Ozone Reactions with Olefins and Alkynes: Kinetics, Activation Energies, and Mechanisms

Abstract

The temperature dependence of the kinetics and the mechanisms of ozone reactions with 19 olefins and 3 alkynes were investigated. The second-order rate constants (kO3) for ozone reactions with olefins were mostly in the range of 103-106 M-1 s-1, with activation energies of 17.4-37.7 kJ mol-1. In comparison, alkynes had lower kO3 (∼102 M-1 s-1) and higher activation energies (36.7-48.1 kJ mol-1). Reactivities of both olefins and alkynes are mainly influenced by inductive effects of substituents, with steric effects observed for cyclic olefins. 2-Buten-1,4-dial (BDA), synthesized with a novel method, is a toxic olefinic oxidation product from phenols. Its cis- and trans-isomers show distinct reactivities with ozone, with kO3 (20 °C) of 3.0 × 103 and 1.2 × 104 M-1 s-1, respectively. Two mols of glyoxal were formed per mol of ozonated BDA, with a slow release of the second mol from an α-hydroxyalkylhydroperoxide intermediate. 2-Ethynylbenzaldehyde reacts with ozone with a stoichiometry of 1:1 and kO3 (20 °C) = 1.6 × 102 M-1 s-1. Ozone attacks the ethynyl group, yielding a carboxyl product (2-carboxybenzaldehyde, 54%), an aldehyde product (phthaldialdehyde), and a dicarbonyl product with a stoichiometric release of H2O2 (21%). This study provides kinetic and mechanistic information for assessing the abatement of olefin- and alkyne-containing micropollutants by ozonation at various temperatures.