Rate constants for the gas-phase reactions of chlorine atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene at 298 K

Article abstract of DOI:10.1002/kin.20567

Rate constants for the reactions of Cl atoms with two cyclic dienes, 1,4-cyclohexadiene and 1,5-cyclooctadiene, have been determined, at 298 K and 800 Torr of N₂, using the relative rate method, with n-hexane and 1-butene as reference molecules. The concentrations of the organics are followed by gas chromatographic analysis. The ratios of the rate constants of reactions of Cl atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene to that with n-hexane are measured to be 1.29 ± 0.06 and 2.19 ± 0.32, respectively. The corresponding ratios with respect to 1-butene are 1.50 ± 0.16 and 2.36 ± 0.38. The absolute values of the rate constants of the reaction of Cl atom with n-hexane and 1-butene are considered as (3.15 ± 0.40) × 10^-10 and (3.21 ± 0.40) × 10^{- 10} cm ³ molecule^-1s^-1, respectively. With these, the calculated values are k(Cl + 1,4-cyclohexadiene) = (4.06 ± 0.55) × 10^-10 and k(Cl + 1,5-cyclooctadiene) = (6.90 ± 1.33) × 10^-10 cm³ molecule^-1 s^-1 with respect to n-hexane. The rate constants determined with respect to 1-butene are marginally higher, k(Cl + 1,4-cyclohexadiene) = (4.82 ± 0.80) × 10^{- 10} and k(Cl + 1,5-cyclooctadiene) = (7.58 ± 1.55) × 10^{- 10} cm³ molecule^-1 s^-1. The experiments for each molecule were repeated three to five times, and the slopes and the rate constants given above are the average values of these measurements, with 2σ as the quoted error, including the error in the reference rate constant. The relative rate ratios of 1,4-cyclohexadiene with both the reference molecules are found to be higher in the presence of oxygen, and a marginal increase is observed in the case of 1,5-cyclooctadiene. Benzene is identified as one major product in the case of 1,4-cyclohexadiene. Considering that the cyclohexadienyl radical, a product of the hydrogen abstraction reaction, is quantitatively converted to benzene in the presence of oxygen, the fraction of Cl atoms that reacts by abstraction is estimated to be 0.30 ± 0.04. The atmospheric implications of the results are discussed.

Full text of DOI:10.1002/kin.20567

Rate Constants for the
Gas-Phase Reactions of
Chlorine Atoms with
1,4-Cyclohexadiene and
1,5-Cyclooctadiene 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 4 November 2011; revised 15 February 2011, 15 March 2011; accepted 15 March 2011
DOI 10.1002/kin.20567
Published online 23 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Rate constants for the reactions of Cl atoms with two cyclic dienes, 1,4-
cyclohexadiene and 1,5-cyclooctadiene, have been determined, at 298 K and 800 Torr of N2,
using the relative rate method, with n-hexane and 1-butene as reference molecules. The con-
centrations of the organics are followed by gas chromatographic analysis. The ratios of the rate
constants of reactions of Cl atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene to that with
n-hexane are measured to be 1.29 0.06 and 2.19 0.32, respectively. The corresponding ratios
with respect to 1-butene are 1.50 0.16 and 2.36 0.38. The absolute values of the rate con-
stants of the reaction of Cl atom with n-hexane and 1-butene are considered as (3.15 0.40) ×
10−10 and (3.21 0.40) × 10−10 cm3 molecule−1s−1, respectively. With these, the calculated
values are k(Cl + 1,4-cyclohexadiene) = (4.06 0.55) × 10−10 and k(Cl + 1,5-cyclooctadiene) =
(6.90
1.33) × 10−10 cm3 molecule−1 s−1 with respect to n-hexane. The rate constants
determined with respect to 1-butene are marginally higher, k(Cl + 1,4-cyclohexadiene)
= (4.82
0.80) × 10−10 and k(Cl + 1,5-cyclooctadiene) = (7.58
1.55) × 10−10 cm3
molecule−1
s
−1. The experiments for each molecule were repeated three to five times,
and the slopes and the rate constants given above are the average values of these mea-
surements, with 2σ as the quoted error, including the error in the reference rate con-
stant. The relative rate ratios of 1,4-cyclohexadiene with both the reference molecules are
found to be higher in the presence of oxygen, and a marginal increase is observed in
the case of 1,5-cyclooctadiene. Benzene is identified as one major product in the case
of 1,4-cyclohexadiene. Considering that the cyclohexadienyl radical, a product of the hy-
drogen abstraction reaction, is quantitatively converted to benzene in the presence of
oxygen, the fraction of Cl atoms that reacts by abstraction is ꢀestimated to be 0.30
C
0.04. The atmospheric implications of the results are discussed.
Inc. Int J Chem Kinet 43: 431–440, 2011
2011 Wiley Periodicals,
INTRODUCTION
Most of the volatile organic compounds continuously
emitted to the earth’s troposphere from biogenic and
Correspondence to: S. Dhanya; e-mail: sdhanya@barc.gov.in.
2011 Wiley Periodicals, Inc.
c
ꢀ

432
SHARMA ET AL.
anthropogenic sources undergo reactions with highly
reactive species, such as OH radical, ozone, and NO3.
It is well established that oxidation reactions of unsat-
urated hydrocarbons with these species play a major
role in the tropospheric photochemistry and generation
of aerosols [1,2]. Previous studies have shown that, for
many organic molecules, reactions with Cl atoms are
also important in the troposphere, due to higher rate
constants of their reactions [3], and the reactions of
Cl atoms with volatile organic compounds (VOCs) are
suggested to play an important role in the atmospheric
chemistry of the remote marine boundary layer [4], as
well as the polluted urban areas with significant an-
thropogenic emission of Cl2 [5]. A recent field study
suggests that the role of the reactions of Cl atoms in
polluted noncoastal areas may be more significant than
what was thought earlier [6]. Hence, there is consid-
erable interest in measuring the rate constants of the
reactions of Cl atoms with different VOCs, including
unsaturated molecules [7,8].
Recently, we measured the rate constants of the re-
actions of Cl atoms with cyclic alkenes, namely cy-
clopentene, cyclohexene, and cycloheptene, and ob-
served an increase in the rate constant with the number
of carbon atoms in the ring. This was attributed to
the increase in the number of abstractable hydrogen
atoms [9]. Unlike the reactions of OH radicals, unsat-
uration does not appear to increase the rate constant
of the Cl atom reaction considerably. To investigate
the effect of unsaturation further, we have now mea-
sured the rate constants of the reactions of Cl atoms
with cyclic hydrocarbons having two double bonds,
namely 1,4-cyclohexadiene and 1,5-cyclooctadiene, at
298 K and 1 atm pressure, using the relative rate
method. Cyclic alkenes and dienes are constituents of
automobile exhausts and are also emitted to the at-
mosphere from various other sources such as forest
fire and incinerator. Many biogenic molecules such as
pinenes and carene have the same cyclic ring struc-
ture as that of cyclohexene and terpenes, such as γ-
terpinene, have the same ring structure as that of 1,4-
cyclohexadiene.
are not found to interfere with the relative rate measure-
ments, even in unsaturated molecules, as indicated by
the similar rate constants estimated using Cl2, SOCl2,
and CCl3COCl as Cl atom precursors [10]. The reac-
tion mixture, consisting of cyclodienes (CD), which
is either 1,4-cyclohexadiene (250–350 ppm) or 1,5-
cyclooctadiene (90–200 ppm), reference compound
(50–100 ppm), and trichloroacetyl chloride (typically
∼300 ppm), was prepared in the reaction chamber, us-
ing a vacuum manifold, and the total pressure was
maintained at 800
3 Torr, by adding N2/N2–O2
mixture/air. The pressure was measured, using a ca-
pacitance manometer (Pfeiffer Vacuum). Before pho-
tolysis, the prepared reaction mixture was equilibrated
for 60–80 min for uniform distribution, which was con-
firmed by the reproducibility of the gas chromatogram.
Experiments were performed at room temperature
(298
2 K). The mixture was photolyzed for a pe-
riod of 6–8 min, in steps of about 1 min, and after
each photolysis step the decrease in the concentration
of the two organics, cyclodiene (CD) and the reference
molecule (R), was determined, using gas chromato-
graph (Chemito GC 8610), with (5%/10%) SE 30 col-
umn, in conjunction with a flame ionization detector.
The stability of the reaction mixture with respect to
wall losses and dark reactions was checked for about 7
h, which is more than the total duration of a relative rate
measurement. Noninterference of any reaction product
of CD and R in the GC analysis was also checked in-
dependently. Assuming that both the molecules react
only with chlorine atoms, the kinetic data were ob-
tained from the fractional loss of CD and R, using the
standard expression
ꢀ
ꢁ
ꢀꢀ
ꢁ
ꢀ
ꢁꢁ
(R)t
(CD)t
kCD
kR
0
0
ln
=
ln
(I)
(CD)t
(R)t
where [R]t and [CD]t are the concentrations of R and
0
0
CD, respectively, at time t0, [R]t , and [CD]t are the
corresponding concentrations at time t, and kCD and kR
are the rate constants of their reactions with chlorine
atoms. In the present study, both n-hexane and 1-butene
were used as the reference molecules (R).
1,4-Cyclohexadiene (97%, from Aldrich (Stein-
heim, Germany)) and 1,5-cyclooctadiene (99.5%,
from Fluka (Basel, Switzerland)) were subjected to
freeze–pump–thaw cycles before use. The nitrogen
and oxygen were of 99.9% purity, from INOX Air
Products, Ltd. (Mumbai, India), and zero-grade air
was from Chemtron Science Laboratories (Mumbai,
India).
EXPERIMENTAL
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. Since molecular chlorine (Cl2) reacts with these
cyclodienes, Cl atoms were generated by photolysis of
trichloroacetyl chloride (CCl3COCl) at 254 nm, using
a UV lamp (Rayonett). This molecule is regularly used
as a source of Cl atoms [8], and the counter fragments
The products of the reaction of chlorine atoms
with cyclohexadiene and cyclooctadiene, formed in
the presence of air at atmospheric pressure, were
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE CONSTANTS FOR THE REACTIONS OF CHLORINE ATOMS WITH CYCLIC DIENES
433
characterized by a gas chromatograph coupled with a
0.6
0.5
0.4
0.3
0.2
0.1
0.0
mass spectrometer (Shimadzu), using BPX50 column
(30 m × 0.25 mm × 0.25 μm) and CP WAX 52CB
(30 m × 0.25 mm × 0.25 μm) columns, indepen-
dently. The observed spectral features of the products
were compared with the reported mass spectra.
RESULTS
0.0
0.1
0.2
0.3
0.4
ln((n-hexane)t /(n-hexane)t )
Determination of Rate Constants
0
t
(a)
To see the possible contribution from direct photolysis
of CD and R molecules, these compounds were pho-
tolyzed in N2, in the absence of added chlorine source
for about 8 min, in steps of 1 min, as in the actual
measurements, described above in the Experimental
section. There was no decrease in the concentration of
CD, n-hexane, and butene, which confirmed the ab-
sence of any significant loss due to direct photolysis.
Reactions of n-hexane, butene, and the CDs with Cl
atoms were also individually monitored, using GC, to
see whether any product buildup occurs that interferes
with the analysis.
0.8
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
0.5
ln((butene)t /(butene)t )
0
t
(b)
The experimental results, with n-hexane as the stan-
dard molecule, are plotted according to Eq. (1), in
Fig. 1a. The plots are linear, with near zero intercept, as
expected for a valid relative rate measurement. From
the linear least-square fit, slopes of the lines are ob-
tained, which are the ratios of the rate constants of
these cyclodienes to that of n-hexane. Since the possi-
bility of addition to double bond also exists in the reac-
tions of cyclodienes with Cl atom, another unsaturated
molecule, 1-butene, with the possibility of the similar
addition reaction, was also used as a reference for bet-
ter comparison. The plots, obtained with 1-butene as
reference, are given in Fig. 1b. The experiments were
repeated a minimum of three times, and the experimen-
tal conditions and the slopes obtained are given in Ta-
ble I. The slope and the error given for each experiment
are obtained from the linear least-square fitting of the
data. The average values of these individual measure-
ments are shown in bold in Table I for each set, with the
quoted error of two standard deviations. The measure-
ments at lower total pressures, 450 and 250 Torr, indi-
cate that the slopes are independent of the total pressure
in this range. In our earlier work, the rate constant of
the reaction of Cl atom with n-hexane was consid-
ered to be (3.03 0.06) × 10−10 cm3 molecule−1 s−1
[3], for calculating the rate constants of cyclic alkenes
[9]. In the present work, this is modified to (3.15
Figure 1 Logarithmic plot of the relative decrease in
the concentration of 1,4-cyclohexadiene ( ) and 1,5–
cyclooctadiene ( ) due to reaction with Cl atom at 298
2 K in N2; with (a) n-hexane and (b) 1-butene as reference
ꢀ
ꢁ
compounds.
reaction used by Atkinson et al. [3], to be (2.05
0.25) × 10−10 cm3 molecule−1 s−1. Similarly, (3.21
0.41) × 10−10 cm3 molecule−1 s−1 is used as the rate
constant for the reaction of Cl atom with 1-butene, cal-
culated on the basis of the experimental relative rate
ratio with respect to n-hexane [12], 1.02 0.02, and the
above rate constant for n-hexane. The calculated rate
constants are listed in Table I, and the errors quoted
include the errors in the reference rate constants.
Experiments were also carried out in the presence
of air; the typical plots are shown in Fig. 2. In both
the cyclodienes, a marginal increase in the average
slope is observed consistently in the presence of air
(Table I), suggesting some additional reactions in this
condition. This increase is found to be more in the case
of cyclohexadiene. To see the effect of oxygen further,
the experiments were conducted in the presence of
different partial pressures of oxygen. In the case of
cyclohexadiene, a systematic increase in the relative
rate ratio was observed up to 25% of oxygen, as shown
in Fig. 3.
0.40) × 10−10 cm3 molecule−1
s
−1, considering the
best current IUPAC estimate of the rate constant for
the Cl atom reaction with n-butane [11], the reference
International Journal of Chemical Kinetics DOI 10.1002/kin

434
SHARMA ET AL.
Table I Relative Rate Measurements of Cl Atom Reactions with 1,4-Cyclohexadiene and 1,5-Cyclooctadiene under
Different Conditions
Concentration
(ppm)
Concentration of
Reference Reference (ppm)
k (× 1010
)
Cyclic Diene
Buffer Gas
Ratio
(cm3 molecule−1 s−1
)
1,4 Cyclohexadiene
226
252
210
n-Hexane
n-Hexane
n-Hexane
72
72
66
N2
N2
N2
1.28 0.03
1.32 0.02
1.27 0.04
1.29 0.06
1.49 0.06
1.43 0.06
1.58 0.09
1.50 0.16
4.03 0.52
4.16 0.53
4.00 0.52
4.06 0.55
4.78 0.63
4.59 0.61
5.07 0.70
4.82 0.80
4.44 0.75
4.19 0.53
518
549
496
1-Butene
1-Butene
1-Butene
99
78
67
N2
N2
N2
183
316
233
231
231
n-Hexane
n-Hexane
n-Hexane
n-Hexane
n-Hexane
76
120
72
55
63
N2(400 Torr) 1.41 0.16
N2(250 Torr) 1.33 0.02
Air
Air
Air
1.50 0.02
1.79 0.04
1.69 0.03
1.66 0.30
1.87 0.02
1.78 0.06
1.82 0.10
2.34 0.26
2.17 0.24
1.99 0.07
2.36 0.12
2.07 0.05
2.19 0.32
2.23 0.06
2.58 0.13
2.26 0.07
2.36 0.38
2.64 0.08
2.64 0.06
2.45 0.08
2.39 0.03
2.40 0.03
2.50 0.26
215
112
1-Butene
1-Butene
75
120
Air
Air
1,5-Cyclooctadiene
141
115
117
117
119
n-Hexane
n-Hexane
n-Hexane
n-Hexane
n-Hexane
77
84
84
77
88
N2
N2
N2
N2
N2
7.37 1.24
6.84 1.15
6.09 0.80
7.43 1.01
6.52 0.84
6.90 1.33
7.16 0.93
8.28 1.13
7.25 0.94
7.58 1.55
155
137
132
1-Butene
1-Butene
1-Butene
99
78
67
N2
N2
N2
127
89
183
146
190
n-Hexane
n-Hexane
n-Hexane
n-Hexane
n-Hexane
48
43
29
57
59
Air
Air
Air
Air
Air
Stable Product Analysis in the Chlorine
Atom Initiated Oxidation
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Attempts were made to characterize the stable products
formed during the chlorine atom initiated oxidation of
both the dienes in atmospheric conditions. For this,
a mixture of cyclodiene/CCl3COCl/air, at a total pres-
sure of 800 Torr, was photolyzed at 254 nm, for a period
ranging from 1 to 8 min. The products were analyzed
by GC-MS, with different columns. Figure 4 shows the
total ion chromatogram obtained in the case of cyclo-
hexadiene. The only products clearly identified here are
benzene and phenol. The mass spectrum of one prod-
uct, seen in both carbowax (retention time of 12.2 min)
and BPX50 columns (retention time of 10.4 min),
matches with that of an unsaturated acyclic alcohol, ei-
ther 2,4-hexadiene-1-ol or 5-hexyn-1-ol. The intensity
0.0 0.1 0.2 0.3 0.4 0.5 0.6
ln((reference)t /(reference)t )
0
t
Figure 2 Logarithmic plot of the relative decrease in the
concentration of 1,4-cyclohexadiene, : with respect to n-
hexane, : with respect to 1-butene and : 1,5-cyclooctadiene
with respect to n-hexane due to reaction with Cl atom at
◦
ꢀ
ꢁ
298 2 K in air.
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE CONSTANTS FOR THE REACTIONS OF CHLORINE ATOMS WITH CYCLIC DIENES
435
2.5
1.5
1.0
0.5
0.0
50
2.0
1.5
1.0
40
30
20
10
0
0
20 40 60 80 100
O2 %
−10
0
40
80
120
160
Depletion of c-hexadiene (ppm)
Figure 5 Typical plots of amounts of benzene formed
against the amounts of 1,4-cyclohexadiene reacted with Cl
0.0
0.2
0.4
0.6
atom. and : in air; : in N , shifted by –5 units for clarity.
ꢂ
ꢁ
•
2
ln((n-hexane)t /(n-hexane)t )
0
t
and 11.9 min, indicate that these are formed by addition
of Cl atom to cyclohexadiene, but the exact structure
could not be assigned. In the case of cyclooctadiene
also, the products could not be characterized, without
ambiguity, by matching with the mass spectra available
in the library. The depletion of cyclohexadiene and for-
mation of benzene were followed quantitatively in the
presence of air, and two typical plots of their linear
dependence are shown in Fig. 5. From 10 indepen-
dent experiments, the average yield of benzene was
Figure 3 Logarithmic plot of the relative decrease in
the concentration of 1,4-cyclohexadiene with respect to n-
hexane at varying partial pressure of oxygen due to reaction
ꢀ
with Cl atom at 298 2 K. : 5%, : 10%, and : 20% oxy-
◦
•
gen. Inset: Variation of the observed rate constant ratio of
cyclohexadiene, with reference to that of hexane at different
partial pressures of oxygen.
ratios for 35Cl and 37Cl isotopes in the recorded mass
spectra of the products, with retention times of 11.2
50
40
A
Benzene
30
20
c
b
a
10
0
3
4
5
6
7
8
9
10
11
12
13
14
15
16
100
80
60
40
20
0
B
Phenol
b
a
6
8
10
12
14
16
18
20
Time (min)
Figure 4 Total ion chromatograms of the products of Cl atom initiated oxidation of 1,4-cyclohexadiene with (A) CP WAX
column; a, unphotolyzed; b, photolyzed sample, both at 30◦C isothermal; and c, photolyzed with temperature programming,
maintained at 100◦C for 10 min and raised the temperature to 150◦C at the rate of 20◦C per min (plots shifted vertically for
clarity) and (B): BPX50 column with temperature programming, maintained at 50◦C for 5 min and with the temperature raised
to 180◦C at the rate of 10◦C/min; a, unphotolyzed and b, photolyzed.
International Journal of Chemical Kinetics DOI 10.1002/kin

436
SHARMA ET AL.
measured to be 30 4% of cyclohexadiene consump-
tion. The dependence in the absence of air, also shown
in Fig. 5, was found to deviate marginally from lin-
earity. Owing to very low yield of phenol, quantitative
estimation was not possible in the present experiments.
Another reason for the very close rate constants
of the reactions of cyclohexane, cyclohexene, and 1,4-
cyclohexadiene with Cl atoms could be that these reac-
tions proceed with very small or almost no activation
barrier and hence could be collision controlled [10].
The bimolecular rate constant for collision, k, is given
by
DISCUSSION
k = νσ
(II)
The rate constants for the reactions of Cl atom with
1,4-cyclohexadiene and 1,5-cyclooctadiene have not
been reported so far. The reaction of Cl atom with
cyclohexadiene has been used as a source of cyclo-
hexadienyl radicals [13], but there is no mention of the
rate constant of the reaction. The values of the rate con-
stants obtained in the present work, with n-hexane and
1-butene as references, are found to differ marginally,
the latter being always higher. We have considered the
rate constants determined with n-hexane as the refer-
ence molecule for comparison of different cyclic unsat-
urated compounds, since it was also used as the refer-
ence in our earlier study on cycloalkenes [9]. However,
the reported rate constants in [9] are modified, using the
corrected rate constant value for n-hexane, whenever
quoted here.
where ν is the speed at which the Cl atom and the cy-
cloalkene approach each other and σ is the collision
cross section in a hard sphere model, given by πd2 and
where d is the collision diameter, 1/ (d + d ), where
2
1
2
d1 and d2 are the diameters of Cl atom and the alkene,
respectively. The rate constant is expected to be pro-
portional to d2/μ1/2, where μ is the reduced mass of Cl
and alkene. Thus, the higher rate constant in the case of
cyclooctadiene as compared to the six-membered rings
could be due to a larger molecular size. The calculated
collision-controlled rate constant for the reaction of Cl
atom with cyclopentene is 2.2 × 10−10 cm3 molecule−1
s−1 and for the reactions with cyclohexane, cyclohex-
ene, and cyclohexadiene, is about 18% higher, 2.6 ×
10−10 cm3 molecule−1 −1. Even though these calcu-
s
As mentioned in the Introduction, the results of our
earlier measurements of the rate constants of the reac-
tions of Cl atom with cyclopentene, cyclohexene, and
cycloheptene, indicated an increase in the rate constant
with increasing number of carbon atoms [9]. Unlike the
reactions of OH radicals, where the rate constant in-
creases by an order of magnitude with unsaturation, the
rate constants of the reactions of Cl atoms with cyclic
alkenes were only marginally higher than those of the
corresponding saturated cyclic alkanes. The products
observed in the reactions of cyclic alkenes with Cl
atoms indicated the contribution of both addition and
abstraction reactions [9]. This implied that the rate
constants for addition and abstraction reactions are
comparable in the case of the Cl atom reaction with
cyclic alkenes. The present results show that the rate
constant for the reaction of Cl atom with cyclohexa-
diene, (4.06 0.55) × 10−10 cm3 molecule−1s−1, is
very close to that of cyclohexene [9] and cyclohex-
lated values are marginally lower than the experimental
values [8,9 and the present work], the increase observed
in the experimental value for cyclohexene as compared
to that for cyclopentene is about 15%, comparable to
the increase in the calculated collision-controlled rate
constants. However, the percentage increase calculated
in the collision-controlled rate constant for cycloocta-
diene as compared to the six-membered rings is less
than that observed. The rate constants of the reaction
of Cl atoms with biogenic molecules such as α-pinene,
β-pinene, and limonene [10], which have a basic struc-
ture of a six-membered unsaturated ring with alkyl
groups attached, are also reported to be high, >5 ×
10−10 cm3 molecule−1 −1, as in the case of cyclo-
s
heptene and cyclooctadiene. Here also, the percentage
increase in the rate constant as compared to cyclohex-
ene or cyclohexadiene is higher than that expected for
collision-controlled rate constants.
An approximate linear correlation has been ob-
served between the rate constants of reactions with OH
and Cl atoms for many unsaturated and saturated hy-
drocarbons, including many halocarbons [14–16] and
chlorofluoroesters [17], with different slopes for satu-
rated and unsaturated compounds. Such a linear cor-
relation indicates similarity of the mechanisms of the
reaction of these hydrocarbons with the OH radical
and Cl atom. Since addition as well as abstraction is
involved in the reactions of the unsaturated molecules,
the parameters are different from those of the saturated
ane [3], (4.13
0.68) × 10−10 and (3.26
0.42)
× 10−10 cm3 molecule−1s−1, respectively. This fur-
ther confirms that replacing two CH2 groups with one
double bond in these cyclic molecules has no pro-
nounced effect on the rate constant of their reaction
with Cl atom. However, in cyclooctadiene, with two
more CH2 groups, there is an increase in the number
of abstractable hydrogen atoms and hence there is an
increase in the rate constant as compared to that of
cyclohexadiene.
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE CONSTANTS FOR THE REACTIONS OF CHLORINE ATOMS WITH CYCLIC DIENES
437
the increase observed in air is more clearly seen. In
cyclooctadiene, considering the large error bars, the
increase in air can be considered marginal (Table I),
similar to cyclopentene, cyclohexene, and cyclohep-
tene. Earlier studies on the reactions of Cl atoms with
acyclic alkenes have shown that the observed rate con-
stant ratios are identical, within experimental error, in
air and nitrogen [12,19]. In the case of the reactions
of Cl atom with biogenic molecules such as limonene,
3-carene, β-pinene, and myrcene the relative rate ra-
tios were found to be marginally higher in the presence
of air but were still considered as identical within ex-
perimental errors except in the case of myrcene [10].
However, as seen from Fig. 3, our study shows a sys-
tematic increase in the observed relative rate ratio for
the reaction of Cl atom with cyclohexadiene, with in-
creasing concentration of oxygen. Very recently, dur-
ing the measurement of the rate constant of the Cl atom
reaction with methacrolein with reference to propane,
addition of 2–20% oxygen was found to increase the
relative ratio by a statistically significant amount [20].
Such difference in the rate constant in the presence of
air was suggested to be due to interference of the OH
reaction, generated during the reactions of peroxy radi-
cals with HO2, as observed in the Cl-initiated oxidation
of acetaldehyde [21,22]. Similar reactions may also be
responsible for the marginal increase observed in the
present study. Detailed understanding of the reaction
mechanism, based on the product studies, can probably
confirm the occurrence of such reactions. The present
study, where benzene and phenol only could be con-
firmed as products, is not able to clearly explain the
oxygen dependence of the relative rates.
1E−10
1E−11
1E−12
1E−13
1E−10
kCl / cm3 molecule−1 s−1
1E−9
Figure 6 Correlation plots between the rate constants for
the reactions of Cl and OH radicals with unsaturated hy-
drocarbons at room temperature. - - -: For molecules and
rate constants taken from [16] shown as (C H , C H Cl,
ꢁ
2
4
2 3
1,1-C2H2Cl2, cis-C2H2Cl2, trans-C2H2Cl2, C2Cl4, C3H6,
1-C4H8, and 1- C5H10). —: After including the data for
unsaturated cycloalkenes from the present work, [9], [10],
shown as (cyclopentene, cyclohexene, cycloheptene, 1,4-
•
cyclohexadiene, 3-carene, α-pinenes, β-pinene, limonene,
isoprene, and myrcene).
molecules. It was observed that the linearity in the
case of unsaturated molecules holds good within the
error limits, even after inclusion of the three cyclic
alkenes [9]. The rate constants of reactions of OH and
Cl atom with unsaturated molecules, including 1,4-
cyclohexadiene and biogenic unsaturated molecules
[6,18] are plotted in Fig. 6. (Since the rate constant
for the reaction of the OH radical with cyclooctadiene
is not reported, this molecule could not be included.) It
is observed that even in these cases, where the rate con-
stants with Cl atoms are going higher than the calcu-
lated collision-controlled limits, the linear correlation
between the constants of Cl and OH is maintained. The
correlation obtained for unsaturated molecules consid-
ered in [16] yields a straight line,
The formation of benzene can be understood by con-
sidering the probable reaction scheme as given below.
The reaction of Cl atom with cyclohexadiene leads to
cyclohexadienyl radical as well as chlorocyclohexenyl
radical. The cyclohexadienyl radical gives benzene by
disproportionation reaction (reaction (3)) in the ab-
sence of oxygen [23,24]. In the presence of oxygen,
reactions (4) and (5) are possible, of which formation
of benzene along with a HO2 radical, by reaction (4),
is identified as the dominant channel [13,25,26]:
log kOH = (2.4 0.3) log kCl + (12.9 2.8) (III)
with a correlation coefficient of 0.95. After inclusion of
cyclopentene, cyclohexene, cycloheptene, cyclohexa-
diene, and biogenic molecules reported in [10], this is
modified marginally to
+
HCl
+
Cl
log kOH = (2.1 0.1) log kCl + (9.6 1.2) (IV)
(1)
(2)
H
with a correlation coefficient of 0.97.
In all the cyclic alkenes, the measured relative rate
ratios of the sample and standard molecules, which give
the ratios of the rate constants, are found to increase
marginally in the presence of air [9]. In the case of
cyclohexadiene, where two double bonds are present,
Cl
+
Cl
International Journal of Chemical Kinetics DOI 10.1002/kin

438
SHARMA ET AL.
study [9], considering the increase in the rate constant
of cyclopentene, cyclohexene, and cycloheptene to be
due to the increase in the number of abstractable hy-
drogen atoms, we had estimated the rate constant for
the reaction of Cl atom per CH2 group to be (8.7
+
+
(3)
(4)
(5)
H
H
1.6) × 10−11 cm3 molecule−1
s
−1. This approximate
+ HO2
+ O2
estimation, which does not differentiate between allyl
and nonallyl CH2 groups, is close to that estimated now
for allyl CH2 groups in cyclohexadiene.
H
The reason for the predominance of reaction (4), as
compared to the formation of a peroxy radical (reaction
(5)), normally observed in other cases, is the formation
of a stable aromatic ring. In the case of molecules like
cyclooctadiene, formation of the peroxy radical and its
subsequent reactions are expected to be more important
[27]. Hence, it is possible that subsequent reactions of
HO2 radicals, formed significantly in cyclohexadiene
only, are responsible for the dependence of the ob-
served relative rate ratio on the oxygen concentration,
which is also observed to be more significant in cyclo-
hexadiene. However, as mentioned earlier, the limited
data do not lead to a clear understanding of this oxygen
dependence. It was also difficult to assign the reactions
responsible for the formation of phenol, due to a lack
of quantitative data.
+ O2
O2
H
H
In the case of the reaction of OH with cyclohexa-
diene, which also leads to cyclohexadienyl radical,
a yield of 15% is reported for benzene, independent
of the partial pressure of oxygen varying from 5 to
760 Torr, by Ohta [26], and 12.5%, by Tuazon et al
[27]. Thus, considering that cyclohexadienyl radical is
quantitatively converted to benzene and that addition
reaction does not lead to benzene formation, the per-
centage contribution of the abstraction reaction to the
total reaction of OH radicals with 1,4-cyclohexadiene
was considered to be around 15% [25]. In the present
study of the Cl atom reaction, the yield of benzene is
30%, higher than that for the OH radical reaction. Con-
sidering this as the yield of cyclohexadienyl radical,
percentage contribution toward hydrogen abstraction
is 30% in this case. Our measured total rate constant
for the reaction of Cl atom with 1,4-cyclohexadiene,
(4.06 0.55) × 10−10 cm3 molecule−1 s−1, consists of
abstraction of one of the four allylic hydrogen atoms
(two CH2 groups), along with addition to either of the
two double bonds. Thus, the rate constant of the hy-
drogen abstraction reaction from each allylic group in
the symmetric 1,4-cyclohexadiene is calculated to be
Atmospheric Lifetimes
The main sink of these cyclic dienes is expected to be
reactions with tropospheric oxidants, such as ozone,
OH radical, and NO3, because these are sparingly sol-
uble in water and do not absorb considerably at wave-
lengths above 320 nm, as would be required for pho-
tolysis in the troposphere. The reactions of Cl atom
are not very important in ambient conditions because
the upper limit for the chlorine atom concentration is
as low as 103 atoms cm−3 in the northern hemisphere
[29]. However, the peak concentration during sunrise
in the marine boundary layer is reported [4] to be as
high as 1.3 × 105 atoms cm−3 and the concentration
in the polluted noncoastal areas is also expected to be
high. To understand the contribution of the Cl atom
reaction in the atmospheric oxidation of cyclic dienes,
tropospheric lifetimes (τX) of these cyclic dienes with
respect to removal by species X, which can be OH, O3,
NO3, and Cl, were estimated from the respective rate
constants, kX, as
(6.1 1.2) × 10−11 cm3 molecule−1
s
−1. In a simi-
lar manner, the rate constant of the addition reaction
to each double bond is calculated to be (1.4
0.3)
× 10−10 cm3 molecule−1
s
−1. The rate constant for
allylic abstraction is higher, whereas that for addition
is considerably lower, than those estimated for sec-
ondary allylic hydrogen abstraction (∼ 4 × 10−11) and
addition (∼ 3 × 10−10 ) in simple noncyclic alkenes
[19,28]. From the rate constant of reaction of Cl atom
with cyclohexane [3], the rate constant of the hydro-
gen abstraction reaction from nonallylic CH2 groups
1
τ =
(V)
can be estimated to be 5.4 × 10−11 cm3 molecule−1 s−1
,
kX[X]
very similar to that of the allylic hydrogen abstraction
reaction, calculated above. These values confirm that
replacing two CH2 groups by CH CH changes the
rate constant values only marginally. In our previous
where [X] is the concentration of the above reactive
species at ambient conditions or marine boundary layer
conditions. The calculated values are shown in Table II.
International Journal of Chemical Kinetics DOI 10.1002/kin

RATE CONSTANTS FOR THE REACTIONS OF CHLORINE ATOMS WITH CYCLIC DIENES
439
Table II Tropospheric Lifetimes (τ) Calculated for the Cyclodienes with Respect to Reactions with Different Species
1,4-Cyclohexadiene
1,5-Cyclooctadiene
kCl (cm3 molecule−1
τCl (ambient conditions)
τCl (marine boundary layer)
kOH (cm3 molecule−1
s
−1) (present work)
4.06 × 10−10
11.4 days
5.3 h
6.9 × 10−10
6.7 days
3.1 h
Not available
–
s
−1) [7]
−1) [7]
−1) [7]
9.9 × 10−11
τOH
2.8 h
kNO (cm3 molecule−1
s
6.6 × 10−13
1.7 h
Not available
–
3
τNO
3
kO (cm3 molecule−1
s
4.6 × 10−17
1.4 × 10−16
3
τO
8.6 h
2.8 h
3
The ambient concentrations used for Cl [31], OH, NO3, and O3 [32] are 2.5 × 103, 1 × 106, 2.5 × 108, and 7 × 1011 molecules cm−3
,
respectively.
The typical ambient concentrations (in units of
the case of reaction of Cl atom with cyclohexadiene,
molecule cm−3) used are 5 × 103, 2 × 106, 5 ×
108, and 7 × 1011 for Cl [30], OH, NO3, and O3
[31], respectively. The concentrations of Cl and OH,
present only in the daytime, are 12-h daytime average
concentrations and that of NO3 is a 12-h nighttime av-
erage. Hence, these are corrected by dividing by 2, to
get the average 24-h concentration, whereas the con-
centration of O3, given above is 24-h average and is
used directly. The rate constants obtained in nitrogen,
with n-hexane as the reference, were used for these
estimations. From Table II, it can be seen that in the
case of 1,4-cyclohexadiene, removal by NO3 radical is
very efficient in the nighttime. The daytime removal by
OH is comparable with that by O3 for this molecule,
because the rate coefficient of reaction of ozone with
this molecule is slower than that with other cyclic di-
enes [7]. In marine boundary layer conditions, where
concentration of Cl atom is 50 times higher than that
in ambient conditions, the reaction with Cl atoms be-
comes competitive with the reactions with OH and
ozone, for both cyclohexadiene and cyclooctadiene.
is estimated quantitatively, and the yield is found to be
(30 4)% of the cyclohexadiene consumption. Based
on this, the rate constant for abstraction of H atom from
the allylic group of cyclohexadiene is calculated to be
(6.1
1.2) × 10−11 cm3molecule−1
s
−1, assuming
that cyclohexadienyl radicals formed by H atom ab-
straction are quantitatively converted to benzene. Sim-
ilarly, the rate constant for addition of Cl atom is esti-
mated to be (1.4 0.3) × 10−10 cm3 molecule−1 s−1
in 1,4-cyclohexadiene. The results also indicate that
the reaction with Cl atom becomes a competitive chan-
nel for both cyclohexadiene and cyclooctadiene, in the
conditions of the marine boundary layer and polluted
urban industrial areas, especially in the case of 1,4-
cyclohexadiene, which reacts slowly with ozone.
The authors are thankful to Dr. S. K. Sarkar, Head, Radiation
and Photochemistry Division, and Dr. T. Mukherjee, Direc-
tor, Chemistry Group, for their keen interest and encourage-
ment during the course of this work. The authors thank H. D.
Alwe and M. Gurav for assistance in the experiments.
CONCLUSION
BIBLIOGRAPHY
The rate constants for the reactions of Cl atoms with
1,4-cyclohexadiene and 1,5-cyclooctadiene have been
determined at room temperature, 298 2 K, using the
relative rate method. The rate constants determined in
nitrogen are (4.06 0.55) × 10−10 and (6.90 1.33)
× 10−10 cm3 molecule−1 s−1 with respect to n-hexane
and (4.82 0.80) × 10−10 and (7.58 1.55) × 10−10
cm3 molecule−1 s−1 with respect to 1-butene. The ob-
served depletion ratio of the cyclic diene to that of the
reference molecule is found to increase in the presence
of oxygen, the effect being more prominent in the case
of 1,4-cyclohexadiene. Benzene, one major product in
1. Calvert, J. G.; Atkinson, R.; Kerr, J. A.; Madronich, S.;
Moortgat, G. K.; Wallington, T. J.; Yarwood, G. The
Mechanisms of Atmospheric Oxidation of the Alkenes;
Oxford University Press: New York, 2000.
2. Hatakeyama, S.; Ohno, M.; Weng, J.; Takagi, H.;
Akimoto, H. Environ Sci Technol 1987, 21, 52–57.
3. Atkinson, R.; Aschmann, S. M. Int J Chem Kinet 1985,
17, 33–41.
4. Spicer, C. W.; Chapman, E. G.; Finlayson-Pitts, B. J.;
Plastridge, R. A.; Hubbe, J. M.; Fast, J. D.; Berkowitz,
C. M. Nature 1998, 394, 353–355.
5. Tanaka, P. L.; Oldfield, S.; Neece, J. D.; Mullins, C. B.;
Allen, D. T. Environ Sci Technol 2000, 34, 4470–4473.
International Journal of Chemical Kinetics DOI 10.1002/kin

440
SHARMA ET AL.
6. Thornton, J. A.; Kercher, J. P.; Riedel, T. P.; Wagner,
N. L.; Cozic, J.; Holloway, J. S.; Dube, W. P.; Wolfe,
G. M.; Quinn, P. K.; Middlebrook, A. M.; Alexander, B.;
Brown, S. S. Nature 2010, 464, 271–274.
16. Garib, A.; Timerghazin, Q. K.; Ariya, P. A. Can J Chem
2006, 84, 1686–1695.
17. Blanco, M. B.; Teruel, M. A. Chem Phys Lett 2007, 441,
1–6.
7. Atkinson, R.; Arey, J. Chem Rev 2003, 103, 4605–4638.
8. Manion, J. A.; Huie, R. E.; Levin, R. D.; Burgess
Jr., D. R.; Orkin, V. L.; Tsang, W.; McGivern, W. S.;
Hudgens, J. W.; Knyazev, V. D.; Atkinson, D. B.; Chai,
E.; Tereza, A.M.; Lin, C.-Y.; Allison, T. C.; Mallard,
W. G.; Westley, F.; Herron, J. T.; Hampson, R. F.;
Frizzell, D. H. NIST Chemical Kinetics Database, NIST
Standard Reference Database 17, Version 7.0 (Web Ver-
sion), Release 1.4.3, Data version 2008.12; National
Institute of Standards and Technology: Gaithersberg,
MD. Available at http://kinetics.nist.gov/. Accessed 17
September 2010.
9. Sharma, A.; Pushpa, K. K.; Dhanya, S.; Naik, P. D.;
Bajaj, P. N. Int J Chem Kinet 2010, 42, 98–105.
10. Finlayson-Pitts, B. J.; Keoshian, C. J.; Buehler, B.; Ezell,
A. A. Int J Chem Kinet 1999, 31, 491–499.
11. Evaluated Kinetic Data from IUPAC Sub Committee
for Gas Kinetic Data Evaluation. Available at www
.iupac-kinetic.ch.cam.ac.uk/. Accessed 17 September
2010.
12. Coquet, S.; Ariya, P. A. Int J Chem Kinet 2000, 32,
478–484.
13. Shiga, Y.; Koshi, M.; Tonokura, K. Chem Phys Lett
2009, 470, 35–38.
14. Stutz, J.; Ezell, M. J.; Ezell, A. A.; Filayson-Pitts, B. J.
J Phys Chem A 1998, 102, 8510–8519.
15. Andersen, M. P. S.; Nielsen, O. J.; Wallington, T. J.;
Hurley, M. D.; DeMore, W. B. J Phys Chem A 2005,
109, 3926–3934.
18. Peeters, J.; Boullart, W.; Pultau, V.; Vandenberk, S.;
Vereecken, L. J Phys Chem A 2007, 111, 1618–1631.
19. Ezell, M. J.; Wang, W.; Ezell, A. A.; Soskin, G.;
Finlayson-Pitts, B. J. Phys Chem Chem Phys 2002, 4,
5813–5820.
20. Kaiser, E. W.; Pala, I. R.; Wallington, T. J. J Phys Chem
A 2010, 114,6850–6860.
21. Jenkin, M. E.; Hurley, M. D.; Wallington, T. J. Phys
Chem Chem Phys 2007, 9, 3149–3162.
22. Hasson, A. S.; Tyndall, G. S.; Orlando, J. J. J Phys Chem
A 2004, 108, 5979–5989.
23. Sauer, M. C., Jr.; Ward, B. J Phys Chem 1967, 71, 3971–
3983.
24. Berho, F.; Rayez, M-T.; Lesclaux, R. J Phys Chem A
1999, 103, 5501–5509.
25. Ohta, T. Int J Chem Kinet 1984, 16, 1495–1503.
26. Estupinan, E.; Villenaeve, E.; Raoult, S.; Rayez,
J. C; Rayez, M. T.; Lesclaux, R. Phys Chem Chem Phys
2003, 5, 4840–4844.
27. Tuazon, E. C.; Aschmann, S. M.; Nguyen, M. V.;
Atkinson, R. Int J Chem Kinet 2003, 35, 415–426.
28. Tyndall, G. S.; Orlando, J. J.; Wallington, T. J.; Dill, M.;
Kaiser, E. W. Int J Chem Kinet 1997, 29, 43–55.
29. Rudolph, J.; Koppmann, R.; Plass-Dulmer, Ch. Atmos
Environ 1996, 30, 1887–1894.
30. Pszenny, A. A. P.; Keene, W. C.; Jacob, D. J.; Fan,
S.; Maben, J. R.; Zetwo, M. P.; Springer-Young, M.;
Galloway, J. N. Geophys Res Lett 1993, 20, 699–702.
31. Atkinson, R. Atmos Environ 2000, 34, 2063–2101.
International Journal of Chemical Kinetics DOI 10.1002/kin