Carbonyl products of the gas phase reaction of ozone with C-5-C-7 alkenes

The gas phase reaction of ozone with alkenes, which is of critical importance in atmospheric chemistry, is still poorly understood. Major uncertainties regarding the reaction mechanism include the nature and the formation yields of the carbonyl products, the formation yields of the biradicals R(1)R(2)COO, and the subsequent reactions of these biradicals. In this study, the gas phase reaction of ozone with l-pentene, l-hexene, 1-heptene, 2,3-dimethyl-1-butene, cyclopentene, and 1-methylcyclohexene has been studied with sufficient cyclohexane added to scavenge OH. Carbonyl products were identified as their DNPH derivatives by liquid chromatography and chemical ionization mass spectrometry. Primary carbonyl formation yields for the 1-alkenes and 2,3-dimethyl-1-butene were close to the value of 1.0 that is consistent with the mechanism O-3 + R(1)R(2)C=CH2 --> alpha(HCHO + R(1)R(2)-COO) + (1 - alpha)(R(1)COR(2) + H2COO), where HCHO and R(1)-COR(2) are the primary carbonyls. Data for the 1-alkenes, alpha = ca. 0.50, were consistent with about equal formation yields for the biradicals H2COO and RCHOO. Data for 2,3-dimethyl-1-butene, a = 0.64 +/- 0.01, were consistent with modest preferential formation of the disubstituted biradical. Carbonyls other than the primary carbonyls were measured, e.g., propanal from l-pentene, consistent with the following reaction sequence: biradical --> hydroperoxide hydroxycarbonyl --> alpha-hydroxyalkyl radical --> carbonyl + HO2. The alpha-dicarbonyls CH3(CH2)(n)COCHO, n = 1, 2, 3, e.g., 2-oxobutanal from 1-pentene, and/or the corresponding beta-hydroxycarbonyls CH3(CH2)(n)CHOHCHO were also observed to form from the l-alkenes, and this with low yields of 0.02-0.03. Only limited information was obtained regarding carbonyl products of the cyclic compounds cyclopentene (butanal, yield 0.12 +/- 0.00; glyoxal, yield 0.15 +/- 0.01) and 1-methylcyclohexene (formaldehyde, yield 0.04 +/- 0.01; 2-hexanone, yield ca. 0.04 +/- 0.01). Butanal and 2-hexanone form by loss of CO2 from the carbonyl-bearing biradicals.