The effect of hydrogen bromide on the products of the cool-flame combustion of n -pentane

Abstract

Detailed studies have been made of the products of the cool-flame combustion of n -pentane in the absence and presence of small concentrations (2-6 vol. %) of added hydrogen bromide. In the uncatalysed reaction, acetone and acetaldehyde are the principal products formed at low temperatures during the induction period preceding the first cool flame but increasing amounts of C 5 alkenes and O-heterocycles start to be formed as the initial temperature is increased. The main effect of hydrogen bromide is to increase dramatically the yields of C 5 ketones at the expense of almost all the other products. The results indicate that in the absence of the halogen compound the principal fate of the initially formed pentylperoxy radicals is isomerisation to hydroperoxypentyl radicals. At 250 °C, the latter radicals mainly add on further oxygen and are eventually converted to pentanedihydroperoxides; at higher temperatures, the hydroperoxypentyl radicals tend increasingly to decompose directly to give principally pentenes and C 5 O-heterocycles. Hydrogen bromide alters the mechanism operating with binary mixtures primarily by providing a source of readily abstractable hydrogen and thus enhancing the formation of pentenemonohydroperoxides. Control experiments on the homogeneous breakdown of pentane-2-monohydroperoxide show that the principal decomposition product is pentan-2-one and thus confirm the probable importance of pentanemonohydroperoxides as intermediates in the HBr-promoted reaction. Studies of the chemical changes accompanying the passage of cool flames show that these vary considerably with the prevailing conditions as well as with the number of previous cool flames which have propagated through the mixture. Hydrogen bromide causes well-defined differences in the nature and distribution of the products of the combustion of n -pentane, although these changes are not as great as those brought about by the passage of cool flames which generally lead to considerable transient temperature rises in the system.