Product distributions from the OH radical-induced oxidation of but-1-ene, methyl-substituted but-1-enes and isoprene in NOx-free air

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 (gamma=0.90+/-0.03 for 2-Me-but-1-ene and gamma=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 beta-hydroxyperoxyl radicals (k(ssb)/k(ss)=0.29+/-0.07 for but-1-ene and k(ssb)/k(ss)=0.19+/-0.06 for 3-Me-but-1-ene), and for the ratio of the reaction with oxygen vs. decomposition of beta-hydroxyalkoxyl radicals, k(3)[O-2]/(k(4)+k(3)[O-2])=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).