Gas phase reaction of alkenes with ozone: Formation yields of primary carbonyls and biradicals

The gas phase reaction of ozone with alkenes plays a major role in tropospheric chemistry including urban air quality. The reaction of trans-2,2-dimethyl-3-hexene and 2,4-dimethyl-2-pentene with ozone has been studied at ambient temperature and p = 1 atm of air (RH = 55% +/- 10%) with sufficient cyclohexane added to scavenge the hydroxyl radical. Carbonyl products have been identified, and their formation yields are reported. The results are compared to those previously obtained for 35 other alkenes under the same conditions. For these alkenes, the sums of the formation yields of the primary carbonyls are close to the value of 1.0 that is consistent with the following reaction mechanism: O-3 + R1R2C=CR3R4 --> alpha(R1COR2 + R3R4COO) + (1 - alpha)(R1R2COO + R3COR4), where R1COR2 and R3COR4 are the primary carbonyls and R1R2COO and R3R4COO are the corresponding biradicals. For 26 nonsymmetrical alkenes, the coefficients alpha range from 0.28 to 0.82 and indicate preferential formation of the more substituted biradicals and of the biradicals that bear the less bulky substituents. The results are directly relevant to the atmospheric chemistry of alkenes and to their role in oxidant and aerosol formation.