OZONOLYSIS OF TRANS-2-BUTENES AND CIS-2-BUTENES IN LOW PARTS-PER-MILLION CONCENTRATION RANGES

Ozonolysis of 1-5 ppm concentrations of trans- and cis-2-C4H8 was carried out in a 580 l spherical glass reaction vessel at 730 +/- 5 torr and 296 +/- 2 K. The yields of CH3CHO, HCHO, CO, CO2, CH4, and CH3OH were determined by long-path FTIR spectroscopy. About 60% of C4H8 that reacted with O3 decomposed via the formation of the excited CH3CHO2* intermediates into the following pathways: (4a) CO2 + CH4, (4b) CO2 + H + CH3, (4c) CO + OH + CH3, and (4d) CO + CH3OH. The branching ratios for each channel, expressed as the percent of the total pathways, were determined for trans isomer: 20, 30, 40, and 10, and for cis isomer: 29, 35, 24, and 12, respectively. The conversion of C4H8 relative to the reacted O3 was about 1.6 and 1.4 for trans and cis isomers, respectively. These results were explained by the reactions of OH radicals formed in (4c) with C4H8, in which secondary OH radicals were generated: C4H8 + OH + O2 --> CH3-CH(OH)-CH(CH3)OO, followed by CH3-CH(OH)-CH(CH3)OO --> 2 CH3CHO + OH. About 40% of C4H8 that reacted with O3 yielded a mixture of a carbonyl and a noncarbonyl product, assigned as hydroxyethyl formate, CH3CH(OH)-O-CHO, and secondary butene ozonide, respectively. The addition of HCHO increased the formation of the former while the latter was unaffected. These results were consistent with the mechanism proposed by Cremer et al. [Chem. Phys. Lett., 187, 491 (1991)], where the primary ozonide rearranges, before dissociation, to the carbonyl oxide-aldehyde complex (the dipole complex) which is the precursor of the secondary ozonide. (C) 1994 John Wiley & Sons, Inc.