Measurements of Kinetics and Equilibria for the Condensed Phase Reactions of Hydroperoxides with Carbonyls to Form Peroxyhemiacetals

Heterogeneous/multiphase reactions can influence the formation, composition, and chemical–physical properties of secondary organic aerosol (SOA), but data describing their kinetics and equilibria remain sparse. Here, we synthesized and utilized a probe molecule to investigate the condensed phase reactions of hydroperoxides with ketones and aldehydes, including those in SOA generated from the ozonolysis of α-pinene in an environmental chamber. The probe molecule, which contained a hydroperoxide group and a UV-absorbing nitrate group, was mixed with a ketone (3-decanone) or aldehyde (nonanal) and monitored over 24 h using liquid chromatography with UV–vis detection to determine the rate and equilibrium constants for each reaction. The probe molecule did not react with the ketone but reacted reversibly with the aldehyde to form a peroxyhemiacetal, a process that was also catalyzed by carboxylic acid. The rate constant for the reversible decomposition of the peroxyhemiacetal was also measured using attenuated total reflectance Fourier transform infrared spectroscopy. The forward (f) and reverse (r) rate constants for uncatalyzed (u) and catalyzed (c) peroxyhemiacetal formation were k_f,u = 1.5 ± 0.4 M^–1 h^–1, k_r,u = 0.16 ± 0.001 h^–1, k_f,c = 0.62 ± 0.07 M^–2 h^–1, and k_r,c = 0.055 ± 0.006 M^–1 h^–1; and the equilibrium constant was K_eq = 9.1 ± 2 M^–1. No evidence of Baeyer–Villiger decomposition of the peroxyhemiacetal was observed. When mixed with α-pinene/O₃ SOA, the probe molecule reached reaction equilibrium within 20 min, indicating that atmospheric timescales for peroxyhemiacetal formation can be short. Using the results of the nonanal experiments and measured carbonyl content of the SOA, we estimate that up to 25% of the carbonyls in this SOA was aldehydes.