Product distributions from the OH radical-induced oxidation of but-1-ene, methyl-substituted but-1-enes and isoprene in NOx-free air†
Abstract
Product
distributions resulting from the OH-induced oxidation of but-1-ene, 2-methylbut-1-ene, 3-methylbut-1-ene
and isoprene in air were measured in the absence of nitrogen oxides and compared with predictions
based on currently accepted oxidation mechanisms. In the case of butenes, the observed distributions of
carbonyl compounds, hydroxyketones, hydroxyalkanals and diols were evaluated to obtain probabilities for
the initial attack of OH radical on the outer position of the double bond (γ = 0.90 ± 0.03 for 2-Me-but-1-ene
and γ = 0.76 ± 0.05 for both but-1-ene and 3-Me-but-1-ene), for the probability of formation of stable products in the self-reaction of secondary β-hydroxyperoxyl radicals (kssb/kss = 0.29 ± 0.07 for but-1-ene and kssb/kss = 0.19 ± 0.06 for 3-Me-but-1-ene), and for the ratio of the reaction with oxygen s. decomposition of
β-hydroxyalkoxyl radicals, k3[O2]/(k4 + k3[O2]) = 0.25 ± 0.04 for but-1-ene and = 0.38 ± 0.04 for 3-Me-but-1-ene. The
last two values disagree with other published data, which suggest a smaller effect of oxygen. The oxidation
of isoprene produced methacrolein and methyl vinyl ketone with a ratio 0.93 ± 0.10, the ratio of methyl
vinyl ketone and 3-methylfuran was 7.3 ± 1.0. Other products were 1-hydroxy-3-methylbut-3-en-2-one (identified
by mass spectrometry) and 3-methyl-3-oxo-butane (tentatively identified). The overall product
distribution was complex and could not be fully elucidated. Computer simulations based on several mechanisms
applied the relative probabilities for OH addition found for the but-1-enes. Comparison with the experimental
data suggests probabilities for OH addition to the methylated double bond of 0.504 ± 0.027 (outer
position) and 0.056 ± 0.003 (inner position), and to the non-methylated
double
bond of 0.335 ± 0.023
(outer position) and 0.105 ± 0.008 (inner position).