No abstract

The stable products of the low-pressure (4-8 torr (1 torr = 133.33 Pa)) gas-phase reactions of ozone with ethene, propene, 2-methylpropene, cis-2-butene, trans-2-butene, trans-2-pentene, 2,3-dimethyl-2-butene, and 2-ethyl-1 -butene have been identified by using a photoionization mass spectrometer coupled to a stirred-flow reactor. The products observed are characteristic of (i) a primary Criegee split to an oxoalkane (aldehyde or ketone) and a Criegee intermediate,of the Criegee intermediates such as unimolecular decomposition, secondary ozonide formation, etc., and (iii) secondary alkene chemistry involving OH and other free-radical products formed by the unimolecular decomposition of the Criegee intermediates. The secondary OHalkene412 reactions account for a significant fraction of the alkene ( C a b ) consumed and lead to characteristic products such as C, dioxoalkanes [R'C(O)C(O)R'', at m/e (C,Hb + 30)], C, acyloins [R'C(O)CH(OH)R", at m/e (C,H2, + 32)], and C, alkanediols [R'CH(OH)CH(OH)R", at m/e (C,H2, + 34)]. C, oxoalkanes and C, epoxyalkanes observed at m/e (C,H2, + 16) are probably formed primarily via epoxidation of the alkene by 03. A general mechanism has been proposed to account for the observations.There is considerable evidencel-10 that the gas-phase reactions of ozone with alkenes can be described, at least in part, by the Criegee mechanism shown in eq 1 and 2 for solution-phase ozo-

CH3)C(=0)CH2' radical as well as the accompanying HO' radical was formed in the 03-TME system via reaction 5.To further examine the oxidation of CH3C(=0)CH2' radicals, the photolysis of mixtures containing parts-per-million concentrations of Cl2 and (CH3)2C=0 in air yielded methylglyoxal H3C(=0)CH0 as a major product, i.e., ca. 30% of A-[(CH3)2C=0], which can be attributed to the oxidation of the CH3C(=0)CH2' radical. Thus, the observation of CH3C(= OjCHO in the 03-TME-02 system (cf. Tables I and II) is consistent with the formation of this radical via reaction 5. A plausible mechanism for the formation of CH3C(=0)CH0 is CH3C(=0)CH2' 4-02 -CH3C(=0)CH200'; 2CH3C(=0)-CH200' -2CH3C(=0)CH20' 4-02; and CH3C(=0)CH20' 4-02 -* CH3C(=0)CH0 4-HOO'. Under atmospheric conditions, CH3C(=0)CH20' may react predominantly with 02 rather than undergoing unimolecular dissociation to CH3CO" and HCHO.12 No further quantitative determination of these competing reactions was pursued in the present study. Instead, at-tention was focused on the detection of a carbonyl product arising from the reaction CH3C(=0)CH200' 4-HOO* -CH3C(= 0)CH2OOH 4-02, since this compound was suspected to be the unidentified species exhibiting the C=0 band at ca. 1740 cm'1 in the 03-TME reaction (cf. Figure 2C). A broad band in this

Steady state concentrations of formyl radicals were measured with a photoionization mass spectrometer. The reaction of ethylene with oxygen atoms in a system free of OCHO+Oas was suggested by Groth and coworkers in 1938. The rate constant for this reaction was calculated to be (5.7±1.2)×10

The chemistry and energetics of the Criegee intermediate, a primary product of the ozonolysis of alkenes, are discussed in terms of recent ab-initio calculations and laboratory studies. The experimental observations in 03-alkene systems can be rationalized on the basis of a general mechanism: