Rate coefficients and products for gas-phase reactions of chlorine atoms with cyclic unsaturated hydrocarbons at 298 K

Article abstract of DOI:10.1002/kin.20467

Rate coefficients for the reaction of Cl atoms with cycloalkenes have been determined using the relative rate method, at 298 K and atmospheric pressure of N₂. Reference molecule was n -hexane, and the concentrations of the organics were followed by gas chromatographic analysis. Cl atoms were prepared by photolysis of trichloroacetyl chloride at 254 nm. The relative rates of reactions of Cl atoms with cycloalkenes, with, respect to M-hexane, are measured as 1.12 ±0.38, 1.31 ±0.14, and 1.69±0.18forcyclopentene, cyclohexene, and cycloheptene, respectively. Considering the absolute value of the rate coefficient of the reaction of Cl atom with n-hexane as 3.03 ± 0.06 × 10^-10 cm³ molecule^-1 s ^-1, the rate coefficient values for cyclopentene, cyclohexene, and cycloheptene are calculated, to be (3.39 ± 1.08) × 10 ^-10, (3.97 ±0.43) × 10^-10, and (5.12 ± 0.55) × 10^-10 cm³ molecule^-1 S ^-1', respectively. The experiments for each, molecule were repeated six to eight times, and the slopes and the rate coefficients given above are the average values of these measurements, and the quoted error includes 2σ as well as all other uncertainties in the measurement and calculations. The rate coefficient increases linearly with the number of carbon atoms, with an increment per additional CH₂ group being (8.7 ± 1.6) × 11^-12 cm³ molecule^-1 s^-1. Chloroketones and chloroalcohols, along with unsaturated, ketones and alcohols, were found to be the major products of Cl-atom-initiated oxidation of cycloalkenes in the presence of air. The atmospheric implications of these results are discussed, along with a comparison with the reported structure activity relationships.

Full text of DOI:10.1002/kin.20467

Rate Coefficients and
Products for Gas-Phase
Reactions of Chlorine Atoms
with Cyclic Unsaturated
Hydrocarbons at 298 K
A. SHARMA, K. K. PUSHPA, S. DHANYA, P. D. NAIK, P. N. BAJAJ
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Received 15 July 2009; revised 23 September 2009; accepted 24 September 2009
DOI 10.1002/kin.20467
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Rate coefficients for the reaction of Cl atoms with cycloalkenes have been de-
termined using the relative rate method, at 298 K and atmospheric pressure of N2. Reference
molecule was n-hexane, and the concentrations of the organics were followed by gas chromato-
graphic analysis. Cl atoms were prepared by photolysis of trichloroacetyl chloride at 254 nm.
The relative rates of reactions of Cl atoms with cycloalkenes, with respect to n-hexane, are
measured as 1.12 ± 0.38, 1.31 ± 0.14, and 1.69 ± 0.18 for cyclopentene, cyclohexene, and cyclo-
heptene, respectively. Considering the absolute value of the rate coefficient of the reaction of Cl
−
10
3
−1 −1
s , the rate coefficient values for
atom with n-hexane as 3.03 ± 0.06 × 10
cm molecule
−10
cyclopentene, cyclohexene, and cycloheptene are calculated to be (3.39 ± 1.08) × 10 , (3.97
−
10
−10
3
−1 −1
±
0.43) × 10 , and (5.12 ± 0.55) × 10
cm molecule s , respectively. The experiments
for each molecule were repeated six to eight times, and the slopes and the rate coefficients
given above are the average values of these measurements, and the quoted error includes 2σ
as well as all other uncertainties in the measurement and calculations. The rate coefficient in-
creases linearly with the number of carbon atoms, with an increment per additional CH2 group
being (8.7 ± 1.6) × 10− cm molecule
12
3
−1 −1
s . Chloroketones and chloroalcohols, along with
unsaturated ketones and alcohols, were found to be the major products of Cl-atom-initiated
oxidation of cycloalkenes in the presence of air. The atmospheric implications of these re-
sults are discussed, along with a comparison with the reported structure activity relationships.
ꢀ
C
2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 98–105, 2010
INTRODUCTION
the lower atmosphere. In the troposphere, source ap-
portionment studies show that reactions of alkenes
with ozone are responsible for jump-starting photo-
chemistry after sunrise [1]. The oxidation reactions of
alkenes are considered as important source of aerosols
in the troposphere [2]. The atmospheric oxidation of
these compounds involves reactions with OH radi-
cals, O , NO , NO , etc. The rate coefficients for the
Alkenes, one of the major pollutants in the urban am-
bient air, are an important source of free radicals in
Correspondence to: S. Dhanya; e-mail: sdhanya@barc.gov.in.
ꢀ
c 2009 Wiley Periodicals, Inc.
3
2
3

RATE COEFFICIENTS AND PRODUCTS FOR GAS-PHASE REACTIONS OF CHLORINE ATOMS
99
gas-phase reactions of OH, NO2, NO3, O3, etc., with
many alkenes have been determined recently [3–6].
Previous studies have suggested that, in addition to the
above oxidants, Cl atoms are also important in the ma-
rine boundary layer at midlatitudes, especially at dawn.
Although the upper limit for the chlorine atom concen-
tration in the northern hemisphere is estimated to be
ments were also conducted in ultrapure air (zero grade;
Chemtron Science Laboratories, Mumbai, India). The
reactant mixture contained typically ∼300 ppm of
CCl3COCl and 250–350 ppm of the sample and the ref-
erence molecules. The prepared reaction mixture was
kept for 60–80 min for equilibration, before photoly-
sis, which was confirmed by reproducibility of the gas
chromatogram. Experiments were performed at room
temperature (298 ± 2K). Concentrations of the sam-
ple and the reference were monitored, prior to, and
after photolysis, using gas chromatograph (Chemito
GC 8610), with 10% SE 30 (7 × 1/8 ) column, with
temperature programming, in conjunction with flame
ionization detector. The mixture was photolyzed for a
period of 10 min, in steps of 1–2 min, and after each
photolysis step, the decrease in the concentration of
the two organics, cycloalkene and n-hexane, were fol-
lowed. A sample of 500 μL was transferred from the
reaction chamber to GC, using a gas-tight syringe. The
decrease in the concentration of the sample and the
reference was determined at each step.
The fractional loss of the CA molecules and the
reference molecules, due to reaction with Cl atoms,
is monitored to obtain the kinetic data. In the present
study, n-hexane, for which the rate coefficient is well
known [9], was the reference molecule (R), and cy-
clopentene, cyclohexene, and cycloheptene were the
sample molecules (CA). The relative rate constants
were then determined, using the following expres-
sion, assuming that both molecules react only with
chlorine:
3
−3
less than 10 atoms cm [7], the peak concentration
during sunrise in the marine boundary layer is reported
5
−3
[
8] to be 1.3 × 10 atoms cm . Reactions of Cl atoms
with many volatile organic compounds (VOCs) pro-
ceed with high rate coefficients [9] and, therefore, may
play an important role in the atmospheric chemistry
of remote marine boundary layer [8] as well as the
polluted urban areas, with significant anthropogenic
emission of Cl2 [10]. Hence, measurement of rate co-
efficients of reactions of Cl atoms with different VOCs
has become important [4], and the same has been re-
ported for a large number of saturated and unsaturated
organic molecules with different substitutions [11–15].
In the case of alkenes, the rate coefficients of reactions
of Cl atoms have been measured by different groups
ꢁ
ꢁꢁ
[11,16]. In addition to these straight chain alkenes,
many cyclic alkenes have also been detected in the tro-
posphere; the major source being automobile exhausts,
gasoline vaporization, forest fire, rubber abrasion, etc.
[17]. However, there are not many studies on the rate
coefficients, or mechanism of reactions of Cl atoms
with these cyclic alkenes, except for a measurement of
Cl atom reactions with cyclopentene by Anderson et al
[18]. In the present study, the rate coefficients of reac-
tions of Cl atoms with three cyclic alkenes, cyclopen-
tene, cyclohexene, and cycloheptene, are determined
using the relative rate method, to assess the signifi-
cance of these reactions in the troposphere. The major
stable products of the Cl-atom-initiated oxidation of
these molecules are also identified.
ꢀ
ꢁ
ꢀꢀ
ꢁ
ꢀ
ꢁꢁ
[
CA]t0
CA]t
kCA
kR
[R]t0
[R]t
ln
=
ln
(I)
[
where [R]t0 and [CA]t0 are the concentrations of
n-hexane and CA, respectively, at time t0, [R]t and
[CA]t are the corresponding concentrations at time t,
EXPERIMENTAL
and kCA and kR are the rate coefficients of their reac-
tions with chlorine atoms.
All the reactions were carried out in a quartz reaction
chamber of 3-L volume, fitted with vacuum stopcocks
and a sealed port for taking out samples for GC anal-
ysis. Chlorine atoms were generated by photolysis of
trichloroacetyl chloride (CCl3COCl) at 254 nm, using
a UV lamp (Rayonett). The reaction mixture, consist-
ing of cycloalkene (CA), n-hexane as reference com-
pound, and trichloroacetyl chloride, was prepared in
the reaction chamber, using a vacuum manifold, and
the total pressure was maintained at 800 ± 3 Torr,
by adding 99.9% nitrogen (INOX Air Products Ltd.,
Mumbai, India). The pressure was measured, using
a capacitance manometer (Pfeiffer Vacuum). Experi-
All the reagents were obtained from Sigma-Aldrich
(Steinheim, Germany), with purities >95%. The reac-
tants and n-hexane were subjected to freeze–pump–
thaw cycles, before use.
The products of the reaction of the chlorine atoms
with cyclic alkenes in the presence of air at atmospheric
pressure were characterized by a gas chromatograph
coupled with a mass spectrometer (Shimadzu), using
BPX50 (30 m × 0.25 mm × 0.25 μm) and CP WAX
52CB (30 m × 0.25 mm × 0.25 μm) columns in-
dependently. The products were identified by match-
ing the spectral features with the reported mass
spectra.
International Journal of Chemical Kinetics DOI 10.1002/kin

1
00
SHARMA ET AL.
RESULTS
to secondary reactions. From the linear least-square fit,
slopes of the lines are obtained, which are the ratios of
the rate coefficients of the samples to that of the ref-
erence molecule. Considering k1, k2, k3, and kR as the
rate coefficients for the reactions of Cl atoms with cy-
clopentene, cyclohexene, cycloheptene, and n-hexane,
respectively, the mean values of the slopes, obtained in
N2 are k1/kR = 1.12 ± 0.38, k2/kR = 1.31 ± 0.14, and
k3/kR = 1.69 ± 0.18. From these slopes, using kR =
Determination of Rate Coefficients
The relative rate method relies on the assumption that
both the reactant and the reference compounds are
removed solely by reaction with chlorine atoms. To
verify this assumption, mixtures of CCl3COCl, CA,
n-hexane, and nitrogen/air were prepared and allowed
to stand in the dark for about 6 h, and the concentration
of the components were monitored by GC at regular
time intervals. There was no significant decrease in the
concentration of any of the molecules, indicating the
absence of dark reactions as well as wall losses. To
test for possible photolysis of the reactants, a mixture
of CA, n-hexane, and nitrogen/air was photolyzed for
−10
3
−1 −1
s [9], the rate
3.03 ± 0.06 × 10
cm molecule
−
10
coefficients are calculated as k1 = 3.39 ± 1.08 × 10
,
−10
k2 = 3.97 ± 0.43 × 10 , and k3 = 5.12 ± 0.55 ×
−10
3
−1 −1
10
cm molecule
of the slopes in air are 1.22 ± 0.40, 1.58 ± 0.28, and
.76 ± 0.28 and the rate coefficients calculated from
s . The corresponding values
1
−10
these values are 3.69 ± 1.24 × 10 , 4.79 ± 0.86 ×
10 min in the absence of added chlorine source. There
−10
−10
3
−1 −1
10
, and 5.33 ± 0.86 × 10
cm molecule s ,
was no decrease in the concentration of the sample or
the reference compound, which confirmed the absence
of any significant loss due to direct photolysis.
The experimental data, the fractional loss of CA ver-
sus the fractional loss of the reference compound, are
plotted, according to Eq. (I), in Fig. 1. In the presence
of nitrogen (Fig. 1A) as well as air (Fig. 1B), a lin-
ear plot is obtained, with near zero intercept, for each
CA, suggesting the absence of any complications due
for cyclopentene, cyclohexene, and cycloheptene, re-
spectively. The experiments for each molecule were
repeated six to eight times, and the slopes and the rate
coefficients given above are the average values of these
measurements, with two standard deviation as quoted
error, along with a 2% error added to take care of the
error in the rate coefficient of the reference reaction.
Product Study of the
Chlorine-Atom-Initiated Oxidation
t
t
0.6
The stable products formed during the chlorine-atom-
initiated oxidation of cyclic alkenes in atmospheric
conditions were characterized. For this, a mixture of
CA/CCl3COCl/air was photolyzed at 254 nm, for a pe-
riod ranging from 3 to 9 min. The main observed prod-
ucts were chloro-ketones, chloro-alcohols, enones, and
enols for all CAs. The total ion chromatograms ob-
tained for cyclopentene, cyclohexene, and cyclohep-
tene are shown in Fig. 2. The products characterized
by GC-MS, for each CA, are listed in Table I. The
yield of the major products was found to increase with
duration of the photolysis up to 9 min, as shown in the
inset of Fig. 2.
A
0
0
0
.4
.2
.0
0
t
0
.0
0.1
0.2
0.3
0.4
ln((n-hexane) /(n-hexane) )
t
t
0
t
t
t
B
0.6
0
0.4
t
DISCUSSION
0
.2
Except for one rate coefficient measurement for the
reaction with cyclopentene [18], there are no previous
rate coefficient data, at least to our knowledge, for the
reactions of chlorine atoms with cycloalkenes. Even in
the case of cyclopentene, the reported value (Table II)
is at least two times higher than the presently mea-
sured rate coefficient. The reason for this discrepancy
is not clear. As mentioned earlier, we observed that
the rate coefficient in the presence of air is marginally
higher than that in the presence of nitrogen. A probable
0.0
0
.0
0.1
0.2
0.3
0.4
0.5
ln((n-hexane) /(n-hexane) )
t
t
t
0
Figure 1 Logarithmic plot of relative decrease in the
concentration of cycloheptene (ꢀ), cyclohexene ( ), and
cyclopentene (ꢁ) with respect to n-hexane (reference com-
pound), due to reaction with Cl atom at 298 ± 2 K. (A) in
N2 and (B) in air.
◦
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE COEFFICIENTS AND PRODUCTS FOR GAS-PHASE REACTIONS OF CHLORINE ATOMS
101
and the reference molecule is not the same as the ra-
tio of the rate coefficients of the reactions of Cl with
CA and reference molecule. The former is expected to
be much higher than the latter, based on the data for
alkenes. This difference can lead to an overestimation
of the rate coefficient of reaction of Cl atom with CA.
3
3
3
3
3
2
2
2
2
2
1
1
8
6
4
2
0
Cycloheptene
9
'
27
26
6'
3
'
2
5
8 15 16 17
18
3
19
Hence, the rate coefficient obtained in N is considered
2
Time/min
6
4
2
0
8
6
as the true rate coefficient of the reaction of Cl atom
A
B
2
1
with CA. However, it can be noted that the increase in
the rate coefficient in the presence of air is not very
pronounced, except in the case of cyclohexene, proba-
bly indicating a smaller extent of OH formation from
these peroxy radicals as compared to that in the case
of acetoxy peroxy radicals [19,20].
5
4
1
4
4a
3
2
C
2
1
8
7
5
6
3
7
8
9
10
11
12
13
14
15
Time/min
The rate coefficients of reactions of OH with these
cyclic alkenes, reported by various groups [21,22],
along with the present results for the reactions of
Cl atoms, are listed in Table II, for comparison. The
present results show that the rate coefficients of reac-
tions of Cl atoms with cyclic unsaturated hydrocarbons
are about five to seven times higher than those of OH
reactions. The rate coefficients for the reactions of both
OH [4] and Cl atom [23] with cyclic alkanes are also in-
cluded in Table II. In all the cases, the rate coefficients
are higher for the unsaturated cyclic hydrocarbons than
those for the saturated ones. This indicates that the pos-
sibility of addition reaction of OH/Cl in unsaturated
molecules makes them more reactive as compared to
saturated molecules, where only abstraction reaction is
possible.
Figure 2 Total ion chromatograms of the products of Cl-
atom-initiated oxidation of A: cyclopentene, B: cyclohexene,
and C: cycloheptene. The identities of the peaks, which could
be characterized by GC-MS, are listed in Table I. Inset: Total
ion chromatograms of the products after 3, 6, and 9 min of
photolysis of the reaction mixture in the case of cycloheptene.
The column temperature is lower than that in C.
reason is the formation of OH in the presence of air,
as observed in the Cl-initiated oxidation of acetalde-
hyde, where reaction of acetyl peroxy radical with HO2
[
19,20] leads to OH formation. If OH is generated dur-
ing the reaction, it will affect the determination of the
rate coefficient of Cl atom reaction, because the ratio
of the rate coefficients of the reactions of OH with CA
Table I Major Products of Atmospheric Oxidation, Characterized by GC-MS
Sr. No.
Cycloalkene
Products
1
2
Cyclopentene
Cyclohexene
2-Chlorocyclopentanone (1), 2-chlorocyclopentanol (2), 2-cyclopenten-1-one (3)
2-Chlorocyclohexanone (1), 2-chlorocyclohexanol (2), 2-cyclohexen-1-one (3), 3-cyclohexen-1-
ol (4a), 3-chlorocyclohexene (5); peak 4 unidentified
3
Cycloheptene
2-Chlorocycloheptanone (1), cycloheptanol (2), 2-cycloheptene-1-one, (5), 4-cycloheptene-1-one
(7), 1,4-dichlorocycloheptane or 1,3-dichlorocycloheptane (8); peaks 3, 4, and 6 unidentified
The corresponding peak number in Fig. 2 is given in parenthesis.
Table II Rate Coefficients of Reactions of Cycloalkenes and Cycloalkanes with OH and Cl Atoms
Rate Coefficient for Reaction with
Rate Coefficient for Reaction with Cl
3
−1 −1
3
−1 −1
Molecule
OH at 298 K (cm molecule
s
)
Atom at 298 K (cm molecule
s
)
−
−
−
−
−
−
−
11
11
11
11
11
12
12
−10
−10
−10
a
Cyclpentene
6.7 × 10
.7 × 10
6.77 × 10
[22]
[21]
[22]
[21]
[22]
[4]
3.39 ± 1.08 × 10
7.32 ± 0.88 × 10
3.97 ± 0.43 × 10
(present work)
[18]
5
a
Cyclohexene
(present work)
6.1 × 10
−10
a
Cycloheptene
Cyclopentane
Cyclohexane
7.4 × 10
4.97 × 10
6.97 × 10
5.12 ± 0.55 × 10
(present work)
−10
3.05 × 10
[23]
−10
[4]
3.83 × 10
[23]
a
In the present work, the rate coefficients of CA with Cl atom are measured relative to n-hexane, considering kn-hexane = 3.03 ± 0.06 ×
−
10
3
−1 −1
1
0
cm molecule
s .
International Journal of Chemical Kinetics DOI 10.1002/kin

1
02
SHARMA ET AL.
Different predictive structure activity relationships,
ical with CAs with C > 4, obtained from the literature
based on frontier molecular orbital calculations, have
been developed for reactions of OH with many fluoro-
substituted hydrocarbons [24]. In the case of alkenes,
correlations with HOMO (highest occupied molecular
orbital) energy have been developed for predicting rate
coefficients of reactions of OH, O3, and NO3 [22]. Em-
pirical correlations with position of substituents, dou-
ble bonds, etc., have also been developed for reactions
with OH [25]. These relations grossly predict the rate
coefficient values of thedifferent CAs tobeverysimilar
and do not reflect the marginal increase with increasing
number of carbon atoms. In a recent study, Aschman
and Atkinson [3] have shown that the rate coefficients
for the reactions of OH with C4–C14 1-alkenes increase
linearly with the number of carbon atoms, and from
the slope of the linear plot, the increase in the rate
coefficient per CH2 group in noncyclic alkenes was
(Table II), yields the increase of the rate coefficient per
−12
3
−1 −1
CH2 group as 3.5 × 10
cm molecule s . This is
smaller as compared to the increment for the reactions
of Cl atom with cyclic alkenes, observed in the present
work, but comparable to the increment for the reac-
tions of OH with noncyclic alkenes [3]. For a similar
comparison between the increment per CH2 group in
the case of Cl atom reactions of noncyclic and cyclic
alkenes, the reported rate coefficients of the Cl atom re-
actions with noncyclic alkenes are also plotted in Fig. 3.
In this case, there are two different sets of data reported
at room temperature and atmospheric pressure [16,27],
shown by different symbols in Fig. 3. Except for hex-
ene, all other values are for molecules with terminal
double bond, with no branching. There are no reports
on 1-hexene and any other alkenes with higher carbon
number. A linear variation with carbon number is ob-
served up to C = 5, for both sets of data, with slopes
−12
3
−1 −1
derived as (2.02 ± 0.32) × 10
cm molecule s .
−10
−10
3
−1 −1
This was found to be 1.4–1.5 times higher than that of
n-alkane series, where only H abstraction takes place.
Based on this, the increase in the rate coefficient with
carbon number is assigned to H atom abstraction from
the alkyl groups and to enhancement of addition at
C C bond with additional alkyl group upto C = 8 [26].
Although the number of CAs studied here is limited to
three, a similar plot of the rate coefficient of Cl atom
reaction with number of carbon atoms yields an ap-
proximate linear correlation (Fig. 3), with (8.7 ± 1.6) ×
of 0.9 × 10
and 1.34 × 10
cm molecule s
for data from [16] and [27], respectively. In the case of
Cl atom reaction with n-alkanes, the rate coefficients
[13] increase with the carbon number and levels off at
C = 7, as shown in Fig. 3, and the slope is less than
that for straight chain alkenes and CAs. Similar data for
C = 4 to C = 14 was used to separate and estimate the
contribution of addition and abstraction reactions for
OH reactions in alkenes [26]. However, due to lack of
sufficient data, it is difficult to make similar estimations
in the case of Cl atom reactions.
−11
3
−1 −1
10
cm molecule s as the increase in the rate co-
efficient of Cl atom reaction per CH2 group. A similar
plot of the rate coefficients for the reaction of OH rad-
It can be noticed that in the case of reactions of
OH with CAs, the rate coefficient increases by an or-
der of magnitude with unsaturation (Table II). How-
ever, in the case of Cl atom reaction with CAs, the in-
crease in the rate coefficient with unsaturation is only
marginal, though the nature of the products (Table I)
suggests the importance of addition, along with ab-
straction reaction. One reason for this could be that
the rate coefficients of the reactions of saturated com-
pounds are very close to diffusion-controlled limits,
and thus there is not much scope for further increase
due to unsaturation. However, we do find an increase in
the rate coefficient with the increase in the number of
carbon atoms. Another reason for not observing sub-
stantial increase with unsaturation could be that allylic
hydrogen atoms react less rapidly with Cl atoms than
the analogous alkyl hydrogens in alkanes, as observed
earlier in a series of noncyclic alkenes [16].
9
8
7
6
5
4
3
2
1
k
k
0
–
1
1
2
3
4
5
6
7
8
Number of Catoms
An approximate linear correlation between the rate
coefficients of reactions with OH and Cl atoms has
been observed for many unsaturated [11] and satu-
rated [28] hydrocarbons, including many halocarbons
Figure 3 Variation of rate coefficients of Cl atoms and OH
radicals with the number of carbon atoms. Reactions of Cl
atoms with cyclic alkenes from the present work (ꢁ); with
straight chain alkenes from [16] (ꢂ) and from [27] (
); with
•
[15], with different slopes for saturated and unsatu-
n-alkanes from [13] (ꢃ); OH radicals with cyclic alkenes
from [22] (ꢀ).
rated compounds. Such a linear correlation indicates
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE COEFFICIENTS AND PRODUCTS FOR GAS-PHASE REACTIONS OF CHLORINE ATOMS
103
similarity of the mechanisms of the reaction of these
hydrocarbons with OH radical and Cl atom. Recently,
during their study on the reactions of fluorochlorinated
esters with OH radical and Cl, Blanco and Teruel [29]
have also shown a linear correlation between the rate
coefficients of reactions of OH and Cl atom with satu-
rated VOCs. The least-square fitting of the data gives
a relationship similar to that in [28]:
2-chloroketones, 2-chloroalcohols, and enones. The
former two are the products of addition of Cl atom
at the double bond, whereas the latter is due to the
H abstraction reaction. In either case, the first step
of addition or abstraction leads to a radical, which
reacts with oxygen, leading to the formation of per-
oxy radicals. Peroxy radicals are known to undergo
reactions among themselves, leading to alcohol and
ketone [30].
log kOH = 0.485 log kCl − 7.00
(II)
ꢁ
RO2 + RO2/R O2 → alcohol + aldehyde/ketone + O2
3
with the rate coefficients (k) in the units of cm
−1
−1
molecule
s . The correlation is expected to be dif-
(IV)
ferent for unsaturated compounds, because the reac-
tion mechanism is different, involving both abstraction
and addition reactions. The rate coefficient values for
alkenes, given in [15], are plotted, along with the val-
ues reported here for CAs, in Fig. 4. The least-square
fitting of the data yields the following relationship for
alkenes:
Although the ketones have a weak absorption at
254 nm, the experimental results suggest that under
the present conditions, there is not much degradation
of ketone, as indicated by the increase in their concen-
tration with duration of the photolysis. Very recently,
enol formation and ring opening is reported during pho-
tolytic OH-initiated oxidation of cyclohexene, inves-
tigated by tunable synchrotron photoionization mass
spectrometry [31]. We could not confirm the forma-
tion of such products in our studies on cyclohexene. It
is quite likely that the yield of such products is very
low. The number of products in the case of cyclohex-
ene and cycloheptene is more than that in the case of
cyclopentene, with keto or OH groups at different po-
sitions, indicating abstraction of H atom from different
C atoms. The interference due to OH formation in the
presence of air is probably responsible for the forma-
tion of cycloheptanol, an addition product of OH with
cycloheptene. Equivalent products could not be identi-
fied in the case of cyclopentene and cyclohexene. There
are a few unidentified peaks in GC-MS, suggesting the
complexity and the variety of the products, especially
in the case of cycloheptene. This could be probably
due to the increased role of abstraction reactions, al-
lowing ring opening as well as interference of OH or
HO2 reactions.
log kOH = 2.39 log kCl + 12.97
(III)
The overall linearity appears to hold good within error
limits, even after inclusion of the three CAs, shown
as filled squares in Fig. 4. The linearity indicates the
similarity of the reaction mechanism in the case of OH
and Cl atom reactions.
In the present study, the most common prod-
ucts observed in all the three cycloalkenes are
1
E–10
E–11
1
k
1E–12
Atmospheric Lifetimes
1E–13
1E–10
1E–9
Henry’s law coefficients of these cyclic alkenes stud-
ied in the present work are not expected to be high,
due to their low solubility in water. These molecules
are not expected to undergo photodissociation in the
troposphere, since they do not absorb at wavelengths
above 320 nm, which is available in the troposphere.
Hence, the main sink of the cyclic alkenes studied
is the chemical reaction with oxidizing species avail-
able in the troposphere. Tropospheric lifetimes (τ) of
these CAs with respect to removal by OH, Cl, O3,
–
1
3
–1
kCl / molecule cm s
Figure 4 Correlation plots between the rate coefficients
for the reactions of Cl and OH radicals with unsaturated
hydrocarbons at room temperature. —: for molecules and rate
coefficients taken from [15] shown as ꢃ. - -: after including
the data for unsaturated cycloalkenes from the present work,
shown as ꢀ. The molecules from [15], included in fitting, are
C2H4, C2H3Cl, 1,1-C2H2Cl2, cis-C2H2Cl2, trans-C2H2Cl2,
C2Cl4, C3H6, 1- C4H8, and 1- C H10.
5
International Journal of Chemical Kinetics DOI 10.1002/kin

1
04
SHARMA ET AL.
Table III Tropospheric Lifetimes (τ) Calculated for the Cycloalkenes with Respect to Reactions with Different Species
Cyclopentene
Cyclohexene
Cycloheptene
3
−1 −1
−10
−10
−10
kCl (cm molecule
s
) (present work)
) [22]
3.39 × 10
3.97 × 10
5.12 × 10
τCl (days)
13.7
11.7
9.0
3
−1 −1
−11
−11
−11
kOH (cm molecule
τOH (h)
s
6.7 × 10
6.77 × 10
7.4 × 10
4.2
4.1
3.8
3
−1 −1
−13
−13
−13
kNO (cm molecule
s
) [4]
4.2 × 10
5.1 × 10
5.1 × 10
3
τNO (h)
2.6
2.2
2.2
3
3
−1 −1
−16
−17
−16
kO (cm molecule
s
) [4]
5.7 × 10
8.1 × 10
2.5 × 10
3
τO3
41.8 min
4.9 h
1.6 h
3
6
8
11
−3
The ambient concentrations used for Cl [31], OH, NO
respectively.
3
, and O
3
[32] are 2.5 × 10 , 1 × 10 , 2.5 × 10 , and 7 × 10 molecules cm
,
and NO3 can be estimated from the rate coefficients,
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