Atmospheric Photo-oxidation of Diethyl Carbonate: Kinetics, Products, and Reaction Mechanism

Article abstract of DOI:10.1021/acs.jpca.9b09887

The rate coefficient for the gas phase of diethyl carbonate with chlorine atoms has been determined at 298 K using a relative method, employing ethyl formate and ethyl acetate as reference compounds. The experimental value, (1.0 ± 0.2) × 10^-11 cm³ molecule^-1 s^-1, is in good correlation with the one estimated by the SAR (Structure-Activity Relationship) method. The photo-oxidation mechanism of diethyl carbonate initiated by chlorine atoms was also studied at 298 K and atmospheric pressure as a function of the oxygen partial pressure. The main products identified by infrared spectroscopy were CH₃CH₂OC(O)OCHO, CH₃CH₂OC(O)OCH₂CHO, CH₃CH₂OC(O)OC(O)CH₃, CO₂, CO, HCOOH, and CH₃COOH. The results reveal that the oxidation process occurs by the abstraction of a hydrogen atom from the methyl (43%) and methylene (57%) groups. The relative importance of each reaction path from the primary radicals formed in photo-oxidation and the identity of CH₃CH₂OC(O)OCHO, CH₃CH₂OC(O)OC(O)CH₃, and CH₃CH₂OC(O)OCH₂CHO were determined using computational methods. The activation energy of reaction paths for the main oxygenated radicals formed during photo-oxidation was determined using Gaussian09 Program.

Full text of DOI:10.1021/acs.jpca.9b09887

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A: Kinetics, Dynamics, Photochemistry, and Excited States
The Atmospheric Photo-Oxidation of Diethyl Carbonate:
Kinetics, Products, and Reaction Mechanism
Guido Noe Rimondino, Walter José Peláez, and Fabio Ernesto Malanca
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b09887 • Publication Date (Web): 04 Dec 2019
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The Atmospheric Photo-Oxidation of Diethyl
Carbonate: Kinetics, Products, and Reaction
Mechanism
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Guido N. Rimondino*, Walter J. Peláez, and Fabio E. Malanca
INFIQC - CONICET - Instituto de Investigaciones en Fisicoquímica de Córdoba.
Departamento de Fisicoquímica, Facultad de Ciencias Químicas. Universidad Nacional de
Córdoba, Ciudad Universitaria (X5000HUA), Córdoba, República Argentina.
*Corresponding author
Guido Noé Rimondino
INFIQC - CONICET
Departamento de Fisicoquímica - Facultad de Ciencias Químicas
Universidad Nacional de Córdoba
grimondino@fcq.unc.edu.ar
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ABSTRACT
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The rate coefficient for the gas-phase of diethyl carbonate with chlorine atoms has
been determined at 298 K using a relative method, employing ethyl formate and ethyl
acetate as reference compounds. The experimental value, (1.0 ± 0.2) × 10−11 cm3 molecule-1
s-1, is in good correlation with the one estimated by the SAR (Structure-Activity
Relationship) method. The photo-oxidation mechanism of diethyl carbonate initiated by
chlorine atoms was also studied at 298 K and atmospheric pressure as a function of the
oxygen partial pressure. The main products identified by infrared spectroscopy were:
CH3CH2OC(O)OCHO, CH3CH2OC(O)OCH2CHO, CH3CH2OC(O)OC(O)CH3, CO2, CO,
HCOOH, and CH3COOH. The results reveal that the oxidation process occurs by the
abstraction of a hydrogen atom from the methyl (43%) and methylene (57%) groups. The
relative importance of each reaction path from the primary radicals formed in photo-
oxidation and the identity of CH3CH2OC(O)OCHO, CH3CH2OC(O)OC(O)CH3, and
CH3CH2OC(O)OCH2CHO were determined using computational methods. The activation
energy of reaction paths for the main oxygenated radicals formed during photo-oxidation
was determined using Gaussian09 Program.
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INTRODUCTION
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Diethyl carbonate (DEC, CH3CH2OC(O)OCH2CH3) is the carbonate ester of
carbonic acid and ethanol. It is the second homologue of the symmetric dialkyl carbonates
family ROC(O)OR. As dimethyl carbonate, DEC has attracted a lot of attention in both
industrial and research fields due to its potential eco-friendliness.1,2
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Although DEC is industrially produced in several ways, it has been synthesized
from costless and renewable resources in the last few years by processes with high
conversion levels and which avoid the use of toxic reagents.3 DEC has been considered a
bio-based solvent due to the fact that it could be obtained from the ethanolysis of urea or
carbon dioxide.4–6 The profit of this kind of “green” synthetic procedure is even much
bigger if we take into account that CO2 molecules could be used to produce fuel.7
Due to its low toxicity and high biodegradability, DEC is widely used as a solvent
in the mid-boiling range, replacing more harmful organic substances,8 and as a raw material
in the polymer industry to produce polycarbonates and urethanes.9,10 Its high oxygen
content and good solubility in fuels place it as a fuel additive, reducing the diesel engine
emissions of CO2 and particulates to the environment.11,12 Additionally, in the last decades,
with the development and massive production of rechargeable batteries, DEC was widely
employed as electrolyte for lithium cells.4 For this reason, it is expected that, if the use of
carbonates and the rate of production are maintained, their atmospheric emissions will
increase drastically. However, to the best of our knowledge, no studies of the gas-phase
photochemistry of DEC have been carried out. In this work, we present the results of the
determination of the rate coefficient of DEC with chlorine atoms (used as surrogate of OH
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radicals) and its photo-oxidation mechanism, which are both supported by experimental
measurements and computational studies.
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EXPERIMENTAL
General Information
Diethyl carbonate (98%, Sigma Aldrich), ethyl formate (97%, Riedel-deHaën), and formic
acid (90%, Dorwil) were used as purchased, while ethyl acetate (Sintorgan) was distilled
and stored in molecular sieves prior to use.
Commercially available oxygen (4.8, AGA), nitrogen (4.8, AGA), and carbon
monoxide (2.0, Praxair) were used as received. Chlorine (>98% purity) was synthesized by
direct reaction between HCl and KMnO4 at inert atmosphere and further distilled, while
NO2 was synthesized by the thermal decomposition of Pb(NO3)2 in the presence of oxygen
and further distilled.
Procedures
Gaseous samples were manipulated using a glass vacuum line equipped with two
different capacitance pressure gauges (0 to 760 Torr, MKS Baratron and 0 to 70 mbar, Bell
and Howell).
Typical experiments were carried out as follows: a gas-phase mixture of DEC and
Cl2 were prepared in a 5 L previously evacuated glass reactor. Then, either N2 (for the
determination of rate coefficients) or O2 and NO2 (for photo-oxidation experiments) were
added to reach atmospheric pressure. Finally, a fraction of the gas mixture was transferred
to the thermostated infrared glass cell and the photolysis was carried out using two black
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lamps. The reaction progress was followed using a Fourier Transform Infrared
Spectrometer (Bruker IFS-28) following the temporal variation of reactants and products.
All spectra were recorded in the range of 500 to 4000 cm−1 with a resolution of 2 cm−1.
Control experiments were performed in darkness in order to check for possible
heterogeneous reactions.
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The kinetics of DEC with chlorine atoms were obtained using the relative method,
which employs a reference compound with a well-known rate coefficient and presents no
complications such as unwanted secondary reactions.13 Ethyl acetate (EAc) and ethyl
formate (EFor) were used as reference compounds and the results were plotted using
Equation 1.





DEC

kDEC

reference 0







0 
 kwall t 
ln
ln
Eq. 1


DEC

kreference

reference t

t




The terms [DEC]0, [DEC]t, [reference]0 and [reference]t represent the concentrations of
DEC and reference compound before and after the irradiation time t, respectively. The rate
coefficients kDEC and kreference correspond to the reactions of chlorine atoms with DEC and
with the reference compound, respectively. A linear fit of the ln([DEC]0/[DEC]t) vs.
ln([reference]0/[reference]t) gives the slope corresponding to the ratio kDEC/kreference. The
kwall value (which accounts for overall non-photolytic processes) in typical experiments was
9 × 10−5 s−1.
In order to identify the new products observed as reaction products, their infrared
spectrum was calculated using Density Functional Theory with the B3LYP/6-311+G(d,p)
basis set and Gaussian09 Program. Frequencies were corrected according to Equation 2 to
account for the anharmonicity of the molecule.14
νexp/νcalc = 1.0 − 0.00001356 νcalc
Eq. 2
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The symbols νexp and νcalc correspond to the experimental and calculated wavenumbers,
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respectively.
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RESULTS AND DISCUSSIONS
The rate coefficients for the reaction of DEC with chlorine atoms at 298 K
(Reaction 1) relative to the reaction of EAc and EFor with chlorine atoms (Reactions 2 and
3) were derived from the slope of the linear plot of ln([DEC]0/[DEC]t) vs.
ln([reference]0/[reference]t) as showed in Figure 1.
CH3CH2OC(O)OCH2CH3 + Cl
 Products
(1)
CH3C(O)OCH2CH3 + Cl
CH3CH2OC(O)H + Cl
 Products
(2)
 Products
(3)
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Figure 1. Kinetic data (total pressure 1000 mbar at 298 K) for the reaction of DEC with
chlorine atoms relative to EAc (solid circles) and EFor (open circles).
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The following slopes, which represent kDEC/kreference, were obtained from the plot presented
in Figure 1: kDEC/kEAc = 0.74 and kDEC/kEFor = 1.05. Using the rate coefficient for EAc (1.37
× 10−11 cm3 molec−1 s−1)15 and for EFor (9.92 × 10−11 cm3 molec−1 s−1), available in
bibliography,16 values of 1.01 × 10−11 cm3 molec−1 s−1 and 1.04 × 10−11 cm3 molec−1 s−1 for
kDEC were obtained, respectively. According to the errors derived from the linear fit of our
experimental data (5%) and those informed for the rate constants of reference compounds
(15% for EAc and 20% for EFor),15,16 the error rate is estimated at around 20-25%.
Structure-Activity Relationship
The Structure-Activity Relationship (SAR) method was used to corroborate the
experimental data. This method, widely used,17,18 allows the calculation of the rate
coefficients of organic molecules based on the estimation of methyl, methylene, and
methine group rate constants (kprim, ksec, and ktert, respectively) and parameters (F)
associated to the functional groups bonded to each one. The values available in
bibliography for these mathematical terms are: kprim = 3.32 x 10-11 cm3 molecule-1 s-1, ksec
=
8.34 x 10-11 cm3 molecule-1 s-1, F(-CH3) = 1.00, F(-CH2-) = 0.79, and F(-CH2OC(O)) =
0.075.19 Value for F(-OC(O)O) = 0.035 was derived from the attack of chlorine atoms on
dimethyl carbonate, according to the experimental rate coefficient measured by Bilde et
al.20
Considering the values presented above, the rate coefficient for DEC can be
estimated as 1.08 × 10−11 cm3 molec−1 s−1, showing an excellent concordance with the
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experimental value obtained in this work. This result also corroborates the value of the F(-
CH2OC(O)) factor obtained by Vila et al. for the study of the gas phase degradation of n-
butyl formate.19
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From the SAR method, the relative reactivity trend for each hydrogen atom in the
molecule can also be estimated. For instance, two possible attack positions are available at
DEC moiety: the abstraction of an H-atom from the -CH2- groups and the corresponding
one to -CH3 groups, herein named “via α” and “via β” respectively, according to their
position with regards to the carbonate moiety. Using the above parameters, a relative
reactivity trend of 54% for via α and 46% for via β were obtained, which shows that both
are competitive vias.
Reaction mechanism of photo-oxidation
Photo-oxidation products were identified by infrared spectroscopy. The mixture
containing DEC (0.41 mbar), Cl2 (0.30 mbar), and O2 (1013 mbar) was photolyzed during
60 minutes and two infrared spectra were obtained: one immediately after the lights are
turned off, and another after 18h of darkness to study the stability of the photochemical
products. The first trace of Figure 2 shows the infrared spectrum obtained immediately after
irradiation, in which signals of remaining DEC were already subtracted (“Products” trace).
There are clear signals at 1105, 1775 cm-1, and 667 cm-1 (corresponding to HCOOH and
CO2 respectively) and 1005, 1152, 1260, and 1828 cm-1 (indicated with an asterisk) whose
assignment to products will be discussed in the next paragraphs. The subtraction of the
spectra of HCOOH and CO2 to the first trace leads to Trace “A”.
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Figure 2. Infrared spectra obtained in the photo-oxidation of DEC. Traces from top
to bottom: photo-oxidation products; HCOOH reference; spectrum resulting from the
subtraction of HCOOH and CO2 to the first trace (Trace “A”); dark products; H3CCOOH
reference; spectrum resulting from the subtraction of HCOOH, H3CCOOH, and CO2 to
Trace “A” (Trace “B”).
The fourth trace, “Dark products”, shows the spectrum of the irradiated sample after
the dark period (unreacted DEC had already been subtracted). As can be seen, in addition to
the presence of CO2 and HCOOH, there were signals of H3CCOOH indicating its formation
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during darkness, while the absorbance of the peaks marked with asterisks in the first trace
decreases. The subtraction of the identified products leads to Trace “B”.
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According to the reaction mechanisms reported for carbo-oxigenated compounds
with similar structures to DEC,20-22 the formation of CH3CH2OC(O)OC(O)CH3,
CH3CH2OC(O)OCHO, and CH3CH2OC(O)OCH2CHO as stable photo-degradation
products is possible. Using Density Functional Theory, spectra of those species were
calculated and compared to our experimental data. The results are presented in Figure 3. A
comparison of the theoretical spectrum of CH3CH2OC(O)OCH2CHO with Trace “B” (taken
from Figure 2) shows a good agreement, indicating the correspondence of the experimental
product to this product. Finally, the subtraction of Trace “B” to Trace “A” leads to the
spectrum of a new unstable product that disappears during the dark period (Trace “C”).
This product was assigned to CH3CH2OC(O)OC(O)CH3, taking into account the agreement
between the experimental and calculated infrared spectra showed in the last trace of Figure
3.
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Figure 3. Infrared spectra obtained in the photo-oxidation of DEC. Traces from top
to bottom: Trace “B” taken from Figure 2; theoretical spectrum of
CH3CH2OC(O)OCH2C(O)H; spectrum resulting from the subtraction of Trace “B” to
Trace “A” (Trace “C”); theoretical spectrum of CH3CH2OC(O)OCH2CH3.
The formation of the unstable CH3CH2OC(O)OC(O)CH3 was also proved by the
appearance of acetic acid during the dark period, which comes from the heterogeneous
degradation of the –OC(O)CH3 fragment. Similarly, the appearance of formic acid during
photolysis was attributed to the formation of a more unstable anhydride containing the –
C(O)OC(O)H group, which decomposes completely during photo-oxidation:
CH3CH2OC(O)OCHO.
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Photo-oxidation experiments in the presence of NO2 were also performed. Mixture
of DEC (0.41 mbar), Cl2 (0.30 mbar), NO2 (0.40 mbar), and O2 (1013 mbar) was
photolyzed and analyzed. The most remarkable result is the appearance of a set of signals at
794, 1163, 1741, and 1842 cm-1, corresponding to the characteristic peaks of peroxy acetyl
nitrate (PAN, CH3C(O)OONO2).23 This result suggests the formation of the CH3C(O)
radical during the photo-oxidation, which leads the formation of PAN according to
reactions 4 and 5.
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CH3C(O) + O2  CH3C(O)OO
(4)
(5)
CH3C(O)OO + NO2  CH3C(O)OONO2
Finally, a set of experiments varying the oxygen partial pressure (from 70 to 1013
mbar) inside the photo-reactor were performed. Spectra were recorded at 60 minutes of
photolysis and after 18 hours in darkness. The quantification of carbon monoxide, carbon
dioxide, formic acid, and acetic acid was performed using their corresponding calibration
curve at the same experimental conditions used in the experimental set-up.
CH3CH2OC(O)OC(O)CH3 and CH3CH2OC(O)OCHO were quantified from their
decomposition product (i.e. acetic and formic acid, respectively) after the dark period to
ensure their complete decomposition. Formaldehyde was quantified from its photo-
oxidation product (CO) taking into account the rate coefficient of reaction between chlorine
atoms and CH2O (7.3 x 10-11 cm3 molecule-1 s-1) is higher to the rate coefficient of reaction
with DEC (1.02 x 10-11 cm3 molecule-1 s-1). Figure 4 shows the percentage of products
relative to the disappearance of DEC, determined as a function of the oxygen partial
pressure. As can be seen, when O2 partial pressure increases, the concentrations of CO and
CO2 decrease while CH3CH2OC(O)OC(O)CH3 increases. It is interesting to note that the
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concentration of CH3CH2OC(O)OCHO does not change in the analyzed range of O2 partial
pressures. The high percentage of CO2 formation is a consequence that it is formed in
several ways, as will be discussed later. PAN quantification (not showed in the plot) was
also performed in the photo-oxidation in the presence of NO2 at 1013 mbar total pressure,
leading to a percentage formation of 25%.
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0
0
250
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Oxygen pressure (mbar)
Figure 4. Products percentages relative to DEC disappearance as a function of O2
pressure (70 − 1013 mbar): CO2 (down triangles), H2CO (upper triangles),
CH3CH2OC(O)OC(O)CH3 (solid circles), and CH3CH2OC(O)OCHO (solid squares).
According to the experimental results, the following photo-oxidation mechanism
was postulated (Scheme 1):
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Scheme 1. Proposed mechanism for the Cl-atoms initiated oxidation of DEC.
Essentially, chlorine atoms attack DEC at both possible positions (-CH3 and -CH2-
groups) abstracting an H-atom from each one. Both resulting radicals react with O2 to form
the corresponding peroxy radical, which subsequently reacts either with chlorine atoms or
another peroxy radical, leading to the secondary radical S1 (CH3CH2OC(O)OCHOCH3)
and the primary radical P1 (CH3CH2OC(O)OCH2CH2O) radicals (via α and β,
respectively).
It is well known that the main path for radicals with a chemical structure similar to
S1 (R-C(O)OCH2O) is the α-ester rearrangement24,25 and the reaction with O2 and their
relative importance are dependent of the oxygen pressure in the system.26 This agrees with
the large amount of CH3CH2OC(O)OC(O)CH3 formed in the experiments carried out at
higher oxygen pressures (Figure 4), as it is depicted in Scheme 1, Path B.
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On the other hand, the α-ester rearrangement of S1 radical leads finally to the
formation of CO2 (via the degradation of unstable CH3CH2OCOOH and the CH3C(O)O
radical), formaldehyde, and additionally PAN when the photolysis is carried out in the
presence of nitrogen dioxide. It is important to note that monoethyl carbonic acid ester
(CH3CH2OCOOH) was not observed in our system due to its extreme instability and
decomposed to an equimolar amount of CO2.27
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For the P1 radical (CH3CH2OC(O)OCH2CH2O), the reaction with O2 and rupture
are both available degradation paths. The first one leads to the formation of
CH3CH2OC(O)OCH2CHO (Scheme 1, Path C), while the second leads to the formation of

CH3CH2OC(O)OCH2 and formaldehyde (Scheme 1, Path D).

The CH3CH2OC(O)OCH2 radical formed follows the same sequence previously
mentioned for the primary step, leading to the formation of the CH3CH2OC(O)OCH2O
radical, named P2 radical. At this point, two paths are available as discussed for the S1
radical above: α-ester rearrangement and reaction with O2.21,24 The reaction with O2
(Scheme 1, Path E) was corroborated by the formation of CH3CH2OC(O)OCHO. The α-
ester rearrangement was inferred considering that it is a competitive main path for the
similar CH3CH2C(O)OCH2O radical.28 According to this, the final products of path F are
CO2 (originated from the decomposition of the unstable CH3CH2OCOOH) and CO.
From the previous analysis and the percentages of product formation presented in
Figure 4, the percentages of attack of the chlorine atom on the methyl group and methylene
group were determined. The via α percentage can be determined from the sum of
CH3CH2OC(O)OC(O)CH3 (32%) and PAN (25%). The percentage of via α (57%) was
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determined according to these values and inferred the values for via β (43%) in agreement
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with those obtained from the SAR method (54% and 46%, respectively).
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Computational Studies
Quantum chemical calculations using the DFT level of theory (B3LYP/6-
311+G(d,p) functional, Gaussian09 Program system)29 have been carried out in order to
investigate the main reaction paths of DEC. Full geometry optimizations were performed,
followed by harmonic frequency calculations at the same level of theory, which also
allowed the characterization of the nature of the stationary points.30,31
As depicted above in the mechanism reaction showed in Scheme 1, we suggest two
main ways (vias  and β). Besides the fact that the H abstraction from DEC by Cl are quite
energetically similar for both positions, calculations show that, after oxidation, the resulting
S1 radical is about 39 kJmol-1 lower in energy than the P1 radical.
Figure 5 shows the energy corresponding to the main path reactions of S1 and P1
radicals. As can be seen, the formation of unstable CH3CH2OC(O)OH from α-ester
rearrangement of S1 (path A, left panel) is favored over the formation of the stable
CH3CH2OC(O)OC(O)CH3 coming from the reaction between S1 and O2 (path B, right
panel). Unraveling the different possibilities to achieve CH3CH2OC(O)OH, calculation
showed that it could be obtained through two possible processes that occur in a concerted
but slightly asynchronous manner, with an energy difference of only 2 kJ mol-1. On the
other hand, conformational analysis for path B allows to identify the conformer that leads to
the formation of CH3CH2OC(O)OC(O)CH3 through the H atom subtraction with a relative
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energy barrier of 108 kJmol-1, which is higher than those which lead to a
CH3CH2OC(O)OH formation.
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Figure 5. Partial reaction coordinates for S1 radical: path A (left) and path B (right).
In the same way, reaction paths for P1 radical (Scheme 1, via β) were also analyzed
theoretically. Theoretical calculations (Figure 6, left panel) show that either energy
activation that leads to CH3CH2OC(O)OCH2CHO from its reaction with molecular oxygen

(Scheme 1, Path C) or CH3CH2OC(O)OCH2 radical generated by C-C bond fragmentation
(Scheme 1, Path D) are high enough to explain the low occurrence of these paths. However,
a comparison of the activation energy of both paths shows that the formation of
•
CH3CH2OC(O)OCH2 radical and formaldehyde is 62.7 kJmol-1 and is favored over the
formation of CH3CH2OC(O)OCH2CHO.
Computational calculations were also extended to determine the most favorable
reaction pathway for the CH3CH2OC(O)OCH2O• radical (P2) originated from the
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CH3CH2OC(O)OCH2 radical. The energy required for both available paths (E and F), that
is for the reaction with O2, and for the α-ester rearrangement, are very different indicating
that path E is favored. Moreover, as it was described above for path A, the formation of
CH3CH2OC(O)OH could be achieved through two possible processes which are, at least, 64
kJmol-1 lesser in energy than path E.
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Figure 6. Partial reaction coordinates for P1 radical: path C and D (left panel) and path E
and F (right panel).
CONCLUSIONS
The rate coefficient for DEC with chlorine atoms was determined using the relative
method and a mean value of (1.0 ± 0.2) × 10−11 cm3 molecule-1 s-1 was obtained. This value
is in good agreement with those calculated using the Structure-Activity Relationship
method (1.08 × 10−11 cm3 molec−1 s−1).
The reaction mechanism for the Cl-atoms initiated by photo-oxidation was
determined from the products identification and numerical calculations performed for the
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main paths reactions. Results show that the chlorine attacks both methyl (43%) and
methylene groups (57%), leading to the formation of CH3CH2OC(O)OCHO,
CH3CH2OC(O)OCH2CHO, CH3CH2OC(O)OC(O)CH3, CH2O, CO2, and CO. Theoretical
calculations show that decomposition via α-ester rearrangement and reaction with
molecular oxygen are competitive atmospheric reaction paths for both
CH3CH2OC(O)OCH2OCH3 and CH3CH2OC(O)OCH2O radicals, while rupture is the
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main path for the CH3CH2OC(O)OCH2CH2O radical.
ACKNOWLEDGMENTS
Financial support from SECYT-UNC, ANPCYT, and CONICET is gratefully
acknowledged. This work used computational resources from CCAD-Universidad Nacional
de Córdoba, in particular the Mendieta Cluster, which is part of SNCAD-MinCyT,
República Argentina. FEM thanks Gil Michel for English language assistance.
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