| Abstract: | The kinetics of ozonation of C2H4 and C2H2 have been studied in the gas phase from ?40 to ?95°C (C2H4) and +10 to ?30°C (C2H2). The O3 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 O3 absorption at 2537 Å. Reaction stoichiometries and effects of added O2 were investigated. The second-order rate constant for C2H4 was log k(M?1 sec?1) = (6.3 ± 0.2) – (4.7 ± 0.2)/θ (θ = 2.3RT). The rate was independent of the presence of excess O2. Rate measurements for C3H6 were less accurate because of aerosol interference. Combined with room temperature measurements of other workers, the C3H6 rate constant was log k(M?1 sec?1) = (6.0 ± 0.4) – (3.2 ± 0.6)/θ. The C2H2 rate constant was log k(M?1 sec?1) = (9.5 ± 0.4) – (10.8 ± 0.4)/θ. In the case of C3H6 the major product was propylene ozonide. Ethylene did not yield the ozonide, and the products of the O3–C2H4 and O3–C2H2 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 O3. For C2H2, however, the much higher observed A factor suggests a different mechanism. Possible transition states for the O3–C2H2 reaction are discussed. |
