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

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 (k_O) 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 k_O (∼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 k_O (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 k_O (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.