The peroxides formed during hydrocarbon slow combustion and their role in the mechanism

Abstract

Abstract The peroxides formed during the slow combustion of five hydrocarbons in a flow system have been identified and the approximate yields estimated. With propane at 327°C and 2, 2, 3-trimethylbutane at 365 to 385°C the products contained only traces of hydrogen peroxide, its addition compounds with aldehyde and (with C3H8) peracetic acid. With n-butane between 310 and 345°C and cylohexane between 290 and 316°C appreciable yields of peroxide were obtained (~ 10 to 20% of the hydrocarbon oxidized). These consisted of the monohydroperoxides, hydrogen peroxide and their addition compounds with aldehydes. With n-C4H10 the relative amount of H2O2 free and combined ( ~ 50% of the total peroxide yield) was much higher than with C6H12 and some perpropionic acid was also detected. With n-heptane between 240 and 310°C the yield of peroxide in the products was also con­ siderable ( ~ 20% of the hydrocarbon reacted), and consisted mainly of dihydroperoxyheptane and its addition compounds with aldehydes (mainly formaldehyde), with much smaller amounts of monohydroperoxide and hydrogen peroxide (free and combined with aldehyde), diheptylperoxide and possible trihydroperoxyheptane. Packing the vessel increased the relative amount of aldehyde-addition compounds but did not affect the yield of free aldehyde, which apparently depended only on the temperature, being zero at 240°C. All aldehydes up to C5H11CHO were formed at higher temperatures, but those from C3 to C6 only in small yield. A little β-dicarbonyl compound and carboxylic acids were also detected in the products. The modes of formation and decomposition of the peroxides is discussed. It is suggested the dihydroperoxyheptane resulted from the abstraction of a hydrogen atom internally in the C7H15O2 radical from the CH2 group β or γ to the point of original attack, that aldehydes were produced partly by heterogeneous hydroperoxide decomposition and partly by decomposition of RO2 radicals, and that with n-heptane the aldehyde-hydroperoxide compounds were formed mainly on the walls of the reaction vessel. Chain branching in the oxidation of propane and 2, 2, 3-trimethylbutane was presumably due exclusively to the oxidation of aldehydes formed, whereas with the other three hydrocarbons branching due to homo­geneous peroxide decomposition was probably important up to about 350°C.