Kinetics and mechanism of the tropospheric reaction of 3-hydroxy-3-methyl- 2-butanone with Cl atoms

Article abstract of DOI:10.1021/jp5054343

The relative rate coefficient for the gas-phase reaction of 3-hydroxy-3-methyl-2-butanone (3H3M2B) with Cl atoms was determined under atmospheric conditions (298 ± 2 K, 720 ± 2 Torr). The products of the reaction were identified and quantified. This work

Full text of DOI:10.1021/jp5054343

Article
pubs.acs.org/JPCA
Kinetics and Mechanism of the Tropospheric Reaction of 3‑Hydroxy-
3
-methyl-2-butanone with Cl Atoms
†
,†
‡
‡
‡
†
§
C. Sleiman, G. El Dib,* B. Ballesteros, A. Moreno, J. Albaladejo, A. Canosa, and A. Chakir
†
́ ́ ́
Departement de Physique Moleculaire, Institut de Physique de Rennes, UMR 6251 du CNRS - Universite de Rennes 1, Bat. 11C,
Campus de Beaulieu, 35042 Rennes Cedex, France
‡
Facultad de Ciencias y Tecnologías Químicas, Departamento de Química Física, Universidad de Castilla La Mancha, , Campus
Universitario, 13071 Ciudad Real, Spain
§
́
Laboratoire GSMA-UMR 6089 CNRS, Universite de Reims, Campus Moulin de la Housse, BP 1039, 51687 Reims Cedex 02, France
ABSTRACT: The relative rate coefficient for the gas-phase reaction
of 3-hydroxy-3-methyl-2-butanone (3H3M2B) with Cl atoms was
determined under atmospheric conditions (298 ± 2 K, 720 ± 2 Torr).
The products of the reaction were identified and quantified. This work
provides the first kinetic and mechanistic determinations of the gas-
phase reaction of Cl atoms with 3H3M2B. The rate measurements
and the products studies were performed in two simulation chambers
coupled to the gas chromatography−mass spectrometer (GC−MS)
and the Fourier transform infrared (FTIR) techniques, respectively.
−10
3
The obtained average rate coefficient was (1.13 ± 0.17) × 10 cm
molecule−
1
s
−1
using propene and 1,3-butadiene as reference
compounds. The major primary reaction products observed in this
study were (with % molar yields): acetic acid (42.6 ± 4.8) and 2,3-
butanedione (17.2 ± 2.3). Results and mechanism are discussed in
terms of the structure−reactivity relationship and compared with the reported reactivity with the other atmospheric oxidants.
The atmospheric implications derived from this study are discussed as well.
1
. INTRODUCTION
mechanistic studies of the reaction of hydroxyacetone with OH
and Cl have been carried out at different temperatures.
However, very little information exists concerning the
9
−13
Carbonyl compounds play an important role in atmospheric
chemistry and in urban air pollution. In fact, these species
widely contribute to the formation of free radicals that are
responsible for the oxidation of hydrocarbons. They are also
precursors of other oxidants such as ozone, peroxyacyl nitrates
atmospheric fate of longer chain C -hydroxyketones. In fact,
4
1
only four kinetic studies on the gas-phase reaction of C4-
hydroxyketones can be found in the literature. Three of these
studies have been carried out at atmospheric pressure using a
(
PANs), and nitric acid and are important intermediates in
14
2
relative technique. Aschmann et al. studied the reaction of a
aerosol formation. Carbonyl compounds are directly emitted
into the troposphere from biogenic and anthropogenic
sources
atmospheric oxidation of unsaturated hydrocarbons and other
volatile organic compounds (VOCs).
Among the carbonyl compounds released into the
atmosphere, hydroxycarbonyls are a variety of multifunctional
organic molecules that are formed from the atmospheric
degradation of anthropogenically or naturally emitted VOCs.
Furthermore, hydroxycarbonyls are used in a number of
series of hydroxyketones including four C -hydroxyketones,
4
3
−5
with OH, NO , and O . The obtained results indicated that the
3
3
and are also major reaction products in the
degradation of these hydroxyketones was dominated by their
gas-phase reaction with the OH radical. In the study carried out
6
1
5
by Baker et al. two C -hydroxyketones (1-hydroxy-2-
4
butanone and 4-hydroxy-2-butanone) were identified as
products of the reactions of butanediols with OH. The kinetics
of these hydroxyketones with OH was measured as well. Very
16
recently, Messaadia et al. investigated the gas-phase reactions
of 3-hydroxy-2-butanone and 4-hydroxy-2-butanone with OH
radicals as a function of temperature and the reaction with Cl
atoms at room temperature using a relative rate method. A
slight negative dependence of the rate coefficients behavior was
observed for reactions with OH. Moreover, an absolute rate
7
industrial sectors namely in food, chemicals, and pharmaceut-
icals synthesis.
8
Hydroxyketones constitute a large category of hydroxycar-
bonyls. Despite the importance of these species in atmospheric
chemistry, so far most of the performed studies concerning the
reactivity of these compounds have been focused on
hydroxyacetone HOCH C(O)CH , which is formed during
Received: June 2, 2014
Revised: July 25, 2014
Published: July 28, 2014
2
3
the atmospheric oxidation of isoprene, the most important
biogenically emitted non-methane hydrocarbon. Kinetic and
9
©
2014 American Chemical Society
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Article
coefficient for the reaction of 4-hydroxy-2-butanone with OH
The first environmental chamber consists of a multipass 16 L
borosilicate glass cylinder reaction cell, homogeneously
surrounded by 4 symmetrically arranged UV fluorescent
lamps (Philips TL-K 40 W, λmax = 365 nm) used to
photochemically initiate the reaction. This chamber is coupled
to a Nexus Thermo Nicolet FTIR spectrometer equipped with
a liquid nitrogen cooled mercury cadmium telluride (MCT)
detector for online infrared spectroscopy. The total path length
of 96 m is obtained by using silver coated mirrors. The spectra
1
7
was measured by El Dib et al., by vapor pressure
measurements of 4-hydroxy-2-butanone. This study was carried
out at room temperature and over the pressure range 10−330
Torr in He and synthetic air as diluents gases. In addition, the
removal of these species due to photolysis has been studied by
1
8
Messaadia et al. and upper limits of the UV photolysis rates of
a series of C -hydroxyketones have been estimated by the
4
authors. The obtained values showed that photolysis could be
an important loss process of these species in the Earth
atmosphere.
−
1
were obtained by coadding 64 scans recorded at 2 cm
−1
instrumental resolution in the 650−4000 cm range. The
second environmental chamber is equipped with GC−MS
detection and consists in a 200 L Teflon bag surrounded by 8
UV lamps (Philips TL-K 40 W). Gas samples were analyzed by
GC (Thermo Electron Co., model Trace GC Ultra) coupled to
a mass spectrometer (Thermo Electron Co., model DSQ II).
Chlorine atoms were produced by the broad-band UV
The present work reports the first kinetic and mechanistic
studies of the gas-phase reaction of Cl atoms with 3-hydroxy-3-
methyl-2-butanone 3H3M2B, ((CH ) C(OH)C(O)CH ) a C
4
3
2
3
carbon chain hydroxyketone where the OH group is in the α
position with respect to the carbonyl group.
In the atmosphere, like other carbonyl compounds, 3H3M2B
is expected to be removed by chemical reactions with the
atmospheric photooxidants and by photolysis. The atmospheric
degradation of 3H3M2B is, however, not well-known. In fact,
only one kinetic study on the gas-phase reaction of 3H3M2B is
photolysis of Cl in the bath gas (synthetic air). UV radiation
2
was produced by the series of lamps symmetrically arranged
around the outside of the FTIR cell and the Teflon bag.
Reactants and Cl precursor were introduced directly into the
chambers by expansion from a glass manifold system and mixed
in synthetic air at 298 ± 2 K and 720 ± 2 Torr of total pressure.
Two reference compounds were used during our kinetic
measurements: propene in the FTIR measurements and 1,3-
butadiene in the GC−MS measurements. Concentrations in
the reaction chambers for 3H3M2B and the reference
compounds ranged from 10 to 15 ppm, whereas the
14
found in the literature. In this study, the degradation of
H3M2B due to its reaction with OH, NO , and O has been
3
3
3
investigated at room temperature and atmospheric pressure
using a relative technique. A rate coefficient of (0.94 ± 0.37) ×
−
12
3
−1 −1
1
0
cm molecule
s
was obtained for the reaction of
−16
3
−1
3
H3M2B with OH. Upper limits of 2 × 10 cm molecule
−
1
−19
3
−1 −1
s
and 1.1 × 10
cm molecule
s
were obtained for
concentrations of the precursors of Cl atoms (Cl ) ranged
reactions with NO radicals and O , respectively. In addition,
2
3
3
from 15 to 25 ppm. Reactants were left to homogenize in the
chamber for 60 min prior to irradiation. The kinetics of
degradation of both the reference and the analyte inside the
chamber was followed by monitoring the evolution of the areas
of the absorption bands (by FTIR) by using the “abstraction”
method based on calibrated reference spectra. In the GC−MS
method, the degradation of 3H3M2B and the reference
compound was monitored by following the decrease of the
chromatographic peaks of both compounds as explained in the
Results section. Samples were analyzed in situ into the pyrex
cell by FTIR whereas in the Teflon bag, the concentrations
were monitored by using the technique of solid-phase
microextraction (SPME). SPME is a simple and effective
adsorption and desorption technique, which eliminates the
need of using solvents or complicated apparatus for
concentrating volatile or nonvolatile compounds in liquid
the tropospheric lifetime of 3H3M2B due to photolysis has
18
been estimated by Messaadia et al. to be at least 0.4 day using
an upper limit of the photolysis rate assuming a quantum yield
of unity.
Despite the significant role of Cl atoms in the atmospheric
chemistry, there are no data reported in the literature on the
atmospheric oxidation of 3H3M2B with Cl atoms. In fact, the
chemistry of Cl atoms plays an important role in the oxidizing
capacity of the troposphere particularly in the early morning in
continental regions, in coastal and marine air environments and
1
9−24
in the Arctic troposphere during springtime.
Although the
5
−3 25
peak of Cl atom concentrations (10 atoms cm ) is much
lower than that of OH radicals (10 atoms cm ),
6
−3 26,27
the two
reactions can compete with one another in areas where the
chlorine atom concentration is sufficiently high since rate
coefficients for the reactions of Cl atoms with organic
compounds are generally a factor of 10 higher than the
3
2,33
samples or headspaces.
In this work, a 50/30 μm
28
divinylbenzene/carboxen/polydimethylsiloxane fiber was used.
The coated fiber was inserted into the Teflon chamber and
exposed during an optimized time of 15 min to the chamber
contents. Then, SPME fiber samples were thermally desorbed
in the heated (250 °C) GC injection port and the products
were analyzed by mass spectrometry, thus the irradiation time
was not continuous for the GC−MS system and the analysis
was performed when the lamps were off.
corresponding OH rate coefficients. The aim of the present
study is to assess the importance of the reaction of Cl atoms as
an atmospheric loss process of 3H3M2B. The results are used
to calculate the effective lifetime of 3H3M2B in the
troposphere and to better define the reactivity of the studied
compound toward the Cl atoms and for a better understanding
of its atmospheric fate.
Prior to analyses by using both techniques, a calibration of
the measured concentration of the analyte and the detected
products commercially available was carried out.
In addition to the standard constraint that consists of having
a well-documented kinetic rate coefficient, the reference
compound was chosen in such a way that its FTIR spectrum
and/or chromatographic peak do not interfere with those of
3H3M2B and vice versa, and with the products of the reaction.
The rate coefficient used in this work for the reaction of Cl with
2
. EXPERIMENTAL SECTION
2.1. Relative Rate Coefficient Measurements. The
relative Cl-kinetic measurements were carried out in two
environmental chambers equipped with FTIR and GC−MS in
the presence of reference compounds as explained below. A
detailed description of the experimental setup and the kinetic
analysis performed has been previously presented.
the experimental system will be only briefly described here.
2
9−31
Thus,
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The Journal of Physical Chemistry A
propene was that reported by Ceacero-Vega et al., (2.23 ±
Article
3
4
secondary losses for 3H3M2B and the reference compound,
respectively. Figure 1 shows an example of this plot. A good
10
3
−1 −1
0
.31) × 10− cm molecule s . For the rate coefficient of Cl
−10 3
atoms with 1,3-butadiene, the value of (4.2 ± 0.4) × 10 cm
molecule−
1
s
−1
at 760 Torr of air and 298 K was used.
35
2
.2. Nature and Quantification of the Reaction
Products. The products formed in the reaction of Cl atoms
with 3H3M2B were investigated using both simulation
chambers. These studies were performed under the same
experimental conditions as those for the kinetic studies, but in
the absence of reference compounds.
Samples were analyzed in situ by FTIR into the pyrex cell.
The identification of the products was made by comparison
with a library of IR spectra and with the IR spectrum of a pure
sample. In the Teflon bag, the identification of the products was
made by analysis of the mass spectrum by comparison with a
library of spectra and the retention time and mass spectrum of a
pure sample of the detected product. Under the conditions
used here the SPME/GC−MS response was linear with the
analyte concentration. No carryover was observed for any of the
compounds upon a second injection, indicating a complete
recovery of the fiber. The fiber was used immediately after
desorption for the next sampling.
After each experiment, the reaction chamber was cleaned out
by repeated purge-pump cycles until the reactants and/or
products were not detected.
The purity of the used chemicals as stated by the
manufacturer were 3H3M2B (95%), propene (99%), 1,3-
butadiene (99%), 2,3-butanedione (97%), and acetic acid
Figure 1. Examples of relative rate plots for the reaction of Cl with
3
H3M2B with propene and 1,3-butadiene as reference compounds,
according to eq I.
linearity is observed with a correlation coefficient greater than
9
9% and an intercept close to zero. The obtained rate
coefficients are summarized in Table 1. For each point in
Figure 1, the error bars indicate the total uncertainties on the
experimental conditions as explained in the Discussion section.
Table 1. Rate Coefficient Ratios and the Obtained Relative
Rate Coefficients for the Reaction of Cl Atoms with
(99%) from Sigma-Aldrich and synthetic air (99.99%) from Air
3H3M2B at 298 ± 2 K and 720 ± 2 Torr
liquide.
detection
k
3H3M2B
3
10
−1
3
. RESULTS
.1. Relative Rate Coefficient Measurements. In the
ref compd
technique
k3H3M2B/kref
(10− cm molecule s−1
)
a
c
3
propene
FTIR
0.53 ± 0.10
0.58 ± 0.06
0.49 ± 0.06
1.10 ± 0.30
1.30 ± 0.20
1.09 ± 0.20
1.10 ± 0.20
c
conducted relative kinetic experiments the following reactions
take place:
c
c
0
.49 ± 0.05
Cl + 3H3M2B → products
Cl + reference → products
k3H3M2B
kref
(R1)
(R2)
c
1,3-
butadiene
GC−MS
0.25 ± 0.04
1.06 ± 0.20
b
d
average
1.13 ± 0.17
cm3 molecule−1
a
−10
−1
k(propene+Cl) = (2.23 ± 0.31) × 10
s
where k3H3M2B is the rate coefficient of the reaction of 3H3M2B
with Cl and kref is the known rate coefficient of the reference
compounds with Cl. Prior to kinetic measurements, experi-
ments were performed in the absence of Cl precursor under
radiation to determine the loss rate of the reactants (analyte
and reference compound) due to photolysis. These tests were
performed under light intensities and radiation time similar to
those employed during our kinetic experiments in both
simulation chambers. Further tests were performed in the
34
b
−10
(
Ceacero-Vega et al. ). k(1,3-butadiene+Cl) = (4.2 ± 0.4) × 10
3
−1 −1
35 c
cm molecule
s
(Ragains and Finlayson-Pitts. ). Uncertainty
d
calculated according to eq II (see text). Twice standard deviations.
As mentioned before, the kinetics of degradation of both the
reference and the analyte inside the IR cell was followed by
monitoring the evolution of the IR absorption bands areas of
both compounds. The IR spectral features used for analysis
−1
−1
were 1211−1149 cm for 3H3M2B and 1005−971 cm for
absence of radiation for the 3H3M2B/Cl /air mixture to check
2
propene.
for dark reactions and wall losses. The global secondary losses
were 6%, 3%, and 4% for 3H3M2B, propene, and 1,3-
butadiene, respectively, mainly coming from wall losses.
The 3H3M2B and reference compound concentration time
evolution was described using the following expression taking
into account these secondary loss processes:
In the Teflon bag chamber coupled to a GC−MS, the
measurements were performed by following the evolution of
the chromatographic peak areas of 3H3M2B and 1,3-butadiene
with time at retention times of 4.75 and 1.95 min, respectively.
The blank was analyzed before and after each sample analysis
so as to minimize any uncertainties originating from the
deviation of the MS signal and GC response.
3.2. Reaction Products and Mechanism. Products of the
studied reaction were identified and quantified in the FTIR cell
by using the FTIR spectroscopy and in the Teflon bag coupled
to GS−MS.
The major primary oxidation products observed in the
experiment carried out by FTIR were acetic acid, and CO. The
⎡
× ⎢ln
⎣
⎤
[
[
3H3M2B]0
3H3M2B]t
k3H3M2B
kref
[ref]0
[ref]t
ln
− k t =
− k′ t⎥
w
w
⎦
(I)
where [3H3M2B] , [3H3M2B] , [ref] , and [ref] are the
concentrations of the reagents before irradiation and at time t,
respectively and k and k′ are the decay rates resulting from
0
t
0
t
w
w
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Article
concentration−time profiles of 3H3M2B and acetic acid were
−1
obtained from integration of the IR spectra 1211−1149 cm
−
1
for 3H3M2B and 1890−1740 cm for acetic acid. 2,3-
Butanedione and formaldehyde were observed as well in FTIR
analyses but were not quantified due to interferences with
absorption peaks of other molecules occurring in the chemical
mechanism of the studied reaction. 2,3-Butanedione was
quantified by GC−MS as a major primary product. In addition
to the above products, GC−MS analysis indicated the presence
of chlorinated secondary reactions products and a number of
small peaks that could not be identified. 2,3-Butanedione and
secondary products were identified by comparison of their mass
spectrum with the corresponding spectra from a spectral library
and also the mass spectrum and retention time of a commercial
sample. Figures 2 and 3 show an example of an IR spectra and a
Figure 3. Gas chromatograms of samples from a Cl /3H3M2B
2
mixture in 720 Torr of synthetic air in the Teflon bag before and 60
min after the start of irradiation.
Figure 2. Example of infrared absorption spectra recorded in this
work. The upper trace shows the infrared spectrum of the Cl2/
3H3M2B mixture in 720 Torr of synthetic air in the FTIR cell before
irradiation, and the lower trace shows the spectrum 60 min after the
start of irradiation.
Figure 4. Plot of the concentration of 3H3M2B and acetic acid versus
the time of reaction. Analyses were carried out by FTIR.
gas chromatogram taken before irradiation and 60 min after
irradiation of a 3H3M2B/Cl /air mixture in the FTIR cell and
2
the Teflon bag, respectively. The temporal evolution of
4
. DISCUSSION
3
H3M2B and main reaction products is shown in Figures 4
and 5. The concentration profiles of acetic acid and 2,3-
butanedione show that both are primary products. The
formation yields of the major products detected in this study
represent the slopes of the plot of [product] /[3H3M3B] vs 1
4.1. Error Analysis. The overall error in the rate coefficient
values reported in this work (Table 1) varies from 17 to 26%.
This error originates from the uncertainty on the rate
coefficient of the reference compounds used in this study and
the errors on the determination of the slope k/kref shown in
Figure 1. The error on the rate coefficient is therefore
calculated according to eq II:
t
0
−
[3H3M2B] /[3H3M2B] , where [product] is the product
t 0 t
concentration at a time t and [3H3M3B] and [3H3M3B] are
0
t
the 3H3M2B concentrations before irradiation and at time t,
respectively (Figures 6 and 7). The formation yields of the
products can be derived from the slopes of these plots, and the
obtained values were 42.6 ± 4.8% and 17.2 ± 2.3% for acetic
acid and 2,3-butanedione, respectively. The carbon balance
obtained was 33.1%, showing that additional formed oxidation
products could not be detected or quantified.
2
2
⎡
⎤
⎦
⎡
Δkref
kref
Δk = k ×
⎢
⎣
⎣ slope
⎦
(II)
The errors on kref as found in the literature were 13.9 and 9.5%
for propene and 1,3-butadiene, respectively.
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Article
handling 3H3M2B because of its low vapor pressure, and the
purity degree of the used compounds. To minimize these
uncertainties, several preliminary tests were carried out such as
the GC column temperature and the selection of the reference
compound to have relatively intense chromatograms and
spectroscopic peaks.
Individual vertical and horizontal error bars on each point in
Figure 1 correspond to 14%. The error reported on the average
rate coefficient given in Table 1 is two standard deviations.
In addition to the systematic errors cited above, the error in
the formation yield values reported in this work originate from
statistical errors that result from a least-squares analysis of the
[
product] [3H3M2B] vs 1 − [3H3M2B] [3H3M2B] plot,
t
0
t
0
multiplied by the Student’s t-factor appropriate for the 95%
confidence interval and the number of degrees of freedom. The
overall error on the formation yield obtained in this work was
about 11% and 13% for acetic acid and 2,3-butanedione,
respectively.
Figure 5. Plot of the concentration of 3H3M2B and 2,3-butanedione
versus the time of reaction. Analyses were carried out by SPME/GC−
MS.
4
.2. Comparison with the Literature and the Effect of
the Structure on the Reactivity. This work provides the first
kinetic study of the gas-phase reaction of 3H3M2B with Cl
atoms. As mentioned before, the degradation of 3H3M2B by
14
OH, NO , and O has been investigated by Aschmann et al.
3
3
The measured rate coefficient for the reaction of 3H3M2B with
−10
3
Cl atoms obtained in this work, (1.13 ± 0.17) × 10 cm
molecule− s , is about 120 times that reported in the literature
1
−1
−
12
3
for the reaction with OH, (0.94 ± 0.08) × 10
cm
molecule− s .
1
−1
The calculated value for the reaction of Cl atoms with
−
11
3
−1
3
H3M2B by the SAR method is 5.7 × 10 cm molecule
1
36
−
s , using a factor F(CO) = 0.12 derived from the
experimental value for the reaction of Cl atoms with butanone
of (4.08 ± 0.21) × 10− cm molecule s . This calculated
value is about 50% lower than the present measurement. This
discrepancy is due to the fact that activating effects are not
considered in this calculation.
11
3
−1 −1 37
Figure 6. Plot of the ratio of acetic acid formed over the initial
concentration of 3H3M2B versus reacted 3H3M2B.
The comparison of the rate coefficient for 3H3M2B obtained
in this work to that of 3-methyl-2-butanone (CH ) CHC(O)-
3
2
−11
3
−1 −1
CH (6.2 ± 0.5) × 10 cm molecule
s
reported by Kaiser
and Wallington suggests that the reaction with Cl proceeds
3
38
mainly via H-abstraction of the weakest C−H bond on the two
−
CH groups activated by the presence of the −OH
3
substituent group. The same behavior has been observed for
reactions of 2-butanone, 3H2B, and 4H2B with Cl
1
6,37,39
atoms.
Our value is compared to that of reaction of Cl with 3-
hydroxy-2-butanone (3H2B) where the OH group is in the α
position with respect to the carbonyl group similarly to
3
H3M2B. The rate coefficient for the gas-phase reactions of Cl
atoms with 3-hydroxy-2-butanone was measured by Messaadia
et al. using an atmospheric simulation chamber made of
Teflon at 298 ± 3 K and 760 Torr. The obtained rate
16
Figure 7. Plot of the ratio of 2,3-butanedione formed over the initial
concentration of 3H3M2B versus reacted 3H3M2B.
coefficient was (4.90 ± 0.45) × 10− cm molecule s . This
comparison shows that the reactivity of these species with Cl
increases by a factor of 2.3 when an H atom attached to the
carbon atom α to the >CO group is replaced by a methyl
11
3
−1 −1
The errors on the slope originate from
(
i) statistical errors that result from a least-squares analysis
of Figure 1, multiplied by the Student’s t-factor
appropriate for the 95% confidence interval and the
number of degrees of freedom and
39−41
group. As for other ketones,
the >CO group seems to
deactivate the C−H bonds attached to carbon atom α to the
>
CO group for hydroxyketones and to activate the C−H
(
ii) systematic errors that are estimated to 7%.
bonds attached to carbon atom β to the >CO group as for
The systematic errors originate from the difficulties in
measuring the concentrations of the studied reagents with good
accuracy due to the temperature instability, the difficulties in
4H2B.
4.3. Mechanism. As seen in Figures 6 and 7, a good
linearity is observed for the plot of [product] [3H3M2B] vs 1
t
0
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Article
Scheme 1. Proposed Mechanism for the Reaction of Cl Atoms with 3H3M2B, Where Detected Products Are Shown in a Box
•
−
[3H3M2B] [3H3M2B] with a correlation coefficient higher
The CH C (OH)C(O)CH radical may react with O to form
t
0
3
3
2
than 99% showing that acetic acid and 2,3-butanedione are the
major primary oxidation products of the studied reaction. This
result suggests that the reaction of 3H3M2B with Cl may
proceed mainly via H-abstraction of the weakest C−H bond on
2,3-butanedione CH C(O)C(O)CH identified and quantified
by GC−MS analysis.
In addition to 2,3-butanedione, acetic acid CH COOH was
observed in this work as a major product. We suggest here a
mechanism that could explain the formation of acetic acid as a
primary product of the reaction of 3H3M2B with Cl.
3 3
3
the two −CH groups in the β position with respect to the
3
carbonyl group. In Scheme 1 is shown the mechanism of the
oxidation of 3H3M2B by the chlorine atoms. This scheme
includes different reaction pathways that may occur and may
explain the formation of different products which have been
identified in this work.
•
The CH C (OH)COCH radical formed after the decom-
3
3
•
position of the CH C(OH)CH O C(O)CH radical may
decompose to lead to the formation of the CH C (O) radical
and ethanal CH CHO. The CH C (O) radical may decompose
to lead to the formation of CO (observed at the beginning of
3
2
3
•
3
•
3
3
Initially, the abstraction mainly proceeds through the
following channel:
the reaction) and the CH radical, which may generate the
3
methylperoxy radical CH O after reaction with O . The
3
2
2
Cl + (CH ) C(OH)C(O)CH
•
3
2
3
CH C (O) radical may also react with O to yield the
3
2
•
•
CH C(O)OO radical. This radical may react through three
→
CH C(OH)CH C(O)CH + HCl
3
3
2
3
possible pathways: It might react with itself to give the
•
•
CH C(O)O radical, which decomposes to generate the CH
The primary radical formed CH C(OH)CH C(O)CH will
3
3
3
2
3
42
react with O followed by reaction with RO to lead to the
radical and CO
2
.
Another possible path leading to the
2
2
•
corresponding radical CH C(OH)CH O C(O)CH . This
production of the observed acetic acid can result from the
3
2
3
•
radical may react with O or decompose. Decomposition may
reaction of the CH C(O)OO radical with HO . The yield of
3
2
2
•
lead to the CH C (OH)C(O)CH radical and to formaldehyde
acetic acid for this reaction is 20 ± 8%, as reported by Orlando
3
3
4
3
•
HCHO, which was detected by FTIR but not quantified due to
interferences with absorption peaks of other molecules
occurring in the chemical mechanism of the studied reaction.
and Tyndall. The radical−radical reaction of CH C(O)OO
3
•
42,44−46
with CH O has been previously studied
and shown to
3
2
proceed through two main channels: the first one (a) leads to
6
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The Journal of Physical Chemistry A
Article
•
the formation of CH C(O)O , CH O, and O and the second
Table 2. Tropospheric Lifetimes (days) of 3H3M2B Due to
Photolysis and Its Gas-Phase Reactions with Atmospheric
Photooxidants
3
3
2
one (b) leads to the formation of acetic acid, formaldehyde and
O . Different values of the branching ratios of channel (a) have
been reported in the literature, varying from 0.48 to 0.90.
Therefore, the yield of acetic acid for this reaction should be in
2
42
a
b
b
b
c
τphotolysis
τOH
τNO3
τO3
τCl
the range 10−52%.
>0.76
12
>230
>150
102
a
Calculated using the photolysis rate for hydroxyacetone determined
by Orlando et al. and the absorption cross sections determined by
Messaadia et al. (see text). Calculated by Aschmann et al., with a
OH 24 h average concentration of 1 × 10 molecules cm , a 24 h
1
2
5. TROPOSPHERIC IMPLICATIONS
18 b 14
It is known that volatile organic compounds, in general, are
chemically removed from the troposphere mainly by reaction
with the tropospheric oxidants: OH, NO , O , and halogen
atoms. The tropospheric lifetime (τ) of any volatile organic
compound due to its reaction with an oxidant X is commonly
defined by
6
−3
8
−3
average NO concentration of 2.5 × 10 molecules cm , and a 24 h
3
11
−3
c
3
3
average O concentration of 7 × 10 molecules cm . Calculated by
3
using the rate constant determined in this work with an upper limit of
3
−3
41
1 × 10 molecules cm reported by Wingenter et al.
3
H3M2B in the presence of NO can be a source of other toxic
x
1
τ =
molecules with high atmospheric interest such as PAN which
could be formed from the reaction of CH C(O)OO with NO .
kx[X]
(III)
•
3
2
where k is the rate coefficient of the organic compound with
the X oxidant which is present in the troposphere in an average
concentration [X].
x
6
. CONCLUSION
This work constitutes the first kinetic and mechanistic studies
of the reaction of Cl with 3H3M2B. The kinetic measurements
were investigated using a relative method at room temperature
and at atmospheric pressure. The obtained data showed that
the reaction of the studied compound with Cl atoms proceeds
more rapidly than that with OH radicals. Our results are
consistent with the expectation that the presence of the
carbonyl group deactivates the CH bonds attached to the
carbon atom α to the >CO group and activates those
attached to the carbon atom β to the >CO group.
A tropospheric lifetime for the reaction of Cl atoms with
H3M2B of 102 days was calculated according to eq III where
3
[
Cl] represents the 24 h daytime average global tropospheric
3
−3 47
concentration of Cl atoms of 1 × 10 molecules cm .
Therefore, despite their high reactivity, Cl atoms concen-
trations are too low to compete with OH radicals in influencing
the oxidizing capacity of the global troposphere. The reactions
with Cl atoms, however, could be an important loss process for
these species in coastal areas where the concentration of Cl
5
−3 24
The reaction of 3H3M2B with Cl is suggested to mainly
proceed via H-abstraction of the weakest C−H bond on the
atoms reaches 1 × 10 molecules cm , and the tropospheric
lifetime is therefore reduced to about 1 day. The lifetimes due
to the reactions with OH, NO , and O as reported by
two −CH groups in the β position with respect to the carbonyl
3
3
3
1
4
group. The major products quantified in this work were acetic
acid and 2,3-butanedione.
Aschmann et al. were (in days) 12, >230, and >150,
respectively. These values were calculated by using a 24 h
average concentration of 1 × 10 molecules cm for OH, 2.5 ×
0 molecules cm for NO , and 7 × 10 molecules cm for
6
−3
The calculated tropospheric lifetime obtained in this work
suggests that the reactions with Cl atoms could be an important
loss process for these species in coastal areas and contributes to
the photochemical pollution in these areas.
It is important to note that the above analysis does not take
into account any heterogeneous loss processes for 3H3M2B.
Moreover, there is a need to determine the photolysis quantum
yields of the studied compound and to measure its photolysis
rates under simulated atmospheric conditions.
8
−3
11
−3
1
3
O .
3
Until now, there has been no study concerning the wet and
dry deposition of 3H3M2B. However, the photolysis of a series
of hydroxyketones including 3H3M2B has been studied by
1
8
Messaadia et al. An upper limit of photodissociation rate
coefficient (J ) for the studied compound of 0.4 day
p
corresponding to a zenith angle of 30° and between 200 and
45 nm was calculated by these authors considering a quantum
3
AUTHOR INFORMATION
Corresponding Author
G. El Dib. Tel.: +33 2 23 23 56 80. Fax: +33 2 23 23 67 86. E-
mail address: gisele.eldib@univ-rennes1.fr.
yield of unity. It should be noted that the overall quantum yield
in the actinic region for 3H3M2B is not known. However, the
quantum yield for smaller hydroxyketones such as hydrox-
■
*
12
yacetone has been measured by Orlando et al. who obtained
an upper value of 0.6 for wavelengths greater than 290 nm. In
the present work, a lifetime of 3H3M2B due to photolysis of
Notes
The authors declare no competing financial interest.
0
.76 day is calculated by using the upper limit of quantum yield
ACKNOWLEDGMENTS
of hydroxyacetone (Table 2).
■
Reactions with Cl in marine areas and photolysis seem to be
the main loss processes for 3H3M2B provided that the
assumption with respect to the quantum yield is correct.
In the light of the products detected in this work, the
reactions of 3H3M2B with Cl may contribute to the
photochemical pollution in contaminated coastal urban areas.
In addition, the developed mechanism to explain the products
observations via FTIR or GC−MS in the present work suggests
the formation of CH C(O)OO radical (Scheme 1). Therefore,
it is important to stress that the atmospheric degradation of
The authors gratefully thank the INSU-LEFE French
programme, the Brittany Region and the doctoral school
SDLM at the University of Rennes 1 for financial support. They
would like to thank as well the Spanish Ministerio de Ciencia e
Innovacion (Project CGL2010-19066) for the financial support
́
of this research work.
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