Ozone-Assisted Oxidation of C₅ Alkenes: Structural Effects on Criegee Intermediate Reaction Networks

Criegee intermediates (CIs), generated in alkene-ozone reactions, play a key role in atmospheric, synthetic organic, and combustion chemistry. However, challenges in detecting these intermediates and their subsequent reactions have hindered a thorough understanding of their chemical behavior in complex reactive environments. Here, we characterize the reactive behavior of CIs formed in the ozone-assisted oxidation of two C₅-alkenes, i.e., trans-2-pentene and cyclopentene. Photoionization efficiency curves obtained using a jet-stirred reactor and molecular-beam high-resolution mass spectrometry with tunable synchrotron radiation, in conjunction with ab initio calculations, enable the identification of elusive intermediates. The CIs formed in the ozone-assisted oxidation of trans-2-pentene, acetaldehyde oxide (CI-1) and propanal oxide (CI-2), are observed to undergo unimolecular isomerization to produce glycolaldehyde and 2-hydroxypropanal, respectively. Additionally, these CIs react bimolecularly with aldehydes and alkenes, resulting in the formation of products containing up to four oxygen atoms. Conversely, ozone-assisted cyclopentene oxidation produces only glutaraldehyde oxide (CI-3), which forms no detectable bimolecular adducts under our experimental conditions. Instead, CI-3 chemistry is dominated by isomerization and decomposition, forming 2-hydroxypentanedial and several lower-molecular-weight oxygenates, including 1,2,5-pentanetrione, glutaraldehyde, formic 5-oxopentanoic anhydride, 2-hydroperoxybut-3-enal, and 3-hydroperoxypropanal. These findings show that alkene structure governs CI fate, providing key constraints for modeling ozone-driven alkene oxidation processes.