Temperature dependence of the alpha-ester rearrangement reaction

Article abstract of DOI:10.1021/jp048537c

Measurements have been made of the oxidation products of methyl formate and methyl acetate as a function of temperature (253-324 K). Both processes show the occurrence of the α-ester rearrangement channel RC(O)OCH ₂O→ RC(O)OH + HCO, which has an activation energy of ～10 kcal mol^-1. Reaction of hydroxyl radicals with methyl formate occurs with roughly equal efficiency at both of the carbon atoms, while OH-reaction with methyl acetate leads to 30-40% attack at the acetate group.

Full text of DOI:10.1021/jp048537c

6850
J. Phys. Chem. A 2004, 108, 6850-6856
Temperature Dependence of the Alpha-Ester Rearrangement Reaction
Geoffrey S. Tyndall,* Andre S. Pimentel,† and J. J. Orlando
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, P.O. Box 3000,
Boulder, Colorado 80307
ReceiVed: April 2, 2004; In Final Form: May 27, 2004
Measurements have been made of the oxidation products of methyl formate and methyl acetate as a function
of temperature (253-324 K). Both processes show the occurrence of the R-ester rearrangement channel
RC(O)OCH2O• f RC(O)OH + HCO, which has an activation energy of ∼10 kcal mol-1. Reaction of hy-
droxyl radicals with methyl formate occurs with roughly equal efficiency at both of the carbon atoms, while
OH-reaction with methyl acetate leads to 30-40% attack at the acetate group.
Introduction
reported by Tuazon et al. in 1998.12
The atmosphere contains a rich suite of oxygenated organic
compounds, which can be either emitted directly or formed by
in situ oxidation of primary emissions.1 Esters are potential
oxygenated atmospheric constituents. While esters are naturally
occurring compounds in fruits and flowers, their main source
in the atmosphere probably comes from their use as solvents in
paints, adhesives, and cleaning agents, and from the food
industry. The use of esters as solvents is attractive, since they
appear to have low ozone creation potentials.2 Esters may also
be produced in the atmospheric oxidation of ethers.3,4 Methyl
formate has a potentially significant source from the use of linear
methyl ethers CH3OR and in particular dimethyl ether which
has been proposed as an additive in diesel fuel. Methyl acetate,
on the other hand, is a common product from branched methyl
ethers such as methyl tert-butyl ether and methyl tert-amyl ether
which have been used as gasoline additives. Few (if any)
measurements of esters in the atmosphere have been published,
although methyl acetate has recently been tentatively identified
in the atmosphere in Houston, Texas (E. Atlas, University of
Miami, personal communication, 2003). Methyl acetate and
methyl formate, the two simplest esters, have relatively long
atmospheric lifetimes, since they react slowly with OH radicals5-10
and are not expected to be subject to photolysis.11
CH3C(O)OCH2O• f CH3C(O)OH + HCO
(3)
(4)
HC(O)OCH2O• f HC(O)OH + HCO
For both molecules, the ester rearrangement is slow enough to
allow competition with the O2 reaction, and so valuable kinetic
information can be obtained by measuring relative product yields
as a function of oxygen partial pressure.13-15
CH3C(O)OCH2O• + O2 f CH3C(O)OCHO + HO2 (5)
HC(O)OCH2O• + O2 f HC(O)OCHO + HO2
(6)
The R-ester rearrangement reaction is rather unusual in that
it involves a five-membered H-atom transfer to the oxygen atom
of the carbonyl group. Currently, the dynamics of this reaction
are not well understood. Three papers have addressed the
energetics of the transition state theoretically, although the high
degree of delocalization of the transition state makes calculation
difficult.16-18 In an attempt to obtain a better understanding of
these reactions, we have conducted a study of the products of
the oxidation of methyl acetate and methyl formate as a function
of O2 partial pressure and temperature. Product studies were
carried out using chlorine atoms to initiate the reaction. The
results lead to an estimate of the activation energy for the R-ester
rearrangement, which will allow for a better constraint on
theoretical estimates. The mechanism of the reaction of OH
radicals with methyl formate and methyl acetate was also studied
at room temperature.
OH + CH3C(O)OCH3 f products
k1 ) 3.5 × 10-13 cm3 molecule-1 s-1 (1)
OH + HC(O)OCH3 f products
k2 ) 1.8 × 10-13 cm3 molecule-1 s-1 (2)
Experimental Section
The position of attack by OH has not been determined
experimentally for either molecule, and estimates of the
selectivity using either structure-activity relationships6,8,10 or
theoretical calculations for methyl formate9,10 are not conclusive.
Mechanistic studies of both methyl formate and methyl
acetate have been published in the past few years, and both
oxidations involve the R-ester rearrangement reaction, first
The experiments were all conducted in a 47-L stainless steel
chamber.14,19 Chilled ethanol was circulated through the outer
jacket to reach temperatures below 296 K, or heated water could
be circulated for higher temperatures. Gas mixtures were
analyzed via FTIR spectroscopy using a resolution of 1 cm-1
.
Standard spectra obtained in-house were used, except for formic
anhydride (FAN) and acetic formic anhydride (AFAN), the
room-temperature spectra of which were provided digitally by
Dr. T. Wallington at Ford Motor Company. Whenever possible,
standards were taken at the appropriate temperatures for the
* Corresponding author. E-mail: tyndall@ucar.edu; phone: (303) 497-
1472.
† Present address: Laboratory for Extraterrestrial Physics, Code 691,
NASA Goddard Space Flight Center, Greenbelt, MD 20771.
10.1021/jp048537c CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/27/2004

Alpha-Ester Rearrangement Reaction
J. Phys. Chem. A, Vol. 108, No. 33, 2004 6851
experiments. For FAN and AFAN, product spectra obtained at
temperatures other than 296 K were calibrated against the room-
temperature standard spectra using integrated band strengths.
Concentrations of formic and acetic acid (AcOH) were corrected
for the presence of dimers using the equilibrium constants given
by Dagaut et al.20 The corrections were applied both to the
amounts of acid in the calibration bulb (at 296 K) and the
products formed in the reaction chamber (most important at the
lowest temperature studied). Calibration curves were used for
CO and CO2 to account for curvature caused by saturation of
the lines at the concentrations used. Absolute concentration
measurements at 296 K are considered to be accurate to better
than 5%. At other temperatures, the uncertainty is expected to
be a little higher (7-8%).
Irradiation of gas mixtures was via a xenon arc lamp, filtered
to give output from 250 to 420 nm. The majority of the
experiments involved the photolysis of Cl2 in the presence of
methyl formate (MF) or methyl acetate (MeAc), with various
partial pressures of O2 (6-600 Torr), in a total of 700 Torr
(balance N2). Mixtures were photolyzed for several periods of
up to 5 min, and IR spectra were taken initially and after each
Figure 1. Production of acetic formic anhydride (AFAN; filled
symbols) and acetic acid (AcOH; open symbols) as a function of methyl
acetate loss due to reaction with chlorine atoms at 253 K. Circles, 6
Torr O2; triangles, 15 Torr O2; squares, 70 Torr O2. The acetic acid
concentrations have been corrected for equilibration with the dimer.
successive irradiation. Typical concentrations (molecule cm-3
)
used were ester (3-4) × 1014, Cl2 (7-12) × 1014, NO when
used 7 × 1014. At the lower temperatures, where reduced
pressures of O2 were required, the Cl2 concentration was reduced
to maintain a ratio [O2]/[Cl2] > 100, thus avoiding loss of alkyl
radicals to reaction with Cl2. Further experiments were done at
253 K using reduced total pressure or with NO present to test
whether chemical activation of the oxy radicals could affect
the product distribution.
The mechanisms of the reactions of OH with methyl formate
and methyl acetate were also investigated at 296 K. For these
experiments, methyl nitrite (1.5-2.1) × 1015 molecule cm-3
was photolyzed in the presence of NO (2.8-4.1) × 1014
molecule cm-3 and methyl formate 7 × 1015 molecule cm-3 or
methyl acetate (2.1-3.5) × 1015 molecule cm-3 in the presence
of 60-540 Torr O2 at 700 Torr total pressure. The duration of
the irradiation periods was ∼10 min. As a consequence of the
slow rates of the OH reactions, high concentrations of the esters
had to be used, making measurement of the consumption of
starting material unreliable. Hence, in several experiments a
small amount (1.5 × 1014 molecule cm-3) of C2H4 was added
as a tracer, and the consumption of ester was calculated from
the measured loss of C2H4.
results before studying the mechanisms in more detail at
temperatures of 253, 273, and 324 K.
Methyl Acetate Oxidation. Methyl acetate was stable in the
chamber, with a wall-loss rate in the dark of less than 10-6 s-1
.
The experiments were carried out in the absence of NO, since
at low temperatures the formation of nitrates complicates the
product yields and also slows down the rate of conversion.
Reaction of chlorine atoms with methyl acetate is thought to
result only in abstraction at the methoxy group, leading to
production of CH3C(O)OCH2O2 radicals.13 These peroxy radi-
cals react to give either oxy radicals, or molecular products.
CH3C(O)OCH2O2 + CH3C(O)OCH2O2 f
CH3C(O)OCH2O• + CH3C(O)OCH2O• + O2 (8a)
CH3C(O)OCH2O2 + CH3C(O)OCH2O2 f
CH3C(O)OCHO + CH3C(O)OCH2OH + O2 (8b)
The oxy radicals either decompose by the R-ester rearrange-
ments or react with O2 to give acetic formic anhydride (AFAN).
CH3C(O)OCH2O• f CH3C(O)OH + HCO
(3)
•
OH + C2H4 f HOCH2CH2
(7)
CH3C(O)OCH2O• + O2 f CH3C(O)OCHO + HO2 (5)
The loss can be expressed in terms of the relative rates.
Assuming a small extent of conversion for the ester (i.e., ln(1
- x) ∼ -x), results, for methyl formate, in equation E1.
Yields of AFAN and acetic acid were measured as a function
of the partial pressure of O2 at each temperature studied. Figure
1 shows the formation of acetic acid and AFAN at several
different oxygen partial pressures at 253 K, while Figure 2
shows the same for 324 K. The slopes of these plots give the
corresponding yields. The product yields from methyl acetate
measured in this work are shown in Figure 3. The acetic acid
has been corrected for formation of dimer at 253 K; at higher
temperatures the correction was negligible. The maximum
correction to the concentration was 30%, at the lowest O2 (2
Torr); this led to a 20% change in the slope at that oxygen
pressure. However, the effect on the value of k5/k3 is less than
10%, even if the dimer is totally ignored. The sum of the yields
of acetic acid and AFAN was independent of O2 at a given
temperature and were typically 75-80%. The remainder is
thought to be either the alcohol from 8b or hydroperoxides
formed in the reaction of the peroxy radicals with HO2. The
k
[C2H4]0
[C2H4]t
∆MF ≈ [MF]0 × 2 ln
(E1)
(
)
k7
The rate coefficient for the reaction of OH with ethene is well-
known,21 k7 ) (7.9 ( 0.4) × 10-12 cm3 molecule-1 s-1, and
ethene has convenient, sharp infrared features that enable its
loss to be measured accurately in the presence of large
concentrations of the esters.
Results
The oxidation mechanisms of methyl formate and methyl
acetate have previously been studied at 296 K.13,14 Experiments
were first conducted at room temperature to confirm the previous

6852 J. Phys. Chem. A, Vol. 108, No. 33, 2004
Tyndall et al.
Figure 2. Same as Figure 1, except at 324 K. The data for acetic acid
have been moved upward by 1.5 mTorr for clarity. Circles, 15 Torr
O2; triangles, 150 Torr O2; squares, 450 Torr O2.
Figure 4. Arrhenius plot for the rate coefficients ratios k3/k5 and k4/k6
for the R-ester rearrangements of methyl acetate and methyl formate.
product. The infrared features correspond closely to those of
CH3C(O)OCH2OOH, reported by Neeb et al.22 It may be the
source of the extra CO and CO2 seen, although the oxidation
of CH3C(O)OCH2OOH is more likely to give AFAN as a
product than CO or CO2. Thus, at present the identity of this
product remains unknown.
Assuming that the yield of AFAN which did not come from
oxy radicals was independent of oxygen, the O2 dependence of
the yields YAcOH and YAFAN can be given by the following
expressions, where YRAD is the yield of the alkoxy radical, and
INT the O2-independent intercept.
-1
k
YAcOH ) YRAD × 1 + 5[O2]
(E2)
(
)
k3
YAFAN ) INT + YRAD × k5[O2](k3 + k5[O2])-1 (E3)
The room-temperature data, k3/k5 ) (5.4 ( 0.6) × 1018 molecule
Figure 3. Plot of product yields from the oxidation of methyl acetate
by chlorine atoms as a function of oxygen pressure as a function of
temperature. Circles, 253 K; triangles, 273 K; squares, 296 K; diamonds,
324 K. Acetic formic anhydride is given by closed symbols, acetic
acid by open ones. The lines are a simultaneous fit to all the data using
equations E2 and E3, with a difference in activation energies ∆E of
cm-3 (2-σ precision), agree very well with those of Christensen
et al., (6.0 ( 2.4) × 1018 molecule cm-3 13
.
To obtain an
activation energy for the decomposition reaction, all the data
were fit simultaneously to equations E2 and E3. The room-
temperature value of k3/k5 was fixed to the average of our and
Christensen’s values, and the difference in the activation
energies, ∆E, was optimized until the best fit to all the data
was obtained. The resulting fits are shown in Figure 3. In fitting
the data, the intercepts in the AFAN yields were taken directly
from the measured yields at the lowest O2 pressures. As can be
seen, all the data can be fitted simultaneously, using a value of
∆E ) (9.6 ( 1.0) kcal mol-1. Reactions of oxy radicals with
O2 such as reaction 5 usually have an activation energy in the
range 0.5-1.0 kcal mol-1, suggesting that the activation energy
for the decomposition channel is in the range 9.1-11.6 kcal
mol-1, with a most probable value of 10.1 kcal mol-1. Figure
4 shows the ratios k3/k5 in Arrhenius form derived by fitting
the data with equations E2 and E3 individually at each temper-
ature, and the results are also summarized in Table 1. Although
the precision on a given fit is about 10%, the uncertainty is
reduced by simultaneously fitting both the CH3COOH and
AFAN. Including calibration uncertainties, we estimate the
overall uncertainty to be about 12% at room temperature, in-
creasing to 15% at the extreme temperatures as a result of the
increased uncertainties in calibration.
9.6 kcal mol-1
.
occurrence of the molecular channel 8b of the self-reaction of
the CH3C(O)OCH2O2 radicals leads to a production of AFAN
independent of the O2 concentration, resulting in the intercept
in Figure 3. The reactions of alcohols and peroxides with chlor-
ine atoms are expected to be more rapid than that of the starting
material, and probably give AFAN as a product, which may
also contribute to the intercept in the AFAN yield in Figure 3.
Yields of CO and CO2 were also measured. The oxygen
dependence of the CO yield paralleled that of acetic acid, but
its yield lay about 10% higher (an effect also noted by
Christensen et al.13). The CO2 yield was independent of O2 and
was in the range 8-10%, with indications of upward curvature
at longer reaction times. Thus, it is possible that a single channel
exists leading to the formation of both CO and CO2, since the
“excess” CO is roughly equal to the CO2. It is possible that
both these compounds form in the reaction of Cl atoms with
one of the primary products, resulting in curvature in the CO2
plots, which is not observed in the CO yield because of the
strong primary source of CO. Christensen et al.13 noted the
formation of an unknown product which absorbs near 1030 cm-1
and is consumed rapidly by Cl atoms. We also observed this
Christensen et al.13 also did experiments at 296 K in the
presence of NO, finding that the yield of the decomposition

Alpha-Ester Rearrangement Reaction
J. Phys. Chem. A, Vol. 108, No. 33, 2004 6853
TABLE 1: Ratio of Rate Coefficients for Dissociation versus
Reaction with O2 at Various Temperatures for Methyl
Acetate (k3/k5) and Methyl Formate (k4/k6)a
Cavalli et al. observed the formation of methyl glyoxylate in
high yield from the photolysis of methyl bromoacetate.23 The
observation that the •OCH2C(O)OCH3 radical reacts with oxygen
rather than decomposing is expected from the thermochemistry
for that radical.18 Loss of methyl glyoxylate could occur through
reaction with OH radicals, leading to the production of two CO2
molecules.
temperature (K)
253
k3/k5 (2.7 ( 0.4) 1017 (1.3 ( 0.2) 1018 (5.4 ( 0.7) 1018 (2.3 ( 0.3) 1019
k4/k6
(3.1 ( 0.4) 1017 (1.5 ( 0.2) 1018 (4.8 ( 0.7) 1018
a The ratios have units of molecule cm-3
273
296
324
.
OH + HC(O)C(O)OCH3 f H2O + •C(O)C(O)OCH3 (11)
product, CH3C(O)OH, was higher when NO was present. From
an analysis of the product yields as a function of O2 pressure,
they deduced that chemical activation of the oxy radical CH3C-
(O)OCH2O• was occurring about 20% of the time. We conducted
an experiment in the presence of NO at 253 K to explore the
possibility that chemical activation of CH3C(O)OCH2O• radicals
could enhance the production of acetic acid. The experiment
used 70 Torr O2, which is sufficient to suppress the decomposi-
tion of thermalized radicals at that temperature. The analysis
of the experiment was complicated by the formation of
peroxynitrates, RO2NO2, in reaction 9, which reduced the overall
yield of AFAN + CH3COOH.
•C(O)C(O)OCH3 + O2 f O2C(O)C(O)OCH3 (12)
O2C(O)C(O)OCH3 + NO f f CO2 + CO2 + HCHO (13)
It is also feasible that the •C(O)C(O)OCH3 radical decomposes
to CO + CH3OCO, which oxidizes to give CO + CO2.14 Methyl
glyoxylate could also be lost by photolysis, to give HCO +
CH3OCO. The overall impact of these reactions would be to
produce one CO2 per molecule lost.
Figure 5 shows the formation of products as a function of
methyl acetate loss. The CO2 yields are consistent with
35-40% attack at the CH3C(O) group leading to two molecules
of CO2. Under the conditions of the experiment, the steady-
state concentration of the methyl glyoxylate would only be ∼1
× 1013 molecule cm-3, and since most of the bands reported
by Cavalli et al.23 would be obscured by the starting material,
quantification of this product would be impossible. In the
absence of further information regarding the fate of methyl
glyoxylate, further analysis of the CO2 yield is not warranted.
Methyl Formate Oxidation. As detailed in previous stud-
ies,14,15 CO, CO2, FAN, and formic acid are all observed in the
oxidation of methyl formate. Formic acid, CO, and FAN are
from abstraction at the methyl group, while HCHO and CO2
are produced following abstraction of the formyl hydrogen.
CH3C(O)OCH2O2 + NO2 + M f
CH3C(O)OCH2O2NO2 + M (9)
However, the ratio [CH3COOH]/[AFAN] should still be a good
indicator of the relative yields. In the absence of NO, the ratio
[CH3COOH]/[AFAN] was 1/11, but increased to 1/4 in the
presence of NO, indicating clearly that activation was occurring
at 253 K. This is consistent with the 20% yield of chemically
activated radicals found by Christensen at 296 K.
Experiments were conducted at 296 and 273 K at reduced
pressure to see whether a pressure dependence related to the
alpha-ester rearrangement could be discerned. These experiments
were done at 120 Torr total pressure using 60 Torr O2 at 296 K
and 14 Torr O2 at 273 K and were compared to experiments
containing the same amount of O2 at 700 Torr total pressure.
No difference in product yields could be found compared to
the experiments at 700 Torr, indicating that the ester rearrange-
ment does not depend strongly on pressure, assuming that the
other reactions leading to these products are also independent
of pressure.
Three experiments were performed to investigate the reaction
of OH with methyl acetate. Oxygen pressures of 140 and 300
Torr were used, with methyl acetate concentrations in the range
(2.1-3.5) × 1015 molecule cm-3. The presence of NO was
unavoidable in these experiments, since it is used to generate
OH radical. Ethene was used as a tracer to calculate the methyl
acetate loss, as described in the Experimental Section by
equation E1. There is good agreement between the three
measurements of k1,5-7 and the average value 3.5 × 10-13 cm3
molecule-1 s-1 was used. The product distributions were
qualitatively different from those obtained using Cl atoms. The
sum of CH3COOH and AFAN was only 50% and a large yield
of CO2 was observed, approaching 60%. Upon closer inspection,
the CO2 behaved like a secondary product and appeared after
an initial delay. Since very low yields of CO2 are obtained with
Cl atoms, which only react at the methoxy group, it is concluded
that OH attack also occurs at the acetyl group. The large
secondary yield of CO2 is consistent with production, and
subsequent loss, of methyl glyoxylate, HC(O)C(O)OCH3, from
the oxy radical formed at the acetyl group.
HC(O)OCH2O2 + HC(O)OCH2O2 f
HC(O)OCH2O• + HC(O)OCH2O• + O2 (14)
HC(O)OCH2O• f HC(O)OH + HCO
(4)
HC(O)OCH2O• + O2 f HC(O)OCHO + HO2
HCO + O2 f HO2 + CO
(6)
(15)
CH3OC(O)O2 + CH3OC(O)O2 f
CH3OC(O)O• + CH3OC(O)O• + O2 (16)
CH3OC(O)O• f CH3O + CO2
CH3O + O2 f HCHO + HO2
(17)
(18)
As a result of the slow rate of reaction of methyl formate with
Cl atoms,24 HCHO is rapidly removed by Cl atoms to form
CO.
Cl + HCHO f HCl + HCO
(19)
Since decomposition of the HC(O)OCH2O• radical is com-
petitive with the O2 reaction, the yields of CO, HC(O)OH, and
FAN should be oxygen-dependent, while that of CO2 is expected
to be constant.14 The yields of formic acid and formic anhydride
were fitted with equations analogous to E2 and E3 at each
temperature. Product yields are shown in Figure 6 for the
experiments at 324 K. The expected dependence on O2 is found,
•OCH2C(O)OCH3 + O2 f HO2 + HC(O)C(O)OCH3 (10)

6854 J. Phys. Chem. A, Vol. 108, No. 33, 2004
Tyndall et al.
is more efficient at low temperature. Wall effects may also play
a role at low temperature. An extrapolation of the results
obtained at higher temperature gives a ratio k4/k6 which is
equivalent to 1.6 Torr O2 at 253 K, so little dependence is
expected for 10 Torr O2 and above. An experiment was done
in air at 253 K in the presence of NO to look for the occurrence
of chemical activation. The yields of FAN (22%) and HCOOH
(3%) were both reduced from what was found in the absence
of NO, because of the formation of nitrates and peroxyni-
trates.14,25 Because of the difficulty in measuring and interpreting
the low yields of HCOOH, along with the reduced total yields,
no firm conclusions can be reached regarding the occurrence
of chemical activation in this experiment.
Products from the OH-initiated reaction of methyl formate
were compared with those from the Cl reaction at 296 K, to
ascertain the relative rates of attack at the two carbon atoms.
Ethene was again included in the mixtures as a tracer. Measure-
ments of rate coefficients for reaction 2 at 296 K range from
1.7 × 10-13 in ref 8 to 2.3 × 10-13 cm3 molecule-1 s-1 in ref
7, although the most recent measurements8-10 agree to within
a few percent. Thus, we used a rate coefficient of 1.8 × 10-13
cm3 molecule-1 s-1 to calculate the loss. The product yields
could also be normalized relative to the sum of the major
products and compared to the calculated loss of methyl formate
when C2H4 was present. This reduces any uncertainties associ-
ated with the rate coefficient for OH with methyl formate.
Products identified were HCOOH, HC(O)OCHO, and CO2. CO
was also observed, but it was not quantified, since it is also
produced by oxidation of HCHO formed from OH + ethene
and from methyl nitrite photolysis. Toward the end of the
reaction, the PAN-analogue CH3OC(O)OONO2 was also ap-
parent in the spectra.14,25 Figure 7 shows part of the spectrum
from an experiment with 300 Torr O2. Since a large part of the
spectrum is saturated, only certain windows could be used for
analysis. Nevertheless, the production of FAN and HCOOH can
clearly be seen on the wing of the large MF absorption near
Figure 5. Product yields from the reaction of OH with methyl acetate
in 300 Torr O2. Initial conditions: MeAc, 66 mTorr; CH3ONO, 43
mTorr; NO, 13 mTorr; C2H4, 2.7 mTorr. The data for CO2 have been
offset vertically by 0.3 mTorr, and the line through the CO2 data is a
quadratic fit, for visual purposes.
1100 cm-1
.
Clear evidence was found for abstraction at each end of the
molecule by OH. Formic acid and acetic formic anhydride result
from abstraction at the methyl group, while CO2 and the PAN
Figure 6. Yields of products as a function of partial pressure of O2
from the OH-oxidation of methyl formate at 324 K. Open circles, formic
acid; closed circles, formic anhydride; open triangles, CO2; closed
triangles, CO. The lines represent a simultaneous fit to the data using
analogue are indicative of abstraction at the acyl group.14
A
series of runs was performed in which the oxygen partial
pressure was varied from 60 to 300 Torr. Typical data are shown
for an experiment in air (140 Torr O2) in Figure 8, normalized
to the sum of the measured products (which was 109% for that
run). Product yields for all the experiments are summarized in
Table 2. The measured branching ratios for abstraction varied
from 60:40 to 40:60. Taking into account the occurrence of
chemical activation at 296 K, the product distributions from
OH and Cl were essentially identical, indicating that OH radicals
abstract with roughly equal efficiency from the H-C(O) and
-CH3 groups in methyl formate.
a value k4/k6 ) 4.8 × 1018 molecule cm-3
.
and the data can be fitted with a value for k4/k6 of (4.8 ( 0.5)
× 1018 molecule cm-3. Values of k4/k6 obtained at different
temperatures are given in Table 1 and shown in Figure 4. The
magnitudes of the uncertainties are similar to those for methyl
acetate, 12-15% depending on temperature. The branching ratio
for abstraction by Cl atoms does not appear to have a particularly
strong temperature dependence, since the combined yield of
formic acid and formic anhydride only varies from 45% at 273
K to 37% at 324 K.
In our previous study, we reported the presence of an
Discussion
absorption feature at 1308 cm-1 14
and tentatively suggested
,
that this product came from abstraction at the H-C(O) group,
possibly the ester monomethyl carbonate CH3OC(O)OH. The
presence of this band was noted at all the temperatures studied.
The absence of a similar feature in the methyl acetate experi-
ments supports the tentative assignment.
At 253 K, the decomposition of the oxy radical (reaction 4)
was sufficiently slow that no dependence of the product yields
on O2 was found. Yields of 41% and 5% were obtained for the
FAN and HCOOH, respectively. The source of HCOOH is
uncertain, but may be the reaction of HO2 with HCHO, which
The R-ester rearrangement was first discovered in 1998 and
has not yet been the subject of a large number of studies. Tuazon
et al. first demonstrated its occurrence for the oxidation of ethyl
acetate.12 Since then, there have been further studies of ethyl,
isopropyl, and isobutyl acetates in air,26,27 along with investiga-
tions of the oxidation of methyl acetate,13 methyl propionate,23
and methyl formate14 which used varying oxygen pressures.
However, all these studies were done at room temperature and
do not give any information about the thermochemical param-
eters for the reaction.

Alpha-Ester Rearrangement Reaction
J. Phys. Chem. A, Vol. 108, No. 33, 2004 6855
TABLE 2: Percentage Product Yields for the Reaction of
Hydroxyl Radicals with Methyl Formate as a Function of
Oxygen Partial Pressure at 296 Ka
P(O2)
HCOOH
FAN
HCOOH + FAN
CO2 + PAN
60
140
300
29 (24)
18 (16)
19 (16)
25 (20)
29 (27)
43 (36)
54 (44)
47 (43)
62 (51)
69 (56)
62 (57)
59 (49)
a Yields in parentheses have been normalized to the total of
measurable products.
composition reaction for methyl acetate is faster relative to the
O2-reaction. It is not clear whether this difference represents a
small difference in the activation energy, the A-factor, or both.
The measured activation energy is about 10-11 kcal mol-1
.
This is consistent with the observation of a limited degree of
chemical activation,4,28 since the available energy from the
reaction RO2 + NO is typically around 10 kcal mol-1
.
Calculations on the energetics of the R-ester rearrangement
were made by Good and Francisco,16 Rayez et al.,17 and Ferenac
et al.18 Good and Francisco calculated a barrier of about 13
kcal mol-1 for the methyl formate radical, a little larger than
what was measured here. Rayez found a relatively low barrier
for ethyl acetate, 6-7 kcal mol-1. The larger Ea of 10.1 kcal
mol-1 reported herein is consistent with the slower rate
coefficients for the radicals studied (ethyl acetate undergoes
100% decomposition, methyl acetate and methyl formate
competitive decomposition, and O2 reaction at 296 K). Rayez
et al. also calculated the rate coefficient as a function of pressure
using RRKM theory. At 296 K, the rate coefficient was
predicted to be in the falloff region at atmospheric pressure
(roughly a factor of 2 below the high-pressure limit) with a value
of 3.7 × 107 s-1. The high-pressure A-factor was calculated as
4.7 × 1012 s-1. The current experiments did not show any
pressure dependence for the reaction of methyl acetate. Assum-
ing a rate coefficient for k5 of 8 × 10-15 cm3 molecule-1 s-1
gives a room-temperature rate coefficient of 4.6 × 104 s-1 and
an A-factor at 1 atm of 4.2 × 1012 s-1, on the basis of the
activation energy measured here. The reaction is about 3 orders
of magnitude slower than that estimated for ethyl acetate,
although the A-factor is very similar.
Figure 7. Sequence of spectra showing the oxidation of methyl formate
by OH radicals in the presence of 300 Torr O2. Initial concentrations
were MF (6.95 × 1015), CH3ONO (2.1 × 1015), ethene (1.3 × 1014),
and NO (5.5 × 1014 molecule cm-3). A: starting mixture; B: spectrum
after 36 min photolysis; C: residual spectrum after subtraction of MF
and CH3ONO; D: reference spectra of HCOOH (7.5 × 1013) and formic
anhydride (6.1 × 1013 molecule cm-3).
Ferenac et al.18 found that the barrier to dissociation of
CH3C(O)OCH2O• was very sensitive to the level of theory used,
obtaining 7-8 kcal mol-1 with B3LYP and ∼11 kcal mol-1
with G2(MP2). The latter result is clearly in better agreement
with the experimental results obtained in the present work.
Ferenac et al. also pointed out that tunneling may make a small
contribution to the reaction rate (∼30%) and so the measured
barrier could be lower than the true one by up to 1 kcal mol-1
,
which would bring the experimental and calculated results into
very good agreement.
The present study also indicates that both chlorine atoms and
OH radicals abstract from the two different carbon atoms in
methyl formate with roughly equal rates. The structure additivity
relationships (SAR) of Le Calve´ et al.8 also indicate that the
branching ratio for OH should be 50:50. However, further
conclusions from that paper (and also experimental results from
our laboratory on isopropyl and tert-butyl formates) indicate
that strict group additivity cannot be used in calculating the rate
coefficients for larger formates and that caution should be
exercised in making these extrapolations. Good et al.9 made ab
initio calculations of the reaction thermochemistry and activation
barriers and implied that abstraction by OH from both ends of
the molecule should occur, with an overall rate coefficient very
similar to that measured. However, their calculations suggested
that the majority of the reaction (86%) should occur at the
Figure 8. Product distribution from the reaction of OH radicals with
methyl formate at 296 K in air. The yields have been normalized to
100%, to account for difficulties in quantifying the loss of methyl
formate. Initial conditions: methyl formate, 217 mTorr; methyl nitrite,
66 mTorr; ethene, 4 mTorr; NO, 13 mTorr. Yields are shown for CO2,
formic acid anhydride (FAN), and formic acid (HCOOH).
The present study is the first to investigate the temperature
dependence of the ester rearrangement and as such offers
information on the activation energy. As seen in Figure 4, the
activation energies for methyl acetate and methyl formate are
identical, within experimental uncertainties, although the de-

6856 J. Phys. Chem. A, Vol. 108, No. 33, 2004
Tyndall et al.
(2) Derwent, R. G.; Jenkin, M. E.; Saunders, S. M. Atmos. EnViron.
H-C(O) group, although such calculations are clearly very
sensitive to the magnitudes of the barriers (which were both
less than 5 kcal mol-1) and the extent of tunneling. For
comparison, the local activation energy measured at 296 K by
Le Calve´ et al. was only 0.9 kcal mol-1, which is probably lower
than the uncertainty in the thermochemical parameters involved.
We also demonstrate that OH attacks methyl acetate at both
carbon atoms, with up to 40% occurring at the acetyl group.
This is again consistent with the structure-activity relationship
developed by Mellouki and co-workers.6,8 Szila´gyi et al. recently
attempted to express the SAR in terms of the groups HC(O)O-
and CH3C(O)O-, rather than modifying the rates for carbonyl
groups.10 While their predicted rate coefficients are reasonable,
the method is not successful in predicting the branching ratio
found here for methyl formate with OH.
The present study enables the atmospheric fate of methyl
formate and methyl acetate to be predicted over a range of
conditions. It is not expected that the relative fractions of
abstraction by OH will vary strongly with temperature, because
of the low overall barrier to reaction. The branching of the
acyloxy-substituted methoxy radicals should be described well
by the temperature-dependent decomposition rates reported here.
In methyl formate, approximately half of the reaction occurs
at the formyl group. While the reactions of the oxy radical
CH3OC(O)O• probably do not depend strongly on temperature
(with rapid decomposition dominating), the potential for forma-
tion of the PAN-analogue CH3OC(O)O2NO2 will of course
depend on the ratio NO:NO2, and the thermal stability of the
PAN-analogue will also be temperature dependent. Interestingly,
this PAN-analogue may have been tentatively identified in
ambient air using chemical ionization mass spectrometry (F.
Flocke, NCAR, personal communication). For methyl acetate,
abstraction at the acetyl group will probably lead to the
formation of methyl glyoxylate, which could react with OH,
photolyze, or be removed by aqueous particles.
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Acknowledgment. NCAR is operated by the University
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