respectively. Although structura lly possible, no evidence was found for the formation of conjugate alkene i n the initial products of methyl ethyl ether, diethyl ether and ethyl t-but yl ether, in marked contrast to the high yields found from structurally-rel ated alkanes under identical conditions. The explanation lies in the very h igh rate constants for C-O homolysis in the ether radicals which are a fact or of 10(2) higher than for C-C homolysis in alkyl radicals at 753 K.