Atmospheric chemistry of 3-pentanol: Kinetics, mechanisms, and products of Cl atom and OH radical initiated oxidation in the presence and absence of NOx

Article abstract of DOI:10.1021/jp803637c

Smog chamber/FTIR techniques were used to study the atmospheric chemistry of 3-pentanol and determine rate constants of k(Cl+3-pentanol) = (2.03 ± 0.23) × 10^-10 and k(OH+3-pentanol) = (1.32 ± 0.15) × 10^-11 cm³ molecule

Full text of DOI:10.1021/jp803637c

J. Phys. Chem. A 2008, 112, 8053–8060
8053
Atmospheric Chemistry of 3-Pentanol: Kinetics, Mechanisms, and Products of Cl Atom and
OH Radical Initiated Oxidation in the Presence and Absence of NOX
M. D. Hurley,*,† T. J. Wallington,† M. Bjarrum,‡ M. S. Javadi,‡ and O. J. Nielsen‡
Systems Analytics and EnVironmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122,
Dearborn, Michigan 48121-2053, and Department of Chemistry, UniVersity of Copenhagen,
UniVersitetsparken 5, DK-2100 Copenhagen, Denmark
ReceiVed: April 25, 2008
Smog chamber/FTIR techniques were used to study the atmospheric chemistry of 3-pentanol and determine
rate constants of k(Cl+3-pentanol) ) (2.03 ( 0.23) × 10-10 and k(OH+3-pentanol) ) (1.32 ( 0.15) × 10-11
cm3 molecule-1 s-1 in 700 Torr of N2/O2 diluent at 296 ( 2 K. The primary products of the Cl atom initiated
oxidation of 3-pentanol in the absence of NO were (with molar yields) 3-pentanone (26 ( 2%), propionaldehyde
(12 ( 2%), acetaldehyde (13 ( 2%) and formaldehyde (2 ( 1%). The primary products of the Cl atom
initiated oxidation of 3-pentanol in the presence of NO were (with molar yields) 3-pentanone (51 ( 4%),
propionaldehyde (39 ( 2%), acetaldehyde (44 ( 4%) and formaldehyde (4 ( 1%). The primary products of
the OH radical initiated oxidation of 3-pentanol in the presence of NO were (with molar yields) 3-pentanone
(58 ( 3%), propionaldehyde (28 ( 2%), and acetaldehyde (37 ( 2%). In all cases the product yields were
independent of oxygen concentration over the partial pressure range 10-700 Torr. The reactions of Cl atoms
and OH radicals with 3-pentanol proceed 26 ( 2 and 58 ( 3%, respectively, via attack on the 3-position to
give an R-hydroxyalkyl radical, which reacts with O2 to give 3-pentanone. The results are discussed with
respect to the literature data and atmospheric chemistry of 3-pentanol.
1. Introduction
Concerns regarding energy security and climate change have
renewed interest in alternative transportation fuels derived from
biogenic sources. The two classes of biofuels currently receiving
attention are methyl esters of long chain fatty acids for blending
with diesel fuel and alcohols for blending with gasoline. Currently,
ethanol is the dominant alcohol biofuel; however, there is interest
in moving to larger alcohols to reduce water induced phase
separation of alcohol-gasoline blends.1 The use of alcohols as
transportation fuels will result in their release into the atmosphere.
Once in the atmosphere, the fate of alcohols will be photochemical
oxidation initiated by the OH radical.2 Prior to any large scale
industrial use of larger alcohols, an assessment of their atmospheric
chemistry and environmental impact is needed. There is a
substantial kinetic and mechanistic database for small alcohols such
and methanol and ethanol,3 and the atmospheric oxidation mech-
anisms of these species are well understood. However, the kinetic
and mechanistic database for larger alcohols is rather sparse and
detailed descriptions of their atmospheric oxidation mechanisms
are, in general, not available. For example, the reaction of
3-pentanol with OH radicals has been the subject of just one kinetic
study and this was conducted at just one temperature (ambient);
there have been no product studies of the atmospheric oxidation
of 3-pentanol. To improve our understanding of the atmospheric
chemistry of alcohols, the kinetics and mechanism of the simulated
atmospheric oxidation of 3-pentanol was studied using the smog
chamber at Ford Motor Company.
Figure 1. Loss of 3-pentanol versus ethene (filled symbols) and
propene (open symbols) in the presence of Cl atoms in 700 Torr air at
296 ( 2 K. Different symbols are results from different experiments.
2. Experimental Section
Experiments were performed in a 140 L Pyrex reactor interfaced
to a Mattson Sirius 100 FTIR spectrometer.4 The reactor was
surrounded by 22 fluorescent blacklamps (GE F40T12BLB) that
were used to photochemically initiate the experiments. Chlorine
atoms were produced by photolysis of molecular chlorine.
Cl2 + hν f Cl + Cl
(1)
* Corresponding author, email address mhurley3@ford.com.
† Ford Motor Company.
OH radicals were produced by photolysis of CH3ONO in the
presence of NO in air.
‡ University of Copenhagen.
10.1021/jp803637c CCC: $40.75
2008 American Chemical Society
Published on Web 08/12/2008

8054 J. Phys. Chem. A, Vol. 112, No. 35, 2008
Hurley et al.
CH3ONO + hν f CH3O + NO
CH3O + O2 f HO2 + HCHO
HO2 + NO f OH + NO2
(2)
(3)
(4)
Relative rate techniques were used to measure the rate constant
of interest relative to a reference reaction whose rate constant
has been established previously. The relative rate method is a
well established technique for measuring the reactivity of Cl
atoms and OH radicals with organic compounds.5 Kinetic data
are derived by monitoring the loss of 3-pentanol relative to one
or more reference compounds. The decays of 3-pentanol and
the reference are then plotted using the expression
[reactant]0
[reactant]t
kreactant
[reference]0
[reference]t
ln
)
ln
(I)
(
)
(
)
kreference
where [reactant]0, [reactant]t, [reference]0 and [reference]t are
the concentrations of 3-pentanol and the reference compound
at times “0” and “t”, and kreactant and kreference are the rate
constants for reactions of Cl atoms or OH radicals with the
3-pentanol and the reference compound. Plots of ln([reactant]0/
[reactant]t) versus ln([reference]0/[reference]t) should be linear,
Figure 2. Loss of 3-pentanol versus ethene (filled symbols) and
propene (open symbols) in the presence of OH radicals in 700 Torr air
at 296 ( 2 K. Different symbols are results from different experiments.
pass through the origin and have a slope of kreactant/kreference
.
CH3ONO was synthesized by the dropwise addition of
concentrated sulfuric acid to a saturated solution of NaNO2 in
methanol. 3-Pentanol was obtained from Sigma-Aldrich at a
purity of 98%. Experiments were conducted in 700 Torr total
pressure of high purity O2/N2 diluent at 296 ( 2 K. Concentra-
tions of reactants and products were monitored by FTIR
spectroscopy. IR spectra were derived from 32 coadded inter-
ferograms with a spectral resolution of 0.25 cm-1 and an
analytical path length of 27.1 m. To check for unwanted loss
of reactants and reference compounds via heterogeneous reac-
tions, reaction mixtures were left to stand in the chamber for
60 min. With the exception of CH3CCl(OH)CH3, there was no
observable (<2%) loss of any of the reactants or products in
the present work. Unless stated otherwise, quoted uncertainties
are 2 standard deviations from least-squares regressions.
mTorr C3H6 in 700 Torr of air diluent. The observed loss of
3-pentanol versus the loss of the reference compounds is plotted
in Figure 1. Linear least-squares analysis of the data in Figure
1 gives k5/k6 ) 2.20 ( 0.20 and k5/k7 ) 0.76 ( 0.08. Using k6
) 9.29 × 10-11 6 and k7 ) 2.64 × 10-10 7 gives k5 ) (2.04 (
0.19) × 10-10 and (2.01 ( 0.21) × 10-10 cm3 molecule-1 s-1
.
We cite a final value which is the average of the individual
determinations together with error limits which encompass the
extremes of the determinations, k5 ) (2.03 ( 0.23) × 10-10
cm3 molecule-1 s-1
.
There have been no previous studies of k5; however, we
can compare our result with the rate constants reported for
other alcohols. As shown in Table 1, there is a general trend
of increasing reactivity of alcohols with increasing molecular
size.8-11 The reactivity of 3-pentanol measured in the present
work is consistent with this trend. Also, as seen from the
literature data for 1- and 2-propanol and 1- and 2-butanol in
Table 1, secondary alcohols are less reactive than primary
alcohols. Inspection of the data in Table 1 shows that the
reactivity of 3-pentanol, although indistinguishable from that
for 2-pentanol, is significantly less than that for 1-pentanol.
Our kinetic data for 3-pentanol are consistent with the
available database for the reactions of chlorine atoms with
alcohols.
3. Results and Discussion
3.1. Kinetics of the Cl + 3-Pentanol Reaction. The rate of
reaction 5 was measured relative to reactions 6 and 7
Cl + CH3CH2CH(OH)CH2CH3 f products
Cl + C2H4 f products
(5)
(6)
(7)
Cl + C3H6 f products
Reaction mixtures consisted of 7.4-66.3 mTorr 3-pentanol,
100-120 mTorr Cl2 and either 2.4-8.0 mTorr C2H4 or 4.4-7.6
TABLE 1: Summary of the Rate Constants for the Reaction of Cl Atoms with Alcohols at Ambient Temperature Reported in
This Work and Selected Previous Studies
k(Cl) at 298 K (10-11 cm3 molecule-1 s-1
)
this work
Garzo´n et al.8
Ballesteros et al.9
Wu et al.11
Nelson et al.10
IUPAC3
methanol
ethanol
5.44 ( 0.34
9.93 ( 0.98
15.3 ( 1.2
4.79 ( 0.36
10.1 ( 0.6
14.9 ( 0.7
8.4 ( 0.4
5.5
10.0
16.0
8.6
9.95 ( 0.7
15.0 ( 1.3
7.8 ( 0.7
21.7 ( 1.1
1-propanol
2-propanol
1-butanol
2-butanol
1-pentanol
2-pentanol
3-pentanol
1-hexanol
19.6 ( 1.9
23.7 ( 2.9
20.4 ( 1.4
12.6 ( 2
25.8 ( 2.5
25.1 ( 1.3
21.8 ( 3.6
20.3 ( 2.3
29.5 ( 0.9

Atmospheric Chemistry of 3-Pentanol
J. Phys. Chem. A, Vol. 112, No. 35, 2008 8055
TABLE 2: Summary of the Rate Constants for the Reaction of OH Radicals with Alcohols at Ambient Temperature Reported
in This Work and Selected Previous Studies
k(OH) at 298 K (10-12 cm3 molecule-1 s-1
)
this work
Wallington et al.13
Cavalli et al.14,15
Wu et al.11
Nelson et al.10
IUPAC3
methanol
ethanol
0.90 ( 0.09
3.04 ( 0.85
5.64 ( 0.48
5.69 ( 1.09
7.80 ( 0.20
0.9
3.2
5.8
5.1
8.5
8.7
3.4 ( 0.25
5.47 ( 0.44
5.31 ( 0.39
8.66 ( 0.66
1-propanol
2-propanol
1-butanol
2-butanol
1-pentanol
2-pentanol
3-pentanol
1-hexanol
8.28 ( 0.85
11.1 ( 1.1
12.3 ( 1.0
12.0 ( 1.6
11.8 ( 0.8
12.2 ( 0.7
12.4 ( 0.7
13.2 ( 1.5
12.2 ( 2.4
3.2. Kinetics of the OH + 3-Pentanol Reaction. The rate
9.7-13.2 mTorr C3H6 in 700 Torr total pressure of air diluent.
Figure 2 shows the loss of 3-pentanol plotted versus loss of the
reference compounds. Linear least-squares analysis gives k8/k9
) 1.61 ( 0.15 and k8/k10 ) 0.47 ( 0.50. Using k9 ) 8.52 ×
10-12 12 and k10 ) 2.9 × 10-11 3 gives k8 ) (1.37 ( 0.13) ×
of reaction 8 was measured relative to reactions 9 and 10:
OH + CH3CH2CH(OH)CH2CH3 f products
OH + C2H4 f products
(8)
(9)
10-11 and (1.36 ( 0.15) × 10-11 cm3 molecule-1 s-1
.
OH + C2H2 f products
(10)
Indistinguishable values of k8 are obtained using the two
different references. We cite a final value which is the average
of the individual determinations together with error limits which
Reaction mixtures consisted of 74.3-81.0 mTorr 3-pentanol,
100-153 mTorr CH3ONO, and either 4.6-7.8 mTorr C2H4 or
Figure 3. IR spectra obtained before (A) and after (B) a 250 s irradiation of 65.7 mTorr, 3-pentanol and 99 mTorr Cl2 in 700 Torr air. Panel C
shows the product spectrum obtained by subtracting 55% of panel A from panel B. Panel D shows the residual product spectrum after the features
attributable to CH3CHO (E), CH3CH2CHO (F), and 3-pentanone (G) have been subtracted from panel C.

8056 J. Phys. Chem. A, Vol. 112, No. 35, 2008
Hurley et al.
compounds. Panel D shows the IR features that remain after
subtraction of the features from acetaldehyde, propionaldehyde,
and 3-pentanone. In addition, there are features in the residual
spectrum at 3500-3700 cm-1, which are characteristic of
alcohols. These residual IR features are ascribed to unidentified
product(s). Figure 4 shows the formation of 3-pentanone,
acetaldehyde, and propionaldehyde versus the loss of 3-pentanol
for experiments with 10 Torr O2, 50 Torr O2, 140 Torr O2 and
700 Torr O2 in 700 Torr total pressure made up with N2 in the
absence of NO. The data have been corrected for secondary
reactions of the products with Cl atoms using the procedure
described elsewhere.18 Inspection of the data in Figure 4 reveals
that there was no discernible impact of [O2] on the product
yields. Linear least-squares analysis of the composite 3-pen-
tanone data set gives a molar yield of 26 ( 2%. The yield plots
of acetaldehyde and propionaldehyde are curved, suggesting that
there are both primary and secondary sources of these com-
pounds. The initial yields of acetaldehyde and propionaldehyde
are 13 ( 2% and 12 ( 2%, respectively, and represent primary
product yields. Formaldehyde is present in a yield of 2 ( 1%.
Given the very small amounts of formaldehyde observed, within
the experimental uncertainties, we are not able to exclude the
possibility that a significant fraction, perhaps all, of the observed
HCHO may come from secondary reactions.
Figure 4. Formation of 3-pentanone (black), propionaldehyde (gray),
acetaldehyde (open) and formaldehyde (crossed) versus the loss of
3-pentanol following UV irradiation of 3-pentanol/Cl2 with 10 Torr
O2 (circles), 50 Torr O2 (triangles up), 140 Torr O2 (triangles down),
or 700 Torr O2 (diamonds) in 700 Torr total pressure of nitrogen diluent.
Formation of 3-pentanone following the UV irradiation of 3-pentanol/
Cl2 in N2 is shown with stars. The 3-pentanone data is offset for clarity.
The reaction of Cl atoms with 3-pentanol proceeds via three
channels
Cl + CH3CH2CH(OH)CH2CH3 f
CH3CH2C(OH)CH2CH3 + HCl (5a)
Cl + CH3CH2CH(OH)CH2CH3 f
CH3CH2CH(OH)CHCH3 + HCl (5b)
Cl + CH3CH2CH(OH)CH2CH3 f
CH3CH2CH(OH)CH2CH2 + HCl (5c)
By analogy to the behavior of other hydroxyalkyl radicals,3 the
R-hydroxyalkyl radical formed via attack at the 3-position is
expected to react exclusively with oxygen to form 3-pentanone
and HO2.
encompass the extremes of the determinations, k8 ) (1.32 (
0.15) × 10-11 cm3 molecule-1 s-1. Wallington et al.13 measured
k8 using an absolute technique and report k8 ) (1.22 ( 0.07) ×
10-11 cm3molecule-1 s-1, which is in agreement, within the
experimental uncertainties, with our result.
From inspection of Table 2 we can make two general
observations regarding the reactivity of 3-pentanol toward OH
radicals measured in the present work compared to the kinetic
database for alcohols.10,11,13-15 First, the magnitude of k8 is
consistent with expectations based upon the trend of increasing
reactivity with size of the alcohol. Second, unlike the situation
with chlorine atoms described in the previous section, there is
no discernible difference in reactivity of primary and secondary
alcohols toward OH radicals (e.g., compare 1- and 2- propanol,
and 1- and 2-butanol in Table 2) and our results for 3-pentanol
show that this holds true for pentanol.
CH3CH2C(OH)CH2CH3 + O2 f CH3CH2C(O)CH2CH3 +
HO2 (11)
The structure-activity relationship (SAR) method of Kwok
and Atkinson16 is useful for estimating the rate constants for
the gas-phase reactions of the OH radical with organic com-
pounds. Bethel et al.17 have proposed new substituent factors
for use with alcohols and diols. The updated SAR method
predicts k(OH+3-pentanol) ) 1.38 × 10-11 cm3 molecule-1
s-1, which is consistent with the result from the present work.
3.3. Products and Mechanism of Cl Atom Initiated
Oxidation of 3-Pentanol in the Absence of NO. The mech-
anism of Cl atom initiated oxidation of 3-pentanol was
investigated by irradiating mixtures of 34-71 mTorr 3-pentanol
and 99-1010 mTorr Cl2 in 0-700 Torr oxygen. Nitrogen was
added as needed to provide 700 Torr total pressure. Figure 3
shows spectra acquired before (A) and after (B), a 250 s
irradiation of a mixture of 65.7 mTorr 3-pentanol and 99 mTorr
Cl2 in 700 Torr air. The consumption of 3-pentanol in this
experiment was 45%. Panel C shows the product spectrum
derived by subtracting the IR features of 3-pentanol from the
spectrum in panel B. Comparison of the IR features in panel C
with reference spectra of acetaldehyde, propionaldehyde, and
3-pentanone in panels E, F, and G shows the formation of these
The ꢀ and γ hydroxyalkyl radicals formed from the abstrac-
tion of a hydrogen atom at the 2 and 1 positions, respectively,
will add oxygen to give hydroxyalkylperoxy radicals. In the
absence of NO, the hydroxyalkylperoxy radicals will react
with peroxy radicals to give hydroxyalkoxy radicals. As
illustrated in Figures 5 and 6, the fate of the hydroxyalkoxy
radicals is expected to be reaction with O2, decomposition,
or isomerization via a hydrogen shift through a six-member
transition state.
In addition to the above reactions, a variety of other
reactions probably occur in the system. In the discussion
above, it is assumed that RO2 + RO2 reactions result in the
formation of alkoxy radicals. Typically, such RO2 + RO2
reactions also proceed via a molecular channel to form ROH
and R′CHO.3 Furthermore, the formation of HO2 radicals
(e.g., via reaction 12) raises the possibility that reaction of
RO2 radicals with HO2 radicals to form hydroperoxides,19
ROOH, is important in the system. These possibilities are
included in Figures 5 and 6. The isomerization pathways in
Figures 5 and 6 give 1,4-hydroxycarbonyls that can undergo

Atmospheric Chemistry of 3-Pentanol
J. Phys. Chem. A, Vol. 112, No. 35, 2008 8057
reversible cyclization to cyclic hemiacetals which can
dehydrate to form reactive dihydrofurans.20,21
CH3CH2CCl(OH)CH2CH3 f CH3CH2C(O)CH2CH3 + HCl
(13)
Further information on the branching ratio for chlorine
atom attack on 3-pentanol was obtained by conducting
experiments with 3-pentanol and Cl2 in 700 Torr N2. Under
these conditions, attack of Cl at the 3-position results in the
formation of the chlorinated alcohol CH3CH2CCl(OH)CH2-
CH3.
The 3-pentanone yield determined in experiments conducted
in N2 is shown in Figure 4 and was indistinguishable from the
3-pentanone yield observed in the presence of oxygen. From
the yields of 3-pentanone in the presence and absence of oxygen
we conclude that k5a/k5 ) 0.26 ( 0.02. As seen from inspection
of Figures 5 and 6, the only primary reaction that gives
acetaldehyde is decomposition of the CH3CH2CH(OH)CH(O)-
CH3 radical formed from abstraction of a H atom in the
2-position. Furthermore, the channel giving acetaldehyde (see
Figure 5) leads to formation of an equal amount of propional-
dehyde. Consistent with expectations the primary yields of
acetaldehyde and propionaldehyde, 0.13 ( 0.02 and 0.12 ( 0.02,
respectively, were indistinguishable; see Figure 4. Recognizing
two factors, (i) the possibility of RO2 + RO2 and RO2 + HO2
reactions discussed above which reduce the efficiency with
which the peroxy radical CH3CH2CH(OH)CH(OO)CH3 is
converted into the alkoxy radical CH3CH2CH(OH)CH(O)CH3
CH3CH2CH(OH)CH2CH3 + Cl f
CH3CH2C(OH)CH2CH3 + HCl (5a)
CH3CH2C(OH)CH2CH3 + Cl2 f
CH3CH2CCl(OH)CH2CH3 + Cl (12)
R-Chloro-alcohols such as CH2ClOH, CHCl2OH, CCl3OH,
CH3CHClOH, and CH3CClOHCH3 decompose heterogeneously
in the chamber via elimination of HCl22-24 to give the
corresponding carbonyl compound. In the case of CH3CH2-
CCl(OH)CH2CH3 this gives 3-pentanone, which then provides
a marker for reaction 5a.
Figure 5. Fate of the ꢀ-hydroxyalkyl radical in the Cl atom/OH radical initiated oxidation of 3-pentanol.

8058 J. Phys. Chem. A, Vol. 112, No. 35, 2008
Hurley et al.
Figure 6. Fate of the γ-hydroxyalkyl radical in the Cl atom/OH radical initiated oxidation of 3-pentanol.
and (ii) the fact that we are not able to exclude the possibility
that isomerization and decomposition via methyl elimination
are competing fates for the alkoxy radical (see Figure 5), we
conclude that k5b/k5 g 0.13 ( 0.02.
The reaction of OH radicals with 3-pentanol proceeds via
three channels
OH + CH3CH2CH(OH)CH2CH3 f
CH3CH2C(OH)CH2CH3 + H2O (8a)
OH + CH3CH2CH(OH)CH2CH3 f
CH3CH2CH(OH)CHCH3 + H2O (8b)
OH + CH3CH2CH(OH)CH2CH3 f
CH3CH2CH(OH)CH2CH2 + H2O (8c)
3.4. Products and Mechanism of OH Radical Initiated
Oxidation of 3-Pentanol. The mechanism of OH radical
initiated oxidation of 3-pentanol in the presence of NOx was
investigated by irradiating mixtures consisting of 65-67 mTorr
of 3-pentanol, 96-122 mTorr CH3ONO and 0-50 mTorr NO
in 700 Torr air. Figure 7 shows the formation of 3-pentanone,
acetaldehyde and propionaldehyde versus the loss of 3-pentanol
for experiments in 700 Torr air in the presence of NO. The
data have been corrected for losses via secondary reaction of
the products with OH radicals.18 Linear least-squares analyses
of the data in Figure 7 give molar yields of 3-pentanone,
acetaldehyde, and propionaldehyde of 58 ( 3%, 37 ( 2%, and
28 ( 2%, respectively. Quoted uncertainties are two standard
deviations from the regression analyses.
As discussed in section 3.3, reaction 11 is the sole fate of
the CH3CH2C(OH)CH2CH3 radical and hence from the 3-pen-
tanone yield we conclude that k8a/k8 ) 0.58 ( 0.03. The only
source of acetaldehyde as a primary product is decomposition
of the CH3CH2CH(OH)CH(O)CH3 radical formed from abstrac-
tion of a hydrogen atom in the 2-position, which should lead to
equal amounts of acetaldehyde and propionaldehyde. From the

Atmospheric Chemistry of 3-Pentanol
J. Phys. Chem. A, Vol. 112, No. 35, 2008 8059
yield plots in Figure 4, the yields of acetaldehyde and propi-
onaldehyde as primary products are 37 ( 2% and 28 ( 2%,
respectively. Recognizing the possibility of nitrate formation
in reaction of the peroxy radical with NO 3 and isomerization
of the alkoxy radical, we conclude that k8b/k8 g 0.28 ( 0.02.
The observed products account for 86% of the reacted 3-pen-
tanol.
The SAR method is also useful for estimating branching ratios
for the various sites of attack of the OH radicals with an organic
compound. Using the SAR method of Kwok and Atkinson16
with the refinements of Bethel et al.17 gives values of 0.61, 0.35
and 0.02 for k8a/k8, k8b/k8 and k8c/k8, respectively. These values
are consistent with those for k8a/k8 and k8b/k8 determined
experimentally.
3.5. Products and Mechanism of Cl Atom Initiated
Oxidation of 3-Pentanol in the Presence of NO. The mech-
anism of Cl atom initiated oxidation of 3-pentanol in the
presence of NO was investigated by irradiating mixtures
consisting of 66-74 mTorr 3-pentanol, 100-105 mTorr Cl2,
27-58 mTorr NO, and 10-140 Torr oxygen in 700 Torr total
pressure of nitrogen diluent.
Figure 8 shows the formation of 3-pentanone, acetaldehyde,
propionaldehyde, and formaldehyde versus the loss of 3-pentanol
for experiments with 10 Torr O2, 100 Torr O2, or 140 Torr O2
in 700 Torr of nitrogen diluent in the presence of NO. The data
in Figure 8 have been corrected for losses via secondary reaction
of the products with Cl atoms. Data for different O2 concentra-
tions are indistinguishable. Linear least-squares analysis of the
composite data sets gives initial molar yields of 51 ( 4% for
3-pentanone, 44 ( 4% for CH3C(O)H, 39 ( 2% for
CH3CH2C(O)H, and 4 ( 1% for HC(O)H. Quoted uncertainties
are two standard deviations from the regression analyses.
It is of interest to compare the products observed in the
absence of NO (see section 3.3) and those observed in the
presence of NO. The presence of NO ensures rapid removal of
RO2 (converted into RO radicals and, to a small degree, organic
nitrates, RONO2) and HO2 radicals.
Figure 7. Formation of 3-pentanone (black), propionaldehyde (gray),
and acetaldehyde (open) versus loss of 3-pentanol following UV
irradiation of 3-pentanol /CH3ONO mixtures in 700 Torr of air
diluent.
RO2 + NO f RO + NO2
RO2 + NO + M f RONO2 + M
HO2 + NO f OH + NO2
(14a)
(14b)
(15)
The presence of NO simplifies the mechanistic interpretation
of the product yields because it provides a more direct
conversion of peroxy into alkoxy radicals via reaction 14a and
removes the need to consider RO2 + RO2 and RO2 + HO2
reactions. However, the presence of NO adds complexity to the
analysis because of the need to consider three additional factors:
(i) the possibility that both Cl atoms and OH radicals (formed
in reaction 15) initiate the oxidation of 3-pentanol, (ii) the effect
of chemical activation of the alkoxy radicals formed in reaction
14a25 and (iii) the formation of small amounts of organic nitrates
in channel 14b.
The increase in initial acetaldehyde yield from 13% to 44%
and the increase in initial propionaldehyde yield from 12% to
39% with the addition of NO probably reflects more efficient
peroxy to alkoxy radical conversion in the system as a result of
the elimination of RO2 + RO2 and RO2 + HO2 chemistry. The
increase in the yield of 3-pentanone from 26% to 51% with the
addition of NO probably reflects a substantial contribution of
OH radical initiated oxidation of 3-pentanol. As illustrated in
Figures 5 and 6, for each molecule of 3-pentanol that is lost
via reaction with chlorine atoms an HO2 radical is formed. HO2
radicals react with NO to give OH radicals which will then react
Figure 8. Formation of 3-pentanone (black), propionaldehyde (gray),
acetaldehyde (open) and formaldehyde (dotted) versus loss of 3-pentanol
following UV irradiation of 3-pentanol/Cl2/NO mixtures with 10 Torr
O2 (circles), 100 Torr O2 (triangles), or 140 Torr O2 (inverted triangles)
in 700 Torr of nitrogen diluent. The 3-pentanone data are offset for
clarity.
with 3-pentanol starting another cycle. For typical initial
experimental conditions with [3-pentanol] ) 70 mTorr and [NO]
) 42 mTorr and using k(OH+3-pentanol) ) 1.3 × 10-11 and
k(OH+NO) ) 9.0 × 10-12 cm3molecule-1 s-1 26
,
it can be
calculated that approximately 70% of OH radicals will react
with 3-pentanol with the remaining 30% reacting with NO to
form HONO. The formation of HONO and HNO3 (formed from
addition of OH to NO2) was confirmed by examination of the
IR product spectra. In addition to gas phase chemistry, HONO
and HNO3 are probably also formed in the chamber via the
heterogeneous reaction of NO2 with H2O on the chamber walls.27
4. Implications for Atmospheric Chemistry
We present a large body of self-consistent data concerning
the kinetics and mechanisms of the Cl atom and OH radical

8060 J. Phys. Chem. A, Vol. 112, No. 35, 2008
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initiated oxidation of 3-pentanol, which increases our knowledge
of the atmospheric chemistry of this compound. The atmospheric
oxidation of 3-pentanol will be initiated by reaction with OH
radicals, which occurs with a rate constant of k8 ) (1.32 (
0.15) × 10-11 cm3 molecule-1 s-1 in 700 Torr of air diluent at
296 K. Using an estimate for the 24 h average OH radical
concentration in the atmosphere of 106 molecules cm-3 2 gives
an estimate for the atmospheric lifetime of 3-pentanol of
approximately 1 day. The atmospheric oxidation of 3-pentanol
gives 3-pentanone as the major product in a yield of 58 ( 3%.
The reactivity of 3-pentanone toward OH radicals is ap-
proximately 6 times lower than that of 3-pentanol,28-30 and
hence 3-pentanone has an atmospheric lifetime of approximately
1 week. This lifetime is sufficiently long that most of the
3-pentanone will be transported out of urban areas and will not
contribute further to local air quality issues. CH3CHO and
C2H5CHO are formed in yields of 37 ( 2% and 28 ( 2%. With
respect to reaction with OH radicals, CH3CHO has ap-
proximately the same reactivity as 3-pentanol and C2H5CHO
is approximately 50% more reactive than 3-pentanol. A
substantial fraction of the CH3CHO and C2H5CHO products
will react in urban areas and contribute to local air quality issues.
The present study provides kinetic and mechanistic data that
can be used in future computer model studies of urban air
chemistry to assess the environmental impact of emissions of
3-pentanol. Such studies are beyond the scope of the present
work.
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Acknowledgment. O.J.N. thanks the Danish Natural Science
Research Council and the Villum Kann Rasmussen Foundation
for financial support.
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References and Notes
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JP803637C