Gas-phase reactivity of acyclic α,β-unsaturated carbonyls towards ozone

Abstract

We evaluated different approaches to discuss the reactivity of α,β-unsaturated carbonyls comparative to alkene analogues. It was found that the reactivity factors x_r, defined as the relative ratio between the rate coefficient of the carbonyl and a core structure, allow a semi-quantitative estimation of substituent effects in α,β-unsaturated acids, aldehydes and esters when the carbonyl containing substituent is replaced by a hydrogen atom. By contrast, it can be shown that the reactivity of the corresponding ketones differs from the other carbonyls. A linear correlation is presented between the x_r – values and the number of carbon atoms of the alkyl group of the unsaturated esters, which can be used to predict ozonolysis rate coefficients. For this systematic analysis the following rate coefficients (in 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹) have been determined at 298 ± 2 K and 990 ± 15 mbar and under dry conditions using the relative rate method: k(O₃ + methyl methacrylate) = 7.0 ± 0.9, k(O₃ + methyl crotonate) = 5.5 ± 1.4, k(O₃ + methyl 3-methyl-3-butenoate) = 1.3 ± 0.3, k(O₃ + methyl tiglate) = 65 ± 11, k(O₃ + 3-penten-2-one) = 31 ± 7, k(O₃ + 3-methyl-3-penten-2-one) = 80 ± 19, k(O₃ + 4-methyl-3-penten-2-one) = 8.4 ± 0.8.