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 imp ortance in atmospheric chemistry, is still poorly understood. Major un certainties regarding the reaction mechanism include the nature and th e formation yields of the carbonyl products, the formation yields of t he biradicals R(1)R(2)COO, and the subsequent reactions of these birad icals. In this study, the gas phase reaction of ozone with l-pentene, l-hexene, 1-heptene, 2,3-dimethyl-1-butene, cyclopentene, and 1-methyl cyclohexene has been studied with sufficient cyclohexane added to scav enge OH. Carbonyl products were identified as their DNPH derivatives b y liquid chromatography and chemical ionization mass spectrometry. Pri mary carbonyl formation yields for the 1-alkenes and 2,3-dimethyl-1-bu tene were close to the value of 1.0 that is consistent with the mechan ism 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 carbony ls. Data for the 1-alkenes, alpha = ca. 0.50, were consistent with abo ut 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 othe r than the primary carbonyls were measured, e.g., propanal from l-pent ene, consistent with the following reaction sequence: biradical --> hy droperoxide hydroxycarbonyl --> alpha-hydroxyalkyl radical --> carbony l + HO2. The alpha-dicarbonyls CH3(CH2)(n)COCHO, n = 1, 2, 3, e.g., 2- oxobutanal from 1-pentene, and/or the corresponding beta-hydroxycarbon yls 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 ob tained regarding carbonyl products of the cyclic compounds cyclopenten e (butanal, yield 0.12 +/- 0.00; glyoxal, yield 0.15 +/- 0.01) and 1-m ethylcyclohexene (formaldehyde, yield 0.04 +/- 0.01; 2-hexanone, yield ca. 0.04 +/- 0.01). Butanal and 2-hexanone form by loss of CO2 from t he carbonyl-bearing biradicals.