Products of the gas-phase reactions of the OH radical with 1-methoxy-2-propanol and 2-butoxyethanol

Article abstract of DOI:10.1021/es980455c

Glycol ethers are used as solvents and are hence liable to b;e released to the atmosphere, where they react and contribute to the formation of photochemical air pollution. In this work, products of the gas-phase reactions of the OH radical with 1-methoxy-2-propanol and 2-butoxyethanol in the presence of NO have been investigated at 298 ± 2 K and 740 Torr total pressure of air by gas chromatography, in situ Fourier transform infrared spectroscopy, and in situ atmospheric pressure ionization tandem mass spectrometry. The products observed from 1-methoxy-2-propanol were methyl formate, methoxyacetone, and acetaldehyde with molar formation yields of 0.59 ± 0.05, 0.39 ± 0.04, and 0.56 ± 0.07, respectively. The products observed and quantified from 2-butoxyethanol were n-butyl formate, 2-hydroxyethyl formate, propanal, 3-hydroxybutyl formate, and an organic nitrate (attributed to CHsChbCHaCH₂OCH(ON0₂)CH₂OH and its isomers), with molar formation yields of 0.57 ± 0.05, 0.22 ± 0.05, 0.21 ± 0.02, 0.07 ± 0.03, and 0.10 ± 0.03, respectively. An additional product of molecular weight 132, attributed to one or more hydroxycarbonyl products, was also observed from the 2-butoxyethanol reaction by atmospheric pressure ionization mass spectrometry. For both glycol ethers, the majority of the reaction products and reaction pathways are accounted for, and detailed reaction mechanisms are presented which account for the observed products.

Full text of DOI:10.1021/es980455c

Environ. Sci. Technol. 1998, 32, 3336-3345
have been few reported studies of the products and mech-
Products of the Gas-Phase
Reactions of the OH Radical with
1-Methoxy-2-propanol and
2-Butoxyethanol
anisms of the OH radical-initiated reactions of glycol ethers,
with product studies having been reported for 2-ethoxy-
ethanol (14) and 2-butoxyethanol (15). As part of an overall
study of the tropospheric chemistry of the glycol ethers
1-methoxy-2-propanol [CH3CH(OH)CH2OCH3] and 2-bu-
toxyethanol [CH3CH2CH2CH2OCH2CH2OH], we have inves-
tigated the products of the reactions of these two glycol ethers
with OH radicals in the presence of NO. The kinetics of the
reactions of 1-methoxy-2-propanol and 2-butoxyethanol with
OH radicals, NO3 radicals, and O3 have been reported
elsewhere (12).
E R N E S T O C . T U A Z O N ,
S A R A M . A S C H M A N N , A N D
R O G E R A T K I N S O N * , †
Air Pollution Research Center, University of California,
Riverside, California 92521
Experimental Section
Experiments were carried out at 298 ( 2 K and 740 Torr total
pressure of air in a 5870-L evacuable, Teflon-coated chamber
containing an in situ multiple reflection optical system
interfaced to a Nicolet 7199 Fourier transform infrared (FT-
IR) absorption spectrometer and with irradiation provided
by a 24 kW xenon arc filtered through a 0.25 in. thick Pyrex
pane (to remove wavelengths <300 nm); in a 7900-L Teflon
chamber with analysis by gas chromatography with flame
ionization detection (GC-FID) and combined gas chroma-
tography-mass spectrometry (GC-MS), with irradiation
provided by two parallel banks of blacklamps; and in a 7500-L
Teflon chamber interfaced to a PE SCIEX API III MS/ MS
direct air sampling, atmospheric pressure ionization tandem
mass spectrometer (API-MS), again with irradiation provided
by two parallel banks of blacklamps. All three chambers are
fitted with Teflon-coated fans to ensure rapid mixing of
reactants during their introduction into the chamber.
Hydroxyl radicals were generated in the presence of NO
by the photolysis of methyl nitrite (CH3ONO) in air at
wavelengths >300 nm (12,16):
Glycol ethers are used as solvents and are hence liable
to be released to the atmosphere, where they react and
contribute to the formation of photochemical air pollution.
In this work, products of the gas-phase reactions of the
OH radical with 1-methoxy-2-propanol and 2-butoxyethanol
in the presence of NO have been investigated at 298 (
2 K and 740 Torr total pressure of air by gas chromatography,
in situ Fourier transform infrared spectroscopy, and in
situ atmospheric pressure ionization tandem mass
spectrometry. The products observed from 1-methoxy-2-
propanol were methyl formate, methoxyacetone, and
acetaldehyde with molar formation yields of 0.59 ( 0.05,
0.39 ( 0.04, and 0.56 ( 0.07, respectively. The products
observed and quantified from 2-butoxyethanol were n-butyl
formate, 2-hydroxyethyl formate, propanal, 3-hydroxybutyl
formate, and an organic nitrate (attributed to CH CH CH -
3
2
2
CH OCH(ONO )CH OH and its isomers), with molar formation
2
2
2
CH3ONO + hν f CH3O• + NO
(1)
yields of 0.57 ( 0.05, 0.22 ( 0.05, 0.21 ( 0.02, 0.07 (
0.03, and 0.10 ( 0.03, respectively. An additional product
of molecular weight 132, attributed to one or more
hydroxycarbonyl products, was also observed from the
2-butoxyethanol reaction by atmospheric pressure ionization
mass spectrometry. For both glycol ethers, the majority
of the reaction products and reaction pathways are accounted
for, and detailed reaction mechanisms are presented
which account for the observed products.
CH3O• + O2 f HCHO + HO2
HO2 + NO f OH + NO2
(2)
(3)
and NO was added to the reactant mixtures to suppress the
formation of O3 and hence of NO3 radicals.
Teflon Cham ber with Analysis by GC-FID. For the
experiments carried out in the 7900-L Teflon chamber (at
∼5% relative humidity), the initial reactant concentrations
(in units of molecules per cubic centimeter, molecule cm-3
)
Introduction
were CH3ONO, (2.2-2.3) × 1014; NO, (1.9-2.3) × 1014; and
1-methoxy-2-propanol, (2.27-2.36) × 1013, or 2-butoxyetha-
nol, (2.25-2.55) × 1013. [Note that 1 part per million (ppm)
mixing ratio ) 2.40 × 1013 molecule cm-3 at 298 K and 740
Torr total pressure.] Irradiations were carried out at 20% of
the maximum light intensity for 4-15 min (1-methoxy-2-
propanol) or 3-12 min (2-butoxyethanol), resulting in up to
52% and 59% reaction of the initial 1-methoxy-2-propanol
and 2-butoxyethanol, respectively. The concentrations of
the glycol ethers and selected products were measured during
the experiments by GC-FID. Gas samples of 100 cm3 were
collected from the chamber onto Tenax-TA solid adsorbent,
with subsequent thermal desorption at ∼225 °C onto a DB-
1701 megabore column in a Hewlett-Packard (HP) 5710 GC,
initially held at -40, -20, or 0 °C, depending on the products
to be analyzed, and then temperature programmed to 200
°C at 8 °C min-1. Gas samples were also collected from the
chamber onto Tenax-TA solid adsorbent for thermal de-
sorption with analysis by GC-MS, using a 50 m HP-5 fused
Volatile organic compounds present in the atmosphere can
undergo photolysis and chemical reaction with OH radicals,
NO3 radicals, and O3 (1, 2), with the OH radical reaction
being an important, and often dominant, atmospheric loss
process (1, 2). Glycol ethers are used as solvents (3, 4) and
are hence liable to be released into the atmosphere where
they may contribute to the formation of photochemical air
pollution in urban and regional areas (5). Glycol ethers react
with OH radicals (6-12) and NO3 radicals (12, 13), with the
OH radical reactions being calculated to be the dominant
tropospheric loss process and with calculated lifetimes of
approximately 1 day or less (11, 12). However, to date there
* Corresponding author e-mail: ratkins@mail.vcr.edu; phone:
(909)787-4191; fax: (909)787-5004.
† Also Interdepartmental Program in Environmental Toxicology,
Department of Environmental Sciences, and Department of Chem-
istry.
9
3 3 3 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 21, 1998
S0013-936X(98)00455-6 CCC: $15.00
1998 Am erican Chem ical Society
Published on Web 09/15/1998

silica capillary column in an HP 5890 GC interfaced to a HP
5971 mass selective detector operating in the scanning mode.
GC-FID response factors were determined as previously
described (17).
Evacuable Cham ber Experim ents with FT-IR Analysis.
For the experiments carried out in the 5870-L evacuable,
Teflon-coated chamber (at <1% relative humidity), the initial
concentrations for all runs were 2.5 × 1014 molecule cm-3
each of CH3ONO, NO, and 1-methoxy-2-propanol or 2-bu-
toxyethanol. Vapors of 1-methoxy-2-propanol, methyl ni-
trite, and nitric oxide were measured by a capacitance
manometer (MKS Baratron, 100 Torr sensor) into calibrated
5 and 2-L Pyrex bulbs and introduced into the chamber by
flushing the contents of the bulbs with N2 gas. Vapors of
2-butoxyethanol were introduced into the chamber by
heating a weighed amount of the liquid sample and flushing
with a stream of heated N2 gas. IR spectra of the reaction
mixtures were recorded prior to and during irradiation with
64 scans (corresponding to 2.0-min averaging time) per
spectrum, a full-width-at-half-maximum resolution of 0.7
cm-1, and a path length of 62.9 m (16, 18). The mixtures
were irradiated continuously for a 33-38-min period with
FT-IR monitoring every 2-4 min. One experiment with a
1-methoxy-2-propanol-CH3ONO-NO-air mixture was car-
ried out with concurrent FT-IR and GC-FID measurements
and employed intermittent irradiation to allow for GC
sampling during the intervening dark periods.
FIGURE 1. IR spectra of ethylene glycol diformate, 2-hydroxyethyl
formate, and peroxypropionyl nitrate (PPN). The numbers in
parentheses are equivalent concentrations in units of 1013 molecule
cm-3 for a 62.9-m path length (see text).
Peroxypropionyl nitrate [CH3CH2C(O)OONO2; PPN] was
prepared from the Cl atom-initiated photooxidation of
propanal in the presence of NO2 [with initial concentrations
(in units of 1014 molecule cm-3) of propanal, 2.46; NO2, 1.61;
and Cl2, 9.84]. The spectrum of PPN was derived from that
of the reaction mixture by spectral desynthesis [stepwise
subtraction of absorptions by known components (16)]. The
calibration obtained, based on the amount of propanal which
reacted during the early part of the photolysis (e15%
reaction), was in good agreement with the IR absorptivities
published by Stephens (21), and an infrared spectrum of
peroxypropionyl nitrate is presented in Figure 1.
Teflon Cham ber with Analysis by API-MS. In the
experiments with API-MS analyses, the chamber contents
were sampled through a 25-mm-diameter × 75-cm-length
Pyrex tube at ∼20 L min-1 directly into the API mass
spectrometer source. The operation of the API-MS in the
MS (scanning) and MS/ MS [with collision activated dis-
sociation (CAD)] modes has been described elsewhere (19).
Use of the MS/ MS mode with CAD allows the “daughter ion”
or “parent ion” spectrum of a given ion peak observed in the
MS scanning mode to be obtained (19). The positive ion
mode was used in these API-MS and API-MS/ MS analyses,
with protonated water hydrates (H3O+(H2O)n) generated by
the corona discharge in the chamber diluent gas being
responsible for the protonation of analytes. Ions are drawn
by an electric potential from the ion source through the
sampling orifice into the mass-analyzing first quadrupole or
third quadrupole. For these experiments, the API-MS
instrument was operated under conditions that favored the
formation of dimer ions in the ion source region (19). Neutral
molecules and particles are prevented from entering the
orifice by a flow of high-purity nitrogen (“curtain” gas), and
as a result of the declustering action of the curtain gas on the
hydrated ions, the ions that are mass-analyzed are mainly
protonated molecular ions ([M+H]+) and their protonated
homo- and heterodimers (19). The initial concentrations of
CH3ONO, NO, and 1-methoxy-2-propanol or 2-butoxyethanol
were ∼4.8 × 1013 molecule cm-3 each, and irradiations were
carried out for 5 min at 20% of the maximum light intensity,
resulting in a 33% reaction of the initially present 1-methoxy-
2-propanol (as measured by GC-FID) and a 40-45% reaction
of the initially present 2-butoxyethanol (estimated from the
1-methoxy-2-propanol experiment carried out with similar
initial CH3ONO and NO concentrations and light intensity,
and hence a similar OH radical concentration).
2-Hydroxyethyl formate [ethylene glycol monoformate;
HOCH2CH2OCHO] was prepared by room-temperature, acid-
catalyzed reaction of formic acid with a 20-fold excess of
ethylene glycol. The reaction mixture was extracted with
benzene and, with IR spectroscopic monitoring of the vapors
sampled in a 25-cm path length cell, top fractions were
successively discarded in a vacuum line until the absorption
bands of benzene, water vapor, and formic acid were no
longer detectable. From a middle fraction of the remaining
liquid, vapor samples were obtained which were mixtures of
2-hydroxyethyl formate and ethylene glycol diformate of
varying concentration ratios but with no detectable ethylene
glycol component. Continued extraction of the unequilib-
riated vapor above the liquid sample ultimately yielded only
vapors of the diformate, thus allowing quantitative IR spectra
of ethylene glycol diformate to be recorded. This, therefore,
enabled quantitative spectra of 2-hydroxyethyl formate to
be derived from its mixtures with the diformate. For example,
three successive vapor samples from a separate preparation
were found to contain 69%, 81%, and 89% 2-hydroxyethyl
formate, based on the measured pressures of the vapor
samples and the calibrated IR reference spectrum of ethylene
glycol diformate. An iterative spectral subtraction, initially
based on the 1163 cm-1 band of the diformate, allowed the
distinct spectrum of the monoformate to be revealed and
derived from that of the two-component mixture, and
calibrated spectra of both compounds are shown in Figure
1. Individual analyses of consecutive vapor samples from
the monoformate preparation were essential in obtaining
the above calibrations, since the liquid fraction underwent
changes in composition upon prolonged standing at room
temperature because of the tendency to reestablish an
equilibrium composition between the monoformate, difor-
mate, and ethylene glycol (14). The distinct 1756.8 cm-1
band of 2-hydroxyethyl formate vapor shown in Figure 1
Chem icals. The chemicals used, and their stated purities
(in parentheses), were acetaldehyde (99.5+%), butanal (99%),
2-butoxyethanol (99+%), n-butyl formate (97%), methoxy-
acetone (97%), 1-methoxy-2-propanol (98%), methyl formate
(99%), propanal (97%), and n-propyl nitrate (97%), Aldrich
Chemical Co.; and NO (g 99.0%), Matheson Gas Products.
Methyl nitrite was prepared as described by Taylor et al.
(20).
9
VOL. 32, NO. 21, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 3 3 7

was verified to have an exact correspondence in contour and
position with that of a product formed during photolysis of
a CH3OCH2CH2OH-Cl2-air mixture, where HOCH2CH2-
OCHO is one of the expected products.
3-Hydroxybutyl formate was prepared in a mixture with
its coproducts 3-hydroxy-1-methylpropyl formate and 1,3-
butanediol diformate by the acid-catalyzed reaction of formic
acid with a 10-fold excess of 1,3-butanediol and subsequent
benzene extraction of the mixture. A benzene extract of the
reaction mixture was fractionated in a vacuum line as
described above for 2-hydroxyethyl formate. Vapor samples
were extracted which contained only a mixture of formates,
as indicated by two dominant IR absorption bands at 1184
and 1747 cm-1, but further fractionation of the components
could not be achieved. The gas chromatogram of a liquid
sample injected into the 5870-L chamber showed four peaks
with retention times later than that of 2-butoxyethanol (also
added to the mixture as a retention time marker) by 1.53,
1.86, 2.07 and 2.61 min, with the first peak corresponding to
41% (by weight) of 1,3-butanediol, and with the other three
peaks corresponding, respectively, to 3-hydroxybutyl formate,
3-hydroxy-1-methylpropyl formate, and 1,3-butanediol di-
formate. The GC peak at 1.86 min beyond that of 2-bu-
toxyethanol was the only one which corresponded to GC
peaks in GC-FID analyses of irradiated CH3ONO-NO-2-
butoxyethanol-air mixtures and was the basis of the as-
signment to 3-hydroxybutyl formate and, hence, of the other
two peaks. The relative molar fractions of the three
components were calculated from the GC peak areas using
their calculated Effective Carbon Numbers (ECNs) (22), and
for the above liquid sample were 0.66, 0.27, and 0.07 for
3-hydroxybutyl formate, 3-hydroxy-1-methylpropyl formate,
and 1,3-butanediol diformate, respectively. The GC response
factor for 3-hydroxybutyl formate obtained from these
analyses of weighed liquid sample injections was in excellent
agreement with an average of the response factors obtained
relative to the measured response factors for 2-butoxyethanol
and n-butyl formate using the calculated ECNs for 2-bu-
toxyethanol, n-butyl formate and 3-hydroxybutyl formate
(22).
FIGURE 2. Plots of the amounts of methyl formate and methoxy-
acetone formed, corrected for reactions with the OH radical, against
the amounts of 1-methoxy-2-propanol reacted with the OH radical
in the presence of NO, with analyses by GC-FID. The data for methyl
formate have been displaced vertically by 1 × 1012 molecule cm-3
for clarity.
pyl formate mixture was within ∼0.06 min of a product peak
from the irradiated CH3ONO-NO-2-butoxyethanol-air
mixtures, and we use this GC-FID peak to derive an upper
limit for the 2-hydroxybutyl formate formation yield.
The products observed and quantified also react with the
OH radical (1, 2, 23), and hence secondary reactions of the
products with the OH radical were taken into account as
described previously (24) using OH radical reaction rate
constants of (in units of 10-12 cm3 molecule-1 s-1) 1-methoxy-
2-propanol, 20.9 (12); 2-butoxyethanol, 29.4 (12); acetalde-
hyde, 15.8 (1); propanal, 19.6 (1); butanal, 23.5 (1); methyl
formate, 0.200 (25, 26); methoxyacetone, 6.8 (8); n-butyl
formate, 3.33 (25, 26); 2-hydroxybutyl formate, 13.6 [esti-
mated] (27); and 3-hydroxybutyl formate, 11.9 [estimated]
(27). The multiplicative correction factors F to take into
account secondary reactions increase with the rate constant
ratio k(OH + product)/ k(OH + reactant) and with the extent
of reaction (24). The maximum values of F were 1.35 for
acetaldehyde, <1.01 for methyl formate, and 1.14 for meth-
oxyacetone formation from 1-methoxy-2-propanol, and were
1.06 for n-butyl formate, 1.43 for butanal, 1.35 for propanal,
1.25 for 2-hydroxybutyl formate, and 1.22 for 3-hydroxybutyl
formate formation from 2-butoxyethanol.
2-Hydroxybutyl formate was synthesized by an analogous
procedure from 1,2-butanediol, and GC-FID analyses of the
synthesized mixture (again with added 2-butoxyethanol as
a retention time marker) showed GC peaks with retention
times 0.70, 1.42, and 2.18 min later than that of 2-butoxy-
ethanol, with the first of these being 1,2-butanediol. These
GC-FID analyses show that 2-hydroxybutyl formate and
3-hydroxybutyl formate do not coelute on the DB-1701
column used here.
Figure 2 shows representative plots of the amounts of
methyl formate and methoxyacetone formed, corrected for
reaction with the OH radical, against the amounts of
1-methoxy-2-propanol reacted, and Figure 3 shows similar
plots for the formation of n-butyl formate and propanal from
2-butoxyethanol. The formation yields of the products
quantified as obtained by least-squares analyses of plots such
as those shown in Figures 2 and 3 are given in Table 1 (which
also contains an upper limit to the formation yield for
butanal). Two independent sets of experiments were carried
out to measure the formation yields of methyl formate and
methoxyacetone from the 1-methoxy-2-propanol reaction
and, as shown in Table 1, the agreement between the two
measured methyl formate and methoxyacetone formation
yields is excellent.
Results
Teflon Cham ber with Analysis by GC-FID. GC-FID and GC-
MS analyses of irradiated CH3ONO-NO-1-methoxy-2-
propanol-air and CH3ONO-NO-2-butoxyethanol-air mix-
tures showed, by matching of GC retention times and (apart
from 2-hydroxyethyl formate and 3-hydroxybutyl formate)
mass spectra with those of authentic standards, the formation
of methyl formate [CH3OCHO], acetaldehyde and methoxy-
acetone [CH3C(O)CH2OCH3] from 1-methoxy-2-propanol,
and n-butyl formate, propanal, 2-hydroxyethyl formate, and
3-hydroxybutyl formate from 2-butoxyethanol. Because of
difficulties in obtaining a measured GC-FID response factor
for 2-hydroxyethyl formate, this product was not quantified
from the GC-FID analyses (see FT-IR data below). It should
be noted, however, that using a GC-FID response factor
obtained from the calculated ECN for 2-hydroxyethyl formate
(22) resulted in a 2-hydroxyethyl formate formation yield in
good agreement with that determined by FT-IR spectroscopic
analyses. One of the peaks in the GC-FID analyses of the
synthesized 2-hydroxybutyl formate-1-hydroxymethyl-pro-
Evacuable Cham ber Experim ents with FT-IR Analysis.
In situ FT-IR spectroscopic analyses of irradiated CH3ONO-
NO-1-methoxy-2-propanol-air mixtures showed the for-
mation of methyl formate, acetaldehyde, and methoxy-
acetone as the major products. The formation of these
products, together with peroxyacetyl nitrate (PAN), in the
9
3 3 3 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 21, 1998

TABLE 1. Products and Their Formation Yields Observed from the Reactions of the OH Radical with 1-Methoxy-2-propanol and
2-Butoxyethanol in the Presence of NO at 298 ( 2 K and 740 Torr Total Pressure of Air
formation yield
a
b
glycol ether
product
GC-FID
FT-IR
1-m ethoxy-2-propanol
m ethyl form ate
0.579 ( 0.078c
0.569 ( 0.079c
0.58 ( 0.08d
0.49 ( 0.13
0.59 ( 0.05
acetaldehyde
m ethoxyacetone
0.58 ( 0.08e
0.39 ( 0.05
0.396 ( 0.056c
0.399 ( 0.039c
0.39 ( 0.05d
0.55 ( 0.06
2-butoxyethanol
n-butyl form ate
butanal
propanal
2-hydroxyethyl form ate
3-hydroxybutyl form ate
2-hydroxybutyl form ate
organic nitrate
0.60 ( 0.07
<0.01
0.21 ( 0.02
0.21 ( 0.08e
0.22 ( 0.05
0.07 ( 0.03
<0.03
0.10 ( 0.03e
a
Indicated errors are two least-squares standard deviations com bined with estim ated overall uncertainties in the GC-FID response factors for
the glycol ethers and products of (5% each, except for 3-hydroxybutyl form ate for which the uncertainty in the response factor is estim ated to
b
be (30%. Indicated errors are two least-squares standard deviations com bined with the estim ated errors arising from the m easurem ent uncertainties
of the glycol ethers and products; i.e., (6% for 1-m ethoxy-2-propanol, 2-butoxyethanol, and m ethoxyacetone; (3% for m ethyl form ate; and (15%
c
d
for 2-hydroxyethyl form ate. Independent sets of experim ents, each with separately m easured GC-FID calibration factors. For com bined data
sets. See text.
e
FIGURE 3. Plots of the amounts of n-butyl formate and propanal
FIGURE 4. (A) Product spectrum from the photolysis of a 1-methoxy-
formed, corrected for reactions with the OH radical, against the
2-propanol-CH ONO-NO-air mixture after 25 min of irradiation
3
amounts of 2-butoxyethanol reacted with the OH radical in the
presence of NO, with analyses by GC-FID.
and 47% conversion of the initial 2.46 × 1014 molecule cm-3 of
1-methoxy-2-propanol, absorption byproducts from CH ONO and NO
3
irradiated mixture is evident in the series of IR spectra
presented in Figure 4, where absorptions by the remaining
reactants and by the products arising primarily from the
photooxidation of CH3ONO and NO (i.e., NO2, HNO3, HONO,
HCHO, HC(O)OH, and CH3ONO2) have been subtracted for
clarity. Although the acetaldehyde absorption band at 1746
cm-1 is qualitatively seen in the product spectra (Figure 4D),
its intensity is relatively weak and could not be reliably
subtracted from an unknown residual band. Measurements
of acetaldehyde by GC-FID in a specific experiment are
described below.
Figure 5 shows plots of the methyl formate and meth-
oxyacetone concentrations (combined data from two experi-
ments), corrected for reaction with the OH radical, against
the amounts of 1-methoxy-2-propanol reacted. The maxi-
mum values of the multiplicative factor F to correct for
secondary reactions of the products with the OH radical
during a 38-min irradiation were <1.01 and 1.17 for methyl
formate and methoxyacetone, respectively. Least-squares
analyses of these data lead to the methyl formate and
methoxyacetone formation yields given in Table 1.
subtracted, (B) after subtraction of absorptions by methyl formate,
(C) after subtraction of absorptions by methoxyacetone, and (D)
after subtraction of absorptions by peroxyacetyl nitrate (PAN). The
numbers in parentheses are concentrations in units of 1013 molecule
cm-3.
Although acetaldehyde data were not obtained from these
particular experiments, an additional experiment with in-
termittent irradiation (see Experimental Section) was carried
out which employed both FT-IR and GC-FID analyses, with
acetaldehyde being quantified by GC-FID. The FT-IR
analyses of 1-methoxy-2-propanol and the GC-FID analyses
of acetaldehyde resulted in acetaldehyde formation yields,
corrected for reaction with the OH radical, of 59% after 8.0
min and 58% after 18.0 min of irradiation, with multiplicative
correction factors F of 1.13 and 1.24, respectively. These
acetaldehyde yields were verified to be consistent with those
estimated by FT-IR analysis from residual spectra such as
that shown in Figure 4D, and the acetaldehyde yield from
this experiment is given in Table 1 in the column labeled
“FT-IR”. The PAN concentration profile for the experiments
9
VOL. 32, NO. 21, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 3 3 9

FIGURE 7. Plots of the amounts of products formed against the
amounts of 2-butoxyethanol reacted with OH radicals. The n-butyl
formate and 2-hydroxyethyl formate concentrations have been
corrected for secondary reactions with OH radicals. Analyses were
by FT-IR spectroscopy.
FIGURE 5. Plots of the amounts of products formed against the
amounts of 1-methoxy-2-propanol reacted with OH radicals. The
methyl formate and methoxyacetone concentrations have been
corrected for secondary reactions with OH radicals. Analyses were
by FT-IR spectroscopy.
CH2C(O)OONO2; PPN]. Estimates of RONO2 concentrations
were made from the integrated band area in the range 1255-
1315 cm-1 (after subtraction of all identifiable species,
including PPN) based on an average integrated absorption
coefficient of 1.2 ×10-17 cm molecule-1 (base 10) from RONO2
compounds (16). The analyses of PPN were based on the
1835 cm-1 absorption band but should be considered as
upper limit estimates since they possibly include other
peroxyacyl nitrates, hence the use of the label “PPN” in the
following sections. The residual spectra contained other
weak absorptions by an unidentified product(s), including
a band(s) in the CdO stretch region (with some contribution
from 3-hydroxybutyl formate) which remained after sub-
traction of absorptions by propanal (see API-MS analyses,
below).
The amounts of n-butyl formate and 2-hydroxyethyl
formate formed, corrected for secondary reactions with the
OH radical, are plotted in Figure 7 against the amounts of
2-butoxyethanol reacted. A rate constant for reaction of the
OH radical with 2-hydroxyethyl formate of 6.0 × 10-12 cm3
molecule-1 s-1 was estimated (27) and used to correct for
secondary reactions. For the 33-min experiment represented
in Figure 7, the calculated maximum values of F were 1.05,
1.08, and 1.29 for n-butyl formate, 2-hydroxyethyl formate,
and propanal, respectively. Least-squares analyses of the
data shown in Figure 7 lead to the product formation yields
shown in Table 1. The series of spectra recorded near the
end of the irradiation enabled three measurements of the
propanal concentration to be made, resulting in corrected
propanal yields of 0.19-0.23, with an average of 0.21 (( 40%
estimated uncertainty) [Table 1]. The RONO2 yield has been
estimated as 0.10 ( 0.03, and this is also given in Table 1.
The plot of the measured “PPN” concentrations against the
amounts of 2-butoxyethanol reacted (Figure 7) shows that
“PPN” is formed as a result of secondary chemistry, with the
measured “PPN” concentrations corresponding to a forma-
tion yield of 1-7% for the experimental conditions employed.
FIGURE 6. (A) Product spectrum from the photolysis of a 2-bu-
toxyethanol-CH ONO-NO-air mixture after 29 min of irradiation
3
and 50% conversion of the initial 2.51 × 1014 molecule cm-3 of
2-butoxyethanol, absorptions byproducts from CH ONO and NO
3
subtracted, (B) after subtraction of absorptions by n-butyl formate,
(C) after subtraction of absorptions by peroxypropionyl nitrate (PPN)
and 2-hydroxyethyl formate. The numbers in parentheses are
concentrations in units of 1013 molecule cm-3. The absorption bands
marked by an asterisk are attributed to organic nitrates, RONO
2
(see text).
summarized in Figure 5 show that PAN is formed as a result
of secondary chemistry, with the measured PAN concentra-
tions corresponding to a formation yield of 3-11% for the
experimental conditions employed.
FT-IR spectroscopic analyses of irradiated CH3ONO-NO-
2-butoxyethanol-air mixtures showed the formation of
n-butyl formate and 2-hydroxyethyl formate as the major
products, as illustrated in Figure 6 (absorptions due to
remaining reactants and to the products from the photo-
oxidation of CH3ONO and NO have been subtracted, as
discussed above). Propanal was also observed as a product,
but could only be measured by FT-IR analysis near the end
of the photolysis when sufficiently high concentrations were
present. Other products observed were organic nitrate(s)
(RONO2), as indicated by absorption bands very similar to
those of n-propyl nitrate, and peroxyacyl nitrate(s), which
could be mostly comprised of peroxypropionyl nitrate [CH3-
Teflon Cham ber with Analyses by API-MS. Experiments
were carried out using API-MS for analysis of the products
formed from the reactions of the OH radical with 1-methoxy-
2-propanol and 2-butoxyethanol in the presence of NO. API-
MS/ MS “daughter ion” and “parent ion” spectra were
obtained for ion peaks observed in the API-MS analyses.
Product ion peaks were identified based on the observation
9
3 3 4 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 21, 1998

TABLE 2. Products Formed from the Gas-Phase Reactions of the OH Radical with 1-Methoxy-2-propanol and 2-Butoxyethanol in
the Presence of NO, As Observed by API-MS and API-MS/MS Analyses
product
API-MS data
other evidence
1-Methoxy-2-propanol, MMP (MW 90)
m ethyl form ate
M1 (MW 60) M1 + H ) 61
MS/MS of weak 61 u ion peak identical to that of
authentic standard; parents of 61 u ion observed
at 149 [M1 + M2 + H], 151 [M1 + MMP + H], and
181 [M1 + M1 + M1 + H] u.
MS/MS of 89 u ion peak identical to that of
authentic standard; MS/MS of 179 u ion peak;
parents of 89 u ion observed at
m ethoxyacetone
M2 (MW 88) M2 + H ) 89
M2 + MMP + H ) 179
M2 + MMP + H + H2O ) 197
M2 + MMP + H + 2H2O ) 215
177 [M2 + M2 + H], 179, 197, and 215 u.
2-Butoxyethanol, MBE (MW 118)
MS/MS of 103 u ion peak consistent with that of
n-butyl form ate
M1 (MW 102) M1 + H ) 103
authentic standard; parents of 103 u ion
observed at 161 [M1 + M3 + H],
205 [M1 + M1 + H], 221 [M1 + MBE + H], and
282 [M1 + M4 + H] u.
MS/MS of 91 u ion peak identical to that of
synthesized standard; parents of 91 u ion
observed at 149 [M2 + M3 + H],
2-hydroxyethyl form ate
M2 (MW 90) M2 + H ) 91
181 [M2 + M2 + H], 193 [M2 + M1 + H],
209 [M2 + M5 + H], and 270 [M2 + M4 + H] u.
parent of 59 u ion observed at 238 [M3 + M4 + H] u.
propanal
nitrate
M3 (MW 58) M3 + H ) 59
M4 (MW 179) M4 + H ) 180
+
MS/MS of 180 u ion peak showing strong NO2
M4 + MBE + H ) 298
M4 + M5 + H ) 312
CH3CH2C(O)CH2OCH2CH2OH M5 (MW 132) M5 + H ) 133
or isom ers
fragm ent ion at 46 u; MS/MS of 298 u ion
peak showing [M4 + H] and [MBE + H] ions.
MS/MS of 133 u ion peak; parents of 133 u ion
observed at 251, 265, 269 [M5 + MBE + H + H2O],
and 312 u.
M5 + MBE + H ) 251
M5 + M5 + H ) 265
M5 + M4 + H ) 312
of homo- or heterodimer ions (for example, [(MP1)2+H]+,
[(MP2)2+H]+ and [MP1+MP2+H]+, where P1 and P2 are
products or the glycol ether reactants) in the API-MS/ MS
“parent ion” spectra, and consistency of the API-MS/ MS
“daughter ion” spectrum of a homo- or heterodimer ion with
the “parent ion” spectra of the various [MP+H]+ ion peaks.
Water cluster ion peaks of the product ions, [M+H+H2O]+,
were also occasionally observed. The products observed are
listed in Table 2 and the evidence for the formation of these
products, in the form of API-MS and API-MS/ MS data of
molecular ions, dominant fragment ions, and the presence
of homo- and heterodimers formed in the API-MS under the
experimental conditions employed, is also summarized in
Table 2.
The API-MS and API-MS/ MS spectra obtained from the
1-methoxy-2-propanol reaction provided evidence for the
formation of methyl formate and methoxyacetone (Table 2).
Because the 45-u ion was a fragment of the API-MS/ MS
“daughter ion” spectrum of the 91 u [M+H]+ ion of
1-methoxy-2-propanol and no obvious enhancement of the
very weak 45 u ion peak was observed in the irradiated CH3-
ONO-NO-1-methoxy-2-propanol-air mixture compared to
the unirradiated mixture, the formation of acetaldehyde could
not be confirmed by these API-MS analyses. No evidence
for the formation of other products was obtained from the
API-MS and API-MS/ MS analyses.
API-MS and API-MS/ MS spectra of the 2-butoxyethanol
reaction showed evidence for the formation of n-butyl
formate, 2-hydroxyethyl formate, propanal, an organic nitrate
of molecular weight 179, and a product of molecular weight
132 (Table 2) [note that 2-hydroxybutyl formate and 3-hy-
droxybutyl formate have the same molecular weight as
2-butoxyethanol]. The API-MS/ MS spectra of the molecular
weight 132 and 179 products are shown in Figure 8. The
presence of a prominent NO2+ fragment ion at 46 u indicates
that the product of molecular weight 179 is a nitrate of formula
C6H13NO5 [CH3CH2CH2CH(ONO2)OCH2CH2OH and/ or its
isomers (see discussion below)].
FIGURE 8. API-MS/MS CAD “daughter ion” spectra of the 180 u
(top) and 133 u (bottom) ion peaks observed in the API-MS analyses
of irradiated CH ONO-NO-2-butoxyethanol-air mixtures. The 46
3
u fragment ion in the API-MS/MS spectrum of the 180 u ion peak
is attributed to NO +.
2
9
VOL. 32, NO. 21, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 3 4 1

The observation of ion peaks at 73, 101, 117, 189, and 283
u in the API-MS spectra suggests the presence of other
products, although some of these ion peaks are fragments
of [M+H]+ ion peaks of the products M1 through M5 or of
their homo- and heterodimers (for example, the 189 u ion
peak was a weak fragment of the 251 u heterodimer). The
ion peak at 73 u could be the [M+H]+ of butanal, and an
API-MS/ MS spectrum of the 283 u ion peak suggested a
heterodimer of 2-butoxyethanol and a product of molecular
weight 164. However, no additional evidence in the form of
homo- or heterodimers of these molecular weight species
was obtained.
calculated fractions of the OH radical reaction with 1-meth-
oxy-2-propanol proceeding by H-atom abstraction from the
-OCH3, >CH-, -CH2-, -CH3, and -OH groups of 6%, 43%,
50%, <1%, and <1%, respectively. This prediction (27) of
43% and 50% of the overall reaction proceeding by initial
H-atom abstraction from the >CH- and -CH2- groups,
respectively, is in good agreement with respective percentages
of 39% and 59% obtained from the products observed and
their formation yields.
2-Butoxyethanol Reaction. The products observed by
GC-FID, FT-IR spectroscopy, and API-MS analyses were
n-butyl formate, 2-hydroxyethyl formate, propanal, 3-hy-
droxybutyl formate, an organic nitrate of molecular weight
179 (FT-IR and API-MS analyses), and a product of molecular
weight 132 (API-MS analyses) which may account for part of
the observed C-O stretch absorption in the FT-IR spectrum.
The formation yields of n-butyl formate and propanal
measured by GC-FID and FT-IR analyses are in excellent
agreement (Table 1), with formation yields of 0.57 ( 0.05
and 0.21 ( 0.02, respectively. The formation yield of
2-hydroxyethyl formate is essentially identical to that of
propanal, consistent with these two products being formed
from the same initial reaction pathway.
Discussion
1-Methoxy-2-propanol Reaction. The products observed
by GC-FID, FT-IR spectroscopy, and API-MS analyses were
methyl formate, methoxyacetone, and acetaldehyde. The
formation yields of methyl formate and methoxyacetone
measured by GC-FID and FT-IR analyses are in excellent
agreement (Table 1), with formation yields of 0.59 ( 0.05
and 0.39 (0.04, respectively. As obvious from the data shown
in Table 1, the measurements of the formation yield of
acetaldehyde were less precise, but are totally consistent with
acetaldehyde being a coproduct to methyl formate. The
products observed are explained by reaction of the OH radical
with 1-methoxy-2-propanol proceeding by H-atom abstrac-
tion from the C-H bonds of the >CH- and -CH2- groups:
The formation of the products n-butyl formate and
propanal plus 2-hydroxyethyl formate accounts for 78 ( 6%
of the overall reaction products (Table 1), suggesting that
the major alkoxy radicals formed are CH3CH2CH2CH2OCH-
(O•)CH2OH and CH3CH2CH2CH(O•)OCH2CH2OH. These two
alkoxy radicals arise after H-atom abstraction from the two
-CH2- groups adjacent to the ether linkage followed by
reactions analogous to reactions 6 and 7a shown above for
the CH3CH(OH)C˙ HOCH3 radical formed from the 1-methoxy-
2-propanol reaction. The observed formation of the mo-
lecular weight 179 nitrate is consistent with formation from
the reaction of the peroxy radicals CH3CH2CH2CH2OCH-
(OO•)CH2OH and/ or CH3CH2CH2CH(OO•)OCH2CH2OH with
NO [analogous to reaction 7b], although it should be noted
that RO2• + NO reactions of all of the peroxy radicals formed
after H-atom abstraction from the C-H bonds apart from
those in the -CH2OH group can lead to the formation of a
molecular weight 179 C6H13NO5 nitrate.
The R-hydroxy radical CH3C˙ (OH)CH2OCH3 is expected
to react with O2 (1, 28) to form methoxyacetone:
CH3C˙ (OH)CH2OCH3 + O2 f CH3C(O)CH2OCH3 + HO2
(5)
The CH3CH(OH)C˙ HOCH3 radical adds O2 to form a peroxy
radical which, in the presence of NO, reacts to form an alkoxy
radical plus NO2 or an organic nitrate (1):
The potential reactions of the CH3CH2CH2CH2OCH-
(O•)CH2OH and CH3CH2CH2CH(O•)OCH2CH2OH alkoxy radi-
cals are shown in Schemes 1 and 2, respectively, where the
products shown in boxes were identified and quantified and
those underlined are consistent with the API-MS analyses
(noting that the hemiacetal CH3CH2CH2CH(OH)OCH2CHO
in Scheme 2 may decompose to butanal plus glycolaldehyde
during sampling and GC analysis or when protonated in the
API-MS). The products formed, and their yields, from the
reactions of the CH3CH2CH2CH2O• radical (Scheme 1) and
the HOCH2CH2O• radical (Scheme 2) in air are based on
literature data (1, 28, 29).
CH3CH(OH)C˙ HOCH3 + O2 f
CH3CH(OH)CH(OO•)OCH3 (6)
CH3CH(OH)CH(O•)OCH3 +
CH3CH(OH)CH(OO•)OCH3 + NO
NO2 (7a)
CH3CH(OH)CH(ONO2)OCH3
(7b)
This alkoxy radical formed in reaction 7a is expected to
decompose (1), leading to the formation of methyl formate
plus acetaldehyde:
The products observed are in agreement with the de-
composition pathway
RCH(O•)OR′ f R• + R′OCHO
(9)
shown in Schemes 1 and 2 dominating over the alternative
decomposition pathway,
The sum of the methyl formate and methoxyacetone
formation yields is 0.98 ( 0.07, showing that essentially all
of the reaction pathways have been observed. Other initial
reaction pathways (H-atom abstraction from the -OH or
-CH3 groups) and any formation of the organic nitrate CH3-
CH(OH)CH(ONO2)OCH3 from reaction 7b must then account
for e9% of the overall product yield. The structure-reactivity
estimation method of Kwok and Atkinson (27) leads to
RCH(O•)OR′ f RCHO + R′O•
(10)
for both the CH3CH2CH2CH2OCH(O•)CH2OH and CH3CH2-
CH2CH(O•)OCH2CH2OH radicals, and this is confirmed by
our measured upper limit to the butanal formation yield of
<0.01 (Table 1).
9
3 3 4 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 21, 1998

SCHEME 1
SCHEME 2
The formation route to 3-hydroxybutyl formate must
(O•)CH2OH â-hydroxyalkoxy radical, which can decompose
to form 3-hydroxybutyl formate (15):
involve initial H-atom abstraction to form the CH3C˙ HCH2-
CH2OCH2CH2OH radical, which then (after addition of O2
and reaction with NO) leads to the CH3CH(O•)CH2CH2OCH2-
CH2OH radical, which must then isomerize through a seven-
membered transition state (15):
CH3CH(O•)CH2CH2OCH2CH2OH f
CH3CH(OH)CH2CH2OC˙ HCH2OH (11)
The structure-reactivity estimation method of Kwok and
Atkinson (27) predicts that the percentages of the overall OH
radical reaction arising from H-atom abstraction from the
C-H bonds in CH3CH2CH2CH2OCH2CH2OH are (for the
carbons as written from left to right) <1%, 4%, 5%, 37%, 37%,
and 15%, with <1% H-atom abstraction from the O-H bond.
Our product data show that H-atom abstraction from the
-CH2- groups adjacent to the ether linkage dominates (in
agreement with the above predictions), and accounts for
This isomerization via a seven-membered transition state
must be competitive with the reaction with O2 [to yield
CH3C(O)CH2CH2OCH2CH2OH plus the HO2 radical] and with
decomposition (to yield CH3CHO plus, presumably, ulti-
mately HOCH2CH2OCH2CHO).
Addition of O2 to the CH3CH(OH)CH2CH2OC˙ HCH2OH
radical and reaction with NO (analogous to reactions 6 and
7a shown above) results in the CH3CH(OH)CH2CH2OCH-
9
VOL. 32, NO. 21, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 3 4 3

∼23% abstraction from the italicized -CH2- group in CH3-
CH2CH2CH2OCH2CH2OH and ∼63% abstraction from the
italicized -CH2- group in CH3CH2CH2CH2OCH2CH2OH
(assuming that organic nitrate formation occurs from the
various RO2• + NO reactions with the same yield). Our data
indicate that the -CH2- group with -O- and -CH2OH
substituents is activated over the -CH2- group with -O-
and -C3H7 substituents.
No evidence was obtained from the GC or API-MS analyses
(and FT-IR analyses were not specific) for the presence of
CH3CH2CH2CH2OCH2CHO, the expected product formed
after H-atom abstraction from the C-H bonds in the -CH2-
OH group:
3-hydroxybutyl formate by Stemmler et al. (15) was stated
to be subject to significant uncertainties. The other products
observed by Stemmler et al. (15), but not by us, have relatively
low formation yields and, together with CH3CH2CH2C(O)-
OCH2CH2OH (or its isomers), may contribute to the 4 ( 7%
of products/ reaction pathways not accounted for in our study.
Because we have identified and quantified the majority of
the reaction products and reaction pathways involved in the
OH radical-initiated reaction of 2-butoxyethanol, and have
identified 96 ( 7% of the reaction pathways, no significant
products or reaction pathways are unaccounted for.
The product and mechanistic data obtained here for the
reactions of the OH radical with 1-methoxy-2-propanol and
2-butoxyethanol in the presence of NO account for all or
almost all of the reaction products and pathways. These
data can now be used in detailed chemical mechanisms for
the photooxidations of these compounds in ambient air and
will contribute to the accurate calculation of their ozone-
forming potentials (5).
CH3CH2CH2CH2OCH2C˙ HOH + O2 f
CH3CH2CH2CH2OCH2CHO + HO2 (13)
As discussed above, 3-hydroxybutyl formate must arise from
H-atom abstraction from the C-H bonds of the carbon γ to
the ether oxygen, while the expected products formed after
H-atom abstraction from the C-H bonds of the carbon â to
the ether oxygen are (by analogous reactions to those
discussed above for the formation of 3-hydroxybutyl formate)
2-hydroxybutyl formate (isomerization reaction, and with a
measured formation yield of <3%), CH3CH2C(O)CH2OCH2-
CH2OH (O2 reaction, formation consistent with our API-MS
analyses), and propanal plus 2-hydroxyethyl formate or CH3-
CHO plus HOCH2CH2OCH2CHO (decomposition reaction).
As shown in Scheme 2 and noted above, propanal is
formed as a coproduct to 2-hydroxyethyl formate, via the
intermediary of the 1-propyl radical. After addition of O2,
the CH3CH2CH2OO• radical reaction with NO leads to the
formation of 2% 1-propyl nitrate and 98% 1-propoxy radical
plus NO2 (30), with the 1-propoxy radical reacting with O2
to form propanal (1, 28, 29). The formation yields of n-butyl
formate (plus HCHO coproduct), 3-hydroxybutyl formate
(plus the expected HCHO coproduct) and 2-hydroxyethyl
formate plus propanal, combined with the measured forma-
Acknowledgments
The authors gratefully thank the Chemical Manufacturers’
Association, Ethylene Glycol Ethers Panel, for supporting this
research through Contract EGE-73.0-VOC-ATKIN.
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tion yield of the nitrate (presumed to be largely the C6H13
-
NO5 nitrate observed by the API-MS analyses), account for
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0.35 ( 0.11 (0.57 ( 0.05); propanal, ∼0.2-0.3 (0.21 ( 0.02);
2-hydroxyethyl formate, 0.39 ( 0.18 (0.22 ( 0.05); 3-hy-
droxybutyl formate, ∼0.20 (0.07 ( 0.03); 2-hydroxybutyl
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API-MS and FT-IR analyses, see above); and butanal, <0.011
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0.12 ( 0.09; 2-propyl-1,3-dioxalane, 0.025 ( 0.005; and
1-propyl nitrate, 0.038 ( 0.018. Obviously, there are areas
of broad agreement and some disagreements. The major
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2-hydroxyethyl formate, and 3-hydroxybutyl formate mea-
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ES980455C
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