Proton-Catalyzed Decomposition of α-Hydroxyalkyl-Hydroperoxides in Water

In the atmosphere, most biogenic terpenes undergo ozonolysis in the presence of water to form reactive α-hydroxyalkyl-hydroperoxides (α-HHs), and the lifetimes of these α-HHs are a key parameter for understanding the processes that occur during the aging of atmospheric particles. We previously reported that α-HHs generated by ozonolysis of terpenes decompose in water to give H₂O₂ and the corresponding aldehydes, which undergo hydration to form gem-diols. Herein, we report that this decomposition process was dramatically accelerated by acidification of the water with oxalic, acetic, hexanoic, cis-pinonic, or hydrochloric acid. In acidic solution, the temporal profiles of the α-HHs, detected as their chloride adducts by electrospray mass spectrometry, showed single-exponential decays in the pH range from 4.1 to 6.1, and the first-order rate coefficients (k) for the decays increased with decreasing pH. The lifetime of the α-HH derived from α-terpineol was 128 min (k = (1.3 ± 0.4) × 10^-4 s^-1) at pH 6.1 but only 8 min (k = (2.1 ± 0.1) × 10^-3 s^-1) at pH 4.1. Because the rate coefficients increased as the pH decreased and the increase depended on pH rather than on the properties of the acid, we propose that the decomposition of the α-HHs in water was specifically catalyzed by H⁺. Fast H⁺-catalyzed decomposition of α-HHs could be an important source of H₂O₂ and multifunctionalized compounds found in ambient atmospheric particles.