Rate coefficients for the reactions of Cl atoms with a series of C3-C6 hydroxyalkyl nitrates at 296 ± 2 K

Article abstract of DOI:10.1021/jp020088y

Rate coefficients for the gas-phase reactions of chlorine atoms with a series of C₃-C₆ hydroxyalkyl nitrates of atmospheric interest have been determined at 296 ± 2 K and atmospheric pressure. The experiments were conducted using the relative rate technique combined with solid-phase microextraction (SPME) sampling followed by gas chromatography (GC) analysis with an electron capture detector (ECD). The experiments were performed in a collapsible 100 L PVF-film (Tedlar) reaction chamber. It is shown that the presence of the hydroxy group enhances the reactivity of the hydroxyalkyl nitrates toward the Cl atom as compared to the corresponding alkyl nitrates and alkyl dinitrates. The Cl atom reactivity toward the hydroxyalkyl nitrates increases with the length of the alkyl chain and with increasing separation between the hydroxy and the nitrooxy groups. Tropospheric lifetimes are calculated using the determined rate coefficients and the atmospheric implications are briefly discussed.

Full text of DOI:10.1021/jp020088y

5902
J. Phys. Chem. A 2002, 106, 5902-5907
Rate Coefficients for the Reactions of Cl Atoms with a Series of C3-C6 Hydroxyalkyl
Nitrates at 296 ( 2 K
Keren Treves, Lea Shragina, and Yinon Rudich*
Department of EnVironmental Sciences, Weizmann Institute, RehoVot 76100, Israel
ReceiVed: January 14, 2002; In Final Form: April 11, 2002
Rate coefficients for the gas-phase reactions of chlorine atoms with a series of C
3
6
-C hydroxyalkyl nitrates
of atmospheric interest have been determined at 296 ( 2 K and atmospheric pressure. The experiments were
conducted using the relative rate technique combined with solid-phase microextraction (SPME) sampling
followed by gas chromatography (GC) analysis with an electron capture detector (ECD). The experiments
were performed in a collapsible 100 L PVF-film (Tedlar) reaction chamber. It is shown that the presence of
the hydroxy group enhances the reactivity of the hydroxyalkyl nitrates toward the Cl atom as compared to
the corresponding alkyl nitrates and alkyl dinitrates. The Cl atom reactivity toward the hydroxyalkyl nitrates
increases with the length of the alkyl chain and with increasing separation between the hydroxy and the
nitrooxy groups. Tropospheric lifetimes are calculated using the determined rate coefficients and the atmospheric
implications are briefly discussed.
in the early morning hours have been predicted.1
7-19
The
Introduction
concentration of Cl atoms in the global troposphere is lower
Organic nitrates are reservoir species for reactive nitrogen
oxides. They form during the atmospheric photodegradation of
hydrocarbons in the presence of nitrogen oxides. This process
competes with the chemical cycle leading to ozone production
2
5
-3 16,19-21
and is estimated to be 10 -10 cm .
Considering that
Cl rate coefficients are often a factor of 10 higher than the
corresponding OH rate coefficients, it is possible that the
reactions with Cl atoms may play a significant role in the
degradation of organic compounds in urban coastal air. A
detailed understanding of the atmospheric chemistry of hy-
droxyalkyl nitrates is needed for accurate assessment of their
environmental impact. For this, kinetic and mechanistic data
concerning the reaction of Cl atoms is needed. The kinetics of
the reactions of chlorine atoms with alkyl nitrates have been
1-4
since it sequesters both nitrogen oxides and organic radicals.
A common class of bifunctional organic nitrates are the
hydroxyalkyl nitrates that form in the atmospheric photodeg-
radation of alkanes and alkenes. Several pathways for their
formation exist. â-Hydroxyalkyl nitrates form in the oxidation
of alkenes by OH radicals.5 δ-Hydroxyalkyl nitrates result from
a 1,5-H shift of alkoxy radicals following the reactions of the
,6
2
2,23
studied by several researchers.
However, to the best of our
7-10
OH radical with long chain (g4) alkanes.
Pathways leading
knowledge, the reactions of multifunctional organic nitrates,
such as the hydroxyalkyl nitrates with atomic chlorine, have
not yet been investigated.
to the formation of γ-Hydroxyalkyl nitrates in the atmosphere
are not well-known. However, it was recently proposed that
NO3 reactions with unsaturated alcohols such as 2-methyl-3-
buten-2-ol could form various products including an organic
The purpose of this study is to determine the rate coefficients
for the gas-phase reactions of chlorine atoms with a series of
hydroxyalkyl nitrates of atmospheric interest at atmospheric
pressure and temperature of 296 ( 2 K and to understand the
effect of the hydroxyalkyl nitrates structure on their reactivity.
For this purpose, a relative rate technique was employed,
combined with solid-phase microextraction (SPME) sampling
and analysis by GC equipped with an electron capture detector
1
1
nitrate with the hydroxy and nitrooxy groups in a γ position.
Although organic nitrate formation yield is small for short
1
2
n-alkanes, the high emission flux of their precursors in the
urban atmosphere may lead to significant production of the
corresponding nitrates.
Hydroxyalkyl nitrates have been measured in the atmosphere;
C2-C4 â-hydroxyalkyl nitrates were identified and estimated
(ECD). The SPME technique offers many advantages for such
to account for ∼15% of the total atmospheric organic ni-
kinetic measurements since it is fast, sensitive, selective, solvent-
trates.1
3,14
C3-C5 â-hydroxyalkyl nitrates in the low ppt range
2
4,25
free, easy to use, and inexpensive.
This method has only
15
have been measured in marine air at about 10% of their urban
concentrations.
recently been applied to gas-phase kinetic studies.2
Principles of the Relative Rate Technique. Cl atom rate
coefficients were determined using a relative rate technique,
which has been extensively described in the literature.1
Briefly, the underlying principle of this method is to measure
the decay rate of the selected hydroxyalkyl nitrate (HN) relative
to a reference compound (R), whose Cl atom reaction rate
coefficient is known.
6,27
1
3,14
To our knowledge, γ- and δ-hydroxyalkyl
nitrates have not yet been identified in atmospheric samples,
mainly due to the lack of appropriate standards.
2,28,29
Atomic chlorine is generated by photolysis of molecular
chlorine released to the atmosphere via reactions in sea salt
particles and contributes to oxidation of organic compounds in
16
the marine boundary layer. Peak concentrations of atomic
4
5
-3
chlorine as high as 10 -10 cm in the marine boundary layer
In the presence of atomic chlorine, the reactant hydroxyalkyl
nitrate and the reference compound decay via reactions 1 and
2:
*
Corresponding author. Phone: +972-8-934-4237. Fax: +972-8-934-
4
124. E-mail: Yinon.Rudich@weizmann.ac.il.
1
0.1021/jp020088y CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/25/2002

Cl Reactions with Alkyl Nitrates
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5903
kHN
HN + Cl 8 products
(1)
(2)
kR
R + Cl
98 products
where kHN and kR are the rate coefficients for reactions 1 and
, respectively. Provided that both reactants are removed solely
2
by the reaction with Cl atoms and that they do not re-form in
the reaction chamber (see validation section), the following
relation is obtained:
[
HN]0
kHN
[R]0
[R]t
ln
)
ln
(I)
(
) ( ) (
)
[
HN]t
kR
where [HN]0 and [R]0 and [HN]t and [R]t are the concentrations
of the selected hydroxyalkyl nitrate and the reference compound
at the beginning of the experiment and at time t, respectively.
A plot of ln([HN]0/[HN]t) against ln([R]0/[R]t) yields a straight
line with a slope equal to the ratio of the rate coefficients, kHN/
kR and with zero intercept. Given the known value of kR, the
value of kHN may be calculated.
Experimental Section
1
. Chemicals and Materials. The kinetic experiments were
conducted with the following compounds: Hydroxyalkyl ni-
trates: 2-nitrooxy-1-propanol, 1-nitrooxy-2-propanol, 2-nitrooxy-
1-butanol, 1-nitrooxy-2-butanol, 3-nitrooxy-1-butanol, 4-nitrooxy-
2-butanol, 4-nitrooxy-1-butanol, 2-nitrooxy-1-pentanol, 1-nitrooxy-
2
6
-pentanol, 4-nitrooxy-1-pentanol, 5-nitrooxy-2-pentanol, and
-nitrooxy-1-hexanol. Alkyl dinitrate: 1,4-butyl dinitrate. Alkyl
nitrates (reference compounds): 1-butyl nitrate and 1-pentyl
nitrate. (For simplicity, the nomenclature introduced by Schneider
3
0
and Ballschmiter for nitrates will be used in this paper:
1OH2C3, 2OH1C3, 1OH2C4, 2OH1C4, 1OH3C4, 2OH4C4,
1OH4C4, 1OH2C5, 2OH1C5, 1OH4C5, 2OH5C5, 1OH6C6,
1,4C4, 1C4, and 1C5, respectively). For clarification, Figure 1
shows the hydroxyalkyl nitrates and the alkyl dinitrate studied
and their shorthand notation.
The hydroxyalkyl nitrates (except 1OH6C6) were all synthe-
sized by selective nitration of the parent diols or epoxides. They
were purified (>98%) and characterized according to the
previously developed procedures.3
1-33
1
OH6C6 was synthesized, for the first time, by a new
procedure: nitration of (iodobutoxy)-tert-butyldimethylsilane by
silver nitrate. The reaction occurs via two steps: a replacement
of the halogen atom by a nitrooxy group, while the tert-
butyldimethylsilane tail blocks the hydroxy group. Following
the nitration, the protecting group is removed by hydrolysis (the
detailed procedure, for the preparation of 1OH4C4, is described
in the Appendix). This synthesis method is very specific, results
in high yields, is less complicated, and demands less purification
stages as compared to our previously published synthesis
methods.32 Although the procedure described here was used for
the preparation of 1OHnCn hydroxyalkyl nitrates, other silane
reactants may be chosen for producing the corresponding
hydroxyalkyl nitrates.
Figure 1. Structure of the studied compounds and their shorthand
notation.
as the bath gas. Molecular chlorine (99.999%) was used without
any further purification.
The SPME fiber used for sampling all the nitrates was
Stableflex poly(dimethylsiloxane)/divinylbenzene, PDMS/DVB,
1
,4-Butyl dinitrate and alkyl nitrates were prepared via
3
5
nitration of the parent 1,4-butanediol and n-alcohol, respectively,
65 µm coating thickness. The fibers were conditioned inside
the GC injector port for 30 min at 260 °C before use.
by fuming HNO3 and H2SO4 in dichloromethane.34
The diluent gas used in the kinetic experiments was synthetic
air (>99.999%). Nitrogen (>99.999%) was also used, but since
no difference was observed between the experiments conducted
with either synthetic air or nitrogen, synthetic air was chosen
2. Reference Compounds. Alkyl nitrates, 1-pentyl nitrate
and 1-butyl nitrate, were chosen as reference compounds since
they have known reaction rate coefficients toward Cl atoms,
which are within a factor of 10 of the predicted hydroxyalkyl

5904 J. Phys. Chem. A, Vol. 106, No. 24, 2002
Treves et al.
nitrate rate coefficients. The rate coefficients of 1-pentyl nitrate
-
10
and 1-butyl nitrate at 296 K are kCl ) 1.57 × 10
and kCl )
-
11
3
-1 -1
23,22
9
.24 × 10
cm molecule s , respectively.
It was
verified that these alkyl nitrates do not photolyze or stick to
the walls of the reaction chamber. No chromatographic interfer-
ence between the retention times of these alkyl nitrates and the
hydroxyalkyl nitrates was observed under the conditions em-
ployed.
3
. Experimental Procedure. Kinetic experiments were
carried out at 296 ( 2 K and at atmospheric pressure using a
collapsible 100 L PVF-film (Tedlar) reaction chamber (SKC
Inc.). The reaction chamber was equipped with three inlets
positioned at its upper, central, and lower sections. The reaction
chamber was homogeneously surrounded by 20 UV fluorescent
lamps (Philips TL 40W/05, 300 e λ g 460 nm; λmax ) 365
nm). The irradiation time was controlled by a digital timer. An
electric fan was positioned on top of the reactor chamber to
maintain a uniform reaction temperature during irradiation
periods.
Desired amounts of the hydroxyalkyl nitrate and the reference
compounds were prepared off-line in a separate vacuum
system: known amounts were injected as liquids through a
septum into evacuated Pyrex bulbs. Molecular chlorine was
introduced into another evacuated Pyrex bulb. The bulbs were
then connected one after the other to the reaction chamber, at
the upper and lower inlets, alternatively, to ensure good mixing,
and the reactants were flushed into the chamber by a stream of
the bath gas. The reaction chamber was filled with synthetic
air to 100 L. The reactants in the reaction chamber were left to
homogenize for ∼30 min, prior to irradiation. During that time,
the chamber was kept in the dark to avoid photolysis of the
molecular chlorine. After stabilizing, the reaction chamber was
irradiated for short periods (usually 30 s to 2 min). Between
each irradiation period, two SPME samples were taken.
Chlorine atoms were generated directly by the photolysis of
molecular chlorine:
Figure 2. Relative rate plots for chlorine atoms experiments at 1 atm
pressure and 296 ( 2 K of 1OH4C , 1,4C , and 1C . Pentyl nitrate is
4 4 4
used as the reference compound.
Each compound’s retention time was determined by direct
injection of a liquid solution to the GC. Stock solutions of each
compound in methanol were prepared. Typically, 5 mg was
added to 1 mL of methanol and the solutions were diluted in
the case of saturation of the detector response. The retention
times were the same for the direct liquid and the SPME
injections.
The gas-phase sampling using SPME allows for simultaneous
sampling of both the hydroxyalkyl nitrates and the reference
compounds. The SPME fiber was inserted into the reaction
chamber, through a septum positioned at its center, to a constant
depth of 1 cm. The fiber was exposed to the gas mixture for 30
s. After sampling, the fiber was inserted into the GC injector
for 30 s for thermal desorption, followed by chromatographic
separation and quantification. No carryover was observed for
any of the compounds upon a second injection, indicating a
complete recovery from the fibers. The fiber was used im-
mediately after desorption for the next sampling.
Cl + hν(300 e λ e 460 nm) f 2Cl
(3)
2
The typical initial concentrations of the reactant mixture were
13
-3
Results
(
2-6) × 10 molecule cm for hydroxyalkyl nitrates and alkyl
12 -3
dinitrate, (4-8) × 10 molecule cm for alkyl nitrate reference
The data obtained from the SPME samplings was plotted in
accordance with eq I. Relative rate plots for 1,4C4, 1OH4C4,
and 1C4 are shown in Figure 2, and for 1OH6C6, and 2OH5C5
14
-3
compounds, and (1-2) × 10 molecule cm for molecular
chlorine.
After each experiment, the reaction chamber was pumped
and cleaned out by filling it with nitrogen (>99.999%). The
pumping and filling cycle was repeated several times until the
nitrates were not detected.
are shown in Figure 3 (both figures use pentyl nitrate (1C ) as
a reference compound). The slopes of the plots were obtained
5
by least-squares fit. In all cases, the least-squares intercepts of
the plots were within two standard deviations of zero. The linear
correlation coefficients (r) were always better than 0.98 for all
compounds. Each data point on the curves represents inaccuracy
of ∼10-20%. These errors are associated with the measure-
ments and result mainly from the manual SPME procedure (e.g.,
imprecise sampling time periods, manual injections into the GC)
and reactant preparation (syringe liquid injection to the Pyrex
bulb, vacuum line dilution, pressure measurements, etc.).
The slopes of the relative rate plots have been placed on an
absolute basis using the following rate coefficients for the
4
. Analytical Procedure. The decay of the organic nitrates
due to reaction with atomic chlorine was monitored and
quantitatively analyzed by a Varian Star 3800 GC, equipped
with an electron capture detector (ECD). Data acquisition and
processing was performed using the STAR software (Varian).
Helium (99.999%) was used as the carrier gas at a flow rate of
3
-1
1
cm min . Nitrogen (99.999%) was used as the makeup gas
3
-1
at a flow rate of 30 cm min . The ECD was kept at 300 °C.
The injector was maintained at 200 °C. A DB5-MS capillary
column was used for all experiments (30 m × 0.25 mm i.d.,
-
11
3
reference compounds: kCl ) (9.24 ( 0.2) × 10
molecule
for 1-butyl nitrate; kCl ) (1.57 ( 0.03) × 10
for 1-pentyl nitrate. The resulting rate
cm
-
1
-1
-10
0
.25 µm film thickness, J&W Scientific). The initial oven
s
3
-1 -1
22
temperature was set to 80 °C for 2 min, ramped to 280 °C at a
cm molecule
s
-1
rate of 10 °C min , and maintained at that temperature for 2
min. These conditions resulted in sharp and distinct peaks with
no overlapping of the other reactants nor products present in
the reaction chamber.
coefficients of the hydroxyalkyl nitrates and dinitrate are
presented in Table 1 and in a schematic form in Figure 4
together with literature values for the corresponding alkyl
2
2,29,36-38
nitrates, alkanes, and alcohols.
The errors quoted in

Cl Reactions with Alkyl Nitrates
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5905
Figure 3. Relative rate plots for chlorine atoms experiments at 1 atm
pressure and 296 ( 2 K of 1OH6C and 2OH5C . Pentyl nitrate is
6 5
used as the reference compound.
Figure 4. Schematic presentation of the Cl atom rate coefficients of
the hydroxyalkyl nitrates and dinitrate and a comparison with their
corresponding alkyl nitrates, alkanes, and alcohols: (0) hydroxyalkyl
22,29,36-38
nitrates;(1)n-alkanes;(2)alkylnitrates;(O)alcohols;(b)dinitrate.
TABLE 1: Rate Coefficients (95% Confidence Level) and
Tropospheric Lifetimes for the Reactions of Cl Atoms with
C
3
-C
6
Hydroxyalkyl Nitrates, Alkyl Nitrates, and Alkyl
upon changing the initial concentrations of the reactants used,
the diluent gas, the reference compound, the light intensity, or
the photolysis time.
Dinitrate at 296 ( 2 K and Atmospheric Pressure
-
10 b
k
Cl (×10
)
atmospheric
compounda (cm molecule s-1
3
-1
)
ref compoundc lifetime (days)d
Discussion
1
2
1
2
1
2
1
1
2
1
2
1
1
1
1
OH2C
OH1C
OH2C
OH1C
OH3C
OH4C
OH4C
OH2C
OH1C
OH4C
OH5C
OH6C
3
3
4
4
4
4
4
5
5
5
5
6
0.42 ( 0.08
0.45 ( 0.09
0.98 ( 0.10
0.87 ( 0.09
1.39 ( 0.26
1.40 ( 0.16
1.82 ( 0.36
1.59 ( 0.16
1.25 ( 0.19
1.56 ( 0.31
2.05 ( 0.22
2.44 ( 0.26
0.92 ( 0.09
1.57 ( 0.16
0.39 ( 0.08
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
1C
4
4
4
4
4
4
4
4
4
4
4
4
5
4
4
5.5
5.1
2.4
2.7
1.7
1.7
1.3
1.5
1.9
1.5
1.1
0.9
2.5
1.5
5.9
1. Validation of the Experimental System, Procedure, and
; 1C
; 1C
; 1C
; 1C
; 1C
; 1C
; 1C
; 1C
; 1C
; 1C
5
5
5
5
5
5
5
5
5
5
Measurements. The relative rate technique assumes that both
the reactant and the reference compounds are removed solely
by the reaction with chlorine atoms. A series of control
experiments were performed to verify this assumption for each
of the organic nitrates. Wall losses were confirmed to be
unimportant by letting the reactants, reference compounds, and
molecular chlorine stay in the reaction chamber for about 1 h
and observing the same initial and final concentrations. In
addition, mixtures of the reactant, reference, and molecular
chlorine were allowed to stand in the dark for 2 h, to check for
possible dark reactions. In all cases, no decay of the reactants
or of the reference compounds was observed. In addition,
mixtures of the organic nitrates were irradiated at maximum
light intensity in the absence of molecular chlorine for a couple
of hours. No decrease in their concentrations was observed.
Thus, under the experimental conditions employed, the organic
nitrates were photochemically stable.
Moreover, the very good linearity of the plots passing through
the origin (shown in Figures 2 and 3) suggests that the secondary
reactions are insignificant. Finally, no interference between the
retention times of the reactants, reference, and oxidation products
was observed for the gas chromatography conditions employed.
The same rate coefficients were obtained from experiments using
different reference compounds, implying that the results are
independent of the reference compound.
C
C
4
5
,4C
4
; 1C
5
a
For simplicity, the nomenclature introduced by Schneider and
3
0
b
Ballschmiter for multifunctional nitrates is used in the table. The
rate coefficients, kCl, are calculated according to eq I. Rate coefficients
c
-
11
3
-1 -1
of the reference compounds: k1C4 ) 9.24 × 10 cm molecule
s ;
-
10
3
-1 -1 22
d
k
1C5 ) 1.57 × 10
cm molecule
s
.
Tropospheric lifetimes
for hydroxyalkyl nitrates with respect to removal by Cl atoms were
-
1
calculated using the relationship: τ ) (kCl[Cl]} and assuming Cl
concentration of 5 × 10 atom cm
4
-3 17-21
.
Table 1 reflect the accuracy of the measurements and include
statistical uncertainties of the averaged values and an additional
10% uncertainty to account for uncertainties in the reference
rate coefficients. The estimated overall errors of these rate
constants are ∼20%.
As the ratios of the concentrations are used in the relative
rate calculation (see eq I), peak areas are used directly instead
of absolute concentrations. A linear relationship between the
amount of reactants extracted by the SPME fiber and their initial
concentrations in the reaction chamber was observed for all the
studied compounds. The linearity between the chromatographic
response and the concentrations of the compounds was con-
firmed over 3 orders of magnitude.
To the best of our knowledge, this is the first study of the
rate coefficients of Cl atom reactions with hydroxyalkyl nitrates
and the C4 dinitrate. Thus, a comparison to the literature is not
available. Using two alkyl nitrate reference compounds in each
experiment as an intercalibration experiment validated the
results. The relative rate plots for the two reference compounds
themselves were plotted one against the other, for each
experiment, to give their relative loss rate. Their obtained rate
coefficients were compared to the literature values and an
excellent agreement was found between them (see Table 1).
The reproducibility of the measurements was excellent:
variation between repeated SPME samplings was found to be
<10%. There were no variations in the rate coefficients obtained

5906 J. Phys. Chem. A, Vol. 106, No. 24, 2002
Treves et al.
The agreement between the values obtained here and those from
the literature indicates that the experimental system and
procedure are valid and can be used to determine Cl atom
reaction rate coefficients with the various hydroxyalkyl nitrates
and dinitrate. In addition, it implies that choosing one of the
reference compoundss does not influence the results.
reactivity toward 2OH5C5 is higher than that toward 1OH4C4,
since the former has a longer alkyl chain.
The Cl atom reactivity toward hydroxyalkyl nitrates with the
same chain length increases as the separation between the
hydroxy and the nitrooxy group increases. This is probably
because the hydroxy group better enhances the Cl atom
reactivity and the effect of the nitrooxy group diminishes with
increasing distance. Therefore, the Cl atom reactivity toward
C4 hydroxyalkyl nitrate decreases in the order δ-hydroxy nitrate
2
. Cl Atom Rate Coefficients. This study provides a
systematic investigation of the chlorine atoms reactions with a
series of multifunctional compounds, the hydroxyalkyl nitrates.
The reaction of the electrophilic Cl atoms with substituted
(
1OH4C4) > γ-hydroxy nitrate (1OH3C4; 2OH4C4) > â-hy-
1,16
droxy nitrate (1OH2C ; 2OH1C ). 1OH6C is the longest
alkanes is believed to proceed by H-atom abstraction process.
4
4
6
hydroxyalkyl nitrate and exhibits the largest separation between
the two functional groups. Thus, its Cl atom rate coefficient is
the highest (see Figure 3 and Table 1).
The Cl atom reactivity toward the hydroxyalkyl nitrates is
observed to be closely related to their chemical structure. Two
structural parameters determine the Cl atom reactivity toward
hydroxyalkyl nitrates: the length of the hydrocarbon chain and
the separation between the hydroxy and the nitrooxy groups.
As can be seen in Figure 2 and Table 1, within the C4 organic
nitrates, the Cl atom reactivity toward 1,4C4 dinitrate is the
lowest due to the presence of the two nitrooxy groups. It is ∼4
times lower than the Cl atom reactivity toward the corresponding
The nitrooxy group significantly decreases the Cl atom
reactivity relative to the corresponding unsubstituted alkanes.
This behavior is attributed to its strong electron-withdrawing
nature that exhibits long-range electronic effects to the extent
that might influence the reactivity of C-H bonds that are not
1
OH4C4 hydroxyalkyl nitrates, and ∼2 times lower than the
Cl atom reactivity toward the 1C4 alkyl nitrate.
To summarize, atomic chlorine is highly reactive toward the
hydroxyalkyl nitrates. The differences between the rate coef-
ficients for the Cl atom reaction with the various hydroxyalkyl
nitrates are not very large. This reaction probability is high
almost regardless of the collision site, and the chemical structure
plays a limited role in determining the reaction rate coefficients.
3
9
-10
directly adjacent. For example: kCl for butane is 2.2 × 10
cm molecule s , whereas kCl for 1-butyl nitrate is 0.92 ×
3
-1 -1 37
-1 -1 22
-
10
3
-10
3
1
0
cm molecule s . kCl of pentane is 2.5 × 10
cm
-
1
-1 29
-10
molecule s , whereas kCl of pentyl nitrate is 1.57 × 10
3
-1 -1 22
cm molecule
s
(see Figure 4). The presence of the
nitrooxy group decreases the Cl atom reactivity of the hydroxy-
alkyl nitrate as compared to the corresponding alcohols. For
3. Atmospheric Implications. The atmospheric lifetime of
hydroxyalkyl nitrates, with respect to removal by reaction with
Cl atoms, can be calculated from the measured rate coefficients.
Table 1 presents the tropospheric lifetimes of hydroxyalkyl
nitrates toward reactions with Cl atoms estimated by assuming
-10
3
-1 -1 36
example: kCl for propanol is 1.5 × 10 cm molecule s ,
whereas kCl for C3 hydroxyalkyl nitrate is 0.45 × 10
cm3
molecule (see Table 1 and Figure 4) and kCl for butanol
is 2.04 × 10
reactive C4 hydroxyalkyl nitrate is 1.82 × 10 cm molecule
(see Table 1 and Figure 4).
The presence of the hydroxy group, on the other hand, is
-
10
-
1
-1
s
4
-3
-
10
3
-1 -1 36
a Cl concentration of 5 × 10 atom cm in the marine boundary
layer. The estimated lifetimes of the hydroxyalkyl nitrates
are in the range of 0.9 days for 1OH6C to 5.5 days for 1OH2C .
cm molecule s , and kCl for the most
1
7-21
-10
3
-1
-1
s
6
3
The C4-C5 alkyl nitrate and C4 dinitrate atmospheric lifetimes
are in the range of those of the hydroxyalkyl nitrates, with the
latter exhibiting the longest lifetime of 5.9 days.
expected to somewhat enhance the Cl atom reactivity as
compared to the corresponding alkyl nitrates and dinitrates. This
is indeed observed, as is demonstrated in Figure 4, and is similar
to the trend observed for reactions of Cl atoms with alcohols.
These estimates suggest that in the marine boundary layer,
at peak concentration of Cl atoms, gas-phase reactions of
hydroxyalkyl nitrates (C3-C5 â-hydroxyalkyl nitrates in the low
3
6
Nelson et. al. observed that alcohols are more reactive than
the corresponding alkanes due to lowering of the C-H bond
dissociation energies for carbon bound to a hydroxy group by
1
5
ppt range have been measured in marine air ) with Cl atoms
may be a significant loss process. This is especially relevant
for the larger hydroxyalkyl nitrates and should be taken into
account in addition to their other loss mechanisms. Organic
nitrates are long-lived and can undergo long-range transport to
-
1
about 4-5 kcal mol , thus facilitating the hydrogen abstrac-
tion.36 This trend is similar to the trend observed also for
reactions of hydroxy radical with small alcohols, which is also
explained by the more labile R-hydrogen atom to the OH group.
However, since most of the reaction proceeds via H atom
abstraction from the C-H bond, the effect for alcohols is not
6,32
remote areas. Thus, hydroxyalkyl nitrates, which mainly form
in an urban atmosphere, may be transported toward coastal
regions and interact with marine air masses that can have
significant Cl atoms concentrations. Alternatively, gas-phase
reactions of hydroxyalkyl nitrates with Cl atoms may be a
significant loss process for hydroxyalkyl nitrates produced
directly in urban coastal regions. Since the global tropospheric
concentration of Cl atoms is much lower than within the marine
36,40
expected to be significant for larger alcohols.
There is little difference between the Cl atom reactivity
toward the two isomers (for example: 1OH2C4 and 2OH1C4)
of the hydroxyalkyl nitrates.
As shown in Table 1 and Figure 4, for a constant separation
between the hydroxy and the nitrooxy groups, there is an
increase in the Cl atom reactivity with the increase in the alkyl
chain length. Addition of the CH2 group enhances the overall
reactivity toward Cl atoms because of its high reactivity in the
central position of the molecule. This results from the lowering
of the C-H bond dissociation energies. Hence, the Cl atom
reactivity toward â-hydroxyalkyl nitrates decreases in the order
C5 â-hydroxyalkyl nitrate (1OH2C5; 2OH1C5) > C4 â-hydroxy-
alkyl nitrate (1OH2C4; 2OH1C4) > C3 â-hydroxyalkyl nitrate
2
5
-3 16,19-21
boundary layer (∼10 to 10 atom cm
), the reaction
of hydroxyalkyl nitrates with Cl atoms are probably not globally
3
important (taking an average Cl concentration of ∼10 atom
-
3
cm yields lifetimes in the range of 47 days for 1OH6C to
6
276 days for 1OH2C ). Even so, the rate coefficients presented
3
here show that atomic chlorine is highly reactive with the
hydroxyalkyl nitrates. Therefore, even a small Cl atom con-
centration can compete with the other main atmospheric loss
process of the hydroxyalkyl nitrates such as gas-phase reactions
with OH radicals, photolysis, partitioning into organic aerosols
3
2,35
(1OH2C3; 2OH1C3). For δ-hydroxyalkyl nitrates, the Cl atom
and wet deposition.

Cl Reactions with Alkyl Nitrates
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5907
Acknowledgment. This study was partially supported by
research grants from the Angel Faivovich foundation for
ecological studies, the Minerva Science Foundation, and the
Sussman family center for environmental sciences at the
Weizmann Institute. Y.R. is the incumbent of the William Z.
and Eda Bess Novick career development chair.
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Appendix
(
1
OH4C4 Synthesis: Nitration of (4-Iodobutoxy)-tert-bu-
2
tyldimethylsilane by Silver Nitrate. In a 100 mL flask was
dissolved 2.4 g of (4-iodobutoxy)-tert-butyldimethylsilane in
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4
0 mL of extra dry acetonitrile. The solution was cooled to -10
Atmosphere; Academic Press: San Diego, 1999.
°
C using ethylene glycol/dry ice bath. Silver nitrate (2.6 g, 2
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equiv) was added to the flask. After 5 min, the cooling bath
was removed, and the solution was stirred at room temperature
for 30-50 min. The reaction was monitored by TLC (eluents:
hexane, ethyl acetate/hexane 1:4).32 Afterward, the reaction
mixture was filtered from precipitated yellow AgI. A 4 mL
aliquot of water was added to the filtrate and (4-nitrooxybutoxy)-
tert-butyldimethylsilane was hydrolyzed for 40 min at room
temperature. The reaction was monitored by TLC (eluents:
hexane, ethyl acetate/hexane 1:1). Then the reaction mixture
was filtered. Water and 40 mL of ethyl acetate were added to
the reaction mixture, and as a result, two phases appeared and
were separated by a separation funnel. The organic phase was
further dried over MgSO4, and organic solvents were evaporated
under vacuum. The hydroxyalkyl nitrate residue from the
organic phase was purified by flash chromatography on silica
gel 60 (230-400 mesh) (eluent: ethyl acetate/hexane 1:1). The
purity of the resulting hydroxyalkyl nitrates was confirmed by
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