FT-IR kinetic and product study of the OH radical-initiated oxidation of 1-pentanol

Article abstract of DOI:10.1021/es000016p

A kinetic and product study has been performed on the reaction of OH radicals with 1-pentanol in a 480-L glass chamber and also in the EUPHORE outdoor photoreactor in Valencia, Spain. Analysis of reactants and products was made by long-path in situ FT-IR spectroscopy and gas chromatography with photoionization detection. Using a relative kinetic technique, a rate coefficient of k(OH+1-pentanol) = (1.11±0.11)×10^-11 cm³ molecule^-1 s^-1 has been measured at 298±2 K and 740 Torr total pressure of synthetic air. The products observed in the reaction of OH with 1-pentanol and their fractional molar yields were as follows: pentanal (0.405±0.082); butanal (0.161±0.037); propanal (0.081±0.019); ethanal (0.181±0.042); and HCHO (0.251±0.013). The results support the possible formation of 5-hydroxy-2-pentanone, and residual absorptions indicate the possible presence of compounds such as 3-hydroxypropanal. From the product studies, a detailed atmospheric degradation mechanism has been constructed for the OH radical-initiated oxidation of 1-pentanol. A kinetic and product study has been performed on the reaction of OH radicals with 1-pentanol in a 480-L glass chamber and also in the EUPHORE outdoor photoreactor in Valencia, Spain. Analysis of reactants and products was made by long-path in situ FT-IR spectroscopy and gas chromatography with photoionization detection. Using a relative kinetic technique, a rate coefficient of k(OH + 1-pentanol) = (1.11 ± 0.11) x 10^-11 cm³ molecule^-1 s^-1 has been measured at 298 ± 2 K and 740 Torr total pressure of synthetic air. The products observed in the reaction of OH with 1-pentanol and their fractional molar yields were as follows: pentanal (0.405 ± 0.082); butanal (0.161 ± 0.037); propanal (0.081 ± 0.019); ethanal (0.181 ± 0.042); and HCHO (0.251 ± 0.013). The results support the possible formation of 5-hydroxy-2-pentanone, and residual absorptions indicate the possible presence of compounds such as 3-hydroxypropanal. From the product studies, a detailed atmospheric degradation mechanism has been constructed for the OH radical-initiated oxidation of 1-pentanol.

Full text of DOI:10.1021/es000016p

Environ. Sci. Technol. 2000, 34, 4111-4116
importance of the three possible reaction channels, i.e.,
thermal decomposition, isomerization, and reaction with O2
(2).
FT-IR Kinetic and Product Study of
the OH Radical-Initiated Oxidation of
1-Pentanol
Alcohols are a class of highly volatile oxygenates that are
finding increasing use as both solvents and fuel additives.
The alcohol 1-pentanol (n-butylcarbinol, primary n-amyl
alcohol), a high-volume chemical with production exceeding
1 million lb annually (3), is particularly useful as a solvent
(4) in the reaction of substituted diamines with diisocyanates
and for the production of sulfurized olefins and polyacrylates.
1-Pentanol is also employed as an extracting agent (4) (in the
purification of phosphoric acid and the separation of
strontium chloride from aqueous metal chloride solutions)
and as starting material for the production of lubricant
additives and for auxiliaries in flotation (4). Furthermore,
1-pentanol finds an important application in the pharma-
ceutical industry as a starting material for several synthesis
(5) and in the production of several esters (5) that are applied
in different fields.
The mechanistic database for alcohols is presently very
limited and has recently been summarized in the paper of
Azad and Andino (6). To extend both the atmospheric kinetic
and mechanistic databases for alcohols, we present here a
kinetic and product investigation of the room temperature
gas-phase reaction of OH radicals with 1-pentanol. The
kinetic studies were performed in Wuppertal in a large-
volume photoreactor, and the product investigations were
carried out in the outdoor EUPHORE (7) smog chamber
facility in Valencia, Spain. The reaction mechanism for
1-pentanol was not reported before.
F A B R I Z I A C A V A L L I , I A N B A R N E S , * A N D
K A R L H E I N Z B E C K E R
Bergische Universita¨tsGH Wuppertal, Physikalische Chemie/
FB 9, Gauss Strasse 20, D-42097 Wuppertal, Germany
A kinetic and product study has been performed on the
reaction of OH radicals with 1-pentanol in a 480-L glass
chamber and also in the EUPHORE outdoor photoreactor in
Valencia, Spain. Analysis of reactants and products was
made by long-path in situ FT-IR spectroscopy and gas
chromatography with photoionization detection. Using a
relative kinetic technique, a rate coefficient of k(OH
+ 1-pentanol) ) (1.11 ( 0.11) × 10-11 cm3 molecule-1
s-1 has been measured at 298 ( 2 K and 740 Torr total
pressure of synthetic air. The products observed in the
reaction of OH with 1-pentanol and their fractional molar
yields were as follows: pentanal (0.405 ( 0.082); butanal
(0.161 ( 0.037); propanal (0.081 ( 0.019); ethanal (0.181
( 0.042); and HCHO (0.251( 0.013). The results support the
possible formation of 5-hydroxy-2-pentanone, and residual
absorptions indicate the possible presence of compounds
such as 3-hydroxypropanal. From the product studies, a
detailed atmospheric degradation mechanism has been
constructed for the OH radical-initiated oxidation of
1-pentanol.
Experimental Section
The two experimental systems used for the present study
have been described in detail elsewhere (7, 8) and are only
briefly outlined here.
Kinetic Studies of the Reaction of OH Radicals with
1-Pentanol. Kinetic experiments on the reaction of OH
radicals with 1-pentanol were carried out at 298 ( 2 K and
740 Torr total pressure of synthetic air in a 480-L cylindrical
Duran glass chamber. The chamber is surrounded by 24
fluorescent lamps (Philips T 40W/ 05 320 < λ < 450 nm) (8)
and is equipped with a built-in White mirror system for
long-path (51.6 m) in situ monitoring of reactants and
products.
The rate coefficient for the gas-phase reaction of OH
radicals with 1-pentanol was determined using a relative
rate method in which the decays of the reactant (1-pentanol)
and of a reference compound, whose rate coefficient with
OH is reliably known (9), are monitored parallel in the
presence of OH radicals:
Introduction
Oxygenated organic compounds are currently under con-
sideration as potential replacements for the traditional
aromatic hydrocarbon and chlorinated organic solvents. The
drive behind the search for replacements is the need for
more environmentally acceptable solvents with respect to
ambient air quality, i.e., compounds that will help to reduce
the tropospheric O3 and photooxidant burden. However, even
the use of oxygenated replacements will lead to the release
of these compounds to the atmosphere where they will
undergo photooxidative degradation processes, the most
important and often dominant being reaction with the OH
radical (1, 2). Accurate kinetic data and mechanistic infor-
mation on the atmospheric fate of replacement oxygenated
solvents are therefore essential components in any attempts
to reliably assess the ozone-forming potential of these
substances and their possible contribution to photochemical
air pollution in urban and regional areas.
Although the rate coefficients for the reaction of OH with
many different classes of oxygenated compounds are now
reasonably well established, details pertaining to the complex
reaction mechanisms are still often fragmentary. A major
uncertainty in the degradation schemes, which affects the
final product distribution, concerns the fate of the alkoxy
radicals formed during the photooxidation and the relative
OH + 1-pentanol f products k1
OH + reference f products k2
(1)
(2)
Providing that 1-pentanol and the reference are removed
solely by reaction with the OH radicals, then
[1-pentanol]t
[reference]t
0
k
0
ln
)
1 ln
(I)
{
}
{
}
k2
[1-pentanol]t
[reference]t
where [1-pentanol]t and [reference]t are the concentrations
0
0
of 1-pentanol and the reference compound, respectively, at
time t0; [1-pentanol]t and [reference]t are the corresponding
concentrations at time t; k1 and k2 are the rate coefficients
for reactions 1 and 2, respectively.
* Corresponding author e-mail:
wuppertal.de.
Barnes@physchem.uni-
9
10.1021/es000016p CCC: $19.00
Published on Web 08/25/2000
2000 Am erican Chem ical Society
VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 1 1 1

Hydroxyl radicals were generated by the photolysis of
methyl nitrite (CH3ONO) in synthetic air using the fluorescent
lamps. NO was included in the reactant mixtures to suppress
the formation of O3 and, hence, of NO3 radicals. The kinetic
measurements were made relative to cyclohexane. The
approximate initial reactant concentrations (in molecule
cm-3) were as follows: CH3ONO, 4.1 × 1014; NO, 4.1 × 1014;
1-pentanol, 2.8 × 1014; and cyclohexane, 9.3 × 1013. The
concentration-time behavior of 1-pentanol and cyclohexane
were monitored over a 15-30-min irradiation period by in
situ FT-IR long-path spectroscopy. The FT-IR spectrometer
(Nicolet Magna 520) was operated with a resolution of 1 cm-1
.
A few experiments using 1-pentanol-cyclohexane-air
mixtures, i.e., without the OH source, were also carried out
(i) in the dark and (ii) at maximum light intensity over a
period of 60 min to check for complications associated with
adsorption on the wall or photolysis of these compounds.
No such complications were observed.
FIGURE 1. Plot of eq I for the gas-phase reaction of the OH radical
with 1-pentanol using cyclohexane as the reference compound.
The chemicals used (1-pentanol, cyclohexane, and NO)
all had a stated manufacturer purity of >99%. Methyl nitrite
was prepared and stored as described in the literature (10).
Products of the Reaction of OH Radicals with 1-Pentanol.
All of the product experiments were performed in the
European Photoreactor (EUPHORE), a large-volume outdoor
smog chamber, which is part of the Centro de Estudios
Ambientales del Mediterraneo (CEAM) in Valencia, Spain.
The facility consists of two identical half-spherical fluo-
rine-ethene-propene (FEP) foil chambers mounted on
aluminum floor panels covered with FEP foil. The FEP foil
transmits 85-90% of the light with wavelengths from >500
to 320 nm, dropping to 75% transmission at the atmospheric
threshold of 290 nm. To avoid unwanted heating of the
chambers, the floor panels are connected to a cooling system
allowing realistic atmospheric temperature conditions to be
maintained even during long-term irradiations.
The chamber, used for the experiments described here,
has a volume of ca. 195 m3 and two mixing fans to ensure
homogeneity of the reaction mixtures. An air-drying and
purification system supplies the chamber with an oil vapor,
hydrocarbon, and NOy-free dry air. The solar light intensity
was measured in the chamber with a spectral radiantmeter
(scanning double monochromator Bentham TM300), and
filter band radiometers were used to measure the photolysis
frequencies of J(O1D) and J(NO2) during the days of the
experiments.
The reactant mixture was also analyzed, during the
experiments, by gas chromatography with photoionization
detection (GC-PID). Gas samples were automatically col-
lected from the chamber every 15 min and introduced, via
a heated sampling loop of 2 mL capacity, onto a DB624
column at 80 °C (J & W Scientific, 30 m) mounted in a Fisons
GC 8000 instrument. The GC-PID response factors for the
alcohol and the products were determined by introducing
known concentrations of these compounds into the chamber.
In addition, carbonyl compounds were sampled from the
chamber every 30 min using solid-phase 2,4-dinitrophenyl
hydrazone (DNPH)-silica cartridges. Hydrazones, formed
by derivatization, were separated and quantitatively mea-
sured by HPLC (Hewlett-Packard model 1050, UV detector).
To determine the dilution rate of the chamber due to
small leaks in the FEP foil, 37 × 1010 molecule cm-3 SF6 was
added, as an inert trace compound, to the reaction mixture.
The decay of SF6 was monitored using its infrared absorption
935-955 cm-1. At the time of the experiments, the chamber
had a leak rate of ca. 3% h-1
.
Results and Discussion
Rate Coefficient for the Reaction of OH with 1-Pentanol.
Test experiments on 1-pentanol-cyclohexane-air mixtures
in the 480-L laboratory reactor in Wuppertal showed that
losses of the compounds to the wall or via photolysis were
<3% for both processes over a 60-min period. Compared to
the measured decays for the compounds during the kinetic
experiments, contributions from wall loss and photolysis were
negligible.
The kinetic investigations were performed using the
irradiation of CH3ONO-NO-1-pentanol-cyclohexane-air
mixtures. The kinetic data obtained from three experiments
are plotted, according to eq I, in Figure 1. The data from all
the experiments give a good straight line plot. Least-squares
analysis of these data leads to the rate coefficient ratio k1/ k2
) 1.48 ( 0.03 (2σ). This rate coefficient ratio has been placed
on an absolute basis using a value of k2(OH + cyclohexane)
) 7.49 × 10-12 cm3 molecule-1 s-1 at 298 K (2). This results
in a rate coefficient for the reaction of OH with 1-pentanol
of k1 ) (1.11 ( 0.11) × 10-11 cm3 molecule-1 s-1. The indicated
error contains an additional 10% uncertainty due to the
potential systematic errors associated with the value of the
reference rate coefficient.
1-Pentanol was added to the chamber by gently heating
the required amount in a stream of air entering the chamber.
OH radicals were produced by the photolysis of HONO also
introduced into the chamber in an air stream. HONO was
prepared by dropwise addition of 15 mL of a 1% NaNO2
solution to 30 mL of a 30% sulfuric acid solution at room
temperature. To compensate for the consumption of HONO
during the 5-h duration of the experiments, addition of HONO
was normally repeated at least twice to maintain the level of
OH radicals and also the conversion of RO2 to RO via RO2
+ NO reactions. After allowing for mixing of the reactants,
photooxidation was initiated by opening the light-tight
protective housing of the chamber and exposing the pho-
toreactor contents to sunlight. The initial concentrations of
1-pentanol and HONO were approximately 2.1 × 1013 and
6.8 × 1011 molecule cm-3, respectively. The initial concen-
trations of NO and NO2 were both approximately 10-12 ×
1011 molecule cm-3, and their final concentrations were 2.5
× 1011 and 31 × 1011 molecule cm-3, respectively.
An FT-IR spectrometer (NICOLET Magna 550, MCT
detector, 1 cm-1 resolution) coupled to a White mirror system
(base length 8.17 m, total optical path length 554.5 m) was
used to monitor reactants and products. The absorption cross
sections for the compounds used for the evaluation of the
FT-IR spectra were determined in separate calibration studies.
The value of the rate coefficient measured in the present
study can be compared with previous room temperature
determinations reported in the literature (11, 12). Using an
absolute technique, Wallington and Kurylo (11) obtained
k(OH + 1-pentanol) ) (1.08 ( 0.11) × 10-11 cm3 molecule-1
s-1. Nelson et al. (12) have reported values of k(OH +1-
9
4 1 1 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

pentanol) ) (1.20 ( 0.16) × 10-11 and (1.05 ( 0.13) × 10-11
cm3 molecule-1 s-1, using absolute and relative techniques,
respectively. An unit-weighted least-squares average of these
three rate constants leads to the recommendation of k(OH
+ 1-pentanol) ) 1.11 × 10-11 cm3 molecule-1 s-1 given by
Atkinson (2). The present work is in excellent agreement
with the previous determinations.
Using the structure-reactivity estimation method of Kwok
and Atkinson (13) gives k(OH + 1-pentanol) ) 8.17 × 10-12
cm3 molecule-1 s-1 at 298 K, which is in fair agreement with
the average value from the experimental results. The dis-
crepancy between the calculated and measured rate coef-
ficient presumably indicates that the OH substituent group
has a more significant effect on the -CH2- groups than that
assumed in the estimation method. The rate coefficient k1
can be combined with the estimated ambient tropospheric
OH radicals concentration (14) of 1.0 × 106 molecule cm-3
(24 h average) to obtain a tropospheric lifetime of 1 day for
1-pentanol due to gas-phase reaction with OH radicals.
Products of the OH Radical Reaction with 1-Pentanol.
GC-PID and HPLC analysis of the products formed on
irradiation of HONO-1-pentanol-air mixtures, in the EU-
PHORE photoreactor facility, showed the formation of
pentanal, butanal, propanal, ethanal, and formaldehyde
among the products. These products have been quantified
using GC-PID and HPLC response factors determined from
calibrations with authentic standards. In addition, a product
has been tentatively identified as 5-hydroxy-2-pentanone
on the basis of a comparison of its HPLC retention time and
its infrared spectrum with that of the authentic compound.
Vapor samples of 5-hydroxy-2-pentanone were introduced
into the evacuated chamber by injecting known amounts
liquid samples of the commercially available authentic
compound into a steam of N2 entering the chamber. The
infrared spectra thus acquired showed a relatively rapid (k
) 2.19 × 10-1 s-1) conversion of the δ-hydroxycarbonyl to
4,5-dihydro-2-methylfuran. A calibrated IR spectrum of 4,5-
dihydro-2-methylfuran obtained from the authentic sample
was used to subtract its IR features from the mixed 5-hydroxy-
2-pentanone/ 4,5-dihydro-2-methylfuran spectrum to derive
a pure and semiquantitative spectrum of 5-hydroxy-2-pen-
tanone.
Butanal, propanal, ethanal, and HCHO could also be
formed in secondary reactions of some of the primary
aldehydic products. However, under the conditions employed
in the experiments, the contribution from secondary reactions
is probably very minor. With the high NOx concentrations
employed in the experiments, the main fate of the alkyl
peroxyl radicals, formed in the degradation reactions of the
higher aldehydes, will be formation of the corresponding
stable acyl peroxynitrate and not a lower aldehyde.
To derive the formation yields of the aldehydic products,
their measured concentrations had to be corrected for further
reaction with OH radicals, photolysis, and dilution losses.
Corrections have been performed using the mathematical
procedure described by Tuazon et al. (15), which is based on
the following reaction sequence:
FIGURE 2. Plot of the pentanal yield (4) uncorrected and (O)
corrected for the photolysis and reaction with OH radicals (see
text) against the amount of 1-pentanol consumed.
By making the reasonable assumption that the OH radical
concentration was essentially constant over the relatively
short irradiation period, then
[1-pentanol]t ) [1-pentanol]t e-k [OH](t -t )
(II)
1
2
1
2
1
OH[OH]+khν+kD)(t2-t1)
[RHO]t ) [RHO]t e-(k
+
2
1
Y
[1-pentanol]t k1[OH]
1
t2-t1
[e-k [OH](t -t )
-
1
2
1
(kOH - k1)[OH] + khν + kD
e-(k [OH]+khν+kD)(t2-t1)] (III)
OH
where [1-pentanol]t , [RHO]t , [1-pentanol]t , and [RHO]t are
1
1
2
2
the 1-pentanol and aldehyde concentrations measured at
times t1 and t2, respectively, and Y 1 is the formation yield
t
-t
2
of the individual aldehyde over the period time t2 - t1.
The OH radical concentration was calculated from the
decay of the alcohol using the rate coefficient for k1(OH +
1-pentanol) determined in this study. The dilution rate
constants kD were determined for each experiment as
described above. Rate coefficients for reaction of OH radicals
with the aldehydic products were taken from the literature
(16). The following values of kOH (in units of 10-11 cm3
molecule-1s-1) were used: pentanal, 2.96 ( 0.39; butanal,
2.06 ( 0.30; propanal, 1.71 ( 0.24; and ethanal, 1.22 ( 0.27.
Photolysis frequencies for the photolysis of the aldehydes
k
hν were taken from the literature (17). The mean of J(NO2),
measured during the experiments, was used as an indicator
of the light intensity to select the frequencies. The following
values of khν (in units of 10-5 cm3 molecule-1s-1) were
employed: pentanal, 2.20 ( 0.09; butanal, 0.55 ( 0.03;
propanal, 1.04 ( 0.05; and ethanal, 2.92 (. 0.14.
Use of these constants together with eqs II and III allows
Y
1 to be calculated. The aldehyde concentrations, corrected
t -t
2
for reaction with OH radicals, photolysis, and dilution, are
then given by
OH + 1-pentanol f Y(RHO) k1
OH + RHO f products kOH
RHO f dilution kD
[RHO]tcorr ) [RHO]tcorr
([1-pentanol]t - [1-pentanol]t ) (IV)
+
2
1
Y
t2-t1
1
2
where [RHO]ct orr and [RHO]ct orr are the corrected aldeheyde
1
concentrations at times t1 a2nd t2, respectively.
RHO + hν f products khν
The correction factors, [RHO]ct orr/ [RHO]t were e3.5 for
pentanal, e1.9 for butanal, e1.7 for propanal, and e1.8 for
ethanal. The corrected aldehydes concentrations are plotted
against the amount of 1-pentanol consumed in Figures 2-5.
The measurement errors were principally caused by the low
In this sequence, Y is the formation yield of the aldehyde
from the oxidation of 1-pentanol and kOH, kD, and khν are the
rate coefficients for reaction with OH, chamber dilution, and
photolysis, respectively.
9
VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 1 1 3

TABLE 1. Product Formation Yields from Gas-Phase Reaction
of the OH Radical with 1-Pentanol in the Presence of NO
x
product
formation yielda
pentanal
butanal
propanal
ethanal
HCHOb
0.405 ( 0.082
0.161 ( 0.037
0.081 ( 0.019
0.181 ( 0.042
0.251 ( 0.013
a
Corrected for photolysis and reaction with OH radicals as described
in the text. Indicated errors are 2 least-squares standard deviations of
the data shown in Figures 2-5 com bined with overall uncertainties in
the alcohol and carbonyl GC-PID calibration factors of (10% and (5%,
respectively. Additional uncertainties in the form ation yields due to
uncertainties in the m ultiplicative correction factor F (see text) are
<(15% for the form ation of pentanal, butanal, and ethanal and <(6%
b
for the form ation of propanal. Uncorrected form ation yield of HCHO,
derived in the early stage of the reaction; indicated error lim its are 2
standard deviations.
FIGURE 3. Plot of the butanal yield (4) uncorrected and (O) corrected
for the photolysis and reaction with OH radicals (see text) against
the amount of 1-pentanol consumed.
The estimated overall uncertainties in the OH reaction
rate coefficient of 1-pentanol and in the rate coefficients for
reaction with OH and photolysis of the products lead to
maximum uncertainties in the correction factors of (14%
for the formation of pentanal and butanal, (6% for the
formation of propanal, and (15% for the formation of ethanal.
The corrected values for the formation yields are given in
Table 1. The molar formation yield of 0.251 ( 0.013 given for
HCHO was derived from the uncorrected concentrations
plotted against the consumption of 1-pentanol in the early
stage of the reaction, when secondary processes can largely
be neglected. The carbonyl and the hydroxycarbonyl products
observed and quantified in this work account for 70 ( 14%
of the overall OH radical reaction with 1-pentanol.
Mechanistic Considerations Based on SAR and the
Product Yield Data. The OH radical reaction with 1-pentanol
proceeds by H-atom abstraction from the various -CH3, -CH2-,
and -OH groups. The structure activity relationship (SAR)
method of Kwok et al. (13) allows the percentage of attack
at the individual sites to be estimated. Using this technique
gives: from the -OH group, 1%; from the -CH2- group at the
1-position, 49%; from the -CH2- group at the 2- and 3-position,
17%; from the -CH2- group at the 4-position, 14%; and from
the -CH3 group, 2%.
FIGURE 4. Plot of the propanal yield (4) uncorrected and (O)
corrected for the photolysis and reaction with OH radicals (see
text) against the amount of 1-pentanol consumed.
The R-hydroxyalkyl radical, formed by H-atom abstraction
from the -CH2- group at the 1-position, reacts with O2 to
form pentanal in unit yield (2):
CH3CH2CH2CH2C(•)HOH + O2 f
CH3CH2CH2CH2CHO + HO2 (3)
The alkyl radicals formed after H-atom abstraction from the
-CH2- group at the 2-, 3-, and 4-positions will add O2 to form
alkylperoxy radicals that can react with NO or NO2. Addition
of NO2 will form thermally unstable alkylperoxy nitrates.
Reaction with NO will give alkoxy radicals. The contribution
of the channel forming organic nitrate in these RO2 + NO
reactions is considered to be small (e6%). In previous studies
(2, 19), it has been found that, at room temperature and
atmospheric pressure, hydroxy-alkyl nitrate formation from
the reaction of Cn-hydroxy-alkyl peroxy radicals with NO is
approximately half of the alkyl nitrate formation yields from
the reactions of Cn-alkyl peroxy radical with NO. The yield
of pentyl nitrate (18), relative to the n-pentane consumed,
in CH3ONO-NO-n-pentane air irradiations, is 0.117 (0.013.
The further expected reactions of the alkoxy radicals are
outlined in Schemes 1-3. In these reaction schemes, the
alkyl and alkyl peroxy radicals have been omitted for clarity,
and only the reactions of the alkoxy and acyl radicals involved
are shown.
FIGURE 5. Plot of the ethanal yield (4) uncorrected and (O) corrected
for the photolysis and reaction with OH radicals (see text) against
the amount of 1-pentanol consumed.
concentration of 1-pentanol employed in the experiments
and consequently low concentrations of the products. The
random scatter of the data plotted in Figures 2-5 reflects the
extent of these measurement errors in both ∆[1-pentanol]
and [RHO]ct orr
.
9
4 1 1 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 19, 2000

The â-hydroxyalkoxy radical (I) formed after the H-atom
abstraction from the -CH2- group at the 2 position (Scheme
1) can react with O2, decompose, or isomerize via a 1,5-H
The observations of Choo and Benson (22) support that
the energetics of alkoxy radical decompositions depend on
the specific alkyl leaving group and are governed by the
following expression:
SCHEME 1. Reactions of the â-Hydroxyalkoxy Radical That
Accounts for an Estimated 17% of the Overall Reaction of
1-Pentanol with OH
Ed ) a + 0.36∆Hd
(V)
Ed is the activation energy in kcal mol-1 and ∆Hd is the
decomposition enthalpy in kcal mol-1. The activation energy
is dependent on the alkyl leaving group, represented in eq
V by parameter a. Reported values of a (in kcal mol-1) (23)
•
for the ethyl radical and primary alkyl radicals, RCH2 , are
11.4 and 11.1, respectively. Therefore, both decomposition
channels (Scheme 2) of the γ-hydroxyalkoxy radical (III) will
occur with similar decomposition rates, kd, and hence have
comparable importance:
kd ) Ade-E / RT s-1
(VI)
d
where
Ad ) 2 × 1014 × d s-1
(VII)
shift through a six-membered transition state. The alkoxy
radical OCH2CH2CH2CH(OH)CH2OH (II), formed after the
where A is the preexponential factor, d is the reaction path
degeneracy, R is the ideal gas constant, and T is the
temperature in Kelvin.
d
•
isomerization reaction, is predicted to undergo a further
isomerization to give a polyfunctional product, HOCH2CH2-
CH2C(O)CH2OH. However, the isomerization reaction, which
involves an H-atom abstraction from a -CH3 group, is
expected to be minor (19, 20). Of the remaining pathways,
the experimental data of the present and previous (19, 20)
studies and the estimation method proposed by Atkinson
(2), support that decomposition of the â-hydroxyalkoxy
radical (I) to butanal and HCHO will dominate over reaction
with oxygen.
Since we have measured a 8% molar yield for propanal,
the arguments above support by default an ∼8% molar yield
for 3-hydroxypropanal from 1-pentanol. Considering that
the γ-hydroxyalkoxy radical is calculated to account for 17%
of the overall reaction of 1-pentanol, this implies that its
reaction with O2 will be very minor and that the decomposi-
tion pathways will dominate. It is not presently possible to
ascertain whether 1-hydroxy-3-pentanone is being formed.
The δ-hydroxyalkoxy radical (VI) formed after the H-atom
abstraction from the -CH2- group at the 4-position (Scheme
3) can undergo reaction with O2, decomposition, and
The γ-hydroxyalkoxy radical (III) formed after the H-atom
abstraction from the -CH2- group at the 3-position (Scheme
2) cannot undergo isomerization via a six-membered transi-
SCHEME 3. Reactions of the δ-Hydroxyalkoxy Radical That
Accounts for an Estimated 14% of the Overall Reaction of
1-Pentanol with OH
SCHEME 2. Reactions of the γ-Hydroxyalkoxy Radical That
Accounts for an Estimated 17% of the Overall Reaction of
1-Pentanol with OH
tion state; hence, reaction with O2 and decomposition (for
which there are two channels) are the only possible pathways.
These two decomposition channels can lead to the formation
of either propanal plus the •CH2CH2OH radical (IV) or
3-hydroxypropanal plus the CH3CH2• radical (V). A previous
isomerization by an H-atom abstraction from a -CH2- group
through a six-membered transition state. Isomerization of
this δ-hydroxyalkoxy radical to form the δ-hydroxycarbonyl,
4-hydroxypentanal, is predicted (based on the work of Kwok
et al.) (2, 24) to dominate over its decomposition and reaction
with O2. To our knowledge, there is no direct experimental
evidence (i.e., specific product identification) to support the
occurrence of the isomerization process. Our experimental
product data suggest the occurrence of a decomposition
pathway for the δ-hydroxyalkoxy radical (VI), forming ethanal.
As noted in Scheme 2, ethanal can also be formed from the
decomposition of the CH3CH2CHO(•)CH3CH2OH radical (III).
On the basis of the measured yield of 8% for propanal, the
molar yield of ethanal from this channel cannot exceed ∼8%.
Since the observed total yield of ethanal is 18%, this points
•
study (21) has indicated that the fate of the OCH2CH2OH
radical is 82% decomposition to form HCHO, as a result of
the “hot alkoxy radical effect”. The other possible pathway
is reaction with O2, forming glycolaldehyde. Glycolaldehyde
formation was not observed using any of the analytical
techniques (GC-PID, HPLC, and FTIR absorption spectros-
copy), indicating that its yield must be very low. Under
atmospheric conditions, reaction with O2 to form ethanal is
the sole loss process of the ethyl radical (V). Residual
absorptions in the infrared product spectrum indicate the
possible presence of hydroxyl and aldehydic groups such as
would be expected from 3-hydroxy propanal. Until a
calibrated reference spectrum becomes available, this iden-
tification remains very speculative.
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to the existence of (at least) two sources for this compound.
Decomposition of the δ-hydroxyalkoxy radical (VI) could also
lead to the formation of ethanal (Scheme 3). If there are no
additional sources of ethanal in the experimental system,
then this channel would account for the remaining 10%
ethanal and would also constitute the main reaction channel
for the δ-hydroxyalkoxy radical (VI). The possibility of the
existence of other decomposition channel leading to methyl
radical plus 4-hydroxybutanal cannot be excluded because
of the lack of an authentic sample of the compound.
On the basis of the similarity of its HPLC retention time
and its infrared spectrum with that of an authentic sample,
a product has been tentatively identified as 5-hydroxy-2-
pentanone. This product can be formed from the reaction
of O2 with the δ-hydroxyalkoxy radical (VI) (Scheme 3).
However, due to a lack of any analytical characterization
data for 4-hydroxypentanal and because of the probable
similarity of these data with those available for 5-hydroxy-
2-pentanone, being both δ-hydroxycarbonyl compounds, we
cannot completely exclude that the observed product is
4-hydroxypentanal. It is evident from the above discussion
that our analysis cannot give definitive information on the
relative importance of the decomposition, isomerization, and
O2 reaction channels of the δ-hydroxyalkoxy radical (VI).
There is, however, evidence that the decomposition channel
might dominate. Experiments need to be performed in a
system where reactions of the δ-hydroxyalkoxy radical (VI)
will dominate the product distribution.
support by the “Generalitat Valenciana”. F.C. gratefully
acknowledges the European Commission for funding her
research at the Bergische Universita¨t Wuppertal within the
framework of the Environmental and Climate Research and
the Specific RTD Programme. Support of the work within
the 4th Framework Programme (EUROSOLV) is also gratefully
acknowledged.
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CH3CH2CH2CH2CH2OH + OH + 0.78NO f
0.44CH3CH2CH2CH2C(O)H + 0.16CH3CH2CH2C(O)H +
0.02OHCH2CH2CH2C(O)CHOH + 0.09CH3CH2C(O)H +
0.02OHCH2CH(O)H + 0.18OHCH2CH2C(O)H +
0.05CH3C(O)CH2CH2CH2OH + 0.18CH3C(O)H +
0.3HCHO + 0.87HO2 + 0.72NO2 + 0.06RONO2
In the derivation of the above equation, to achieve 100%
mass balance, we have attributed slightly more significance
to the attack of the OH radical at the R-position of the alcohol
than that found in the experimental results and have included
the possible occurrence of the isomerization pathway of the
â-hydroxyalkoxy radical to form 1,5-dihydroxy-2-pentanone.
Furthermore, a molar formation yield of 0.05 has been
assumed for 5-hydroxy-2-pentanone as estimated from the
concentration of the reference spectrum, and it has also been
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reaction with O2 to give 3-hydroxypropanal.
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Received for review January 18, 2000. Revised manuscript
received June 1, 2000. Accepted June 20, 2000.
Acknowledgments
We thank the technical staff of CEAM for their assistance
with the measurements in EUPHORE and the financial
ES000016P
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