The hydroxyl radical reaction rate constant and products of ethyl 3-ethoxypropionate

Article abstract of DOI:10.1002/(sici)1097-4601(1997)29:8<637::aid-kin9>3.0.co;2-v

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH₃CH₂-O-CH₂CH₂C(O)O-CH ₂CH₃). EEP reacts with OH with a bimolecular rate constant of (22.9 ± 7.4) × 10^-12 cm³ molecule^-1 s^-1 at 297 ± 3 K and l atmosphere total pressure. In order to more clearly define EEP's atmospheric reaction mechanism, an investigation into the OH + EEP reaction products was also conducted. The OH + EEP reaction products and yields observed were: ethyl glyoxate (EG, 25 ± 1% HC(=O)C(=O)-O-CH₂CH₃), ethyl (2-formyl) acetate (EFA, 4.8 ± 0.2%, HC(=O)-CH₂-C(=O)-O-CH₂CH₃), ethyl (3-formyloxy) propionate (EFP, 30 ± 1%, HC(=O)-O-CH₂CH₂-C(=O)-O-CH₂CH₃), ethyl formate (EF, 37 ± 1%, HC(=O)O-CH₂CH₃), and acetaldehyde (4.9 ± 0.2%, HC(=O)CH₃). Neither the EEP's OH rate constant nor the OH/EEP reaction products have been previously reported. The products' formation pathways are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry.

Full text of DOI:10.1002/(sici)1097-4601(1997)29:8<637::aid-kin9>3.0.co;2-v

The Hydroxyl Radical
Reaction Rate Constant
and Products of Ethyl
3
-Ethoxypropionate
1 2 3
J. STEVEN BAXLEY, MICHAEL V. HENLEY, and J. R. WELLS
1
Applied Research Associates, Gulf Coast Division, P.O. Box 40128, Bldg. 1142, Mississippi Road, Tyndall Air Force Base,
Florida 32403
2
,3
Armstrong Laboratory, Environics Directorate, AL/EQL, 139 Barnes Drive, Tyndall AFB, Florida 32403-5323
Received 23 September 1996; accepted 2 December 1996
ABSTRACT: The relative rate technique has been used to measure the hydroxyl radical (OH)
reaction rate constant of ethyl 3-ethoxypropionate (EEP, CH CH –O–CH CH C(O)O–
3
2
2
2
Ϫ12
3
CH CH ). EEP reacts with OH with a bimolecular rate constant of (22.9 Ϯ 7.4) ϫ 10
cm
2
3
Ϫ1 Ϫ1
molecule
s
at 297 Ϯ 3 K and 1 atmosphere total pressure. In order to more clearly define
EEP’s atmospheric reaction mechanism, an investigation into the OH ϩ EEP reaction prod-
ucts was also conducted. The OH ϩ EEP reaction products and yields observed were: ethyl
glyoxate (EG, 25 Ϯ 1% HC("O)C("O)–O–CH CH ), ethyl (2-formyl) acetate (EFA,
2
3
4
.8 Ϯ 0.2%, HC("O)–CH –C("O)–O–CH CH ), ethyl (3-formyloxy) propionate (EFP,
2 2 3
3
0 Ϯ 1%, HC("O)–O–CH CH –C("O)–O–CH CH ), ethyl formate (EF, 37 Ϯ 1%,
2
2
2
3
HC("O)O–CH CH ), and acetaldehyde (4.9 Ϯ 0.2%, HC("O)CH ). Neither the EEP’s OH
2
3
3
rate constant nor the OH/EEP reaction products have been previously reported. The prod-
ucts’ formation pathways are discussed in light of current understanding of oxygenated hy-
drocarbon atmospheric chemistry. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 637–644,
1
997.
INTRODUCTION
oxygenated organic compounds are very volatile, but
investigations into their atmospheric chemistry is by
no means complete. Several hydroxyl radical
(OH) ϩ oxygenated organic bimolecular rate con-
stants are well known, but details of the
OH ϩ oxygenated organic compound reaction mech-
anisms are few. These few mechanistic studies have
illustrated the complexity of oxygenated organic at-
mospheric chemistry [1–5]. These investigations sup-
port hydroxyl radical reaction mechanism patterns
based on structural reactivity [6].
Oxygenated organic compounds are being used in
fuels to promote better combustion and increase oc-
tane without using more aromatic compounds. Their
incorporation into fuels reduces carbon monoxide
and photochemically active volatile exhaust emis-
sions. Oxygenated organics are being used as sol-
vents in paints and paint strippers. With the increased
use of these compounds a better understanding of
their environmental impact is necessary. Typically,
The paucity of atmospheric mechanism data on
oxygenated organics can have a serious impact. Cal-
culation of a compound’s incremental reactivity [7],
Correspondence to: J. R. Wells
1997 John Wiley & Sons, Inc.
©
CCC 0538-8066/97/080637-08

6
38
BAXLEY, HENLEY, AND WELLS
tendency to generate tropospheric ozone (O ), re-
quires detailed knowledge of a compound’s reactive
and mechanistic properties. While not emitted di-
were carried out in a light tight chamber using 2-mil
FEP Teflon -film bags (ca. 100 liters), which were
surrounded by the following mix of lamps: 6-Philips
TL40W/03; 1-GE F40BL; 2-QPANEL UV351; and
7-QPANEL UV340.
3
®
rectly, O is a by-product of the photooxidation of hy-
3
drocarbons [8] and a regulated pollutant. Sources that
contribute to its formation in the troposphere might
also be regulated. Currently, the reactivity models in-
corporate educated assumptions about a compound’s
unknown atmospheric mechanism, but in order to
minimize uncertainties and calculate a compound’s
incremental reactivity more accurately, the atmos-
pheric kinetics and atmospheric transformation mech-
anism of a compound must be well known. As a side
benefit, understanding the compounds’ atmospheric
mechanisms in detail can provide a basis for com-
pound selection based on structural reactivity. Thus,
possibility of synthesizing new compounds incorpo-
rating environmentally and technically beneficial
molecular structures exists. The information gained
from the type of research presented here can lead to
more beneficial use of these and similar compounds
in the future.
All samples were quantitatively monitored using
an Hewlett-Packard (HP) gas chromatograph (GC)
5890 with a flame ionization detector (FID) and HP
series Chem Station software. Gas samples for GC
analysis were cryogenically collected on a Hastelloy
C sample loop (ca. 1.3 mL) and injected onto the GC
column (Restek Stabilwax column (0.53 mm i.d., 30-
m, and 1.0 ␮m film thickness)) with a heated rotary
valve [12]. The GC temperature program used was as
follows: 35°C for 5 min. then 10°C/min to 210°C for
2 min. Helium, the carrier gas, was supplied by Tyn-
dall AFB and passed through drying and hydrocarbon
traps before use.
Product identification experiments were performed
®
in both 3000 L and 100 L Teflon chambers. For the
large chamber, the reactants were irradiated with 34
black lamps (GE F40 BLB) and 14 sun lamps (GE
F40 CW) while the small chamber light bank de-
scribed above was used. The chamber mixture was
sampled by a cryogenic collection method described
above except the chromatographic sample volume
was twice as large and analyzed with an HP 5890 Se-
ries II Plus GC/HP 5971 mass selective detector/HP
5965B infrared detector (GC/MS/FTIR) system.
Compound separation was achieved by either a
Restek Stabilwax (0.53 mm i.d., 30-m length, and
1.0-␮m film thickness) or a Restek 502.2 column
(0.53 mm i.d., 30-m length, and 3.0-␮m film thick-
ness). Helium (UHP), the carrier gas, was supplied
by Air Products, and passed through drying and hy-
drocarbon traps before use. The large chamber con-
tents were also analyzed by long path infrared spec-
troscopy. The infrared beam of a Nicolet 740 Fourier
Transform Infrared (FTIR) spectrometer traversed the
chamber using conventional White cell optics [13].
An absorption path length of approximately 70 m was
used to monitor the entire chamber contents during
the course of the reaction.
Ethyl 3-ethoxypropionate (EEP, CH CH –O
3
2
–
CH CH C(O)O–CH CH ) is one of a series of
2 2 2 3
oxygenated organics present in solvent and coatings
formulations used by the US Air Force. In the work
presented here, the rate constant of OH with EEP was
measured by the relative rate method [9]. The prod-
ucts of the OH ϩ EEP reaction are also reported and
used to derive EEP’s atmospheric reaction mecha-
nism. Because both the OH rate constant and reaction
mechanism for EEP have not been reported before,
this article presents new information regarding the at-
mospheric impact of EEP.
EXPERIMENTAL METHODS
Apparatus and Materials
Experiments to measure the gas phase rate constant
of the OH ϩ ethyl 3-ethoxypropionate (EEP,
CH CH –O–CH CH C(O)O–CH CH )
reaction
3
2
2
2
2
3
were conducted with a previously described appara-
tus [1,10,11]. A brief description is provided here.
Reactants were introduced and samples were with-
drawn through a 6.4-mm Swagelok fitting attached to
Carbonyl compounds produced by gas-phase reac-
tions were measured by impinger sampling in a de-
rivatizing solution of 2,4-dinitrophenylhydrazine
(DNPH). Hydrazones formed by derivatization were
separated and quantitatively measured by HPLC (HP
1050) using a three component gradient solvent pro-
gram as described previously [14].
®
ca. 100 L Teflon film chamber. Dry compressed air
was added as a diluent to the reaction chambers and
Ϫ1
measured with a 0–100 L-min mass flow con-
troller. The filler system was equipped with a syringe
injection port facilitating the injection of both liquid
and gaseous reactants into the chamber in a flowing
airstream. All reactant mixtures and calibration stan-
dards were generated by this system. Irradiations
In order to further characterize uncalibrated and
unidentified HPLC peaks,
a
liquid chroma-
tography/mass spectrometry (LC/MS) apparatus was
employed. LC/MS analysis was accomplished using
an Hewlett Packard (HP) 1050 high-performance liq-

HYDROXYL RADICAL REACTION RATE CONSTANT
639
uid chromatograph (HPLC) and an HP 5987 mass
spectrometer. The two instruments were interfaced
using an HP 59980A Particle Beam Interface. Data
collection and manipulation were accomplished with
an HP 1000-F minicomputer running the RTE-6/VM
operating system and RTE-6/VM GC/MS DATA
SYSTEM software.
Acros Organics (99%+) and used as received. Ethyl
formate (97%), dodecane and n-nonane (both 99%+)
were used as received from Aldrich Chemical. Ethyl
glyoxate was obtained from Fluka Chemical as a 50%
mixture with toluene. Methanol and acetonitrile,
HPLC grade, were used as received from Aldrich
Chemical. The water was distilled, deionized to 18
®
Separation by LC/MS was similar to that previ-
megaohm, and filtered using a Milli-Q filter system.
ously described in ref. [14]. Briefly, separation was
The methane was UHP grade and used as received
from Air Products. The nitric oxide (99%+ pure) was
obtained from Matheson Gases and used as received.
Experiments were carried out at 297 Ϯ 3 K at about
1 atmosphere.
TM
achieved using a Dupont Zorbax
ODS column
25 cm ϫ 4.6 mm ϫ 5 ␮m particle size) with a 0.75
ml/min gradient mobile phase. Sample injection vol-
umes varied from 20 ␮l to 80 ␮l depending on the ex-
pected concentration of the analyte. The HPLC efflu-
ent was connected to the Particle Beam Interface for
desolvation prior to analyte entry into the mass spec-
trometer. The HP 1050 fixed wavelength detector, set
at 360 nm, was used in-line to monitor for DNPH.
The HPLC effluent was diverted away from the Parti-
cle Beam Interface and the mass spectrometer until
after the elution of DNPH.
Experimental Procedures
The experimental procedures for determining the
OH ϩ EEP reaction kinetics were similar to those
described previously [10,11].
kEEP
The Particle Beam Interface was used to separate
the lighter HPLC eluent molecules from the analyte
of interest through a momentum separation process.
A helium nebulization gas, set at 70 psig, produced
an aerosol as the HPLC effluent entered the interface.
The aerosol was desolvated as it passed through the
momentum separators of the Particle Beam Interface.
The desolvated analyte entered the mass spectrometer
through a transfer probe inserted directly into the
source of the mass spectrometer.
EEP ϩ OH 9: Products
(4)
(5)
kR
Reference ϩ OH 9: Products
The rate equations for reactions 4 and 5 are combined
and integrated resulting in the following equation:
[
EEP]0
΂[EEP]t
kEEP
[R]0
ln
ϭ
ln
(6)
΃ ΂ ΃
k
R
[R]
t
The mass spectrometer was operated in the nega-
tive chemical ionization mode using methane as the
reagent gas. Methane flow was set so that the mass
If reaction with OH is the only removal mechanism
for EEP and reference, a plot of ln([EEP] /[EEP] )
0
t
vs. ln([R] /[R] ) yields a straight line with an inter-
0
t
spectrometer source pressure was maintained at
cept of zero. Multiplying the slope of this linear plot
Ϫ4
2
.0 ϫ 10 torr. The source temperature was 200°C,
by k yields k . The OH rate constant experiments
R
EEP
and the interface was maintained at 40°C. The mass
spectrometer was programmed to scan from 50 to 500
amu and tuned using perfluorotributylamine (FC-43).
OH, the primary oxidizing radical in the atmos-
phere, was generated from the photolysis of methyl
employed the use of two reference compounds, n-
nonane and dodecane, for EEP. The use of two differ-
ent reference compounds with different OH rate con-
stants more definitively assured the accuracy of the
EEP/OH rate constant.
nitrite (CH ONO) in the presence of nitric oxide
The typical concentrations of the pertinent species
3
®
(
NO) in air [9].
in the 100 liter Teflon chamber were 1–4 ppm EEP,
1
.5–2.0 ppm reference, 9.0 ppm CH ONO, and
3
CH ONO ϩ h
␯
9: CH O ϩ NO
(1)
(2)
(3)
2.3 ppm NO in air. These mixtures were allowed to
stand for 30–60 min before background chamber
samples were collected. The samples were collected
on a cryogenic (Ϫ100°C) sample loop (described
3
3
CH O ϩ O 9: CH O ϩ HO
2
3
2
2
HO ϩ NO 9: OH ϩ NO
2
2
Ϫ1
above) for 4 min at 25 mL min and then flash in-
CH ONO was prepared in gram quantities using the
method of Taylor et al. [15] and stored in a lecture
jected (260°C) onto the GC column [12]. Typically,
15–30 s irradiation intervals were used on the reac-
tion mixture for a combined total photolysis time of
approximately 90 s. The flame ionization detector
(FID) signal was used to determine EEP and refer-
ence concentrations.
3
bottle at room temperature. The CH ONO purity
3
(Ͼ 95%) was verified by GC/MS/FTIR.
Ethyl 3-ethoxypropionate (EEP, CH CH –
3
2
O–CH CH C(O)O–CH CH ) was obtained from
2
2
2
3

6
40
BAXLEY, HENLEY, AND WELLS
EEP/OH reaction product experimental methods
with zero-air. Measurements of an air filled bag
showed no cross contamination between runs.
The formation of hydrazones from the reaction
of the aldehyde and ketone reaction products with
1,2 dinitrophenylhydrazine (DNPH) was achieved
by passing a known volume of the chamber con-
tents through a standard DNPH solution in an ice
bath [14].
and component concentrations were similar to those
for reaction rate experiments except the reference
compound was excluded from the reaction mixture
and the EEP concentration was approximately dou-
bled. For GC/MS/FTIR measurements the mixture
was irradiated for timed (10, 30, 50 s) intervals fol-
lowed by sample collection. The mass selective de-
tector was tuned before each experimental run using
perfluorotributylamine (FC-43). Full scan electron
impact ionization spectra were collected from
RESULTS
2
5 to 220 mass units. The infrared detector
Hydroxyl Radical/EEP Reaction Rate
Constant
Ϫ1
(GC/MS/FTIR) was operated at eight cm resolu-
tion with each four scans averaged to give a single
IR spectrum every 1.5 s. The long path FTIR was
operated at one cm resolution and an infrared spec-
The OH rate constant for ethyl 3-ethoxypropionate
(EEP, CH CH –O–CH CH C(O)O–CH CH ) was
Ϫ1
3
2
2
2
2
3
trum of the entire chamber contents was collect-
ed after each irradiation. Preliminary compound iden-
tifications from the GC/MS/FTIR data sets were
made by searching the Wiley/NBS Mass Spectra
Library and the EPA vapor library. Pure samples
of the identified products were obtained to check
for matching spectra (MS and FTIR) and retention
time.
The loss of the parent EEP was plotted against the
formation of products generating a straight line with
a slope equal to the product yield. Product quantita-
tion was performed using the same GC, elution para-
meters, and column mentioned above with the col-
umn effluent analyzed by an FID. Pure samples, if
available, of identified products were obtained to
check for matching retention times and generate a
quantitative calibration curve.
obtained using the relative rate method described
above. Typically five experimental runs were con-
ducted on each EEP/reference pair. The plot of
a modified version of eq. (6) is shown in Figure 1.
The ln([R] /[R] ) term is divided by the respective
0
t
reference rate constant (dodecane (14.2 Ϯ
Ϫ12
3
Ϫ1 Ϫ1
3.6) ϫ 10 cm molecule s
and
n-nonane
Ϫ12
3
Ϫ1 Ϫ1
(10.2 Ϯ 2.6) ϫ 10 cm molecule s ) [16] and
multiplied by 10 cm molecule s resulting in a
unitless number and yielding a slope that is equal to
Ϫ12
3
Ϫ1 Ϫ1
the hydroxyl radical/EEP rate constant, k , divided
EEP
Ϫ12
3
Ϫ1 Ϫ1
by 10 cm molecule s . This modification allows
for a direct comparison to the two reference
compound/EEP data sets. The slope of the line yields
an hydroxyl radical bimolecular rate constant, k
,
EEP
Ϫ12
3
Ϫ1 Ϫ1
of (22.9 Ϯ 0.8) ϫ 10 cm molecule s . The
data points at the origin are experimental points be-
cause preirradiation, t ϭ 0, data showed no de-
All measurements were at least duplicated. A rela-
tive standard deviation (the data set standard devia-
tion divided by the data set average) of approximately
2
.5% was achieved with the described sampling
method. All compounds were tested for interferences
as follows: methyl nitrite and NO injected into the
bag to determine elution times; methyl nitrite, NO,
and reference injected into bag, initial measurement
taken to locate the reference, photolyzed for 3 min
and further measurements taken to determine elution
time of products, this procedure repeated for the EEP;
the reference and EEP injected into the bag, mea-
sured and photolyzed for 6 min to determine stability;
methyl nitrite, NO, reference, and EEP injected into
the bag, initial measurements taken and left overnight
with further measurements to determine stability.
None of these preliminary experiments yielded chro-
matographic peak overlaps nor observable reactions
occurring without photoinitiation. Both EEP and the
references not lost due to photolysis using the de-
scribed reaction system. At the end of each run, the
Figure 1 Ethyl 3-ethoxypropionate relative rate plot with
both n-nonane and dodecane as reference compounds. The
OH ϩ EEP rate constant, k , measured is 22.9 Ϯ
EEP
Ϫ12
3
Ϫ1 Ϫ1
0
.8 ϫ 10
cm molecule s . The dodecane as a refer-
ence (᭿) data are overlayed with the n-nonane as a refer-
ence (᭺) data.
®
Teflon bag was cleaned by flushing the bag 6 times

HYDROXYL RADICAL REACTION RATE CONSTANT
641
tectable loss of EEP or reference. The error in the rate
constant stated above is the 95% confidence level
from the random uncertainty in the slope. Incorporat-
ing the uncertainties associated with the reference
rate constants (Ϯ25%) used to derive the EEP/OH
OH with reaction product. The measured value for kP
was used when possible, but k was calculated using
P
structure reactivity [6] when no measured value was
available in the literature. It should be noted that all
of the products exhibited linear concentration pro-
files; the lack of curvature strongly suggesting no un-
usual side reactions that generate or remove primary
rate constant yields a final value for k
of
EEP
Ϫ12
3
Ϫ1 Ϫ1
(
22.9 Ϯ 7.4) ϫ 10 cm molecule s . Assuming
6
Ϫ3
an [OH] ϭ 1 ϫ 10 molecules cm , the atmospheric
1/e) lifetime calculated for EEP is 12 h. The
reaction products. For completeness, the k values are
P
(
presented with the respective product.
EEP/OH rate constant, kEEP , has not been previously
reported. The observed rate constant can be compared
with a kEEP calculated using a structure reactivity rela-
Ethyl (3-Formyloxy) Propionate (EFP,
Ϫ12
tionship [6]. The calculated k
was 20 ϫ 10
EEP
HC("O)–O–CH CH –C("O)–O–CH CH )
2
2
2
3
3
Ϫ1 Ϫ1
cm molecule s , in agreement with our measured
EFP is the reaction product retaining most of the par-
ent EEP molecule. Its assignment was based on
analysis of both the FTIR and mass spectrum of the
chromatographic peak. This product was not com-
mercially available and an unequivocal verification
was not possible. The assignment of this product is
reasonable, however, due to the recently published
observation of ethanediol acetate formate (EAF,
CH –C("O)O–CH CH –O–C("O)H) as an
value.
OH/EEP Reaction Products
The reaction products observed are in accordance
with previously observed hydroxyl radical reaction
mechanisms for oxygenated organic species [1–4].
Typically, the oxygenated organic parent compound
reacts with OH to subsequently generate other oxy-
genated organic products. For EEP, the OH/EEP re-
action products observed were: ethyl glyoxate (EG,
HC("O)C("O)–O–CH CH ), ethyl (2-formyl)
3
2
2
OH/2-ethoxyethyl acetate (EEA, CH –C("O)
3
O–CH CH –O–CH CH ) reaction product [1]. Us-
2
2
2
3
ing the same separation parameters, EFP and EAF
behave similarly chromatographically having a reten-
tion time later than the parent compound. In fact, the
calibration of EFP was extracted from the calibration
of EAF by correcting for the additional carbons in the
EFP molecule. The GC/FID system was used to
monitor EFP concentrations. The kEFP calculated us-
2
3
acetate
(EFA,
HC("O)–CH –C("O)–O–
2
CH CH ), ethyl (3-formyloxy) propionate (EFP,
2
3
HC ( " O) – O – CH CH – C( " O) – O – CH CH ),
ethyl formate (EF, HC("O)O–CH CH ), acetalde-
hyde (HC("O)CH ), and diethyl malonate (DM,
2
2
2
3
2
3
3
Ϫ12
3
ing structure reactivity [6] was 8.34 ϫ 10 cm
CH –CH –O–C("O)–CH –C("O)O–CH CH ).
3
2
2
2
3
Ϫ1 Ϫ1
molecule s . From eq. (7) the average [EFP] cor-
rection was 13.3% (maximum correction 31%). The
yield of EFP was 30 Ϯ 1%.
The specific results for each of these products are de-
scribed below. The reported yields are based on the
slopes of the EEP reacted versus product formed
plots. The reported error in the product yield is the
9
5% confidence level from the random uncertainty in
Ethyl Formate (EF, HC("O)O–CH CH )
the slope of these plots.
2
3
Because the OH/EEP reaction products could re-
act with OH, the observed product concentrations had
to be corrected for OH ϩ reaction product reactions.
This correction has been described in detail [17,18]
and has the following form:
Ethyl formate was identified by GC/FTIR/MS and
quantitated by GC/FID. Pure EF was obtained to ver-
ify product assignment. A 37 Ϯ 1% EF yield from
OH ϩ EEP reaction was observed. The rate constant
Ϫ12
3
Ϫ1 Ϫ1
for OH ϩ EF (1.02 ϫ 10 cm molecule s ) mea-
sured by Wallington et al. [19] was used for k in eq.
(7) and resulted in an average [EF] correction of
1.5% (maximum correction 2.6%).
P
[
EEP]t
1
Ϫ
(
kEEP Ϫ kP)
[EEP]0
F ϭ
ϫ
kEEP
[EEP]t kP/kEEP
΂[EEP]0
[EEP]t
[EEP]0
Ϫ
΃
Ethyl (2-Formyl) Acetate (EFA,
(7)
HC("O)–CH –C("O)–O–CH CH )
2
2
3
F, the correction factor, was multiplied by the product
concentration data; kEEP is the OH ϩ EEP rate con-
EFA was derivatized using DNPH, identified using
LC/MS and quantitated using LC. EFA was not com-
mercially available, however hydrazone concentration
stant, and k is the rate constant for the reaction of
P

6
42
BAXLEY, HENLEY, AND WELLS
calibrations are quite similar compound to com-
pound, because the absorption signal comes mainly
ness. Both the mass and infrared spectra observed
matched the respective spectral libraries. This com-
bined with the structural similarity to EEP made the
assignment very conclusive.
from the dinitrophenyl rings on the DNPH. k was
EFA
Ϫ12
3
Ϫ1 Ϫ1
calculated [6] (2.18 ϫ 10 cm molecule s ), and
the average [EFA] correction was on the order of 3%
Table I is a summation of the above results.
(maximum correction 4.6%). The observed EFA yield
was 4.8 Ϯ 0.2%.
DISCUSSION
OH reacts with EEP by H-atom abstraction. EEP is a
large molecule with six possible carbons as hydrogen
abstraction sites. However, the products of the reac-
tion of OH with EEP suggest strongly that the OH
abstracts hydrogen principally from three methylene
groups. The “reactive structure” of EEP can be drawn
as shown in Structure I:
Ethyl Glyoxate (EG,
HC("O)C("O)–O–CH CH )
2
3
EG was derivatized using DNPH, identified using
LC/MS and quantitated using LC. EG is commer-
cially available as a 50/50 mixture with toluene. The
[
EG] analysis was slightly more complicated due to
the splitting of the EG signal into two chromato-
graphic peaks. This observation indicates there may
be two forms of the derivatized hydrazone. For
CH –CH –O–CH –CH –R
3
2
2
2
I
II
III
Structure I
Ϫ12
eq. (7), k was calculated [6] to be 1.66 ϫ 10
EG
3
Ϫ1 Ϫ1
cm molecule s . The EG yield was 25 Ϯ 1% with
an average [EG] correction of 2.2% (maximum cor-
rection 4.5%).
where R ϭ –C("O)O–CH CH . This is consistent
2 3
with the reaction mechanisms proposed for methyl-
enes attached to an ether oxygen [1,3], and structure
reactivity calculations [6]. Also, the agreement be-
tween the calculated versus the measured kEEP sup-
ports these “reactive site” assignments.
Acetaldehyde (HC("O)CH3)
The experimental parameters were set to minimize
other side reactions and highlight the first OH hydro-
gen abstraction step. Nitric oxide (NO) was added to
facilitate the generation of OH and to minimize ozone
(O ) and NO formation preventing other possible
Acetaldehyde was derivatized using DNPH and iden-
tified and quantitated by LC. Acetaldehyde was com-
mercially available and used for calibrations and as-
signment verification. k
is reported in the
Acetaldehyde
3
3
Ϫ11
3
Ϫ1 Ϫ1
literature (1.5 ϫ 10 cm molecule s ) [20–22].
The acetaldehyde yield was 4.9 Ϯ 0.2% with an aver-
age [acetaldehyde] correction of 22% (maximum cor-
rection 47%). This large correction was due mainly to
the fast reaction of acetaldehyde with OH.
radical reactions. The proposed OH reaction mecha-
nism is shown in Figure 2. Depending on the nature
of the radical formed in Reaction (4):
For methylene I:
CH –CH –O–CH –CH –R ϩ OH 9:
3
2
2
2
и
CH –(CH )–O–CH –CH –R ϩ H O (8)
Diethyl Malonate (DM,
3
2
2
2
CH –CH –O–C("O)–CH –
3
2
2
CH –(CH )–O–CH –CH –R
и
3
2 2
C("O)O–CH CH )
2
3
ϩ O ϩ NO 9:
2
While not quantitated due to a very small yield, DM,
observed by GC/MS/FTIR, is included for complete-
CH –(CHO )–O–CH –CH –R ϩ NO
и
3
2 2 2
(9a)
Table I OH ϩ EEP Reaction Products and Yields
Yield (Corrected for
Reaction with OH)
Reaction Product
Structure
HC("O)–O–CH CH C("O)OCH CH
3
Ethyl (3-formyloxy)propionate
Ethyl Formate
30 Ϯ 1%
37 Ϯ 1%
2
2
2
HC("O)O–CH CH3
2
Ethyl Glyoxate
HC("O)C("O)–O–CH CH3
25 Ϯ 1%
2
Ethyl (2-formyl) acetate
Acetaldehyde
HC("O)CH –C("O)O–CH CH
4.8 Ϯ 0.2%
4.9 Ϯ 0.2%
2
2
3
HC("O)CH3

HYDROXYL RADICAL REACTION RATE CONSTANT
643
CH –CH –O–CH –C(H
и
)–R ϩ H O
(14)
3
2
2
2
CH –CH –O–CH –C(H
и
)–R ϩ O2
3
2
2
ϩ NO 9: NO ϩ H(O ")C–R (EG)
2
ϩ CH –CH –O–CH
и
(15a)
(15b)
3
2
2
CH –CH –O–CH
и
ϩ O ϩ NO 9:
3
2
2 2
NO ϩ CH –CH –O–C("O)H (EF)
2
3
2
The major product observed was ethyl formate (EF).
From the mechanism proposed, EF is a product of
hydrogen abstraction from both methylene II and
methylene III in Structure I. The combination of
two possible pathways for EF formation could ex-
plain its significantly larger yield than other reaction
products. In Reaction (12a) EF is formed as an initial
product of the chemical transformation of the
Figure 2 Proposed reaction mechanism for hydroxyl radi-
cal with ethyl 3-ethoxypropionate. Major products are in
bold typeface.
CH –(CHO )–O–CH –CH –R 9:
и
3
2 2
H(O ")C–O–CH –CH –R (EFP)
CH –CH –O–(CH )–CH –R radical. But in Reac-
и
2
2
3
2
2
ϩ
и
CH3 (9b)
tion (15b), EF is formed as a daughter product of the
rest of the EEP molecule after EG formation. The
proposed mechanism supports the EF yield data, but
there is no way to differentiate the two possible EF
CH –(CHO
и
)–O–CH –CH –R ϩ O 9:
3
2
2
2
CH –C( "O)H (Acetaldehyde)
3
ϩ H(O ")C–CH –R (EFA) ϩ HO (9c)
formation processes. The disposition of the
fragment could lead to EG formation as well.
и
CH –R
2
2
2
For methylene II:
The next largest product yield was ethyl (3-formy-
loxy) propionate (EFP). The hydrogen abstraction
from methylene I and subsequent reactions are simi-
lar to what was observed in the reaction of OH with
CH –CH –O–CH –CH –R ϩ OH 9:
3
2
2
и
2
CH –CH –O–(CH
)–CH –R ϩ H O
(10)
3
2
2
2
2
-ethoxyethyl acetate (EEA) [1]. After hydrogen ab-
CH –CH –O–(CH
и
)–CH –R
straction, the rest of the parent molecule remains in-
tact, a methyl group is lost, and the formate group is
formed. Unlike what was observed with EEA, there
was no observed splitting between the diacetate form
and the formate form of the reaction product. This
may indicate the effect of the ester oxygen on the
carbon backbone. For EEA, the oxygen is on the
same side of the carbonyl as the radical site and may
have a stabilizing effect allowing for the genera-
tion of both products, while in EEP the ester oxygen
is on the other side of the carbonyl away from the hy-
drogen abstraction site preventing the stabilization of
the radical promoting the loss of the end methyl
group. The experimental technique was such that if
the diacetate had been formed it would have been
detected.
Ethyl glyoxate (EG) was the product with the
third largest yield. Reactions (12b) and (15a) provide
two possible pathways for EG formation. The struc-
ture reactivity calculation [6] predicted that 48% of
the OH rate constant value could come from hydro-
gen abstraction of methylene II and only 4.2% of the
OH rate constant value would come from methylene
3
2
2
ϩ O ϩ NO 9: NO ϩ CH – H –
2
2
3
C
2
O
ϩ H(O ")C–CH –R(EFA) (11a)
2
CH –CH –O ϩ O 9:
3
2
2
CH –C( "O)H (Acetaldehyde) ϩ HO (11b)
3
2
CH –CH –O–(CH )–CH –R
и
3
2
2
ϩ O ϩ NO 9:
2
NO ϩ CH –CH –O–C( "O)H (EF)
2
3
2
ϩ
и
CH –R
(12a)
2
и
CH –R ϩ O 9:
2 2
H(O ")C–R (EG) ϩ HO2 (12b)
CH –CH –O–(CH )–CH –R
и
3
2
2
ϩ O ϩ NO 9: NO
2
2
ϩ CH –CH –O–C( "O)–CH –R (DM)
3
2
2
(13)
For methylene III:
CH –CH –O–CH –CH –R ϩ OH 9:
3
2
2
2

6
44
BAXLEY, HENLEY, AND WELLS
III. This suggests that the Reaction (12b) is the most
likely pathway for EG formation.
BIBLIOGRAPHY
The last two products, ethyl (2-formyl) acetate and
acetaldehyde, most likely come from the hydrogen
abstraction of methylene II in Structure I. The pro-
posed branched mechanistic pathway shown in Reac-
tions 12a and b is supported by the nearly identical
yields observed for these two products. The carbon
skeletal molecular structure of the two products can
be summed together to form an EEP molecule which
also supports the proposed mechanism.
1. J. R. Wells, F. L. Wiseman, D. C. Williams, J. S. Bax-
ley, and D. F. Smith, Int. J. Chem. Kinet., 28, 475
(1996).
2
3
. D. F. Smith, C. D. McIver, and T. E. Kleindienst, Int. J.
Chem. Kinet., 27, 453 (1995).
. D. F. Smith, T. E. Kleindienst, E. E. Hudgens, C. D.
McIver, and J. J. Bufalini, Int. J. Chem. Kinet., 24, 199
(1992).
4
. T. J. Wallington, J. M. Andino, A. R. Potts, S. J. Rudy,
W. O. Siegl, Z. Zhang, M. J. Kurylo, and R. E. Huie,
Environ. Sci. Technol., 27, 98 (1993).
The diethyl malonate observed in very low yield
most likely came from the hydrogen abstraction of
methylene II. The retention of the entire molecular
5. M. Veillerot, P. Foster, R. Guillermo, and J. C. Galloo,
Int. J. Chem. Kinet., 28, 235 (1996).
“
skeleton” of EEP is evident in DM’s structure, and
6. E. S. C. Kwok and R. Atkinson, Atmos. Environ., 29,
1
685 (1995).
. W. P. L. Carter, J. Air and Waste Man. Assoc., 44, 881
1994).
supports its proposed formation.
7
The linear relationship observed between products
formed vs. EEP reacted indicates that EFP, EFA,
EG, EF, and acetaldehyde are not lost or produced
by any other side reactions. Using the atmospheric re-
action mechanism proposed in Figure 2 and Reac-
tions (8)–(16) and the product yields, approximately
(
8
9
. J. H. Seinfeld, Science, 243, 745 (1989).
. R. Atkinson, W. P. L. Carter, A. M. Winer, and J. N.
Pitts, Jr., J. Air Pol. Control, 31, 1090 (1981).
1
1
0. L. N. O’rji and D. A. Stone, Int. J. Chem. Kinet., 24,
03 (1992).
1. D. C. Williams, L. N. O’rji, and D. A. Stone, Int. J.
Chem. Kinet., 25, 539 (1993).
7
7
2% of the carbon of the total EEP reacted is ac-
counted. The primary error occurring from the bal-
ance of products not observed following the forma-
tion of ethyl formate in Reaction (12a) and (15b).
12. D. F. Smith, T. E. Kleindienst, E. E. Hudgens, and J. J.
Bufalini, Intern J. Environ. Anal. Chem., 54, 265
(1994).
1
1
3. J. U. White, J. Opt. Soc. Am., 32, 285 (1942).
4. D. F. Smith, T. E. Kleindienst, and E. E. Hudgens, J.
Chromatog., 483, 431 (1989).
CONCLUSION
1
5. W. D. Taylor, D. Allston, M. J. Moscato, G. D.
Fazekas, R. Kozlowski, and G. A. Takacs, Int. J. Chem.
Kinet., 12, 231 (1980).
The OH radical abstracts hydrogen principally from
three methylene groups on EEP. The EEP/OH bimo-
lecular rate constant, kEEP , was measured using the
relative rate technique. A value for kEEP of
16. R. Atkinson, J. Phys. Chem. Ref Data, Monograph
No. 2, 1994.
17. D. F. Smith, C. D. McIver, and T. E. Kleindienst, Int. J.
Chem. Kinet., 27, 453 (1995).
Ϫ12
3
Ϫ1 Ϫ1
(
22.9 Ϯ 7.4) ϫ 10 cm molecule s
was ob-
6
Ϫ3
served; using [OH] of 1 ϫ 10 molecules cm yields
an atmospheric (1/e) lifetime for EEP of approxi-
mately 12 h.
1
8. R. Atkinson, S. M. Aschmann, W. P. L. Carter, A. M.
Winer, and J. N. Pitts, Jr., J. Phys. Chem., 86, 4563
(1982).
The identification and quantitation of the
OH/EEP reaction products led to the development of
a detailed atmospheric reaction mechanism. The ob-
served yields for EF, EFP, EFA, EG, and acetalde-
hyde are 37 Ϯ 1%, 30 Ϯ 1%, 4.8 Ϯ 0.2%, 25 Ϯ 1
1
2
2
9. T. J. Wallington, P. Dagaut, R. Liu, and M. J. Kurylo,
Int. J. Chem. Kinet., 27, 177 (1988).
0. H. Niki, P. D. Maker, C. M. Savage, and L. P. Breiten-
bach, J. Phys. Chem., 82, 132 (1978).
1. R. Atkinson and J. N. Pitts, Jr., J. Chem. Phys., 68,
%, and 4.9 Ϯ 0.2% respectively. Analysis of product
3
581 (1978).
yield and product molecular structure resulted in a
proposed atmospheric reaction mechanism for EEP.
22. J. V. Michael, D. G. Keil, and R. B. Klemm, J. Chem.
Phys., 83, 1630 (1985).