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

Abstract

In the atmosphere, most biogenic terpenes undergo ozonolysis in the presence of water to form reactive alpha-hydroxyalkyl-hydroperoxides (alpha-HHs), and the lifetimes of these a-HHs are a key parameter for understanding the processes that occur during the aging of atmospheric particles. We previously reported that alpha-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 alpha-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 alpha-HH derived from alpha-terpineol was 128 min (k = (1.3 +/- 0.4) x 10(-4) s(-1)) at pH 6.1 but only 8 min (k = (2.1 +/- 0.1) x 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 alpha-HHs in water was specifically catalyzed by H+. Fast H+-catalyzed decomposition of alpha-HHs could be an important source of H2O2 and multifunctionalized compounds found in ambient atmospheric particles.