Kinetics of the reactions of C2H5, n-C 3H7, and n-C4H9 radicals with Cl2 at the temperature range 190-360 K

Article abstract of DOI:10.1002/kin.20272

The kinetics of the C₂H₅ + Cl₂, n-C ₃H₇ + Cl₂, and n-C₄H₉ + Cl₂ reactions has been studied at temperatures between 190 and 360 K using laser photolysis/photoionization mass spectrometry. Decays of radical concentrations have been monitored in time-resolved measurements to obtain reaction rate coefficients under pseudo-first-order conditions. The bimolecular rate coefficients of all three reactions are independent of the helium bath gas pressure within the experimental range (0.5-5 Torr) and are found to depend on the temperature as follows (ranges are given in parenthesis): k(C ₂H₅+ Cl₂) = (1.45 ± 0.04) × 10^-11 (T/300K)^-1-73±0.09 cm³ molecule^-1 s^-1 (190-359 K), k(n-C₃H₇ + Cl₂) = (1.88 ± 0.06) × 10^-11 (T/300 K)^{-1.57 ± 0.14}cm³ molecule^-1 s ^-1 (204-363 K), and k(n-C₄H₉ + Cl₂) = (2.21 ± 0.07) × 10^-11 (T/300 K) ^{-238 ± 0.14} cm³ molecule^-1 s ^-1 (202-359 K), with the uncertainties given as one-standard deviations. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±20%. Current results are generally in good agreement with previous experiments. However, one former measurement for the bimolecular rate coefficient of C₂H₅ + Cl₂ reaction, derived at 298 K using the very low pressure reactor method, is significantly lower than obtained in this work and in previous determinations.

Full text of DOI:10.1002/kin.20272

Kinetics of the Reactions of
C2H5, n-C3H7, and n-C4H9
Radicals with Cl2 at the
Temperature Range
190–360 K
ARKKE J. ESKOLA, VLADIMIR A. LOZOVSKY,† RAIMO S. TIMONEN
Laboratory of Physical Chemistry, University of Helsinki, P. O. Box 55 (A. I. Virtasen aukio 1),
Helsinki, FIN-00014, Finland
Received 12 March 2007; revised 22 May 2007; accepted 22 May 2007
DOI 10.1002/kin.20272
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the C2H5 + Cl2, n-C3H7 + Cl2, and n-C4H9 + Cl2 reactions has been
studied at temperatures between 190 and 360 K using laser photolysis/photoionization mass
spectrometry. Decays of radical concentrations have been monitored in time-resolved measure-
ments to obtain reaction rate coefficients under pseudo-first-order conditions. The bimolecular
rate coefficients of all three reactions are independent of the helium bath gas pressure within
the experimental range (0.5–5 Torr) and are found to depend on the temperat−u1r.e73as follows
(rangesaregiveninparenthesis): k(C2H5 + Cl2) = (1.45 0.04) × 10−11 (T /300 K)
0.09 cm3
0.14
molecule−1 −1
s
(190–359 K), k(n-C3H7 + Cl2) = (1.88 0.06) × 10−11 (T /300 K)−1.57
cm3
molecule−1 s−1 (204–363 K), and k(n-C4H9 + Cl2) = (2.21 0.07) × 10−11 (T /300 K)−2.38
cm3 molecule−1 s−1 (202–359 K), with the uncertainties given as one-standard deviations. Es-
timated overall uncertainties in the measured bimolecular reaction rate coefficients are 20%.
Current results are generally in good agreement with previous experiments. However, one for-
mer measurement for the bimolecular rate coefficient of C2H5 + Cl2 reaction, derived at 298 K
using the very low pressure reactor method, is significantly lower than obtained in this work
0.14
and in previous determinations. ꢀ 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 614–619,
C
2007
†
Deceased.
Correspondence to: Raimo Timonen; e-mail: raimo.timonen@
helsinki.fi.
Contract grant sponsor: Bioscience and Environmental Research
Council of the Academy of Finland.
Contract grant sponsor: Maj and Tor Nessling Foundation.
Contract grant sponsor: NoNeCK (Nordic Network for Chemical
Kinetics).
c
ꢀ
2007 Wiley Periodicals, Inc.

KINETICS OF THE REACTIONS OF C2H5, n-C3H7, AND n-C4H9 RADICALS WITH Cl2
615
times faster than that of the CH3 + Cl2. This is
in contradiction with the recent determination of
these reactions by Dobis and Benson [6]. They em-
ployed the very low-pressure reactor (VLPR) method
and derived k(CH3 + Cl2) = (3.4 0.3) × 10−12 cm3
molecule−1 s−1 and k(C2H5 + Cl2) = (1.05 0.05) ×
10−12 cm3 molecule−1 s−1 at 298 K by following the
product (CH3Cl, C2H5Cl) formation kinetics [6]. Al-
though the value obtained by Dobis and Benson [6] for
the CH3 + Cl2 is close to that measured by Timonen
and Gutman [5], the bimolecular reaction rate coeffi-
cient for the C2H5 + Cl2 reaction derived by Dobis and
Benson is about one twentieth of the value measured by
Timonen and Gutman. In addition, the value derived by
Dobis and Benson for the C2H5 + Cl2 reaction is about
three times smaller than the value they obtained for the
CH3 + Cl2 reaction, which is in opposite with the ob-
servation by Timonen and Gutman for these reactions,
as discussed above.
INTRODUCTION
The gas phase reactions of carbon-centered free radi-
cals with molecular chlorine are important elementary
steps in chlorination processes [1]. Chlorination reac-
tions of saturated hydrocarbons (RH) involve simple
two-step free-radical chains that substitute chlorine for
hydrogen, constituting an efficient cyclic process for
the production of chlorine-containing hydrocarbons.
Cl + RH → HCl + R
R + Cl2 → RCl + Cl
(A)
(B)
Both these reaction steps appear to be exothermic
and have little or no activation energy [2]. Much is
known about the reactions of chlorine atoms with sat-
urated hydrocarbons (reaction (A)), partly due to their
importance in the atmospheric as well as in the com-
bustion processes and partly because these reactions
can be isolated for detailed study relatively easily.
Contrasting the knowledge available on reactions
(A), less information is available on the kinetics of the
reactions of saturated hydrocarbon free radicals (R)
with molecular chlorine (reaction (B)). This is particu-
larly true for the reactions that have been subjected to
direct studies [2]. Reactions (B) can also be important
in the combustion and incineration processes of chlori-
nated hydrocarbons, especially under the conditions of
incomplete combustion and when the ratio of hydrogen
to chlorine is low [3,4].
Using a relative rate method (UV radiation to pro-
duce radicals and FTIR spectroscopy to detect sta-
ble products), Kaiser et al. [7] measured reaction
rate coefficient ratios k(C2H5 + O2)/k(C2H5 + Cl2) as
a function of helium pressure at 298 K. Combining
the value for this ratio with the bimolecular reac-
tion rate coefficient of the C2H5 + O2 reaction ob-
tained from direct measurements [8] at 298 K and
under about 5 Torr He pressure, Kaiser et al. [7]
derived k(C2H5 + Cl2)298 K = (16 2.5) × 10−12 cm3
molecule−1 −1. This value is in good agreement
s
with that of Timonen and Gutman [5]. Employ-
ing similar method as Kaiser et al. above, Tyndall
et al. [9] determined the rate coefficient ratio k(n-
C4H9 + O2)/k(n-C4H9 + Cl2) at 296 K and pressures
10 and 700 Torr N2. No pressure dependency was
observed. Combining the obtained value for this ra-
tio with the available direct determination of the bi-
molecular reaction rate coefficient for the n-C4H9 + O2
reaction [10] at 298 K, Tyndall et al. derived k(n-
The reactions of carbon-centered hydrocarbon
free radicals with molecular chlorine are interest-
ing also from the view of basic research. Already
some time ago, Timonen and Gutman [5] performed
the first direct measurements of the CH3 + Cl2,
C2H5 + Cl2, i-C3H7 + Cl2, and t-C4H9 + Cl2 re-
actions as a function of temperature employing
laser photolysis/photoionization mass spectrometry
method (LP-PIMS). Importantly, although the ob-
tained temperature dependence of the CH3 + Cl2 re-
action was positive (Ea = 2.2 0.5 kJ/mol), acti-
vation energies of the other reactions were nega-
tive or about zero, i.e., Ea(C2H5 + Cl2) = −1.3 0.5
kJ/mol, Ea(i-C3H7 + Cl2) = −2.0 1.0 kJ/mol, and
Ea(t-C4H9 + Cl2) = 0 0.6 kJ/mol [5]. At room tem-
perature, bimolecular rate coefficients of these re-
actions are k(CH3 + Cl2)298 K = (2.0 0.4) × 10−12
C4H9 + Cl2)298 K = 23 × 10−12 cm3 molecule−1 s−1
.
To obtain deeper understanding of the reasons
affecting the reactivity of radicals, it is profitable
to systematically investigate a series of reactions in
which only one parameter (e.g., radical substitution)
is changed at a time. In the present study, we have
performed the systematic work among the reactions
of alkyl-substituted methyl radicals with Cl2 and we
describe the direct experimental measurements for re-
actions (1)–(3).
cm3 molecule−1
s
−1, k(C2H5 + Cl2)298 K = (19 4)
× 10−12 cm3 molecule−1
s
−1, k(i-C3H7 + Cl2)298 K
=
(57 11) × 10−12 cm3 molecule−1
s
−1, and k(t-
C2H5 + Cl2 → C2H5Cl + Cl
n-C3H7 + Cl2 → n-C3H7Cl + Cl
n-C4H9 + Cl2 → n-C4H9Cl + Cl
(1)
(2)
(3)
C4H9 + Cl2)298 K = (44 9) × 10−12 cm3 molecule−1
s
−1, respectively. It is especially interesting to note
that C2H5 + Cl2 reaction at 298 K is about 10
International Journal of Chemical Kinetics DOI 10.1002/kin

616
ESKOLA, LOZOVSKY, AND TIMONEN
Current study represents the first direct measure-
ments for reactions (2) and (3). All reactions have been
determined as a function of temperature to obtain in-
formation on the temperature dependencies. This work
also extends the temperature range in which R + Cl2
reactions have been studied to 190 K (i.e., significantly
below room temperature).
or from C2H5Br [5] as
C2H5Br + hν(193 nm) → C2H5 + Br
(5a)
→ Other products (5b)
The n-C3H7 radicals were produced either from
n-C3H7NO2 as
EXPERIMENTAL
n-C3H7NO2 + hν(193 nm) → n-C3H7 + NO2 (6a)
→ Other products (6b)
Details of the experimental apparatus used have been
described previously [11], and so only a brief overview
is given here. The radical R(R = C2H5, n-C3H7, or n-
C4H9) was generated from an appropriate precursor
along the flow reactor by pulsed unfocused exciplex
laser (ELI-76E) photolysis at 193 nm. The gas mixture
flowing through the tubular, temperature-controlled re-
actor coupled with a photoionization mass spectrome-
ter (PIMS) contained the radical precursor (<0.10%),
Cl2 in various amounts (<0.10%), and an inert carrier
gas (He) in large excess (>99.8%). The employed reac-
tor tubes with 8- and 17-mm inner diameters (i.d.) were
made of seamless stainless steel and Pyrex-glass and
were coated with halocarbon wax. The gas flow rates
at used pressures (0.5–5 Torr of He) and temperatures
(190–363 K) were typically about 4–6 m s−1 inside
the reactor, which means that the gas mixture passes
the uniformly cooled (heated) zone in about 80 ms.
The gas was continuously sampled through a 0.4-mm-
diameter hole at the side of the reactor and formed into
a beam by a conical skimmer before it entered a vac-
uum chamber containing PIMS. As the gas beam tra-
versed the ion source, a portion was selectively photo-
ionized and the ions formed were mass selected in
the quadrupole mass spectrometer (Extrel, C-50/150-
QC/19 mm rods). The selected ions were detected by
an off-axis electron multiplier.
Ionization radiation in the PIMS was provided by
a chlorine lamp (8.9–9.1 eV) for C2H5, n-C3H7, and
n-C4H9 radicals. Temporal ion signals were recorded
by a multichannel scaler (EG&G Ortec MCS plus)
from 10 ms before each laser pulse up to 80 ms follow-
ing the pulse. Typically, a profile from 3000–10,000
repetitions was accumulated at about 5 Hz frequency
before the least-squares method was used to fit an
exponential function, [R]t = [R]0 × exp(−kꢁt), to the
data. Here, [R]t is the signal proportional to the radical
concentration at time t and kꢁ is the first-order rate
coefficient.
or from n-C3H7Br [12] as
n-C3H7Br + hν(193 nm) → n-C3H7 + Br (7a)
→ Other products (7b)
whereas n-C4H9 radicals were produced from
n-C4H9Br [12] as
n-C4H9Br + hν(193 nm) → n-C4H9 + Br (8a)
→ Other products (8b)
Experiments were conducted under conditions
where only two significant reactions consumed R
R + Cl2 → Products
R → Heterogeneous loss
(B)
(C)
The first-order decay rate of reaction (C), the wall
reaction rate coefficient kwall, consists of all first-order
processes occurring in the reaction mixture and on the
reactor wall without the added molecular reactant. It
was measured by reducing the precursor concentration
and/or laser intensity until the rate obtained for this
reaction no longer depended on these factors, and the
exponential fit to the temporal ion signal showed no
deviation from the first-order decay. When these con-
ditions were achieved, it was presumed that all radical–
radical processes had only negligible rates compared
to the first-order processes occurring in the system.
The first-order rate coefficient (kꢁ) was then mea-
sured as a function of the Cl2 concentration ([Cl2]),
which was always much higher (>15 times) than [R],
resulting in pseudo-first-order reaction kinetics. Since
the only significant processes consuming R during
these experiments were the reaction with Cl2 (B)
and disappearance in the mainly heterogeneous re-
action (C), the bimolecular reaction rate coefficient
k(R + Cl2) could be obtained from the slope of the kꢁ
versus [Cl2] plot. In Fig. 1, typical plots at 203, 298,
and 358 K are shown for the C2H5 + Cl2 reaction. An
The C2H5 radicals were generated either from
C2H5NO2 as
C2H5NO2 + hν(193 nm) → C2H5 + NO2 (4a)
→ Other products (4b)
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE REACTIONS OF C2H5, n-C3H7, AND n-C4H9 RADICALS WITH Cl2
617
reactant concentrations and from the uncertainties in
the first-order rate coefficients. Linear least-squares
n
˜
fits of an expression k = A × (T /300 K) to the
experimental results are also given in Table I. In
˜
this expression, T is temperature in K, and A and n
are empirical parameters. Corresponding fits of an
Arrhenius expression (k = A × exp(−Ea/RT )) are
k(C2H5 + Cl2) = (3.21 0.29) × 10−12 × exp(3.70
0.19 kJ mol−1/RT ) cm3 molecule−1 −1, k(n-C3H7
s
+ Cl2) = (4.43 0.63) × 10−12 × exp(3.54 0.31 kJ
mol−1/RT ) cm3 molecule−1
s
−1, and k(n-C4H9 +
Cl2) = (2.65 0.47) × 10−12 × exp(5.23 0.38
kJ
mol−1/RT ) cm3 molecule−1 s−1, with the uncertainties
given as one-standard deviations. Double logarithmic
plots of the bimolecular rate coefficients for the C2H5,
n-C3H7, and n-C4H9 radical reactions with Cl2 are
shown in Fig. 2. Also shown are results from previous
studies of these reactions for comparison.
Measurements were carried out at various pressures
to investigate possible contributions of three-body pro-
cesses. Changing pressures between 0.5 and 5 Torr
(He) did not change bimolecular rate coefficients in
any of the R + Cl2 reactions studied. Therefore, no
three-body processes are likely to be present in any
significant extent in these reactions.
Figure 1 Plots of first-order C2H5 decay rate coefficients
kꢁ versus [Cl2] at T = 203, 298, and 358 K at about 1 Torr
pressure employing the 17-mm i.d. reactor tube. Inset shows
actual ion signal profile for the C2H5 decay in the presence
of [Cl2] = 1.75 × 1013 cm−3. The corresponding decay rate
is kꢁ = 188 5 s−1 and is shown as the solid square in the
plot. Uncertainty is one-standard deviation (1σ).
Wallington et al. [13] also studied the reactions
of ethyl radicals with O2 and Cl2 at 295 K using
the relative rate method (UV radiation to produce
radicals and FTIR spectroscopy to detect stable
products). At 700 Torr total pressure (mainly N2), they
obtained k(C2H5 + Cl2)/k(C2H5 + O2) = 1.99 0.14.
Combining this result with the preferred value
for the bimolecular rate coefficient of the
C2H5 + O2 reaction at 298 K and 1 bar pressure
(k(C2H5 + O2)298 K,1 bar = 7.0 × 10−12 cm3 molecule−1
example of the C2H5 radical signal decay is shown at
the lower right corner of Fig. 1.
Radical precursors, C2H5Br (Sigma-Aldrich
Finland, Helsinki, Finland; purity ≥98%), C2H5NO2
(Sigma-Aldrich Finland; purity ≥96%), n-C3H7Br
(Sigma-Aldrich Finland; purity ≥99%), n-C3H7NO2
(Sigma-Aldrich Finland; purity ≥98%), and n-C4H9Br
(Sigma-Aldrich Finland; purity >98%) were degassed
before use. Helium (Messer Suomi Oy, Tuusula,
Finland; purity 99.9996%) and chlorine (Messer
Suomi Oy; purity 99.8%) were employed as supplied.
s−1
)
[14] results in k(C2H5 + Cl2)298 K,1 bar =
(13.9 1.0) × 10−12 cm3 molecule−1 s−1 for the
C2H5 + Cl2 reaction under these conditions. This value
is in an excellent agreement with the current result
at 300 K, k(C2H5 + Cl2)300 K = (1.45 0.04) × 10−11
cm3 molecule−1 s−1
,
which clearly indicates
that the rate of this reaction is independent of
buffer gas density over wide pressure range.
Also the value obtained by Kaiser et al. [7],
k(C2H5 + Cl2)298 K,5 Torr = (16 2.5) × 10−12
cm3
RESULTS AND DISCUSSION
molecule−1 −1, is in an excellent agreement with
s
the current result (see Fig. 2). The bimolecular rate
coefficient obtained by Timonen and Gutman [5] at
298 K, k(C2H5 + Cl2)298 K = (19 4) × 10−12 cm3
The measured bimolecular reaction rate coefficients
for the C2H5, n-C3H7, and n-C4H9 radical reactions
with Cl2 are given in Table I along with their statistical
uncertainties (1σ) and experimental conditions. The
estimated overall uncertainties in the measured bi-
molecular reaction rate coefficients are 20%. These
arise mainly from the uncertainties in determining the
molecule−1
s
−1, is slightly higher than the value
measured in the current determination. On the other
hand, the activation energy they have obtained for
this reaction, Ea(C2H5 + Cl2) [5] = −1.3 0.5 kJ/
mol−1, is less negative than obtained in this work,
International Journal of Chemical Kinetics DOI 10.1002/kin

618
ESKOLA, LOZOVSKY, AND TIMONEN
Table I Results and Conditions of the Experimentsa Used to Measure the Bimolecular Rate Coefficients of the
Reaction R + Cl2 → Products (R = C2H5, n-C3H7, and n-C4H9)
T (K)
Pb (Torr)
10−12 [Cl2] (cm−3
)
kc
(s−1
)
10−12 kd (cm3 s−1
)
wall
C2H5 + Cl2 → C2H5Cl + Cl
190
203
223
244
267
298
336g
359g
1.1e
1.2f
1.2f
1.1e,f
1.2
1.1
1.0
1.0
1.5–4.7
1.4–7.2
2.2–8.3
10
14
16
12
12
8
33.3 1.1
28.6 0.9
23.8 1.3
18.5 1.3
18.1 1.0
14.9 0.8
12.5 0.3
10.3 0.3
3.0–13.5
2.7–12.1
1.7–23.0
3.9–20.0
2.4–25.2
11
6
k(C2H5 + Cl2) = (1.45 0.04) × 10−11 (T /300 K)−1.73
cm3 molecule−1 s−1
0.09
n-C3H7 + Cl2 → n-C3H7Cl + Cl
204
220
230
244
267
297
330i
355i
363
1.1
1.0
5.4
1.5– 6.3
1.6–5.0
18
32.6 1.8
30.5 0.9
30.5 1.9
29.0 1.1
19.8 0.8
17.5 0.9
16.4 0.5
14.8 0.4
14.2 1.3
6
4.8–17.2
1.6–5.9
51h
16
1.1e,f
1.0
1.4–10.4
2.1–10.0
2.1–15.4
1.7–14.1
4.9–18.8
16
13
8
5
33h
1.0e,f
1.0
1.0
4.6
k(n-C3H7 + Cl2) = (1.88 0.06) × 10−11 (T /300 K)−1.57
cm3 molecule−1 s−1
0.14
n-C4H9 + Cl2 → n-C4H9Cl + Cl
202
221
244
267
299
324
359
1.0
1.0
1.0
1.0
1.2e,f
1.1
1.0f
1.4–4.1
26
52.8 2.1
50.1 2.9
35.4 1.1
29.5 1.0
22.7 1.0
16.9 0.4
15.0 0.4
1.5–3.9
1.7–4.9
1.4–6.4
2.0–6.9
3.0–12.1
2.4–11.4
17
17
9
7
17
12
k(n-C4H9 + Cl2) = (2.21 0.07) × 10−11 (T /300 K)−2.38
cm3 molecule−1 s−1
0.14
a C2H5NO2, n-C3H7NO2, and n-C4H9Br used as precursors for C2H5, n-C3H7, and n-C4H9 radicals employing 193 nm radiation, unless
otherwise stated. Range of precursor concentrations used: (0.7–2.4) × 1012 molecule cm−3 for C2H5NO2, (0.5–2.1) × 1012 molecule cm−3 for
n-C3H7NO2, and (4.5–27) × 1012 molecule cm−3 for n-C4H9Br.
b Helium used as a buffer gas.
c Pyrex-glass reactor tube with 17-mm inner diameter (i.d.) coated with halocarbon wax used, unless otherwise stated.
d Statistical uncertainties shown are 1σ; estimated overall uncertainties are about 20%.
e A few decay rates measured at three times higher buffer gas pressure (3 × P ); however, no dependence on pressure was observed.
f
A few decay rates measured at 0.5 × P ; however, no dependence on pressure was observed.
g C2H5Br (193 nm) used as a precursor, concentration range (2.1–2.9) × 1013 molecule cm−3
.
h Stainless steel reactor tube with 8-mm i.d. coated with halocarbon wax.
i n-C3H7Br (193 nm) used as a precursor, concentration range (0.6–1.3) × 1013 molecule cm−3
.
Ea = −3.70 0.19 kJ mol−1. Similar differences were
also observed in the context of C2H3 + Cl2 reaction
measurements [11]. We are unable to propose any
probable reason for these differences. However, a sig-
nificantly more pronounced difference exists between
current results, which are in good agreement with
previous determinations discussed above, and that
obtained by Dobis and Benson [6] for the C2H5 + Cl2
reaction (see Fig. 2). Their bimolecular rate coefficient
at 298 K is lower than other values with a factor of 10
or more.
The value derived by Tyndall et al. [9] for the
n-C4H9 + Cl2 reaction, k(n-C4H9 + Cl2)298 K = 23 ×
10−12 cm3 molecule−1 s−1
,
is in an ex-
cellent agreement with the current determi-
nation k(n-C4H9 + Cl2)300 K = (2.21 0.07) × 10−11
cm3 molecule−1 s−1
.
Finally, to gain understanding of the reasons
affecting the reactivity of the alkyl radicals in
R + Cl2 reactions, it is instructive to make com-
parison among the reactions of alkyl-substituted
methyl radicals with Cl2 when R substitution is
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE REACTIONS OF C2H5, n-C3H7, AND n-C4H9 RADICALS WITH Cl2
619
CONCLUSIONS
The bimolecular rate coefficients of the C2H5 + Cl2,
n-C3H7 + Cl2, and n-C4H9 + Cl2 reactions were mea-
sured as a function of temperature, and no experimental
evidences on activation barriers were observed for any
of the reactions studied. Arrhenius activation energies
of these reactions, Ea(C2H5 + Cl2) = −3.70 0.19 kJ
mol−1, Ea(n-C3H7 + Cl2) = −3.54 0.31 kJ mol−1
,
and Ea(n-C4H9 + Cl2) = −5.23 0.38 kJ mol−1, are
more negative than expected and are comparable with
those of R + HBr reactions [12]. The current results
of the direct measurements for the C2H5 + Cl2 and n-
C4H9 + Cl2 reactions performed at few Torr pressures
are in excellent agreement with bimolecular rate coeffi-
cients derived previously using the relative rate method
at low and high pressures.
BIBLIOGRAPHY
1. Kroschwitz, J. I.; Howe-Grant, M. (Eds). Encyclopedia
of Chemical Technology, 4th ed.; Wiley: New York,
1993; Vol. 5.
2. NIST Chemical Kinetics Database (Web version), re-
lease 1.4; National Institute of Standards and Technol-
ogy: Gaithersburg, MD.
3. Tsang, W. Combust Sci Technol 1990, 74, 99–116.
4. Procaccini, C.; Bozzelli J. W.; Longwell, J. P.; Smith,
K. A.; Sarofim, A. F. Environ Sci Technol 2000, 34,
4565–4570.
Figure 2 Double logarithmic plots of bimolecular rate co-
efficients for C2H5 + Cl2, n-C3H7 + Cl2, and n-C4H9 + Cl2
reactions versus T . Fittings to the current data are shown
with solid lines. Bimolecular rate coefficients for compari-
son are taken from Dobis and Benson [6], Kaiser et al. [7],
and Tyndall et al. [9]. For C2H5 + Cl2 reaction, only fitting
of the bimolecular rate coefficients given by Timonen and
Gutman [5] is shown as a broken line for clarity.
systemically changed. A good choice is to perform
comparison at about 300 K temperature. Starting
from the above-mentioned methyl radical + Cl2
reaction with the value given by Timonen and Gutman
[5], k(CH3 + Cl2)298 K = (2.0 0.4) × 10−12 cm3
molecule−1 s−1 (direct measurement of Kovalenko and
Leone [15] at room temperature, (1.5 0.1) × 10−12
5. Timonen, R. S.; Gutman, D. J Phys Chem 1986, 90,
2987–2991.
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cm3 molecule−1
s
−1, also supported this value),
it can be readily noted that the C2H5 + Cl2 reac-
tion
(k(C2H5 + Cl2)298 K = (1.45 0.04) × 10−11
cm3 molecule−1 −1) is about 10 times faster than
s
CH3 + Cl2 reaction. However, the substitution of
β-hydrogen in the ethyl radical with the methyl group
(i.e., forming n-propyl radical) has only a small
effect: k(n-C3H7 + Cl2)300 K/k(C2H5 + Cl2)300 K
≈
1.30. Substituting γ -hydrogen in the n-propyl radical
with the methyl group (i.e., forming n-butyl radical)
has even a smaller effect: k(n-C4H9 + Cl2)300 K/k(n-
C3H7 + Cl2)300 K ≈ 1.18. Additional kinetic studies
are in progress to improve our understandings of
the reactivities of substituted alkyl radicals in their
reactions with Cl2. Currently, we are working with
CH3 + Cl2 and CD3 + Cl2 reactions.
13. Wallington, T. J.; Andino, J. M.; Kaiser, E. W.; Japar, S.
M. Int J Chem Kinet 1989, 21, 1113–1122.
14. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley,
J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.;
Rossi, M. J.; Troe, J. Atmos Chem Phys 2006, 6, 3625–
4055.
15. Kovalenko, L. J.; Leone, S. R. J Chem Phys 1984, 80,
3656–3667.
International Journal of Chemical Kinetics DOI 10.1002/kin