Aspects of the kinetics and mechanism of the gas-phase reactions of ozone with conjugated dienes

Abstract

The relative-rate method was used to measure room-temperature rate constants for the gas-phase reactions of ozone with six conjugated dienes under ‘ OH-free’ conditions. Separate experiments were carried out to determine the yields of hydroxyl radical formed from the ozonolyses of four of these compounds, and for buta-1,3-diene and isoprene, using hydroxyl radical scavenger and tracer-type experiments. The following rate constants were obtained at 298 ± 5 K and 760 ± 10 Torr, in units of 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹: 43.1 ± 9.9 (E-penta-1,3-diene), 27.8 ± 3.7 (Z-penta-1,3-diene), 26.2 ± 6.9 (2,3-dimethyl-buta-1,3-diene), 23.9 ± 5.5 (5-methylhexa-1,3-diene, mixture of E- and Z-isomers), 25.3 ± 2.3 (5,5-dimethylhexa-1,3-diene, mixture of E- and Z-isomers) and 3060 ± 8802,5-dimethylhexa-2,4-diene). The hydroxyl radical formation yields determined, relative to alkene consumed, were: 0.06 ± 0.03 (buta-1,3-diene), 0.53 ± 0.16 (isoprene), 0.82 ± 0.25 (Z-penta-1,3-diene), 0.83 ± 0.25 (E-penta-1,3-diene), 0.66 ± 0.25 (5-methylhexa-1,3-diene, mixture of E- and Z-isomers) and 0.95 ± 0.25 (2,3-dimethylbuta-1,3-diene). The rate parameters are analysed in terms of frontier orbtial theory, and the mechanism of OH formation in the ozonolysis of conjugated dienes is discussed.