The spontaneous ignition of mixtures of n -heptane and 1-heptene in oxygen

Studies of the ignition in oxygen of binary mixtures of n -heptane and 1-heptene at tempera­tures below 400°C show that under all conditions investigated both hydrocarbons are consumed during ignition. However, below ca . 300°C, where 1-heptene is considerably less reactive than n -heptane, the alkene is not involved in the process leading to ignition and the ignitability of mixtures is controlled effectively by the concentration of the alkane. In con­trast, above 300 °C, 1-heptene is at least as ignitable as n -heptane and the alkene appears to contribute to the process leading to ignition. This shows that, although at relatively high temperatures conjugate alkenes maybe among the principal intermediates involved in the ignition of alkanes, this is not the case at lower temperatures. Here, the reaction between heptyl radicals and oxygen is additive leading to the formation of the corresponding heptylperoxy radicals and ignition then results from further reactions of these latter radicals. Comparison of the results obtained in stainless steel and vitreous vessels shows that the nature of the surface has little effect on limiting ignition pressures but does affect the multiplicity of cool flames. It thus appears that the surface exerts a considerable influence on the course of the reaction following the passage of a cool flame but not on the processes leading to the first cool flame and to ignition.