Isotopic tracer studies of the role of butenes in the combustion of n -butane

Abstract

Studies of the oxidation of n-butane, both in the cool flame and slow combustion regions, show that the principal initial products are butene-1 and butene-2 which are formed simultaneously with much smaller quantities of all the possible C$_{4}$ O-heterocycles. The analytical results suggest that the same free radical species are responsible for attack on the hydrocarbon over the whole temperature range investigated (315 to 423 degrees C); since the selectivity of attack is low, these are probably hydroxyl radicals. Experiments in which [1-$^{14}$C]butene-1 and [2-$^{14}$C] butene-2 are added to reacting n-butane plus oxygen mixtures make it possible to assess quantitatively the role of the alkenes in the combustion of the alkane. The results show that tetrahydrofuran and 2-methyloxetan are produced entirely by isomerization and subsequent decomposition of butylperoxy radicals but that at least part of the 2-ethyloxiran and 2,3-dimethyloxiran formed result from a reaction involving the addition of HO$_{2}$ radicals to butene-1 and butene-2 respectively. The importance of butenes in the oxidation of n-butane is demonstrated by the fact that after 50 s reaction at 315 degrees C, at least 35% of the initial alkane has been converted to the two conjugate alkenes and about 60% of these compounds has reacted further. The results show that in general butene-2 is a more reactive intermediate than butene-1.