Tropospheric chemical degradation of vinyl and allyl acetate initiated by Cl atoms under high and low NOx conditions

Article abstract of DOI:10.1039/c5ra04975j

The products of the reactions of Cl atoms with vinyl acetate (VA) and allyl acetate (AA) have been investigated in a 1080 L chamber using in situ FTIR. The experiments were performed at 296 K and atmospheric pressure of synthetic air in the presence and in the absence of NO_x. For the reaction of Cl with VA in the presence of NO_x formic acetic anhydride, acetic acid and formyl chloride are the major reaction products. In the absence of NO_x, the yields of these products are significantly reduced and formation of the carbon-chain-retaining compound CH₃C(O)OC(O)CH₂Cl is observed. For the reaction of Cl with AA in the presence of NO_x acetoxyacetaldehyde and formaldehyde were observed as the main products. In contrast, without NO_x, the observations support that the major reaction pathway is the formation of the carbon-chain-retaining compound CH₃C(O)OCH₂C(O)CH₂Cl. The reaction mechanisms leading to the products are discussed. The formation of the high yields of formyl chloride and formaldehyde in the reactions of Cl with VA and AA, respectively, are at odds with currently accepted mechanistic pathways.

Full text of DOI:10.1039/c5ra04975j

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Tropospheric chemical degradation of vinyl and allyl acetate initiated by
Cl atoms under high and low NOx conditions
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María B. Blancoa, Iustinian Bejanb,c, Ian Barnesb*, Peter Wiesenb and Mariano A. Teruela*
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aInstituto de Investigaciones en Fisicoquímica de Córdoba (I.N.F.I.Q.C.), Dpto. de
Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba. Ciudad
Universitaria, 5000 Córdoba, Argentina.
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bPhysikalische Chemie/FBC, Bergische Universitaet Wuppertal, Wuppertal, Germany.
c“Al. I. Cuza” University of Iasi, Faculty of Inorganic and Analytical Chemistry, Iasi,
Romania.
Corresponding authors:
barnes@uniꢀwuppertal.de (Phone: +49 202 439 2510, Fax: +49 202 439 2505),
mteruel@fcq.unc.edu.ar (Phone: +54 351 4334169/80 int 221, Fax: +54 351 4334188)
Abstract
The products of the reactions of Cl atoms with vinyl acetate (VA) and allyl acetate (AA) have been
investigated in a 1080 L chamber using in situ FTIR. The experiments were performed at 296 K and
atmospheric pressure of synthetic air in the presence and in the absence of NOx. For the reaction of
Cl with VA in the presence of NOx formic acetic anhydride, acetic acid and formyl chloride are the
major reaction products. In the absence of NOx, the yields of these products are significantly
reduced and formation of the carbonꢀchainꢀretaining compound CH3C(O)OC(O)CH2Cl is observed.
For the reaction of Cl with AA in the presence of NOx acetoxyacetaldehyde and formaldehyde were
observed as the main products. In contrast, without NOx, the observations support that the major
reaction pathway is formation of the carbonꢀchainꢀretaining compound CH3C(O)OCH2C(O)CH2Cl.
The reaction mechanisms leading to the products are discussed. The formation of the high yields of
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formyl chloride and formaldehyde in the reactions of Cl with VA and AA, respectively, are at odds
with currently accepted mechanistic pathways.
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Introduction
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In addition to emissions from natural sources (vegetation and biomass combustion) industrial
activities have resulted in the wideꢀspread release of acetates to the atmosphere.1 Emissions from
automobiles represent another source of these compounds; a number of acetates, including vinyl
acetate (VA: CH3C(O)OCH=CH2) and allyl acetate (AA: CH3C(O)OCH2CH=CH2), have been
detected as products in the combustion of rape methyl esters used as fuel alternatives or additive.2
The gasꢀphase removal processes of these esters from the atmosphere, and volatile organic
compounds (VOCs) in general, is mainly through their reactions with OH, NO3, and O3 with
removal by Cl being thought to play, under most circumstances, a very minor role since the global
Cl atom concentration is low (<103 radicals cmꢀ3).3ꢀ5 The general conception has been that any
significant removal of VOCs through reaction with Cl atom is constrained to (i) the marine
boundary layer and coastal regions,6ꢀ7 and (ii) industrial areas where chlorine can be emitted directly
from incineration and power generation.8,9 However, a series of recent observations of nitryl
chloride (ClNO2) in marine and midꢀcontinental air10ꢀ11 and in urban power plants plumes12 have
highlighted the potential importance of ClNO2 as a hither to unconsidered source of Cl and
indicated that the role of Cl in initiating the oxidation of VOCs may be more widespread and
significant than previously thought. Nitryl chloride is formed during the night via the heterogeneous
reaction of N2O5 with particle Clꢀ and photolyzes readily in the early morning to produce Cl and an
NO2 molecule.13 A field and modelling study of Cl2 and ClNO2 in the coastal marine boundary layer
has recently been reported for the Los Angeles region.14 The results indicated that over the course of
an entire model day ClNO2 accounted for 45% of the integrated Clꢀatom production compared to
only 10% for Cl2 with the remaining 45% being attributed to the reaction of OH with HCl. The
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study also showed that Clꢀatom mediated chemistry could contribute quite substantially to the
photooxidation of VOCs in this region particularly in the early morning.
In order, to evaluate the contribution of VA and AA to the oxidative capacity of the atmosphere and
potential environmental and health effects detailed kinetic and mechanistic information on their
tropospheric gasꢀphase degradation pathways are required. There have been several kinetic studies
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of the reactions of VA and AA with OH,15ꢀ17 NO3,17 O3 and Cl.18 In addition, product studies on
the OHꢀradical initiated oxidation of these unsaturated esters have also been reported. Picquetꢀ
Varrault et al. have reported product studies on the reactions of OH radicals with both VA17 and
AA19 performed at atmospheric pressure and room temperature in the presence of NOx using FTIR
spectrometry for the analyses. PicquetꢀVarrault et al.17 reported formaldehyde, formic acetic
anhydride and acetic acid as the main products formed in the reaction of OH with VA. For the
reaction of OH with AA acetoxyacetaldehyde and HCHO were the main products observed.19
We have recently reported room temperature product investigations on the gasꢀphase reactions of
OH radicals with VA and AA in the presence and absence of NOx.20 Our results in the presence of
NOx agreed well with those reported by PicquetꢀVarrault et al.17ꢀ19 for the two compounds.
However, our studies showed that in the absence of NOx there are significant changes in the relative
importance of the possible product channels for the reaction of the 1,2ꢀhydroxyalkoxy radicals
formed after addition of OH to the double bond in the compounds. In the case of VA, an αꢀester
rearrangement to produce CH3C(O)OH and CH2(OH)CO• radicals rather than decomposition to
form formic acetic anhydride dominated. In the case of AA reaction of the 1,2ꢀhydroxyalkoxy
radical with O2 to form the carbon chainꢀretaining product 3ꢀhydroxyꢀ2ꢀoxoꢀpropyl ester
(CH3C(O)OCH2C(O)CH2OH) dominated in the absence of NOx contrasting very sharply with the
decomposition channel of the 1,2ꢀhydroxyalkoxy radicals to form acetoxyacetaldehyde and HCHO,
which dominated when the reaction was studied in the presence of NOx.
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In a continuation of our investigations on the photooxidation mechanisms of unsaturated VOCs for
different NOx scenarios, we report here product studies on the Clꢀatom initiated photooxidation of
VA and AA in the presence and absence of NOx:
Cl + CH2=CHOC(O)CH3 (VA)
→ products
(1)
(2)
Cl + CH2=CHCH2OC(O)CH3 (AA) → products
A computational study on the mechanism and kinetics of the Clꢀinitiated oxidation of VA in the
presence of NOx was reported recently.21 The study found that Cl addition to the double bond is the
dominate pathway followed, in the presence of O2 and NO, by decomposition to give the major
products observed in our work.
To the best of our knowledge this is the first reported experimental product investigation of the Clꢀ
atom initiated photooxidation for both VA and AA performed in the presence and in the absence of
NOx.
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Experimental Section
All the experiments were performed in a 1080 L quartzꢀglass reaction chamber at (298 ± 2) K and a
total pressure of (760 ± 10) Torr of synthetic air (760 Torr = 101.325 kPa). A detailed description of
the reactor can be found elsewhere22 and only a brief description is given here. The photolysis
system consists of 32 superactinic fluorescent lamps (Philips TL05 40W: 320 – 480 nm, λmax = 360
nm), which are spaced evenly around the reaction vessel. The chamber is equipped with a White
type multipleꢀreflection mirror system with a base length of (5.91 ± 0.01) m for sensitive in situ
long path infrared absorption monitoring of reactants and products in the spectral range 4000 – 700
cmꢀ1. The White system was operated at 82 traverses, giving a total optical path length of (484.7 ±
0.8) m. Infrared spectra were recorded with a spectral resolution of 1 cmꢀ1 using a Nicolet Nexus
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FTꢀIR spectrometer equipped with a liquid nitrogen cooled mercuryꢀcadmiumꢀtelluride (MCT)
detector.
Chlorine atoms were generated by the photolysis of Cl2 using the fluorescent lamps:
Cl2 + hν
→
2 Cl
(3)
Experiments were performed on mixtures of Cl2/acetate/air or Cl2/acetate/NO/air which were
irradiated for periods up to 5 minutes during the course of which infrared spectra were recorded at 1
cmꢀ1 resolution with the FTIR spectrometer. Tests showed that photolytic removal of the acetates in
the absence of Cl2 and NO was negligible. The systems, in particular the AA system, are very
reactive and typically 20 interferograms were coꢀadded per spectrum over a period of
approximately 5 min for the experiments with VA and 10 interferograms for the experiments with
AA. The concentrations of the reactants used in the experiments and the infrared frequencies used
for monitoring are given in the Supporting Information (SI).
The quantification of reactants and products was performed by comparison with calibrated
reference spectra contained in the IR spectral data bases of the laboratories in Wuppertal and
Laboratoire Interuniversitaire des Systemes Atmosphériques (LISA), Paris, France.23
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Results
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Cl + Vinyl acetate
NOx-containing system: A typical product spectrum obtained after irradiation of a VA/Cl2/air/NO
mixture and subtraction of residual absorptions due to VA is shown in Figure 1, panel A. Infrared
spectra of formic acetic anhydride, formyl chloride, acetic acid, and carbon monoxide are shown in
Figure 1, panels B, C, D and E, respectively. After subtraction of absorptions from these identified
products absorptions remain at 786, 853, 1193, 1322, 1698, 1801 and 3588 cmꢀ1 as shown in Figure
1, panel F. The residual absorptions are due mainly to nitrosyl chloride (ClNO) and nitryl chloride
(ClNO2) formed from the reactions of Cl atoms with NO and NO2, respectively, and also some
nitric acid (HNO3) formed through reaction of OH with NO2 and heterogeneous reactions of NOx at
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the reactor wall. The reactions of NO with the HO2 radicals formed in the system will produce OH
radicals, however, since the rate coefficients for Cl with VA is nearly an order of magnitude higher
than the corresponding OH reaction and OH is scavenged by NO2, interference by OH chemistry
will be minimal. This also applies to the AA reaction system in the presence of NOx.
Concentrationꢀtime profiles of VA and the identified products, formic acetic anhydride, formyl
chloride, acetic acid and carbon monoxide are shown in Figure S1 of Supplementary Information
(SI). Plots of the concentrations of the identified products as a function of reacted VA are shown in
Figure S2. As can be seen in Figure S1 the increase in the product concentrations scale linearly
with the loss of VA supporting that the products are all primary and that secondary loss or
formation of these products is insignificant. The molar yields of the products observed in the
reaction of Cl with VA in the presence of NOx, obtained from the slopes of the plots in Figure S2,
are listed in Table 1.
NOx-free system: Figure S3, panel A in the SI, shows the IR spectrum obtained after UV irradiation
of a VA/Cl2/air mixture and subtraction of residual absorptions due to VA. Figure S3, panels B, C,
D, and E show reference spectra of formic acetic anhydride, formyl chloride, acetic acid and carbon
monoxide, respectively, all of which have been identified as products. Figure S3, panel F shows the
residual spectrum that is obtained after subtraction of absorptions from the identified products from
the spectrum in panel A; unidentified absorptions remain at approximately 1017, 1059, 1111, 1205,
1372 and 1808 cmꢀ1.
Many of the absorption features in the residual product spectrum correlate very well with those of
acetic acid anhydride and chloroacetone indicating that the compound (or compounds) giving rise to
the IR features contains structural elements common to both acetic acid anhydride and
chloroacetone. Since we are certain that all other possible reaction products from the reaction of Cl
with VA have been accounted for we are confident that the unidentified absorptions are mainly due
to the formation of the VA carbonꢀskeleton retaining compound chloroꢀacetic acid anhydride
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(CH3C(O)OC(O)CH2Cl) although contributions from CH3C(O)OC(OH)CH2Cl can not be excluded.
Further justification for the formation of CH3C(O)OC(O)CH2Cl from the 1,2ꢀchloroalkoxy radicals
formed in the Cl + vinyl acetate reaction are given in the discussion on the formation of
CH3C(O)OCH2C(O)CH2Cl in the reaction of Cl with allyl acetate.
Concentrationꢀtime profiles of VA and the four identified products are shown in the SI in Figure S4.
Plots of the concentrations of the products as a function of reacted VA for the NOxꢀfree reaction
system are shown in Figure 2. The plots are linear with near zero intercepts supporting that the
products are all primary. The molar yields of the products observed in the reaction of Cl with VA in
the absence of NOx, obtained from the slopes of the plots in Figure 2, are listed in Table 1.
Cl + Allyl acetate
NOx-containing system: Panel A of Figure S5 in the SI shows a typical product spectrum acquired
after irradiation of a AA/Cl2/air/NO mixture and subtraction of residual absorptions due to AA.
Panels B, C, D, E and F show the reference spectrum of formic acetic anhydride, formyl chloride,
acetic acid, formaldehyde and acetoxyacetaldehyde (CH3C(O)OCH2C(O)H), respectively. These
compounds have been identified as products in the reaction system along with CO, the absorptions
of which can be seen around 2200 cmꢀ1 in Figure S5, panel A. Panel G shows the residual spectrum
obtained after the subtraction of absorptions due the identified products. In the residual spectra
many absorption bands remain in the fingerprint region and in the region 1671 – 1808 cmꢀ1. As in
the VA system with NOx some of these absorptions can be assigned unequivocally to nitrosyl
chloride, nitryl chloride and nitric acid. In addition, to these compounds, as will be argued in the
Discussion, bands at 1744, 1300, 1047 and 795 cmꢀ1 are assigned to formation of acetoxyacetyl
peroxynitrate (CH3C(O)OCH2C(O)OONO2).
Figure S6 shows the concentrationꢀtime profiles of AA and the identified products formed in the
NOxꢀcontaining system. The profile of AA shows that it is being consumed very rapidly in the
system. The initial concentrationꢀtime profiles of acetoxyacetaldehyde and HCHO are similar and
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indicate that they are both primary products. However, secondary removal of acetoxyacetaldehyde
and especially HCHO in secondary reactions with Cl is clearly evident.
Although the concentrationꢀtime profiles of acetic acid, formic acetic anhydride and CO look as
though they may be primary products, plots of their concentrations against the amount of AA
consumed have nonꢀzero intercepts and upward curvature which increases with increasing reaction
time (see Figure S7 in the SI). This behavior is indicative of formation of these compounds in
secondary reactions. The formation of acetic acid and formic acetic anhydride in the system is
attributed to fast secondary reactions of acetoxyacetaldehyde with Cl (see Discussion). Formation
of formyl chloride was also observed in the system; however, it was only observed in small
quantities toward the end of the reaction and is obviously being formed in secondary reactions.
The initial slopes of the plots of the concentrations of acetoxyacetaldehyde and HCHO against
reacted AA indicate high yields for both compounds. Taking the first few points of a plot of the
concentration of acetoxyacetaldehyde as a function of reacted AA gives a yield of
acetoxyacetaldehyde of close to unity, however, if all the points are taken a yield of around 66% is
obtained caused by curvature in the plot from fast secondary reaction of acetoxyacetaldehyde with
Cl. Correction of yield for formaldehyde for secondary consumption with Cl using the method
outlined in Tuazon et al.24 and a rate coefficient of 7.32 × 10ꢀ11 cm3 moleculeꢀ1 sꢀ1 for the reaction of
Cl with HCHO25 gives a yield of >90 % for HCHO. The rate coefficient for the reaction of Cl with
acetoxyacetaldehyde is not known. Using the rate coefficient for Cl with HCHO for the correction
gave a yield of around 120%. Since the rate coefficient for Cl with acetoxyacetaldehyde is not
likely to be much less than that for Cl with HCHO this would support a yield much higher than 66%
yield for acetoxyacetaldehyde.
NOx-free system: The irradiation of a AA/Cl2/air mixture give rise to the product spectrum shown
in Figure S8, panel A. Formation of minor amounts of acetoxyacetaldehyde, formic acetic
anhydride, acetic acid and formaldehyde were observed but the molar yields for each of these
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products was < 5% (Table 1). Reference spectra of chloroacetone and methyl acetate are shown in
Figure S8, panels B and C, respectively. It can be seen that the product spectrum in Figure S8, panel
A, contains absorption features that match those in both the spectrum of methyl acetate and that of
chloroacetone. In fact, when these two spectra are mathematically added the resulting spectrum
almost completely matches all of the features in the residual product spectrum supporting that the
compound must contain the CH3C(O)Oꢀ and –COCH2Cl entities. Since addition of Cl will be
predominately at the terminal carbon of the double bond and the yields of carbon cleavage products
are minimal we are quite confident that the absorptions in the IR spectrum in Figure S8, panel A,
are predominantly due to the formation of acetic acid 3ꢀchloroꢀ2ꢀoxoꢀpropyl ester
(CH3C(O)OCH2C(O)CH2Cl). Formation of CH3C(O)OCH2CH(Cl)C(O)H and other hydroperoxy
and hydroxyl compounds from peroxyꢀperoxy reactions in the system are obviously also possible,
however, based on the observed infrared product spectra, we estimate that the yields of these
compounds must be very low. For example, CH3C(O)OCH2CH(Cl)C(O)H would not give rise to
the strong absorptions observed at around 1245 and 1050 cmꢀ1. Formation of the compound would
also not be in line with the high observed yield of acetoxyacetaldehyde in the NOxꢀcontaining
system. It is well established that a major channel in the reactions of the 1,2ꢀchloroalkoxy radicals
formed in the addition of Cl to alkenes is reaction of the 1,2ꢀchloroalkoxy radicals with O2 to form a
1,2ꢀchloro carbonyl compound26, for example, Butt et al. (2009)27 have observed carbonꢀskeleton
retaining compound C4F9CH2CH2OC(O)C(O)CH2Cl as a major product from the addition of Cl to
4:2 fluorotelomer acrylate (C4F9CH2CH2OC(O)CH=CH2) both in the presence and absence of NOx.
Further support comes from recent work on long chain Cx to C9 terminal alkenes by Walavalkar et
al.28 which showed that addition of Cl to the inner carbon of the double bond is minimal and that 1ꢀ
chloroꢀ2ꢀketones are the main products in the absence of NOx. The above points serve to strengthen
the arguments offered for the identification of acetic acid 3ꢀchloroꢀ2ꢀoxoꢀpropyl ester as the main
product in the reaction of Cl with ally acetate in the absence of NO.
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The near quantitative formation of acetic acid 3ꢀchloroꢀ2ꢀoxoꢀpropyl ester in the reaction of Cl with
AA in the absence of NOx allows an approximate IR calibration of the compound. Using a band at
3415 cmꢀ1 which is unique to the compound allows a yield of ≤ 5 % to be placed on the formation of
the ester in the reaction of Cl with AA in the presence of NOx.
Discussion
Cl + Vinyl acetate
Reaction with NOx: As can be deduced from Table 1 the carbon balance for the reaction of Cl with
VA in the presence of NOx is, within the error limits, approximately 100 %. The reaction will
proceed via addition of Cl to the double bond followed by consecutive addition of O2 and reaction
with NO form CH3C(O)OCH(O.)CH2Cl alkoxy radicals (Figure 3, channel A). Three channels are
traditionally thought be operative for the radicals, decomposition, an αꢀester rearrangement and
reaction with O2, Figure 3, channels B, C and D, respectively. The later channel is not observed
with NO. Decomposition to form formic acetic anhydride with a yield of (69 ± 8) % is observed.
.
The expected coꢀproduct is the ClꢀCH2 radical which would be expected to add O2 giving peroxy
radicals which react further with NO to form ClꢀCH2O. alkoxy radicals. Further reaction of these
radicals with O2 are a potential pathway to the HC(O)Cl detected in the system.
.
However, Bilde et al.29 have studied the reaction of ClꢀCH2 radicals in N2 with different partial
pressures of O2 in the presence and absence of NOx. They produced the radicals via reaction of Cl
with CH3Cl. They did not measure the decay of CH3Cl since this was too small and determined the
product yields from the sum of the three identified carbonꢀcontaining products HCOCl, HCHO and
CO which they assumed was equal the amount of consumed CH3Cl. They found that in the absence
of NOx the final product was HC(O)H in nearly 100 % yield and that with NOx in 700 Torr of air
HC(O)Cl, HC(O)H and CO were products with yields of (56 ± 10), (32 ± 6) and (12 ± 5)%,
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respectively. The result of Bilde et al. with NOx contrasts sharply with the large yield of HC(O)Cl
and no HC(O)H observed in the reaction of Cl with VA in the presence of NOx presented here.
Bilde et al.29 attributed the differences in the yields of HC(O)Cl with and without NOx to the
formation of excited ClꢀCH2O.* alkoxy radicals in the exothermic reaction of ClꢀCH2OO. peroxy
radicals with NOx whereas in the NOxꢀfree system only thermalized ClꢀCH2O. radicals are formed
through nearly thermo neutral peroxyꢀperoxy reactions.
At present it is difficult to explain why we observe mainly formation of HC(O)Cl from the reactions
.
of the ClꢀCH2 radical formed in the reaction of Cl with VA in the presence of NOx in contrast to the
findings of Bilde et al.29 The situation is made even more curious by the fact that in the
Cl/AA/NO/air system we observe HC(O)H as the coꢀproduct of acetoxyacetaldehyde and not
.
HC(O)Cl. One obvious difference is the source of the ClꢀCH2 radicals in the studies. Our
observations imply that the decomposition channel is not as straightforward as depicted in Figure 3,
.
i.e. the mechanism is much more complex and may not involve formation of ClꢀCH2 radicals.
The only structural difference between vinyl acetate and allyl acetate is the –CH2ꢀ group in ally
acetate between the oxygen atom and the double bond. This difference is structure is apparently
critical in determining the reaction products resulting from the terminal –CH2Cl group after
consecutive addition of Cl and O2 to form 1,2ꢀchloroperoxy radicals. The carbonyl and CꢀCl entities
in the intermediate 1,2ꢀchloroperoxy radical reaction intermediates are strongly electron
withdrawing. It is possible that there are bonding interactions between the peroxy radicals and the
carbonyl and CꢀCl entities in the reaction intermediates which complicate the reactions of NO and
propably also HO2/RO2 radicals with the 1,2ꢀchloroperoxy radical intermediates and thus the
reaction pathways and products. The 1,2ꢀchloroperoxy radical formed in the reaction of Cl with
allyl acetate could form a weakly bound sixꢀmembered ring with the carbonyl group which would
be more favorable than a fiveꢀmembered ring which could be formed in the reaction of Cl with
vinyl acetate. Interaction of the peroxy group with CꢀCl and NO could potentially result in the
product of chemically activated CH2ClO radicals especially in the case of vinyl acetate where
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interaction of the peroxy group with the carbonyl group is probably not as important as for allyl
acetate. This is presently purely speculation and obviously more detailed laboratory studies as a
function of temperature, pressure and O2 and in particular computational studies are needed to shed
light on the mechanistic differences observed in the reactions of Cl with vinyl acetate and allyl
acetate in the presence and absence of NO.
The other important channel in the reaction of Cl with VA involves an ester rearrangement30 to
form CH3C(O)OH with a yield of (27 ± 5) % and the halogenated acyl radical ClꢀCH2C.(O). There
are two known fates of acyl radicals either thermal decomposition or addition of O2 to form
acylperoxy radicals:31ꢀ34
RC.(O) + M → R. + CO + M
RC.(O) + O2 + M → RC(O)OO. + M
For acyl radicals addition of O2 at atmospheric pressure and room temperature is known to
dominate,33,34 however, there is a known monotonic trend towards decomposition, at 298 K and
atmospheric pressure of air, with increase in the number of Cl in the CClxF3ꢀxC.(O) radical.28,29
Although a theoretical study on the atmospheric fate of carbonyl radicals predicts that the main fate
of the ClꢀCH2C.(O) radical is addition of O2 and not decomposition the experimental observations
in this study of primary CO formation suggests rapid decomposition of the the ClꢀCH2C.(O) radical
.
into CO and a ClꢀCH2 radical rather than addition of O2 (Figure 3, channel C).
If addition of O2 to ClꢀCH2C.(O) was an important channel the peroxy radicals formed would, due
to the high NO2 levels, add NO2 to a significant extent to form the thermally stable PAN type
compound ClCH2C(O)OONO2. No evidence could be found in the product spectra for the formation
of a PAN type compound.
During this writing a computational study on the mechanism and kinetics of the Clꢀinitiated
oxidation of VA appeared.21 The study found that Hꢀatom abstraction pathways were negligible and
that Cl addition dominated followed, in the presence of O2 and NO, by decomposition to give
CH3C(O)OC(O)H, CH3C(O)OH and HC(O)Cl as major products as observed in this work. The
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product channels observed for the reaction of Cl with VA in the presence of NOx are similar to
those observed in the analogous OH reaction,20 however, with a higher preference for the channel
forming CH3C(O)OC(O)H in the OH reaction.
Reaction without NOx: In Figure 3 the peroxy radicals formed after reaction of Cl with VA are
shown to form wholly alkoxy radicals via reaction with HO2 or other peroxy radicals. Other
pathways are known to form a hydroperoxy or alcohol, however, the yields of detected products and
residual IR product spectra suggest that these are relatively minor channels for both the VA and AA
systems. In the absence of NOx, the two decomposition channels producing CH3C(O)OC(O)H and
HCOCl (Figure 3, channel B) and the ester rearrangement producing CH3C(O)OH, CO and HCOCl
(Figure 3, channel C) are observed for the CH3C(O)OCH(O.)CH2Cl radicals formed in the reaction
of Cl with VA. However, compared to the system with NOx the relative contributions of these
channels to the overall reaction are much reduced with the channels B and C accounting for only
(25 ± 5) and (12 ± 3) % of the overall reaction, respectively. The combined yield of HC(O)Cl from
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these two channel was (48 ± 8) %.
The results clearly show that in the absence of NOx, only approximately 50% of the reaction is
proceeding by the decomposition and rearrangement/decomposition channels of the chloroalkoxy
radicals (Figure 3, channels B and C). As indicated in the Results section the experimental
observations support the occurrence of a molecular channel with the formation of chloroꢀacetic acid
anhydride (CH3C(O)OC(O)CH2Cl) through the reaction of the CH3C(O)OCH(O•)CH2Cl with O2
(Figure 3, channel D). Based on the carbon balance the yield of chloroꢀacetic acid anhydride could
be potentially as high as 50%. Unfortunately no calibrated reference spectrum is available to verify
this.
This result for the reaction of Cl with VA in the absence of NOx contrasts quite markedly with that
for the analogous OH reaction under the same conditions. For the reaction of OH with VA in the
absence of NOx a molecular channel is not observed. The absence of NOx just results in a stark
switch in the preference of the two decomposition channels, i.e. the ester rearrangement channel
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dominates in the absence of NOx compared to a dominance of the anhydride forming channel in the
presence of NOx.20 The substantially lower yields of primary decomposition products for the
reaction of Cl with VA in the absence of NOx suggests that chemical activation is important in the
atmospheric chemistry of CH3C(O)OCH(O.)CH2Cl alkoxy radicals.
Cl + Allyl acetate
Reaction with NOx: In this reaction system Cl adds mainly to the terminal carbon of the double
bond in AA followed by the addition of O2 to give peroxy radicals which subsequently react with
NO to form 1,2ꢀchloroalkoxy radicals as shown in Figure 4, channel A. Possible reaction channels
include decomposition to form acetoxyacetaldehyde and CH2Cl radicals (channel B), decomposition
to form chloroacetaldehyde and CH3C(O)OCH2 radicals (channel C) or reaction with O2 to form the
chainꢀretaining product CH3C(O)OCH2C(O)CH2Cl (Figure 4, channel D).
There was no indication for formation of chloroacetaldehyde in the product spectrum showing that
channel C is a negligible reaction pathway. As discussed below, from the study on the reaction of
Cl with AA in the absence of NOx we are able to put a yield of ≤ 5 % on the formation of
CH3C(O)OCH2C(O)CH2Cl from the reaction of the 1,2ꢀchloroalkoxy radicals with O2 (channel D)
in the presence of NOx.
In the product study with NOx, acetoxyacetaldehyde, and formaldehyde have been identified as
major primary products. The results indicate that the major reaction channel after addition of is
cleavage of the CꢀC bond to form acetoxyacetaldehyde and potentially CH2Cl. radicals (Figure 4,
channel B).
As discussed in the Results section although a lower limit of 66% is given in Table for the yield of
acetoxyacetaldehyde the observations would suggest that after correction it would be close to unity.
In stark contrast to the reaction of Cl with VA in the presence of NO, HC(O)H is detected as the
main coꢀproduct originating from the decomposition channel and not HC(O)Cl. As in the
VA/Cl/NO/air system, based on the study of Bilde et al.,29 this observation is again in strong
.
contrast to what would be expected from further reactions of the ClCH2 radical in the presence of
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NO. As discussed in the section on the reaction of Cl with NO the decomposition channel as shown
in Figure 4 for the reaction of Cl with AA would appear to be much more complex. Insertion of a –
CH2ꢀ group between the acetoxy and double bond of VA and AA has resulted in the chlorinated
terminal carbon of the compounds producing HC(O)Cl in the case of VA and HC(O)H in the case
of AA. The classically accepted mechanism involving CꢀC cleavage to form CH2Cl. radicals can not
explain the present product observations in the VA and AA reaction systems.
In addition to the fast secondary reaction of HCHO in the reaction system there is obviously also
considerable secondary reaction of acetoxyacetaldehyde with Cl. This reaction will proceed mainly
through Hꢀatom abstraction from the aldehydic hydrogen:
CH3C(O)OCH2C(O)H + Cl → CH3C(O)OCH2C.(O) + HCl
The resulting acyl radicals are expected, under the experimental conditions, to predominantly add
O2 to form peroxy radicals rather than decompose:
CH3C(O)OCH2C.(O) + O2 + (M) → CH3C(O)OCH2C(O)OO. + (M)
In the reaction system the peroxy radicals can react with NO or add NO2. Reaction with NO will
.
result mainly in the formation of RC(O)O. radicals which lose CO2 and form CH3C(O)OCH2
radicals. These radicals undergo further consecutive reactions with O2 and NO to give
CH3C(O)OCH2O. radicals.
There are two possible reaction pathways for the CH3C(O)OCH2O. radical either reaction with O2 to
form formic acetic anhydride or an αꢀester rearrangement to form acetic acid and HC(O) radicals.
CH3C(O)OCH2O. + O2 → CH3C(O)OC(O)H + HO2
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→ CH3C(O)OH + HC(O)
Both acetic acid and formic acetic anhydride have been detected as products in the Cl + AA
reaction system in the presence of NOx. We attribute the formation of both of these compounds to
secondary reactions of acetoxyacetaldehyde with Cl though the sequence of reactions outlined
above. The ratio of the yield of acetic acid to formic acetic anhydride is approximately 4:1 and
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remains fairly constant throughout the reaction. PicquetꢀVarrault et al.35 have obtained yields of 75
and 15 for the formation of acetic acid and acetoxyacetaldehyde in the reaction of OH with ethyl
acetate which also involves an αꢀester rearrangement. The ratio of the two products is quite similar
to that obtained in this study for Cl with acetoxyacetaldehyde and we take this as an additional
endorsement that secondary reaction of Cl with acetoxyacetaldehyde is the source of acetic acid and
formic acetic anhydride in the Cl + AA reaction system. PicquetꢀVarrault et al.19 in their study on
the reaction of OH with AA found acetoxyacetaldehyde as the main product and strongly suspected
that secondary reactions of this compound led to the formation of formic acetic anhydride and
acetic acid that they observed in their reaction system. This study supports that the reactions of OH
and Cl with acetoxyacetaldehyde are similar and result in formation of formic acetic anhydride and
acetic acid.
Reaction of the peroxy radicals formed in the reaction of Cl with acetoxyacetaldehyde with NO2
will lead to the formation of a peroxynitrate.
CH3C(O)OCH2C(O)O2 + NO2 + (M) → CH3C(O)OCH2C(O)O2NO2 + (M)
This type of peroxynitrate is expected to be reasonably stable at room temperature and atmospheric
pressure.36,37 Indeed, bands are present in the product spectrum which can be assigned to a
peroxynitrate. Trace A in Figure 5 shows the spectrum that is obtained at the end of the irradiation
of a AA/Cl2/NOx reaction mixture after subtraction of acetoxyacetaldehyde, acetic acid, formic
acetic anhydride and nitric acid. The * symbols highlight bands that are characteristic for the
formation of a peroxynitrate.36 The band at ~1744 cmꢀ1 can be assigned to an asymmetric stretching
band of NO2 (asym. NO2), that at ~1330 cmꢀ1 to a symmetrical stretch of NO2 (sym. NO2) and that
at ~795 cmꢀ1 to a NO2 deformation. A band at ~1045 cmꢀ1 is also observed. Bands in this region are
observed in the infrared spectra of many peroxynitrates, however, we have no vibrational group
assignment for this band.
Trace B, Figure 5 shows a difference spectrum obtained after subtraction of the first spectrum in
which the AA concentration was zero from the last spectrum in the experiment. This spectrum
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shows the continuing secondary decay of the main product acetoxyacetaldehyde (# symbols) and
the formation of the bands that we assign to a peroxynitrate. The experimental evidence supports
that the peroxynitrate absorptions are arising predominantly from the formation of thermally stable
acetoxyacetyl peroxynitrate.
Reaction without NOx: As discussed in the Results section, the experimental observations support
that the fate of the CH3C(O)OCH2CH(O.)CH2Cl alkoxy radicals formed in the Cl + AA reaction in
the absence of NOx is essentially reaction with O2 to form acetic acid 3ꢀchloroꢀ2ꢀoxoꢀpropyl ester
(CH3C(O)OCH2C(O)CH2Cl). The analogous OH reaction in the absence of NOx has been observed
to form mainly 3ꢀhydroxyꢀ2ꢀoxoꢀpropyl ester (CH3C(O)OCH2C(O)CH2OH)20 through reaction
CH3C(O)OCH2CH(O.)CH2OH alkoxy radicals with O2, however, a ~26 % contribution from the
decomposition channel to form acetoxyacetaldehyde and HCHO was also observed.
Atmospheric Implications
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As discussed in several publications the main tropospheric chemical removal of unsaturated
compounds is reaction with OH radicals and in the case of VA and AA the tropospheric lifetimes
respect to degradation through reaction with OH radicals are approximately 3 and 5 h,
respectively,15ꢀ20 indicating that these VOCs will be rapidly degraded in the gas phase and near to
their emissions sources. The tropospheric lifetimes of the degradation of VA and AA initiated by Cl
atoms are 9 and 4 days, respectively.18 However, in marine and industrialised continental regions,
where high chlorine concentrations can occur especially at dawn, Clꢀ mediated degradation of the
compounds may be able to compete with the OHꢀ initiated oxidation. Under high NOx conditions
both the OH and Cl initiated oxidation of both compounds will give rise to high yields of bond
cleavage products including HCHO, HCOCl, CH3C(O)OH, formic acetic anhydride and
acetoxyacetaldehyde. Further breakdown of acetoxyacetaldehyde will produce more acetic acid and
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formic acetic anhydride whereby formation of a potentially thermally stable peroxynitrate is
possible which might potentially result in the long range transport of NOx. However, although
peroxynitrates of structure ROC(O)OONO2 are thermally stable under atmospheric conditions33 the
CH3C(O)O entity in the peroxynitrate formed from acetoxyacetaldehyde, i.e.
CH3C(O)OCH2C(O)OONO2, might render this particular susceptible to hydrolysis and thus rapid
removal from the atmosphere.
Since the anhydrides produced in the atmospheric degradation of VA and AA are not very reactive
toward either OH or Cl38 and processes such as washout or heterogeneous hydrolysis on aerosols to
formic and acetic acids are probably the major atmospheric loss processes for these compounds.
Under low NOx conditions only the reaction of OH with VA will lead to the types of bond cleavage
products listed above. The reaction of Cl with VA and the reactions of OH and Cl with AA will
produce hydroxy acyl or chloro acyl compounds retaining the parent compound carbon skeleton.
These compounds will not be very reactive toward OH and their atmospheric degradation, wither in
the gas, aqueous or heterogeneous phase will probably result in acid formation, however, this may
not necessarily occur close to the emission source.
It is probably safe to say that the both the OH and Cl mediated atmospheric degradation of VA and
AA will result primarily in acid formation under both low and high NOx conditions but probably on
different time scales.
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Acknowledgments
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The authors wish to acknowledge the Alexander von Humboldt Foundation, the EU project
EUROCHAMP2, DAADꢀPROALAR (Germany), MINCyT (Argentina) and CONICET (Argentina)
for financial support of this research.
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Supporting Information
Extensive supporting information, mainly in the form of diagrams as indicated in the text, is
available free of charge via the Internet at http://pubs.rsc.org.
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Figure Captions
Figure 1. Panel A shows the product spectrum obtained from the irradiation of a VA/Cl2/NO/air
reaction mixture. Panels B, C. D and E show reference spectra of formic acetic anhydride, formyl
chloride, acetic acid, and carbon monoxide, respectively. Panel F shows the residual product
spectrum obtained after the subtraction of the identified products.
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Figure 2: Plot of the concentrations of the products formed in the reaction of Cl with VA in the
absence of NO as a function of the amount of reacted VA.
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Figure 3. Simplified mechanism for the Clꢀatom initiated oxidation of VA (CH2=CHOC(O)CH3)
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via addition of Cl to the terminal carbon of the double bond.
Figure 4.
Simplified mechanism for the Clꢀatom initiated oxidation of AA
(CH2=CHCH2OC(O)CH3) via addition of Cl to the terminal carbon of the double bond.
Figure 5: Trace A shows the spectrum obtained at the end of the irradiation a AA/Cl2/NOx reaction
mixture after subtraction of acetoxyacetaldehyde, acetic acid, formic acetic anhydride and nitric
acid. The * symbols in are attributed to the formation of acetoxyacetyl peroxynitrate. Trace B is the
difference spectrum obtained after subtraction of the first spectrum in which the AA was zero from
the last spectrum in the experiment. This spectrum demonstrates the secondary decay of the main
product acetoxyacetaldehyde (# symbols) and the formation of acetoxyacetyl peroxynitrate (*
symbols).
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Figure 1
2,5
2,0
1,5
1,0
0,5
0,0
F (Residual spectrum)
E (Carbon monoxide)
D (Acetic acid)
C (Formylchloride)
B (Formic acetic anhydride)
(after irradiation)
A
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1200
1700
2200
2700
3200
3700
-1
Wavenumber (cm )
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667
668
Figure 2
669
670
671
5,0
4,0
3,0
2,0
1,0
0,0
Formic acetic anhydride
Acetic acid
Monoxide Carbon
Formylchloride
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
0,0
2,0
4,0
6,0
8,0
10,0
12,0
Reacted Vinyl acetate (ppm)
24

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710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
Figure 3
O
+
Cl
O
A
O
Cl
O
O2,
RO2
O2, NO
NO2
RO + O2
O
O
O
O
D
Cl
Cl
r
reaction with O2
O
e
a
O
r
r
a
n
g
e
m
e
n
t
/
d
B
O
decomposition
C
e
c
o
m
p
o
s
i
t
i
o
n
O
O
O
+
+
CO
+
Cl
Cl
OH
O2,
O2,
RO2
RO2
O2, NO
NO2
O2, NO
NO2
Formic acetic anhydride
Acetic acid
RO + O2
RO + O2
O
O
Cl
Cl
O2
O2
HO2
HO2
Cl
Cl
O
O
Formylchloride
Formylchloride
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758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
Figure 4
O
O
+
Cl
A
O
O
Cl
O2,
RO2
O2, NO
RO + O2
O
NO2
O
reaction with O2
O
Cl
D
O
Cl
O
O
B
decomposition
decomposition
C
O
O
+
O
O
+
Cl
Cl
O
O
O2,
RO2
Acetoxyacetaldehyde
Chloroacetaldehyde
O2, NO
NO2
O2,
O2, NO
NO2
RO2
RO + O2
RO + O2
O
O2
HO2
O
O
Cl
O
O
O
Cl
H
+
Formylchloride
O2
H
Cl
HO2
O
O
O
+
Cl
H
H
O
Formaldehyde
Formic acetic anhydride
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805
806
Figure 5
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
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835
836
837
838
Table 1: Formation yields of the oxidation products identified from the Cl atom oxidation
of vinyl and allyl acetate studied in the presence and the absence of NOx.
The quoted errors are the 2σ statistical errors from the linear regression analyses.
Acetate
Product
Yield (%)
Yield (%)
(NOx ꢀ containing)
(NOx – free)
CH3C(O)OC(O)H
69 ± 8
25 ± 5
(Formic acetic anhydride)
CH3C(O)OC(O)CH2Cl
CH3C(O)OH
HC(O)Cl
ꢀ
~ 50a
12 ± 3
48 ± 8
ꢀ
CH3C(O)OCH=CH2
(vinyl acetate)
27 ± 5
94 ± 12
≤ 5
HC(O)H
CO
30 ± 5
15 ± 4
CH3C(O)OCH2C(O)H
(Acetoxyacetaldehyde)
66 ± 8a
≤ 5
≥ 95b
≤ 5
CH3C(O)OCH2CH=CH2
(allyl acetate)
CH3C(O)OCH2C(O)CH2Cl
≤ 5
CH3C(O)OC(O)H
secondary
(Formic acetic anhydride)
CH3C(O)OH
HC(O)Cl
HC(O)H
CO
secondary
secondary
> 90
≤ 5
≤ 5
≤ 5
≤ 5
secondary
839
840
841
a Lower limit observations and attempted corrections for secondary consumption support much
higher yield (see text); b Estimate based on experimental observations.
28