Photodegradation of butyl acrylate in the troposphere by OH radicals: Kinetics and fate of 1,2-hydroxyalcoxy radicals

Article abstract of DOI:10.1002/poc.1363

The rate constant of the reaction of OH radicals with butyl acrylate was studied for the first time using an atmospheric simulation chamber at 298 K and ～750 Torr of air or nitrogen. The decay of the organics was followed using a gas Chromatograph with a flame ionization detector (GC-FID), and the rate constant was determined using a relative rate method with different references. The obtained average value of (1.80 ± 0.26) × 10^-11 cm³ molecule^-1 s^-1 is in agreement with previous determinations of the rate constants of OH radicals with acrylates and methacrylates in the literature. Additionally, product identification under atmospheric conditions was performed for the first time by the GC-MS technique. Butyl glyoxalate was observed as the degradation product in accordance with the addition of OH to the less substituted carbon atom of the double bond, followed by decomposition of the 1,2-hydroxyalkoxy radicals formed. Room temperature rate coefficient was used to estimate the atmospheric lifetime of the ester studied. Reactivity trends are discussed in terms of the substituent effects and the length of the hydrogenated chain of the ester. The atmospheric persistence of BUAC was calculated taking into account the experimental rate constant obtained. Copyright

Full text of DOI:10.1002/poc.1363

Research Article
Received: 13 November 2007,
Revised: 22 February 2008,
Accepted: 24 February 2008,
Published online in Wiley InterScience: 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1363
Photodegradation of butyl acrylate in the
troposphere by OH radicals: kinetics and fate
of 1,2-hydroxyalcoxy radicals
a
a
*
Marı´a B. Blanco and Mariano A. Teruel
The rate constant of the reaction of OH radicals with butyl acrylate was studied for the first time using an atmospheric
simulation chamber at 298 K and ꢀ750 Torr of air or nitrogen. The decay of the organics was followed using a gas
chromatograph with a flame ionization detector (GC-FID), and the rate constant was determined using a relative rate
method with different references. The obtained average value of (1.80 W 0.26) T 10S11 cm3 moleculeS1 sS1 is in
agreement with previous determinations of the rate constants of OH radicals with acrylates and methacrylates in the
literature. Additionally, product identification under atmospheric conditions was performed for the first time by the
GC-MS technique. Butyl glyoxalate was observed as the degradation product in accordance with the addition of OH to
the less substituted carbon atom of the double bond, followed by decomposition of the 1,2-hydroxyalkoxy radicals
formed. Room temperature rate coefficient was used to estimate the atmospheric lifetime of the ester studied.
Reactivity trends are discussed in terms of the substituent effects and the length of the hydrogenated chain of the
ester. The atmospheric persistence of BUAC was calculated taking into account the experimental rate constant
obtained. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: reaction rate constant; butyl acrylate; OH radicals; unsaturated VOCs; troposphere
INTRODUCTION
Experiments were conducted using the relative rate method
with different OH sources and reference compounds at room
temperature and atmospheric pressure of air or nitrogen.
To the best of our knowledge, this work provides the first
kinetic study for the reaction cited above (1) under atmospheric
conditions. In addition, our work aims to better define the
Acrylate esters containing a double bond and functional carboxyl
group are used chiefly as a monomer or co-monomer in making
acrylic and modacrylic fibers. They are used in formulating paints
and dispersions for paints, inks, and adhesives, in making
cleaning products, antioxidant agents, amphoteric surfactants as
well as in making aqueous resins and dispersions for textiles and
papers. Butyl acrylate (BUAC) is a very useful feedstock for
chemical syntheses, because it readily undergoes addition
—
reactivity of the CH2 CHR compounds toward OH radicals as an
—
extension of our previous work involving O(3P) additions to
haloalkenes,[8–10] OH addition to methacrylates and acrylates[5,7]
and OH radicals and Cl atoms additions to other unsaturated
VOCs.[11] In this sense, the results are also discussed in terms of
the substituent effects to the double bond and the length of the
hydrogenated chain of the ester on the reactivity of the olefinic
carbon. The atmospheric lifetime of BUAC was calculated taking
into account the experimental rate constant obtained.
Finally, product studies using the GC-MS technique under
atmospheric conditions were carried out for the first time for the
reaction of OH radical with the ester studied, therefore the
degradation pathway of BUAC in the atmosphere is also
postulated.
reactions with
compounds.[1,2]
a wide variety of organic and inorganic
Substantial amounts of unsaturated oxygenates are continu-
ally introduced into the atmosphere and the kinetics and
degradation pathways under atmospheric conditions of many of
these compounds are still largely unknown. The atmospheric
degradation of these compounds will be initiated mainly by
chemical reaction with OH and NO3 radicals and O3 molecules
contributing to tropospheric ozone production and the for-
mation of other secondary photo-oxidants in polluted areas.[3]
In order to assess the impact of these species on air quality,
kinetic, and mechanistic information on their tropospheric
degradation is therefore needed.
Up until now, studies on the atmospheric chemistry of
unsaturated esters have not been very extensive. Limited data are
available in the literature on the gas-phase reactions of a few
unsaturated esters.[4–7]
*
Departamento de Fisicoqu´ımica, Instituto de Investigaciones en Fisicoqu´ımica
´
de Cordoba (INFIQC), Facultad de Ciencias Qu´ımicas, Universidad Nacional de
Cordoba, Ciudad Universitaria, 5000 Cordoba, Argentina.
´
´
E-mail: mteruel@fcq.unc.edu.ar
In this work, we report the rate coefficient for the reaction of
OH radicals with BUAC:
a Mar´ıa B. Blanco, Mariano A. Teruel
Departamento de Fisicoqu´ımica, Instituto de Investigaciones en Fisicoqu´ımica
´
de Cordoba (INFIQC), Facultad de Ciencias Qu´ımicas, Universidad Nacional de
Cordoba, Ciudad Universitaria, 5000 Cordoba, Argentina
OH þ CH2 ¼¼ CHCðOÞOðCH2Þ3CH3 ! Products
(1)
´
´
J. Phys. Org. Chem. 2008, 21 397–401
Copyright ß 2008 John Wiley & Sons, Ltd.

M. B. BLANCO AND M. A. TERUEL
Figure 1. Schematic diagram of the simulation chamber apparatus used in the OH þ BUAC reaction study
In the present work, typically four of these lamps were used to
produce OH radicals and the time of photolysis varied from 3 to
35 min.
Periodically, gas samples were removed from the Teflon bag
using calibrated gas syringes. The organics were monitored by
gas chromatography (Shimadzu GC-14B) coupled with flame
ionization detection (FID), using a Porapak Q column (Alltech,
2.3 m) held from 160 to 240 8C.
The analytical technique employed for qualitative identifi-
cation of the products formed after irradiation was GC-mass
spectrometry on a Shimadzu GC-MS QP 5050 spectrometer
equipped with a 30 m–0.12 mm DB-5 MS column.
The chemicals used were: N2 (AGA, 99.999%), synthetic air
(AGA, 99.999%), BUAC (Aldrich, 99%), diethyl ether (Fluka 99%),
n-pentane (Mallinckrodt, 99.9%) and H2O2 (Ciccarelli 60 wt%)
were degassed by repeated freeze-pump-traw cycling. CH3ONO
was synthesized by the dropwise addition of 50% H2SO4 to a
saturated solution of sodium nitrite in methanol and was purified
by vacuum distillation until a sample of 99% purity was obtained,
confirmed by IR spectroscopy and GC-FID.
EXPERIMENTAL
The experimental set-up consisted of a 80 L Teflon bag located in
a wooden box with the internal walls covered with aluminum foil,
and operated at atmospheric pressure (750 ꢁ 10) Torr and
(298 ꢁ 1) K. The schematic diagram of the simulation chamber
employed in this work is shown in Fig. 1.
Measured amounts of the organic reactants were flushed from
calibrated bulbs into the reaction chamber with a stream of ultra
pure air. The bag was then filled to its full capacity at atmospheric
pressure with N2 or ultra pure air. Before each set of experiments
the Teflon bag was cleaned by filling it with a mixture of O2
and N2 which was irradiated for 15–25 min using four germicidal
lamps (Philips 30 W) with a UV emission at 254 nm, to produce O3.
After this procedure, the bag was cleaned again by repeated
flushing with N2 and checked before performing the experiments
by gas chromatography (GC) to verify that there were no
observable impurities.
The initial concentrations used in the experiments were in the
range of 136–282 ppm (1 ppm ¼ 2.46 ꢂ 1013 molecule cmꢃ3 at
298 K and 760 Torr of total pressure) for BUAC, 155–263 ppm for
diethyl ether and n-pentane used as reference compounds,
251–289 ppm for methyl nitrite, 336–379 ppm for NO, and 100 mL
for H2O2.
Methyl nitrite (CH3ONO) in the presence of NO/O2 was used to
generate OH radicals by its photolysis using a set of black lamps
(Philips 30 W) surrounding the Teflon bag, according to the
following reactions:
RESULTS
Relative rate coefficient for the reactions of OH radicals with
BUAC was determined by comparing the OH reaction rate with
the unsaturated ester to that with the reference compounds
OH þ BUAC ! Products kester
OH þ reference ! Products kref
(6)
(7)
CH3ONO þ hv ! CH3O þ NO
CH3O þ O2 ! CH2O þ HO2
HO2 þ NO ! OH þ NO2
(2)
(3)
(4)
Provided that the reaction with OH is the only significant loss
process for both, reactant and reference compounds, then it can
be shown that
Inð½BUACꢄ0=½BUACꢄtÞ ¼ ðkester=krefÞ ꢂ Inð½refꢄ0=½refꢄtÞ (8)
These lamps provide UV radiation with a l maximum around
360 nm.
H2O2 was used as an alternative source of OH radicals by direct
UV photolysis with germicidal lamps emitting at 254 nm (Philips
30 W) surrounding the Teflon bag, according to the following
reaction:
where the subscripts 0 and t indicate concentrations before
irradiation and at time t, respectively. Plots of ln([BUAC]0/[BUAC]t)
versus ln([ref]0/[ref]t) should yield straight lines with slope
k
ester/kref
.
The rate constants for the reaction of OH with BUAC studied
here was measured at (298 ꢁ 1) K and atmospheric pressure,
relative to the rate constant of OH with n-pentane or diethyl
H2O2 þ hn ! 2OH
(5)
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 397–401

ATMOSPHERIC DEGRADATION OF BUTYL ACRYLATE
Figure 3. Relative rate data for the OH reaction with butyl acrylate using
n-pentane as reference compound at 298 K and atmospheric pressure
of N2 (*) and air (*)
Figure 2. Relative rate data for the OH reaction with butyl acrylate using
diethyl ether as reference compound at 298 K and atmospheric pressure
of N2 (*) and air (*)
The linearity of the data points and the fact that the plots show
practically zero intercepts suggest that the contribution of
secondary reactions is negligible. This fact is supported also by
obtaining the same results using different sources of OH radicals
as well as using nitrogen or synthetic air as bath gases. In Table 1,
the rate constant ratios are given along with the derived rate
constant for the OH reaction with BUAC.
The uncertainties are a combination of the 2s statistical errors
from the linear regression analysis and a contribution to cover
errors in the rate constants of the reference hydrocarbons.
ether. The mixtures of the esters and references with CH3ONO/NO
or H2O2 were stable in the dark when left in the chamber for
about 1 h. Moreover, in the absence of CH3ONO/NO or H2O2,
photolysis of the mixtures (BUAC and references in air or
nitrogen) for more than 1 h shown negligible decrease in the
concentrations.
By using this technique, the rate constant for the reaction
studied was obtained from Eqn (8). The data were fitted to
a straight line by the linear least-squares procedure. Figures 2
and 3 show the plot of ln([BUAC]0/[BUAC]t) versus ln([ref]0/[ref]t),
for the reaction (1) using diethyl ether and n-pentane as reference
compounds, respectively. For each organic reactant studied,
several runs were performed for the rate constant determination;
however for the sake of clarity, only one example is presented in
Figs. 2 and 3. The rate constants of the reaction of OH with
the references used in this work were taken as (3.98 ꢁ 0.13) ꢂ
10ꢃ12 cm3 moleculeꢃ1 sꢃ1 for n-pentane[12] and (1.36 ꢁ 0.11) ꢂ
10ꢃ11 cm3 moleculeꢃ1 sꢃ1 for diethyl ether at 298 K.[13]
DISCUSSION
To the best of our knowledge, no kinetic data on the reactions of
OH radicals with BUAC have been reported. The present study, thus,
is the first measurement of the rate constant of the reaction (1) and
therefore no direct comparison with the literature can be made.
However, it is interesting to compare the reactivity of BUAC
toward OH radicals with the corresponding alkene and other
unsaturated esters, since it has been postulated that unsaturated
VOCs react via similar addition mechanisms.[3,14] In Table 2, the
room temperature rate constants of the reactions of OH radicals
The rate constant obtained by averaging the values from
different experiments in the absolute terms was the following:
k1 ¼ ð1:80 ꢁ 0:26Þ ꢂ 10ꢃ11cm3 moleculeꢃ1 sꢃ1
Table 1. Reactant concentrations, slopes k/kref and the obtained rate constants for the reaction of OH radicals with n-butyl acrylate
at 298 K in 760 Torr of air or N2
[Ester]0
(ppm)
kreactant
(cm3 moleculeꢃ1 sꢃ1
)
Precursor of OH
Bath gas
Reference
[Ref]0 (ppm)
kreactant/kreference
H2O2
H2O2
CH3ONO
H2O2
CH3ONO
CH3ONO
N2
N2
Air
N2
Air
Air
136
165
282
265
192
212
Diethyl ether
Diethyl ether
Diethyl ether
n-Pentane
n-Pentane
n-Pentane
155
263
192
179
230
204
1.29 ꢁ 0.02
1.35 ꢁ 0.08
1.36 ꢁ 0.04
4.41 ꢁ 0.11
4.44 ꢁ 0.14
4.63 ꢁ 0.11
Average
(1.75 ꢁ 0.17)ꢂ 10ꢃ11
(1.84 ꢁ 0.26)ꢂ 10ꢃ11
(1.85 ꢁ 0.20)ꢂ 10ꢃ11
(1.76 ꢁ 0.10)ꢂ 10ꢃ11
(1.77 ꢁ 0.11)ꢂ 10ꢃ11
(1.84 ꢁ 0.10)ꢂ 10ꢃ11
(1.80 ꢁ 0.26)ꢂ 10ꢃ11
J. Phys. Org. Chem. 2008, 21 397–401
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

M. B. BLANCO AND M. A. TERUEL
with BUAC are showed together with other unsaturated esters
and the corresponding 1-hexene and n-hexane obtained from
the literature. It is possible to observe that the rate coefficients of
the esters with OH radicals, including OH þ BUAC reaction, are
close to the corresponding alkene (OH þ 1-hexene) reaction in
the order of 10ꢃ11 cm3 moleculeꢃ1 sꢃ1. The same tendency was
also previously observed by Le Calve et al. for unsaturated
acetates and in our previous work for the reactions of
methacrylates.[16,7] The rate coefficients of unsaturated esters
with OH radicals are in general slightly lower than for the
corresponding alkenes. This observation is consistent with
the negative inductive effect of the —C(O)O— group on the
double bond, indicating that the reactions of OH with
Unfortunatly, no kinetic data exist reaction (9). However, the
room temperature rate constants of the reactions
OH þ CH3CH2CðOÞOðCH2Þ2CH3 ! products
OH þ CH3CðOÞOðCH2Þ4CH3 ! products
(10)
(11)
were reported previously to be k10 ¼ 7.4 ꢂ 10ꢃ12 and
k11 ¼ 7.5 ꢂ 10ꢃ12 cm3 moleculeꢃ1 sꢃ1 [17,18]
.
The rate coefficient
of the reaction (9) is expected to be close to the values of k10 and
k11 (around 7 ꢂ 10ꢃ12 cm3 moleculeꢃ1 sꢃ1). This value is con-
siderably lower than the rate coefficient determined in this work
for reaction (1), 1.8 ꢂ 10ꢃ11 cm3 moleculeꢃ1 sꢃ1, providing evi-
dence that both lead to products following different reaction
unsaturated esters proceeds essentially by OH addition to the
[7]
—
mechanisms (e.g., addition to the C C double bond for BUAC
—
—
—
C
C double bond.
and H-atom abstraction for CH3CH2C(O)O(CH2)3CH3).[3,19–21]
On the other hand, H-substitution by electron-donor groups
like —CH3 or other alkyl groups in the olefin increases the
reactivity toward the electrophilic attack of OH radicals. This
can be observed comparing the higher reactivity of methacry-
Product identification
Further experiments were also conducted to identify reaction
products under similar conditions to the kinetic experiments for
reaction (1). The main product was butyl glyoxalate in accordance
with that obtained previously for the OH addition reaction to
methacrylates.[7] We observed the ions with m/e ¼ 29, 41, and 57
characteristic of butyl glyoxalate. The reaction products found,
together with the observed reactivity trends, confirm that the
reaction proceeds via an addition mechanism of the OH radical to
the double bond similar to that proposed by Atkinson for the OH
radical addition to alkenes.[19,20] The 1,2-hydroxyalkyl radical is
oxidized and further reduced in the presence of O2 and NO to
give the 1,2-hydroxyalkoxy radical. This radical decomposes
leading mainly to the corresponding glyoxalate and formal-
dehyde, as shown in the following reaction scheme:
—
—
CHR), i.e., k
—
lates (CH2 C(CH )R) over the acrylates (CH
—
3
2
CH
2
> kCH ¼¼CHCOOðCH Þ CH , kCH ¼¼CðCH ÞCOOCH CH
>
3
¼¼CðCH ÞCOOðCH Þ CH
3
2
3
2
3
2
3
2
3
3
2
kCH ¼¼CHCOOCH CH , and kCH ¼¼CðCH ÞCOOCH >kCH ¼¼CHCOOCH3 . This
2
2
3
2
2
3
3
effect can be attributed to the electron-donor capacity of
the CH3 radical, which increases the charge density on the carbon
atom next to the double bond, as well as the polarizability of the
p electrons, leading to an increase in the rate constant value
consistent with the electrophilic character of the OH radical.
The reactivity of BUAC toward OH radicals can be compared
also with their corresponding saturated ester
OH þ CH3CH2CðOÞOðCH2Þ3CH3 ! products
(9)
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 397–401

ATMOSPHERIC DEGRADATION OF BUTYL ACRYLATE
products formed from the OH-initiated degradation are mainly
carbonyl compounds, which are net sources of HOx (OH, HO2)
radicals that enhance photo-oxidation formation.
Table 2. Comparison of the rate constants of the reactions of
—
—
(CH )R and n-hexane with OH
—
different CH2 CHR, CH
—
2
3
radicals under atmospheric pressure and 298 K
kOH ꢂ 1011
Acknowledgements
VOC
(cm3 moleculeꢃ1 sꢃ1
)
The authors wish to acknowledge SECYT (Argentina), CONICET –
1.3a
1.7a
1.8b
4.2c
4.6c
7.1c
3.0d
3.0d
1.7d
0.7d
´
PIP 2004 (Argentina), ANPCyT (Prestamo BID 1728/OC-AR; PICT
25544, 2004; Prestamo BID 1201/OC-AR; PICT B No 25544);
—
CH2 CHC(O)OCH
—
—
3
CH2 CHC(O)OCH CH
´
—
2
3
—
CH2 CHC(O)O(CH ) CH
´
´
—
SECyT-UNC – Res. 19/06s (Cordoba, Argentina), Fundacion
Antorchas – Reentry Grant 2004 (Argentina), Third World Acad-
emy of Sciences (TWAS) 04-316 RG/CHE/LA (Italy), and The Royal
Society of Chemistry (RSC) RF 2007 (UK) for financial support of
this research.
2 3
3
—
CH2 C(CH )C(O)OCH
—
3
3
—
CH2 C(CH )C(O)OCH CH
3
—
3
2
—
CH2 C(CH )C(O)O(CH ) CH
3
—
3
2 3
—
CH2 CHCH
—
3
—
CH2 CHCH CH
—
2
3
—
CH2 CH(CH ) CH
—
2 3
3
CH3(CH2)4CH3
REFERENCES
a Reference.[5]
b This work.
c Reference.[7]
d Reference.[15]
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According to the products observed, the main fate of the
1,2-hydroxyalcoxy radicals formed seems to be the decompo-
sition channel, where the OH preferably attacks the terminal
carbon of the double bond.[22] We did not find dihydroxycarbonyl
compounds or hydroxycarbonyl acids as reaction products.
Therefore, we can conclude that other channels such as
isomerization or reaction with O2, respectively, proposed by
Atkinson for alkenes,[19,20] are negligible for BUAC under our
experimental conditions.
Regarding nitrate formation in the reactions of NO with
1,2-hydroxyalkyl, 1,2-hydroxyperoxy, and 1,2-hydroxyalcoxy
radicals, we did not see any organic nitrate formation, hence
addition of NO to such radicals is negligible under our
experimental conditions.
[11] M. A. Teruel, M. B. Blanco, G. R. Luque, Atmos. Environ. 2007, 41, 27
5769–5777.
[12] N. M. Donahue, J. G. Anderson, K. L. Demerjian, J. Phys. Chem. A 1998,
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The atmospheric lifetime, tx, of the BUAC was calculated
through the expression: tx ¼ 1/kx[OH]. Unfortunately, no kinetic
data are available for the reactions of BUAC with other
tropospheric oxidants like O3 molecules NO3 radicals or Cl
atoms. A tropospheric lifetime of 8 h is calculated, assuming a
12 h average concentration of OH of 2 ꢂ 106 molecule cmꢃ3 [23]
.
This lifetime indicates that BUAC is likely to be rapidly removed in
the gas phase, the reaction with OH being the major loss process
for the unsaturated ester. Loss by photolysis can be considered
negligible since it is photolytically stable in the actinic region of
the electromagnetic spectrum.
The short lifetime of a few hours indicates that BUAC, like other
acrylates and methacrylates will be oxidized close to the emission
sources. Consequently, this emissions will have mainly a local
impact concerning the photochemical smog production. The
[23] R. Hein, P. J. Crutzen, M. Heimann, Global Biogeochem. Cycles 1997,
11, 43–76.
J. Phys. Org. Chem. 2008, 21 397–401
Copyright ß 2008 John Wiley & Sons, Ltd.
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