Arrhenius constants for the reactions of ozone with ethylene and acetylene

Abstract

Abstract The kinetics of ozonation of C 2 H 4 and C 2 H 2 have been studied in the gas phase from −40 to −95°C (C 2 H 4 ) and +10 to −30°C (C 2 H 2 ). The O 3 concentrations were near 10 −4 M, and the hydrocarbons were present in 2‐ to 25‐fold excess. A few experiments with propylene were also carried out. The reactions were followed by observing the rate of decay of O 3 absorption at 2537 Å. Reaction stoichiometries and effects of added O 2 were investigated. The second‐order rate constant for C 2 H 4 was log k (M −1 sec −1 ) = (6.3 ± 0.2) – (4.7 ± 0.2)/θ (θ = 2.3 RT ). The rate was independent of the presence of excess O 2 . Rate measurements for C 3 H 6 were less accurate because of aerosol interference. Combined with room temperature measurements of other workers, the C 3 H 6 rate constant was log k (M −1 sec −1 ) = (6.0 ± 0.4) – (3.2 ± 0.6)/θ. The C 2 H 2 rate constant was log k (M −1 sec −1 ) = (9.5 ± 0.4) – (10.8 ± 0.4)/θ. In the case of C 3 H 6 the major product was propylene ozonide. Ethylene did not yield the ozonide, and the products of the O 3 –C 2 H 4 and O 3 –C 2 H 2 reactions were not identified. Pre‐exponential factors for the olefin reactions are consistent with a five‐membered ring transition state formed by 1,3 dipolar cycloaddition of O 3 . For C 2 H 2 , however, the much higher observed A factor suggests a different mechanism. Possible transition states for the O 3 –C 2 H 2 reaction are discussed.