A temperature-dependent relative-rate study of the OH initiated oxidation of n-butane: The kinetics of the reactions of the 1- and 2-butoxy radicals

Abstract

The kinetics of the reactions of 1-and 2-butoxy radicals have been studied using a slow-flow photochemical reactor with GC-FID detection of reactants and products. Branching ratios between decomposition, CH₃CH(O˙)CH₂CH₃ → CH₃CHO + C₂H₅, reaction (7), and reaction with oxygen, CH₃CH(O˙)CH₂CH₃ + O₂ → CH₃C(O)C₂H₅ + HO₂, reaction (6), for the 2-butoxy radical and between isomerization, CH₃CH₂CH₂CH₂O˙ → CH₂CH₂CH₂CH₂OH, reaction (9), and reaction with oxygen, CH₃CH₂CH₂CH₂O˙ + O₂ → C₃H₇CHO + HO₂, reaction (8), for the 1-butoxy radical were measured as a function of oxygen concentration at atmospheric pressure over the temperature range 250–318 K. Evidence for the formation of a small fraction of chemically activated alkoxy radicals generated from the photolysis of alkyl nitrite precursors and from the exothermic reaction of 2-butyl peroxy radicals with NO was observed. The temperature dependence of the rate constant ratios for a thermalized system is given by k₇/k₆ = 5.4 × 10²⁶ exp[(−47.4 ± 2.8 kJ mol⁻¹)/RT] molecule cm⁻³ and k₉/k₈ = 1.98 × 10²³ exp[(−22.6 ± 3.9 kJ mol⁻¹)/RT] molecule cm⁻³. The results agree well with the available experimental literature data at ambient temperature but the temperature dependence of the rate constant ratios is weaker than in current recommendations.