Ozone‐Assisted Oxidation of C ₅ 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 C 5 ‐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.