- Importance of Unimolecular HO2 Elimination in the Heterogeneous OH Reaction of Highly Oxygenated Tartaric Acid Aerosol
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Oxygenated organic molecules are abundant in atmospheric aerosols and are transformed by oxidation reactions near the aerosol surface by gas-phase oxidants such as hydroxyl (OH) radicals. To gain better insights into how the structure of an organic molecule, particularly in the presence of hydroxyl groups, controls the heterogeneous reaction mechanisms of oxygenated organic compounds, this study investigates the OH-radical initiated oxidation of aqueous tartaric acid (C4H6O6) droplets using an aerosol flow tube reactor. The molecular composition of the aerosols before and after reaction is characterized by a soft atmospheric pressure ionization source (Direct Analysis in Real Time) coupled with a high-resolution mass spectrometer. The aerosol mass spectra reveal that four major reaction products are formed: a single C4 functionalization product (C4H4O6) and three C3 fragmentation products (C3H4O4, C3H2O4, and C3H2O5). The C4 functionalization product does not appear to originate from peroxy radical self-reactions but instead forms via an α-hydroxylperoxy radical produced by a hydrogen atom abstraction by OH at the tertiary carbon site. The proximity of a hydroxyl group to peroxy group enhances the unimolecular HO2 elimination from the α-hydroxylperoxy intermediate. This alcohol-to-ketone conversion yields 2-hydroxy-3-oxosuccinic acid (C4H4O6), the major reaction product. While in general, C-C bond scission reactions are expected to dominate the chemistry of organic compounds with high average carbon oxidation states (OSC), our results show that molecular structure can play a larger role in the heterogeneous transformation of tartaric acid (OSC = 1.5). These results are also compared with two structurally related dicarboxylic acids (succinic acid and 2,3-dimethylsuccinic acid) to elucidate how the identity and location of functional groups (methyl and hydroxyl groups) alter heterogeneous reaction mechanisms.
- Cheng, Chiu Tung,Chan, Man Nin,Wilson, Kevin R.
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p. 5887 - 5896
(2016/08/06)
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- Formation of Acetaldehyde from L-Ascorbic Acid and Related Compounds in Various Oxidation Systems
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The quantities of acetaldehyde formed from L-ascorbic acid, L-threonic acid, and D-erythrose upon oxidation with Fenton's reagent and upon UV irradiation were determined by gas chromatography.When 10 μmol of L-ascorbic acid was oxidized with Fenton's reagent, 316 nmol of acetaldehyde was recovered.When 10 μmol of L-ascorbic acid was irradiated (λ=300 nm) for 12 h, 297 nmol of acetaldehyde was produced.Upon oxidation with Fenton's reagent, L-threonic acid (10 μmol) and D-erythrose (10 μmol) produced 112 and 81.5 nmol of acetaldehyde, respectively.Acetaldehyde was formed from D-erythrose but not from L-threonic acid by photolysis, suggesting that acetaldehyde formation involved a Norrish type II reaction.Acetaldehyde was also formed from aqueous solutions of L-asorbic acid (122 nmol/10μmol), D-erythrose (37.0 nmol/10μmol), or L-threonic acid (27.9 nmol)/10μmol) with the presence of Fe2+ at 37 deg C under an air stream.This study is the first to report formation of acetaldehyde from ascorbic acid. Keywords: Antioxidant; acetaldehyde; ascorbic acid; low-density lipoprotein
- Miyake, Takashi,Shibamoto, Takayuki
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p. 1669 - 1672
(2007/10/02)
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