Mechanism and Product Distribution of the O3-Initiated Degradation of (E)-2-Heptenal, (E)-2-Octenal, and (E)-2-Nonenal

Article abstract of DOI:10.1021/acs.jpca.7b01857

The O₃-molecule initiated degradation of three 2-alkenals (E)-2-heptenal, (E)-2-octenal, and (E)-2-nonenal has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The experiments were performed in the absence of an OH scavenger. The molar yields of the primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-2-heptenal with O₃, glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2-octenal with O₃, and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2-nonenal with O₃. The residual bands in the infrared product spectra for each of the studied reactions are attributed to 2-oxoaldehyde compounds. Based on the observed products, a general mechanism for the ozonolysis reaction of long chain unsaturated aldehydes is proposed, and the results are compared with the available literature data.

Full text of DOI:10.1021/acs.jpca.7b01857

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Article
3
Mechanism and Product Distribution of the O-Initiated
Degradation of (E)-2-Heptenal, (E)-2-Octenal and (E)-2-Nonenal
Elizabeth Gaona Colmán, María Belén Blanco, Ian Barnes, Peter Wiesen, and Mariano Andrés Teruel
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01857 • Publication Date (Web): 16 Jun 2017
Downloaded from http://pubs.acs.org on June 20, 2017
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Mechanism and Product Distribution of the O -Initiated Degradation of
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(E)-2-Heptenal, (E)-2-Octenal and (E)-2-Nonenal
a
a*
b
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Elizabeth Gaona Colmán , María B. Blanco , Ian Barnes , Peter Wiesen and Mariano A.
a*
Teruel
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a
Instituto de Investigaciones en Fisicoquímica de Córdoba (I.N.F.I.Q.C.), Facultad de Ciencias Químicas,
Universidad Nacional de Córdoba. Ciudad Universitaria, 5000 Córdoba, Argentina.
Bergische Universität Wuppertal, Fakultät für Mathematik und Naturwissenschaften, Institut für
Atmosphären und Umweltforschung, Gauss Strasse 20, 42119 Wuppertal, Germany.
b
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Corresponding authors: mteruel@fcq.unc.edu.ar.
mblanco@fcq.unc.edu.ar.
Abstract
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The O ꢀmolecule initiated degradation of three 2ꢀalkenals (E)ꢀ2ꢀheptenal, (E)-2ꢀoctenal and (E)ꢀ2ꢀ
3
nonenal has been investigated in a 1080 L quartzꢀglass environmental chamber at 298 ± 2 K and
atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and
products. The experiments were performed in the absence of OH scavenger. The molar yields of the
primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-
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2ꢀheptenal with O , glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2ꢀoctenal
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with O and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2ꢀnonenal with
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O . The residual bands in the infrared product spectra for each of the studied reactions are attributed
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to 2ꢀoxoaldehyde compounds. Based on the observed products, a general mechanism for the
ozonolysis reaction of long chain unsaturated aldehydes is proposed and the results are compared
with the available literature data.
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Introduction
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The C7ꢀC9 2ꢀalkenals (E)-2ꢀheptenal, (E)-2ꢀoctenal and (E)-2ꢀnonenal are volatile organic
1,2
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compounds present in cranberries, Citrullus lanatus, and raw potatoes. (E)-2ꢀNonenal has been
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,6
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detected as a volatile organic constituent of Cucumis sativus L. and apples. The emissions of
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these compounds are attributed to lipid oxidation in the vegetal membrane. Biodiesels also emit
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aldehydes, Cahill and Okamoto and do N. Batista et al. have reported among other aldehydes the
emission of 2ꢀheptenal from the combustion of soybeanꢀ and animalꢀbased biodiesel. Additionally,
oils at frying temperatures emit several aldehydes among them (E)-2ꢀalkenal type aldehydes with
1
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C3ꢀC11 carbon chains and also toxic oxygenated (E)-2ꢀalkenal type aldehydes.
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Unsaturated aldehydes once emitted to the atmosphere will be subject to degradation by photolysis
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and reaction with OH radicals during day and by reaction with NO radicals during the late evening
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and night. Reaction of unsaturated aldehydes with O molecules is possible both day and night but
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will only play a significant role in polluted areas with elevated ozone concentrations. Recent studies
have shown that the gasꢀphase and aqueousꢀphase oxidation of these compounds can form
secondary organic aerosols (SOA) and that they are precursors for polar organosulphates which
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have been found in ambient aerosol.
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We have previously reported a kinetic study on the reaction of O with (E)-2ꢀheptenal, (E)-2ꢀoctenal
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and (E)-2ꢀnonenal at 298 K using the relative rate method, the obtained kO3 in units of ×10
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ꢀ1 ꢀ1
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(cm molecule s ) were: (2.47 ±0.73) for (E)ꢀ2ꢀheptenal, (2.37 ± 0.68) for (E)ꢀ2ꢀoctenal and (2.05 ±
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0.20) for (E)ꢀ2ꢀnonenal. These rate coefficients showed that for the studied longꢀchain aldehydes
increasing the alkyl chain length has a virtually negligible influence on the measured rate
coefficients, i.e. above C3 the electronꢀdonating effect of the alkyl chain has no further effect on the
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reactivity of the double bond toward O addition. Kinetic studies for reaction of these aldehydes
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with other atmospheric oxidants have been reported. A room temperature rate coefficient for the
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reaction of OH radicals with (E)-2ꢀheptenal has been reported by Albaladejo et al. and Davis et
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al. have investigated the temperature dependence of the reaction over the range 244 – 374 K. Gao
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et al. have determined the rate coefficients for the reactions of OH radicals with (E)-2ꢀoctenal and
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(E)-2ꢀnonenal at 298K. A room temperature rate coefficient for the reaction of (E)-2ꢀheptenal with
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NO radicals has been obtained by Zhao et al. Kerdouci et al. have reported a value for the same
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reaction at 204 K in addition to a rate coefficient for the reaction of (E)-2ꢀoctenal with NO radicals.
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Finally, Rodríguez et al. have measured the rate coefficient for the reaction of Cl atoms with (E)-
2ꢀheptenal at room temperature.
There have been a number of previous product studies on the reaction of O with unsaturated
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aldehydes, most of them, however, on shortꢀchain aldehydes (<C6)
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Grosjean et al. have studied
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the products resulting from the ozonolysis of the C3 2ꢀalkenal acrolein (CH =CHC(O)H) at
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temperatures between 12ꢀ17°C, Grosjean and Grosjean have investigated the products from the
ozonolysis of the C4 2ꢀalkenal crotonaldehyde (CH CH=CHC(O)H) at room temperature and
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Grosjean et al. the products resulting from the reaction of O with the C6 2ꢀalkenal transꢀ2ꢀ
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hexenal at ambient temperature. More recently, Uchida et al. have studied the products produced
in the ozonolysis of the C6 3ꢀalkenal (Z)ꢀ3ꢀhexenal under atmospheric conditions. To our
knowledge, there are no previous products studies for the reactions of OH radicals with the C7ꢀC9
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2ꢀalkenals under study in this work, however, Kerdouci et al. have reported product analyses for
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the reactions of NO radicals with (E)-2ꢀhexenal, (E)-2ꢀheptenal and (E)-2ꢀoctenal where they
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observed that unsaturated peroxynitrate type compounds were the main products formed via Hꢀatom
abstraction by NO from the aldehyde entity and subsequent consecutive additions of O and NO to
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the resulting CH (CH ) CH=CHC(=O) radicals (n = 2, 3 or 4).
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The primary aim of the present work is to identify and quantify the gasꢀphase products produced in
the room temperature ozonolysis of the longꢀchain C7ꢀC9 2ꢀalkenals (E)-2ꢀheptenal, (E)-2ꢀoctenal
and (E)-2ꢀnonenal. Identification of the primary reaction products generated in the ozonolysis of the
2ꢀalkenals will provide information on the effect of the aldehyde functionality on the fragmentation
of the primary ozonide compared to that observed for alkyl substituted alkenes. The secondary aim
of the study is to postulate mechanisms for the reactions leading to the observed products and to
evaluate potential atmospheric implications of the reactions. To the best of our knowledge, this
work represents the first report of products and product yields for the ozonolysis of the longꢀchain
aldehydes (E)-2ꢀheptenal, (E)-2ꢀoctenal and (E)-2ꢀnonenal.
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Experimental set up
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A 1080 L reaction chamber was used to perform the experiments at (298 ± 2) K in 750 Torr of
synthetic air. The chamber consists of two cylindrical quartz glass vessels, each 3 m in length and
45 cm inner diameter, joined in the middle with both open ends closed by aluminium flanges. The
reactants and the bath gases were introduced through ports located on the metal flanges. To ensure
homogeneous mixing of the reactants three fans with Teflon blades are mounted inside the
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chamber. The reaction chamber can be evacuated to <10 Torr by a pumping system consisting of a
turbomolecular pump backed by a double stage rotary fore pump. Whiteꢀtype mirrors mounted
internally in the chamber are coupled via an external optical transfer system to a Thermo Nicolet
Nexus FTIR spectrometer. The “White” mirror system in the chamber was operated with the total
optical absorption path length set to 484.7 m. The spectrometer is equipped with a liquid nitrogen
cooled mercuryꢀcadmiumꢀtelluride (MCT) detector. This setup enables “in situ” monitoring of the
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reactants and products in the infrared range 4000ꢀ700 cm . Infrared spectra were recorded with a
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spectral resolution of 1 cm .
For the experiments of the ozonolysis of (E)-2ꢀheptenal, 80 interferograms were coꢀadded per
spectrum and 20 such spectra were recorded per experiment, for the ozonolysis of (E)-2ꢀoctenal 75
interferograms were coꢀadded per spectrum and 22 such spectra were recorded per experiment and
for the ozonolysis of (E)-2ꢀnonenal 100 interferograms were coꢀadded per spectrum and 25 such
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spectra were recorded per experiment. The reactor is described in greater details in Barnes et al.
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Ozone was produced by flowing O through an electrical discharge and was added continuously to
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the chamber containing the aldehyde homogeneously mixed in synthetic air. The O concentration
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introduced into the chamber was up to a maximum of 7 ppmV. The experiments were performed in
the absence of an OH radical scavenger since addition of very large concentrations of scavengers
such as CO or cyclohexane would have been necessary to scavenge the OH in the reaction systems
and such large concentrations would have made an FTIR analysis of the system impossible. The
consequences of the nonꢀuse of an OH radical scavenger in the experiments for the results are
discussed in detail further in the Results and Discussion section. Two experiments were performed
for each of the studied reactions.
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Reaction products were quantified by comparison with reference spectra contained in the IR
spectral database of the laboratory in Wuppertal. Pentanal and hexanal were quantified using
absorption cross sections from the database of the laboratory in Wuppertal while glyoxal and
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heptanal were quantified using cross section values reported by Volkamer et al. and Beaver et
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al., respectively. All three unsaturated aldehydes were monitored in the infrared absorption region
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2700ꢀ3000 cm . Identified products were monitored at the following infrared absorption regions (in
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cm ): glyoxal, 2748ꢀ2910; pentanal, 2650ꢀ3000; hexanal, 2600ꢀ3000; heptanal, 2650ꢀ3000. The
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initial concentrations of the unsaturated aldehydes in ppm (1 ppmV = 2.46 × 10 molecule cm at
298 K) were (E)-2ꢀheptenal 0.87ꢀ0.88; (E)-2ꢀoctenal 0.77ꢀ0.83 and (E)-2ꢀnonenal 0.64ꢀ0.66.
The chemicals used in the experiments had the following purities as given by the manufacturer and
were used as supplied: synthetic air (Air Liquide, 99.999%), (E)-2ꢀheptenal (Aldrich 97%), (E)ꢀ2ꢀ
octenal (Aldrich, ≥95%) and (E)-2ꢀnonenal (Aldrich, 97%)
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Results and Discussion
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Figure 1, panel A, shows the infrared spectrum of a (E)ꢀ2ꢀheptenal/O /air gas mixture after reaction
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and subtraction of residual (E)ꢀ2ꢀheptenal. Reference spectra of glyoxal and pentanal are shown in
panels B and C, and panel D shows the residual product spectrum obtained after subtraction of
features due to the reference spectra from the spectrum in panel A. Similarly, Figure S1 (see
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Supplementary Information, SI), panel A, shows the infrared spectrum of a (E)ꢀ2ꢀoctenal/O /air gas
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mixture after reaction and subtraction of residual (E)ꢀ2ꢀoctenal (Panel A). Reference spectra of
glyoxal and hexanal are shown in panels B and C, and panel D shows the residual product spectrum
obtained after subtraction of features due to the reference spectra from the spectrum in panel A.
Figure S2 shows the same infrared spectral information obtained from experiments performed on a
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(E)ꢀ2ꢀnonenal/O /air gas mixture, however, in this case panel C shows a reference spectrum of
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heptanal.
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Formation of pentanal, hexanal and heptanal has been positively identified in the ozonolysis of (E)ꢀ
2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀnonenal, respectively. Glyoxal was identified as product in the
ozonolysis of all three unsaturated aldehydes. Concentration−time profiles of (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀ
octenal and (E)ꢀ2ꢀnonenal and the identified products are presented in Figures 2, S3 and S4,
respectively. Plots of the concentrations of the products as a function of consumed 2ꢀalkenal are
shown in Figures 3, S5 and S6 for (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀnonenal, respectively.
Reasonably good linear correlations were observed in all cases. The molar product yields obtained
from linear leastꢀsquares analyses of the plots in Figures 3, S5 and S6 are listed in Table 1. The
errors given in Table 1 are the 2σ standard deviations from the analyses of the plots.
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Ozonolysis reactions of unsaturated organics are initiated by addition of the O molecule to the
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double bond of the compound to form a 1,2,3ꢀtrioxolane primary ozonide. This primary ozonide is
energy rich and rapidly decomposes by cleavage of a CꢀC and an OꢀO bond to form a primary
carbonyl and an initially vibrationally excited biradical known as a Criegee Intermediate (CI). The
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excited CI can unimolecular decompose ejecting an O( P) atom, isomerize to form a hydroperoxide,
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rearrange to form an ester or acid or can be collisionally stabilized with the bath gas molecules.
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The soꢀcalled hydroperoxide channel is known to form OH radicals to varying degrees through
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decomposition of the hydroperoxide.
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A very simplified general mechanism for the reaction of O with (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and
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(E)ꢀ2ꢀnonenal, not showing all possible reactions channels, is shown in Figure 4. The O molecule
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reacts with the 2ꢀalkenals by addition to the double bond to form a primary ozonide, which
decomposes through channels A and B indicated in Figure 4. Decomposition of the ozonide through
channel A forms glyoxal HC(O)C(O)H and CH (CH ) CꢁHOOꢁ Criegee biradical where “n”
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indicates the number of ꢀCH ꢀ groups, i.e. 3, 4 and 5 for (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀ
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nonenal, respectively. The vibrationally excited biradicals formed through channel A can
potentially react via the hydroperoxide channel. Decomposition of the hydroperoxide can produce
OH radicals and further reactions of the organic radical coꢀproduct can lead to the formation of a 2ꢀ
oxoaldehyde and a hydroperoxide radical as shown in Figure 4. Due to the lack of reference spectra
we are currently not able to unambiguously confirm whether or not the 2ꢀoxoaldehyde formation
166
1
1
1
1
67
68
69
70
channel is operative. The alkoxy radical (CH (CH ) CH(Oꢁ)CH(O)) can also isomerize as it is
3
2 m
shown in Figure 4. We proposed the 1,5 hydrogen shift of the alkoxy radical when m=2 (from the
171
ozonolysis of (E)-2ꢀheptenal). The isomerization and subsequence reaction with O and peroxy
2
1
1
72
73
radical can lead to other 1,5 hydrogen shift of the new alkoxy radical to finally formed a
hydroxydicarbonyl compound through reaction with O . On the other hand, decomposition of the
2
174
primary ozonide through channel B forms the HC(O)CꢁHOOꢁ Criegee biradical and the
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corresponding saturated aldehydes CH (CH ) C(O)H (n = 3, 4, or 5) which in the case of the
3
2 n
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ozonolysis of (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀnonenal will be pentanal, hexanal and
heptanal, respectively.
1
1
1
1
77
78
79
80
Under our experimental conditions the ozonolysis of (E)ꢀ2ꢀheptenal forms glyoxal and pentanal as
primary products with molar yields of (49 ± 4) % and (34 ± 3) %, respectively. In the case of (E)ꢀ2ꢀ
octenal the primary products are glyoxal and hexanal with yields of (41 ± 3) % and (39 ± 3) %,
respectively, and in the case of (E)ꢀ2ꢀnonenal glyoxal and heptanal with yields of (45 ± 3) % and
(46 ± 3) %, respectively. As mentioned previously in the Introduction section, the ozonolysis
products of the three 2ꢀalkenals studied here have not been previously reported in the literature;
therefore, a direct comparison with other studies for the same 2ꢀalkenals is not possible.
0
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1
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87
88
As already stated a number of times the ozonolysis of alkenes is known to produce OH radicals
which can influence the yields of the primary carbonyls measured in the ozonolysis of the 2ꢀ
alkenals studied here. The OH radical yields for the ozonolysis of a large number of alkenes have
36
been reported in the literature, however, none have been reported to date for 2ꢀalkenals under
37
189
study. Rickard et al. have developed a structure activity relationship (SAR) to rationalize the OH
yields in ozoneꢀalkene reactions as a function of the alkene structure, however, unfortunately this
SAR can not be applied to estimate OH yields for 2ꢀalkenals. The Criegee biradicals formed in the
ozonolysis of the unsaturated aldehydes will have the structures CH (CH ) C·HOO· (n = 3, 4 or 5)
1
1
90
91
192
3
2 n
1
1
1
1
93
94
95
96
and HC(=O)C·HOO· and can potentially form the hydroperoxides CH (CH ) CH=CHOOH (n = 2,
3 2 n
3, 4) and O=C=CHꢀOOH, respectively. It is known from studies on the ozonolysis of alkenes that
decomposition of CH (CH ) CH=CHOOH hydroperoxides will undoubtedly produce OH radicals
3
2 n
36
to some degree, however, nothing is currently known about the formation and fate of the
O=C=CHꢀOOH hydroperoxide. If O=C=CHꢀOOH is formed, on decomposition it could form OH
radicals and ethylenedione (O=C=C=O) and/or CO/CO2:
197
1
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98
99
00
O=C=CHꢀOOH → (O=C=CHꢀO· ↔ O=C(·)ꢀC(=O)H) + ·OH
O=C=CHꢀO· + O → O=C=C=O + HOO·
2
201
O=C(·)ꢀC(=O)H + O → → CO + CO + HOO·
2
2
2
2
02
03
3
8
Ethylenedione has only recently been detected spectroscopically and is known to be intrinsically
204
05
shortꢀlived and to dissociate to give two groundꢀstate CO molecules. If formed, and long enough
2
lived, it would absorb in the infrared region between the CO and CO absorptions as is observed for
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carbon suboxide (O=C=C=C=O). In the infrared region from 2000 to 2400 cm , only absorptions
207
due to CO and CO are visible in the product spectra and there is nothing to indicate the
2
2
2
2
2
08
09
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11
intermediary formation of a compound such as ethylenedione. Also the amount of CO that was in
the reaction system initially did not change during the course of the ozonolysis. Unfortunately, it is
not possible to say what the trend is in CO since the signal in the CO absorption region is
0
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2
saturated due to CO in the transfer optics housing between the chamber and FTIR spectrometer.
2
212
However, the infrared and CO concentration observations support that the O=C=CHꢀOOH
hydroperoxide channel and thus OH radical formation is probably not occurring for the
HC(=O)C·HOO· Criegee biradical. Possible other reactions of the excited HC(=O)C·HOO·
biradical include i) ejection of an O atom through the O-atom channel to form glyoxal, ii)
isomerization via the “hot” acid channel with formation of glyoxylic acid (HC(=O)C(=O)OH) and
iii) stabilization of the excited biradical. The product spectra of the three alkenals were compared
with an IR spectrum of glyoxylic acid and we do not observed the formation of this acid (see
Figure S7), therefore, we assumed that ejection of an Oꢀatom to form glyoxal and collisional
stabilization of the biradical are the main fates of the excited HC(=O)C·HOO· biradical. The
stabilized HC(=O)C·HOO· biradical is also quite likely under our experimental conditions to react
further to form glyoxal. This observation has consequences for the interpretation of the preferred
fragmentation channel of the primary ozonide and is discussed further below. Additionally, Uchida
2
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2
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220
2
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27
2
2
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25
26
27
et al. in their study of the ozonolysis of (Z)ꢀ3ꢀhexenal proposed that the alkyl chain biradical can
decompose through two pathway. Based on their work, the alkyl substituted biradical of the
ozonolysis reaction under study CH (CH ) C·HOO· (n = 3, 4 or 5) can undergo the following
3
2 n
decomposition pathway:
228
2
2
29
30
CH (CH ) C·HOO· (n = 3, 4 or 5) → CO + OH + CH (CH )
3
2 n
3
2 n
231
CH (CH ) C·HOO· (n = 3, 4 or 5) → CO + CH (CH ) CH (x= 2, 3 or 4)
3
2 n
2
3
2 x
3
2
2
32
33
However, as stated above, we did not observe variations in the CO or CO concentrations in our
2
234
experimental conditions. For the second reaction, where an alkane is formed, we compare the nꢀ
butane, nꢀpentane and nꢀhexane infrared spectra with the residual product spectra of the ozonolysis
of (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀnonenal, respectively. The CꢀH stretching region from
2
2
35
36
ꢀ1
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38
−CH − and −CH groups (2800ꢀ3032 cm ) of the alkanes was no observed in the residual product
2
3
2
spectra of the studied reactions.
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Paulson et al. have determined an OH yield of 16 % for the ozonolysis of methyl vinyl ketone
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(CH =CHC(=O)CH ) and Uchida et al. have reported an OH formation yields of 32 % for the
2
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2
2
2
41
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43
44
ozonolysis of (Z)ꢀ3ꢀhexenal (CH CH CH=CHCH C(O)H). Since in our ozonolysis systems it would
3 2 2
appear, based on the discussion above, that only the CH (CH ) C·HOO· (n = 3, 4 or 5) Criegee
3
2 n
2
7
biradicals will be producing OH, based on the work of Uchida et al. we estimate that OH radical
0
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production is probably of the order of 15 % or less.
245
Despite the possible effect that OH radical reactions can have on the yields of the primary carbonyls
measured for the ozonolysis of the C7ꢀC9 2ꢀalkenals, the measured yields are very similar to those
2
2
46
47
2
6
reported in the literature for shorter chain 2ꢀalkenals. Grosjean et al. have studied the product
formation from the ozonolysis of the C6 2ꢀalkenal transꢀ2ꢀhexenal and obtained formation yields
for the primary products glyoxal and butanal of (55.9 ± 3.7) % and (52.7 ± 5.5) %, respectively. The
experiments were carried out in a Teflon chamber at 298K and 760 Torr monitoring the O3
concentration by ultraviolet photometry and analyzing the carbonyl compounds by liquid
chromatography with UV detection. In the experiments cyclohexane was added to scavenge any OH
248
2
2
2
2
49
50
51
52
27
253
radicals formed. Uchida et al. have studied the product formation from the ozonolysis of the C6 3ꢀ
alkenal (Z)ꢀ3ꢀhexenal in a chamber at 298 K and 1 atm using inꢀsitu longꢀpath Fourier transform
infrared (FTIR) spectroscopy for the analysis. The authors report a yield of (35 ± 1) % for the
primary product propanal obtained using 1,3,5ꢀtrimethylbenzene and CO as OH radical scavengers.
2
2
54
55
256
24
2
2
2
2
57
58
59
60
Grosjean and Grosjean have studied the products formed in the ozonolysis of the C3 2ꢀalkenal
2
5
acrolein (CH =CHCHO) and Grosjean and Grosjean those formed in the ozonolysis of the C4
2
2
4
alkenal crotonaldehyde (CH CH=CHCHO). The experiments of Grosjean and Grosjean on the
3
3
ozonolysis of acrolein were performed at 17, 14, 13 and 12°C in a 3.5 m Teflon chamber,
261
cyclohexane was used as an OH radical scavenger and the carbonyl products were sampled on
cartridges coated with 2,4ꢀdinitrophenyhydrazine and subsequently analyzed by liquid
chromatography with ultraviolet detection. The main carbonyl compounds detected were
formaldehyde and glyoxal. The authors give amount of the carbonyls formed in ppb, however,
unfortunately they do not provide any yield information. The experiments of Grosjean and
2
2
62
63
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2
2
2
2
65
66
67
68
25
Grosjean on the ozonolysis of crotonaldehyde were similarly performed in a large volume FEP
Teflon chamber at ambient temperature and atmospheric pressure in the presence of cyclohexane as
an OH radical scavenger. The O concentration was monitored by ultraviolet photometry and the
3
269
concentration of the carbonyl compounds were analyzed by liquid chromatography with UV
detection after collection on cartridges. In this study the authors report formation yields of the
primary carbonyl products glyoxal and acetaldehyde of (47 ± 2) % and (42 ± 3) %, respectively.
2
2
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Based on the generally accepted mechanism for the ozonolysis of alkenes the sum of the yields of
the two primary carbonyl products formed from the two possible primary ozonide decomposition
channel (see Figure 4) should be unity. The sum of the primary carbonyls is (83 ± 7) % for the
ozonolysis of (E)ꢀ2ꢀheptenal, (80 ± 6) % for the ozonolysis of (E)ꢀ2ꢀoctenal, (91 ± 6) % for the
ozonolysis of (E)ꢀ2ꢀnonenal. The sums of the yields of the primary carbonyls from the ozonolysis of
the (E)ꢀ2ꢀalkenals studied here is ≥ 80% and there is no apparent preference for one of the two
decomposition routes of the primary ozonide with only a slight basis in the case of (E)ꢀ2ꢀheptenal
for the glyoxal forming channel. However, it should be noted that the sum of the yields of two
273
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76
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81
primary carbonyl products (glyoxal and alkanal) could be higher than unity due to glyoxal
yield include the oxygen ejection pathway contribution. As can be deduced from the above
discussion on other literature studies on the ozonolysis of 2ꢀalkenals similar yields and no trend
toward a particular primary ozonide decomposition channel have also been observed in these
studies.
282
2
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2
83
84
85
86
It is interesting to note that the biradicals formed from the decomposition of the primary ozonide
through channels A and B (see Figure 4) are both monoꢀsubstituted biradicals, the only difference
being the type of the substituent. The biradical formed through channel A has an alkyl chain
substituent and the biradical formed through channel B has an acyl group substituent. Studies have
shown that the ozonolysis of asymmetrically alkylꢀsubstituted alkenes generally results in
287
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89
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43
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preferential formation of the more substituted biradical.
Moreover, Wegener et al. state that
2
2
91
92
after decomposition of the primary ozonide generally electronꢀdonating substituents remain in the
biradical while electron acceptor substituents will be found in the carbonyl compound. However, as
discussed above such a preference has not observed for the 2ꢀalkenals studied here and also for the
other 2ꢀalkenals reported in the literature all of which contain an electronꢀacceptor, acyl, and an
electronꢀdonating, alkyl, substituent. There is conjugation between the carbonyl group and the
double bond in the 2ꢀalkenals and we speculate that the addition of ozone may not form a primary
ozonide in the classical sense as is accepted for alkyl substituted alkenes and that the preferential
fragmentation patterns observed for alkyl substituted alkenes may not apply for 2ꢀalkenals and
possibly also 2ꢀalkenones.
293
94
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2
2
2
96
97
298
2
3
99
00
After subtraction of the infrared adsorptions due to the identified compounds for each of the studied
reactions, the residual infrared spectra (see panel D in Figures 1, S2, and S3) show distinct
301
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3
3
3
3
02
03
04
05
absorption bands in the carbonylꢀstretching region (~1757 cm ) and the CꢀH stretching region from
ꢀ
1
ꢀCH and ꢀCH groups (2880ꢀ3000 cm ). As noted previously, 2ꢀoxoaldehyde compounds are
2
3
potential reaction products, formation of which can not be experimentally validated due to the lack
26
of authentic samples of the compounds. Grosjean et al. have observed the formation of 2ꢀ
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oxobutanal with a yield of ~7.4 % in the ozonolysis (E)-2ꢀhexenal, therefore, formation of 2ꢀ
oxoaldehydes in our study on the ozonolysis of C7ꢀC9 2ꢀalkenals is expected to be likely. In order
to try and verify formation of 2ꢀoxoaldehydes in the ozonolysis reactions infrared spectra have been
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computed for 2ꢀoxopentanal (proposed product from the reaction of (E)ꢀ2ꢀheptenal + O ), for 2ꢀ
3
oxohexanal (proposed product from the reaction of (E)ꢀ2ꢀoctenal + O ) and for 2ꢀoxoheptanal
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(proposed product from the reaction of (E)ꢀ2ꢀnonenal + O ). The computed spectra were obtained
3
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4ꢀ46
312
with the Gaussian 09 package at the B3LYP/6ꢀ311++G (d, 2p) level of theory.
Figure 5 shows a
3
3
13
14
comparison between the computed IR spectrum for the expected product 2ꢀoxopentanal and the
residual experimental product IR spectrum obtained from the ozonolysis (E)-2ꢀheptenal after
subtraction of the primary carbonyl products. Figures S8 and S9 show the corresponding
comparisons for the ozonolysis of (E)-2ꢀoctenal and (E)-2ꢀnonenal where 2ꢀoxohexanal and 2ꢀ
oxoheptanal, respectively, are the potential 2ꢀoxoaldehyde products. The salient features from the
residual spectra for each of the studied reactions show similarity with the computed spectra. This
similarity suggests that many of the absorption features in the residual product spectra obtained
from the ozonolysis of (E)ꢀ2ꢀheptenal, (E)ꢀ2ꢀoctenal and (E)ꢀ2ꢀnonenal are quite possibly due to the
formation of 2ꢀoxopentanal, 2ꢀoxohexanal and 2ꢀoxoheptanal, respectively. However, an infrared
spectrum of each of the oxoaldehyde compounds obtained from an authentic sample of the
compound is necessary for an absolute validation of their formation. The residual spectra may also
contain absorptions from other carbonyl compounds, such as the hydroxydicarbonyl compounds
from the isomerization of the alkoxy radical that have been previously mentioned, however, in
Figures 1, S1 and S2 there are not observed the infrared band corresponding to the stretching of OH
group (3500ꢀ3600cmꢀ1), other possibility is the formation of epoxy compounds, even though the
315
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16
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25
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4
7ꢀ48
328
epoxy pathway in gas phase ozonolysis reactions is considered to be a minor pathway.
3
3
29
30
In summary, the ozonolysis of the C7ꢀC9 (E)-2ꢀalkenal compounds studied in this work leads to the
formation as primary products the dicarbonyl compound glyoxal and the corresponding saturated
aldehydes pentanal, hexanal and heptanal. The yields of the primary carbonyl products were all
quite similar with combined total carbonyl yields of ≥ 80% for each of the studied reactions. No
preference was found for the two possible primary ozonide decomposition in agreement with other
studies on shorter chain 2ꢀalkenals and contrary to what one would expect based ozonolysis studies
of alkyl substituted alkenes.
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Atmospheric implications
341
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45
In a previous work, we have reported the atmospheric lifetimes of the studied 2ꢀalkenals using the
typical ambient concentrations for the tropospheric oxidants (OH, O , Cl). The calculated lifetimes
3
indicated that the reaction with OH and NO (few hours) radicals are the main degradation
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processes for these aldehydes in the atmosphere, reaction with O possibly being of some
3
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346
importance in regions with high levels of O3. The ozonolysis reactions of these unsaturated
aldehydes have been in this study and some other studies to be sources for alkyl aldehydes,
dialdehydes and possibly other oxygenated compounds. Glyoxal can be degraded in the atmosphere
3
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47
48
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51
349
by reaction with OH radicals and via photolysis. Fu et al. have estimated the global production
budget of glyoxal to be ~45 Tg/annum, even though, isoprene oxidation contributes to 47% of this
globally glyoxal budget. Moreover, glyoxal plays an important role in global SOA formation via
3
3
3
3
50
51
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53
5
1ꢀ55
55
irreversible uptake by aqueous aerosols and clouds,
for example, De Haan et al. have estimated
that glyoxal can produce 1 Tg C/year SOA due to selfꢀreactions in a evaporating droplets.
Additionally, it can be removed from the atmosphere through wet deposition due to its high water
354
56
3
3
55
56
solubility.
The alkyl aldehydes products of the studied reactions also have as main degradation processes in
357
the atmosphere the reaction with OH radicals and photolysis. For example, pentanal has a lifetime
6
3
3
58
59
of 5.1 h due to the reaction with OH radical (calculated using a noon time value of [OH] = 2 × 10
ꢀ3
57
molecule cm ) and a lifetime due to photolysis of 17 h. The lifetime of hexanal due to the reaction
5
8
360
with OH radicals has been estimated by Jiménez et al. to be 10 h using a 24 h average of [OH]=
6
ꢀ3
6
ꢀ3
3
3
61
62
1×10 molecule cm at 298 K, which reduces to 4.9 h if [OH]= 2×10 molecule cm is used in the
lifetime calculation. The lifetime of this aldehyde due to photolysis has been estimated to be < 48 h
58
363
by Jiménez et al. Finally, heptanal has an estimated lifetime of 4.7 h due to reaction with OH
6
ꢀ3
ꢀ11
3
3
64
65
when calculated using a noon time value of [OH]= 2×10 molecule cm and k = 2.96×10
OH
ꢀ1
ꢀ3 ꢀ1 18
molecule cm s
and a photolysis lifetime of 17 h, obtained using the photolysis rate of (1.65 ±
ꢀ5
ꢀ1
59
366
0.03)×10 s from Paulson et al. These C5ꢀC7 alkyl aldehydes have been detected in urban, rural
6
0ꢀ63
3
3
3
3
3
3
3
67
68
69
70
71
72
73
and forest environments.
The atmospheric degradation of pentanal, hexanal, heptanal and
glyoxal can produce free radical species that can contribute to O formation and in the presence of
3
NO could form organic nitrates or peroxynitrates which transport NO species to remote
x
x
environments.
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We gratefully acknowledge the financial support for this research provided by the EU project
EUROCHAMP2 (E2ꢀ2013ꢀ02ꢀ27ꢀ0086), CONICET, PIPꢀ GI 2010ꢀ2012ꢀcod: 11220090100623
(Argentina), SECyTꢀUNC 14306/24. (Argentina) and FONCyT Préstamo BID 1201/OCꢀAR
(Argentina). E. Gaona Colmán wishes to thank CONICET for a PhD fellowship. M. B. Blanco
wishes to acknowledge the Alexander von Humboldt Foundation (Germany) for support.
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
56
557
Figure 1: Panel A shows the infrared spectrum of a (E)ꢀ2ꢀheptenal/O /air gas mixture after reaction
3
5
5
5
58
59
60
and subtraction of residual (E)ꢀ2ꢀheptenal. Panels B and C show reference spectra of glyoxal and
pentanal, respectively. Panel D shows the residual product spectrum obtained after subtraction of
features due to the reference spectra from the spectrum in panel A. (A straight line is presented
ꢀ1
561
between 1900ꢀ2400 cm for the spectra in panel A and D avoiding the CO and CO absorption)
2
5
62
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2
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5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
63
(
E)-2-heptenal
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
glyoxal
pentanal
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
after the
addition of O3
0
500
1000 1500 2000 2500 3000 3500 4000 4500
Time (s)
5
64
5
5
5
65
66
67
Figure 2: Concentrationꢀtime profiles of (E)-2ꢀheptenal and the reaction products glyoxal and
pentanal obtained from a (E)-2ꢀheptenal /O /air reaction mixture.
3
.
5
5
68
69
570
5
5
71
72
573
5
5
74
75
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2
3
4
5
6
7
8
9
glyoxal
pentanal
0
0
0
0
0
0
0
0
0
0
.18
.16
.14
.12
.10
.08
.06
.04
.02
.00
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
ꢀ [(E)-2-heptenal] (ppm)
576
5
5
5
77
78
79
Figure 3: Plots of the concentrations of the reaction products glyoxal and pentanal as a function of
reacted (E)-2ꢀheptenal obtained from experiments performed on (E)-2ꢀheptenal /O /air reaction
3
mixtures.
580
5
5
81
82
583
5
5
5
5
84
85
86
87
588
5
5
89
90
591
5
5
5
5
92
93
94
95
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2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
96
Table 1
597
Formation yields of the oxidation products identified in the ozonolysis of (E)-2ꢀheptenal, (E)-2ꢀ
octenal and (E)-2ꢀnonenal at 298K in 750 ± 10 Torr of air.
5
98
Compound
Product
Yield (%)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
E)-2ꢀheptenal
HC(O)C(O)H
Glyoxal
50 ± 4
48 ± 4
average: 49 ± 4
CH (CH ) C(O)H
3
2 3
Pentanal
35 ± 3
33 ± 2
average: 34 ± 3
41 ± 3
41 ± 3
average: 41 ± 3
(E)-2ꢀoctenal
HC(O)C(O)H
Glyoxal
CH (CH ) C(O)H
3
2 4
Hexanal
38 ± 2
39 ± 3
average: 39 ± 3
45 ± 3
44 ± 3
average: 45 ± 3
(
E)-2ꢀnonenal
HC(O)C(O)H
Glyoxal
CH (CH ) C(O)H
3
2 5
Heptanal
47 ± 2
45 ± 3
average: 46 ± 3
5
6
99
00
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9
O
O
C
O
O
H
H C
C
3
+
CH CH
C
C
H
O3
H
2
H
n
H C
O
C
2
C
n
n= 3,4,5
CH3
H
A
B
H
O
C
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
C
O
+
H
3
C
CH
2
O
+
C
C
H
3
n
O
CH
C
H
HC
O
H
H
H
2
n
Ejection of O atom
OH
O
n= 3,4,5
O
H
O
H
H
3
C
C
C
C
+
O
C
C
H
H
2
m
H
m= 2,3,4
H
O
+
OH
H C
C
3
OH
OH
C
C
H
O
OH
OH
H
2
O , RO
2
1,5 H shift
1
,5 H shift
if m=2
2
m
H C
O
2
H C
O
2
O
H C
2
C
C
O2
C
H
H
H
2
2
2
RO
2
2
2
O
O2
RO2
O
CH
O
3
H C
O
C
C
H
3
C
OH
O
H
H
C
2
m
H
+
RO
2
m
HO2
H C
O
2
C
O2
H
2
2
O
O
+
3
H C
HO2
C
H
2
m
601
602
6
6
03
04
Figure 4: Simplified mechanism for the reaction of O with (E)-2ꢀheptenal, (E)-2ꢀoctenal and (E)-
3
2ꢀnonenal.
6
6
6
6
05
06
07
08
609
610
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1
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2
2
2
2
2
2
2
2
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5
5
6
1
.6
1.6
1.2
0.8
0.4
0.0
B (residual spectrum (E)-2-heptenal + O )
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
9
0
1
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9
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
6
7
8
9
0
1.2
0.8
0.4
0.0
A
(2-oxopentanal B3LYP/6-311++G(d,2p))
900
1200 1500 1800 2100 2400 2700 3000 3300
-1
Wavenumber (cm )
6
6
11
12
Figure 5: Comparison of a computed IR spectrum for the expected product 2ꢀoxopentanal (A) with
the residual product IR spectrum (B) obtained from an experiment with a (E)-2ꢀheptenal/O /air
6
13
3
ꢀ1
614
mixture. (A straight line is presented between 1900ꢀ2400 cm for the residual product spectrum
6
6
6
6
6
6
15
16
17
18
19
20
avoiding the CO and CO absorption)
2
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