Decomposition of 4,4-dimethylpent-1-ene in the presence of oxygen between 400 and 500 °C: oxidation chemistry of allyl radicals

Abstract

The decomposition of 4,4-dimethylpent-1-ene (DMP) in the presence of O₂ has been used as a source of allyl radicals over the temperature range 400–500 °C. The reaction has been studied in both KCl-coated (peroxy species destroyed at the surface) and aged boric-acid-coated vessels (peroxy species preserved), and the basic mechanism shown to be: DMP → t-butyl + [graphic omitted] (1), t-butyl + O₂→ i-C₄H₈+ HO₂(2). Previous studies have shown that 99% of the t-butyl radicals undergo reaction (2), so that the system is an excellent source of allyl and HO₂ radicals.

A full product analysis has been carried out over a wide range of mixture composition and associated mechanistic aspects discussed. It is shown that allyl radicals are unreactive towards both DMP and O₂ and that they react mainly in radical–radical processes. [graphic omitted] → CH₂CHCH₂CH₂CHCH₂(3), [graphic omitted] + HO₂→ CH₃CHCH₂+ O₂(8), [graphic omitted] + HO₂→ CO + products (18). Values of k₈/k₁₈= 0.37, 0.38 and 0.42 are obtained at 400, 440 and 480 °C, respectively. From a reasonably reliable estimate of [HO₂] and [allyl], values of k₈=(3.3 ± 0.9)× 10⁹ and k₁₈=(7.4 ± 2.4)× 10⁹ dm³ mol^–1 s^–1 are obtained at 480 °C. No previous estimates of these rate constants are available in the literature.

Values of k₄ have been determined and when combined with independent data at higher temperatures give log(A₄/s^–1)= 14.19 ± 0.25 and E₄= 255.5 ± 5.3 kJ mol^–1 over the range 400–1025 °C. DMP →(CH₃)₂CCH₂+ CH₃CHCH₂(4). A value of k₁₁=(2.5 ± 1)× 10² dm³ mol^–1 s^–1 has been obtained at 480 °C and shown to be consistent with the reaction's high endothermicity. [graphic omitted] + O₂→ CH₂CCH₂+ HO₂(11)