es0c03438_si_001.pdf (192.14 kB)
Proton-Catalyzed Decomposition of α‑Hydroxyalkyl-Hydroperoxides in Water
journal contribution
posted on 2020-08-15, 14:13 authored by Junting Qiu, Kenichi Tonokura, Shinichi EnamiIn 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.