Gas-phase reactions of Cl atoms with propane, n-butane, and isobutane

Article abstract of DOI:10.1002/kin.10096

Using the relative kinetic method, rate coefficients have been determined for the gas-phase reactions of chlorine atoms with propane, n-butane, and isobutane at total pressure of 100 Torr and the temperature range of 295-469 K. The Cl₂ photolysis (λ = 420 nm) was used to generate Cl atoms in the presence of ethane as the reference compound. The experiments have been carried out using GC product analysis and the following rate constant expressions (in cm³ molecule^-1 s^-1) have been derived: (7.4 ±0.2) × 10^-11 exp [-(70 ± 11)/T], Cl + C₃H₈ → HCl + CH₃CH₂CH₂; (5.1 ± 0.5) × 10^-11 exp [(104±32)/T], Cl + C₃H₈ → HCl + CH₃CHCH₃; (7.3±0.2) × 10^-11 exp [-(68 ± 10)/T], Cl + n-C₄H₁₀ → HCl + CH₃ CH₂CH₂CH₂; (9.9 ± 2.2) × 10^-11 exp [(106 ± 75)/T], Cl + n-C₄H₁₀ → HCl + CH₃CH₂CHCH₃; (13.0 ± 1.8) × 10^-11 exp [-(104 ± 50)/T], Cl + i-C₄H₁₀ → HCl + CH₃CHCH₃ CH₂; (2.9 ± 0.5) × 10^-11 exp [(155 ± 58)/T], Cl + i-C₄H₁₀ → HCl + CH₃CCH₃CH₃ (all error bars are ±2ρ precision). The studies provide a set of reaction rate constants allowing to determine the contribution of competing hydrogen abstractions from primary, secondary, or tertiary carbon atom in alkane molecule.

Full text of DOI:10.1002/kin.10096

Gas-Phase Reactions of Cl
Atoms with Propane,
n-Butane, and Isobutane
DARIUSZ SARZYNSKI, BARBARA SZTUBA
Department of Physical Chemistry, Wroclaw Medical University, Nankier Sq. 1, 50-140 Wroclaw, Poland
Received 31 January 2002; accepted 8 August 2002
DOI 10.1002/kin.10096
ABSTRACT: Using the relative kinetic method, rate coefficients have been determined for
the gas-phase reactions of chlorine atoms with propane, n-butane, and isobutane at to-
tal pressure of 100 Torr and the temperature range of 295–469 K. The Cl2 photolysis (λ =
420 nm) was used to generate Cl atoms in the presence of ethane as the reference com-
pound. The experiments have been carried out using GC product analysis and the following
3
−1 −1
−11
rate constant expressions (in cm molecule
s
) have been derived: (7.4 ± 0.2) × 10
exp
exp[(104 ± 32)/T], Cl +
exp[−(68 ± 10)/T], Cl + n-C H → HCl + CH
−
11
[
−(70 ± 11)/T], Cl + C H → HCl + CH CH CH ; (5.1 ± 0.5) × 10
3
8
3
2
2
−
11
C H → HCl + CH CHCH ; (7.3 ± 0.2) × 10
3
8
3
3
4
10
3
−
11
CH CH CH ; (9.9 ± 2.2) × 10
exp[(106 ± 75)/T], Cl + n-C H → HCl + CH CH CHCH ;
2
2
2
4
10
3
2
3
−
11
(
1
13.0 ± 1.8) × 10
exp[−(104 ± 50)/T], Cl + i -C H → HCl + CH CHCH CH ; (2.9 ± 0.5) ×
4
10
3
3
2
−
11
0
exp[(155 ± 58)/T], Cl + i -C H → HCl + CH CCH CH (all error bars are ± 2σ preci-
4
10
3
3
3
sion). These studies provide a set of reaction rate constants allowing to determine the contri-
bution of competing hydrogen abstractions from primary, secondary, or tertiary carbon atom
in alkane molecule. ꢀC 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 651–658, 2002
INTRODUCTION
where the main degradation process of organics is initi-
ated by gas-phase reactions with the OH radicals. Also
measurements of hydrocarbons concentration in the
Arctic troposphere in springtime have confirmed that
the chemistry ofhydrocarbonsisstrongly influenced by
chlorine atoms [6,7]. Although numerous kinetic stud-
ies of Cl atom reactions with a series of simple alkanes
have been reported in the literature [8–20], there are no
detailed studies of hydrogen abstraction reactions from
individual carbon atoms.
Herein we report on the competitive photochlorina-
tion of propane, n-butane, and isobutane in the presence
of ethane as the reference compound. This study rep-
resents part of detailed ongoing investigation on the
kinetics and mechanisms of the reactions of Cl atoms
with simple alkanes. The main aim of this work was
to establish the temperature dependencies of the rate
coefficients for reactions of hydrogen abstractions by
chlorine atoms from carbons C1 and C2 in propane and
Reactions of chlorine atoms with hydrocarbons are of
great importance for understanding the atmospheric
chemistry. It is commonly known that Cl atoms con-
tribute both to the ozone balance and the loss of organ-
ics in the troposphere. The role of interaction between
chlorine and hydrocarbon chemistry has taken on new
importance during last years as the mechanism of chlo-
rine atoms generation in marine troposphere from the
reactions of NaCl in sea salt particules had been un-
derstood [1–4]. The reactions with Cl atoms have been
postulated to be an additional and significant removal
process of ≥C2 alkanes in marine troposphere [1,3,5],
Correspondence to: Dariusz Sarzynski; e-mail: darek@bf.uni.
wroc.pl.
Contract grant sponsor: State Committee for Scientific Research.
Contract grant number: 498.
ꢀ
c 2002 Wiley Periodicals, Inc.

652
SARZYNSKI AND SZTUBA
n-butane and from carbons C1 and C3 in isobutane.
The results comprising the rate coefficients and their
temperature dependencies are compared with other
literature data.
width slit, and a monochromator. The light from the
monochromator was introduced into the cylindrical
reactor through its bottom window and after passing
through the reactor was reflected back by the mirror
placed above the upper window of the reactor. The re-
actioncell, thefeedlines, andtheopticalassemblywere
housed in a light-tight enclosure to prevent photolysis
initiated by room or stray light. The absence of such
reactions was confirmed by repeated blank analyses.
Chlorine atoms were generated in their ground state
(Cl( P3/2) > 99%, Cl( P1/2) < 1% at 298 K) [21] by
the photolysis of Cl2 at 420 nm. The irradiation time
(2–20 min) and the slit width (2–20 nm) were varied
depending on the reaction temperature in order to avoid
higher conversion of reactants.
EXPERIMENTAL
The experiments were carried out in a greaseless, static
3
system, using a cylindrical Pyrex reactor of ca. 250 cm
2
2
volume (dead volume ∼2%) placed at a copper heat-
ing block. Chlorine atoms gas-phase reactions with
propane, n-butane, and isobutane were investigated
with ethane as reference over the temperature range
of 295–470 K. Temperature was controlled by employ-
ing power regulator connected to temperature regulator
equipped with Pt-100 resistance sensor. The tempera-
Product analysis was carried out by thermal con-
ductivity gas chromatography (HP, Model 5890 Se-
ries II), using a 25 m-long × 0.32 mm CP-SIL 13CB
Chrompack capillary column for halocarbons with
◦
ture of the reaction cell was constant to within 0.5 C.
Chrome–nickel thermocouple maintained in the half
length of the side wall of the reactor was used to mea-
sure the temperature. The reactants were introduced
into the cell by expansion starting from a mixture of
◦
temperature–time programming between 30 and 110 C
◦
and the heating rate 10 C/min. In all analysis He was
used as a carrier gas and 2 ml thermostated gas loop as
a sampling unit.
5
% Cl2 in N2 followed by pre-made mixture of alkane
and reference. Preliminary tests showed that ca. 10-
min mixing time was sufficient for the reactants in the
cell to reach ambient temperature and confirmed that
dark reactions were never observed. In order to test for
a possible photolysis or thermal decomposition of the
organic reactants, mixtures of organics were irradiated
in the absence of Cl2 at the highest temperature used for
more than 60 min. No photolysis or thermal reaction of
any of the reactants was observed. Prior to each set of
kinetic experiments at temperature above 295 K, tests
were performed for dark reactions. Mixtures of organic
species and molecular chlorine were prepared and al-
lowed to stand in the dark for at least 60 min. In all the
cases, the reaction of the organic species with molecu-
lar chlorine was negligible in the absence of photolytic
light. In addition, the thermal stability of the reaction
products was examined at the highest temperatures ap-
plied in the experiments. No destruction products were
observed except for 2-chloro-2-methylpropane, which
appeared to be unstable at 466 K. All experiments
were conducted at total pressure of about 100 Torr
while the partial pressure of Cl2 was varied from 1
to 4 Torr. The mixtures prepared at different ratios of
alkane:ethane at least 48 h prior to each set of experi-
ments were stored in a 4 l blackened Pyrex bulbs. Pres-
suremeasurementswerecarriedoutwithaModel127A
MKS Baratron capacitance manometer equipped with
two gauges (0–100 and 0–1000 Torr).
All experiments were conducted below 5% con-
version of the reactants when the only products ob-
served were 1-chloropropane and 2-chloropropane,
1-chlorobutane and 2-chlorobutane, and 1-chloro-2-
methylpropane and 2-chloro-2-methylpropane for the
Cl reactions with propane, n-butane, and isobutane, re-
spectively. Calibration curves of relative peak area ver-
sus pressure for each reaction product at each reaction
temperature were determined by GC analyses of known
amounts of original samples diluted with Ar. The lin-
ear relationships obtained by the least square method
were subsequently used for calculation of the products’
pressure in the reactor.
The reactants used in this study had the following
stated minimum purities: Propane (99.95%), n-butane
(99.95%), isobutane (99.95%), and chloroethane
(99.7%) from Praxair; ethane (99.97%), 1-chloropro-
pane (99.5%), and 2-chloropropane (99.5%) from
Fluka; and 1-chlorobutane (99.5%), 2-chlorobutane
(99%), 1-chloro-2-methylpropane (98%), and 2-
chloro-2-methylpropane (99%) from Aldrich. 5%
Cl2/N2 was prepared by Linde. All reaction products
except for chloroethane were purified three times, us-
ing the freeze-pump-thaw method. All other reactants
were used directly from the containers.
RESULTS AND DISCUSSION
The Xe arc lamp (Osram XBO 150W/1 OFR) served
as the light source. The optical train consisted of a
manually operated shutter, condensing lens, a variable-
The general scheme for gas-phase photochlorination
reactions is well established [22–24]. In the case of

GAS-PHASE REACTIONS OF Cl ATOMS WITH PROPANE, n-BUTANE, AND ISOBUTANE
653
photochlorination of hydrocarbon RH in the presence
of C2H6, the reaction sequence may be represented by
the following mechanism:
molecules of propane, n-butane, and isobutane. All ex-
perimental results for reactions of Cl with propane,
n-butane, and isobutane are summarized in Table I.
Absolute values of k2 were calculated from the rate
constant ratios k2 /k4, k2 /k4, and k2 /k4, using
an Arrhenius expression for the reference reaction of
Cl atoms with ethane
prim
sec
tert
Cl2 → 2Cl
RH + Cl ↔ R + HCl
R + Cl2 → RCl + Cl
(1)
(2, −2)
(3)
k4 = (8.6 ± 0.5) × 10−
11
C2H6 + Cl ↔ C2H5 + HCl
C2H5 + Cl2 → C2H5Cl + Cl
(4, −4)
(5)
3
−1 −1
×
exp[−(135 ± 26)/T ] cm molecule
s
determined for the temperature range of 292–600 K [8],
where RH denotes propane, n-butane, or isobutane.
Application of the steady-state approximation and the
long-chain assumption [25] leads to the following rate
expression for the product formation:
prim
sec
where k2
and k2 denote the rate coefficients of ab-
straction of hydrogen atoms from all primary and sec-
ondarycarbon atoms in propane or n-butane molecules,
tert
respectively; k2 denotes the rate coefficient of the
abstraction of tertiary hydrogen in isobutane. Based
on these values the simple Arrhenius expressions are
derived for each of the reactions studied and listed in
Table II. The Arrhenius plots are shown in Figs. 1–3
for Cl reactions with propane, n-butane, and isobutane,
respectively.
d[RCl]
k3[Cl][Cl2]
=
k2[RH]
(6)
dt
k3[Cl2] + k−2[HCl]
An analogous rate expression for C2H5Cl formation is
given by
d[C H5Cl]
k3[Cl][Cl2]
2
=
k4[C2H6]
(7)
dt
k5[Cl2] + k−4[HCl]
Reaction of Cl with Propane
It is well known from the thermochemical data [26]
that the primary and secondary hydrogen abstraction
from propane leading to the formation of n-propyl
and isopropyl radical are both exothermic at 298 K
The rate ratio for products formation can be obtained
by dividing Eq. (6) by (7):
d[RCl]
dt
d[C2H5]
dt
k−4[HCl]
k5[Cl2]
k−2[HCl]
k3[Cl2]
1
+
k2[RH]
−1
−1
by 8.9 kJ mol and 18.9 kJ mol , respectively. The
difference in the exothermicity of radical formations
indicates greater stability of a secondary radical than
that of a primary radical. Experimental data obtained
show that the attack of chlorine is faster and the
activation energy lower in the case of ꢀ-hydrogen
than those for ꢁ-hydrogen abstraction (Table II). De-
rived simple Arrhenius expressions show very weak
temperature dependence with a positive value of acti-
vation energy for hydrogen abstraction from primary
carbon of propane and negative one for hydrogen ab-
straction from secondary carbon (Table II and Fig. 1).
The data were also fitted to a modified Arrhenius
=
(8)
k4[C2H6] 1 +
For initial conditions or if conversion with respect to
Cl2 is not significant (in other words, concentration of
HCl is very low), the reverse reactions (−2) and (−4)
are unimportant and may be neglected in relation to the
steps (3) and (5). Under these conditions, it can be also
assumed that the ratio [RH]/[C2H6] is almost equal to
the ratio of initial concentrations [RH]0/[C2H6]0 and
Eq. (8) can be integrated giving
k2
k4
[C2H6]0 [RCl]
[RH]0 [C2H5Cl]
=
(9)
n
−Ea/RT
(cm3
expression of the form k = BT e
−
1
−1
prim
molecule
10
s
) yielding k2
= (3.53 ± 8.84) ×
−
10
(−0.227± 0.362)
where k2 denotes the rate coefficient for the reaction of
hydrogen atom abstraction from the primary or sec-
ondary carbon in the molecules of propane and n-
butane, and from the primary or tertiary carbons in the
molecule of isobutane.
Relative rate determinations of the kinetics of the
system described above were conducted using GC
product analysis which allowed to discriminate be-
tween two pathways of hydrogen abstraction from
× T
exp[−(154.49 ± 134.22)/T ]
sec
−10
(−0.274± 0.384)
and k2 = (3.41 ± 9.08) × 10
× T
exp[(3.84 ± 143.08)/T ] (the errors represent 2σ).
The results obtained for both primary and secondary
hydrogen abstractions are shown by dotted line in
Fig. 1. Again, the activation energy for the hydrogen
abstraction from a primary carbon is slightly posi-
tive but practically equal to zero for the hydrogen
abstraction from a secondary carbon.

654
SARZYNSKI AND SZTUBA
Table I Rate Parameters for Competitive Photochlorination of Propane, n-Butane, and Isobutane
No. of Runs
Average Temp (K)
k2prim/k4a
k2sec/k4a
k2prim a,b,c,d
k2sec a,b,c,d
Cl + C3H8/C2H6
−
−
−
−
−
11
11
11
11
11
−11
−11
3
2
2
2
2
0
8
3
5
8
298.6
328.0
365.0
412.3
467.7
1.06 ± 0.03
1.04 ± 0.02
1.02 ± 0.02
1.00 ± 0.04
0.97 ± 0.04
1.31 ± 0.03
1.24 ± 0.03
1.16 ± 0.01
1.08 ± 0.04
0.97 ± 0.04
(5.8 ± 0.2) × 10
(5.9 ± 0.1) × 10
(6.1 ± 0.1) × 10
(6.2 ± 0.3) × 10
(6.3 ± 0.3) × 10
(7.2 ± 0.2) × 10
(7.1 ± 0.1) × 10
−11
(6.9 ± 0.08) × 10
−11
−11
(6.7 ± 0.3) × 10
(6.3 ± 0.2) × 10
Cl + C4H10/C2H6
−
−
−
−
−
11
11
11
11
11
−11
−11
−11
−11
−11
2
2
2
2
2
2
7
3
0
6
295.4
325.5
365.2
411.7
469.4
1.06 ± 0.07
1.06 ± 0.08
1.02 ± 0.04
1.04 ± 0.03
0.97 ± 0.06
2.59 ± 0.13
2.41 ± 0.16
2.26 ± 0.13
2.18 ± 0.12
1.85 ± 0.12
(5.8 ± 0.4) × 10
(6.0 ± 0.4) × 10
(6.1 ± 0.2) × 10
(6.2 ± 0.2) × 10
(6.3 ± 0.4) × 10
(14.1 ± 0.7) × 10
(13.7 ± 0.9) × 10
(13.4 ± 0.8) × 10
(13.5 ± 0.7) × 10
(11.9 ± 0.8) × 10
prim
a
tert
a
prim a,b,c,d
k2
tert a,b,c,d
k2
k2
/k4
k2 /k4
Cl + i-C4H10/C2H6
−
−
−
−
11
11
11
11
−11
−11
−11
−11
2
2
4
1
2
5
5
3
9
6
297.5
327.1
364.0
411.6
466.6
1.71 ± 0.04
1.60 ± 0.06
1.65 ± 0.23
1.65 ± 0.05
1.59 ± 0.11
0.92 ± 0.06
0.81 ± 0.10
0.76 ± 0.10
0.69 ± 0.13
(9.3 ± 0.2) × 10
(9.1 ± 0.3) × 10
(9.8 ± 1.3) × 10
(10.2 ± 0.3) × 10
(10.3 ± 0.7) × 10
(5.0 ± 0.3) × 10
(4.6 ± 0.6) × 10
(4.5 ± 0.6) × 10
(4.3 ± 0.8) × 10
e
−11
e
–
–
a
The indicated errors are experimental uncertainties (± 2σ precision) and do not take into account the uncertainties in the rate constant k
4
.
b
c
d
Placed on an absolute basis by use of an Arrhenius expression for the reaction of the Cl atoms with ethane [8].
3
−1 −1
In cm molecule
s
units.
prim
sec
k
2
and k
2
denote the rate coefficients for the abstraction of hydrogen atom from all primary and secondary carbon atoms in alkane
molecule, respectively.
e
The 2-chloro-2-methylpropane is not stable at 466.6 K.
The first experimental study of Cl + propane reac-
tion and its rate temperature dependence was published
by Lewis et al. [9] who used absolute rate method:
measured rate coefficients for hydrogen abstraction
from primary and secondary carbons, k = (1.3 ±
−
10
3
−1 −1
0.04) × 10
cm molecule s (at 298.6 K) is ap-
proximately 15% lower than the rate factor found by
Lewis. Moreover, the overall rate coefficient was found
to be invariant with temperature over the range 298–
k = (1.36 ± 0.13)
−
10
exp[(44 ± 25)/T ] cm molecule−1
3
−1
×
10
s
412 K as shown in Table I, decreasing only at tem-
perature 467.7 K to slightly lower value of (1.26 ±
−
10
3
−1 −1
They reported a negative activation energy for the over-
all reaction studied by low-pressure discharge flow-
resonance fluorescence technique in temperature range
of 220–607 K. Our value of an overall rate coeffi-
cient for Cl + C3H8 reaction calculated as the sum of
0.05) × 10
cm molecule s . Our experimental
observations are consistent with those reported recently
by Taatjes and co-workers [8] who observed no tem-
perature dependence of overall rate coefficient in the
temperature range of 292–700 K.
Table II A Summary of Arrhenius Parameters
Reactiona
A (1011 cm molecule
3
−1 −1 b
s
)
Ea/R (K)b
Cl + C3H8 → CH3CH2CH2 + HCl
Cl + C3H8 → CH3CHCH3 + HCl
7.4 ± 0.2
5.1 ± 0.5
7.3 ± 0.2
9.9 ± 2.2
13.0 ± 1.8
2.9 ± 0.5
70 ± 11
−104 ± 32
68 ± 10
Cl + C4H10 → CH3CH2CH2CH2 + HCl
Cl + C4H10 → CH3CH2CHCH3 + HCl
Cl + i-C4H10 → CH3CHCH3CH2 + HCl
Cl + i-C4H10 → CH3CCH3CH3 + HCl
−106 ± 75
104 ± 50
−155 ± 58
a
Cl + C
2
H
6
→ C
2
H
5
Cl + HCl as a primary standard.
b
The indicated errors are two least-squares standard deviations (± 2σ precision).

GAS-PHASE REACTIONS OF Cl ATOMS WITH PROPANE, n-BUTANE, AND ISOBUTANE
655
Figure 1 Simple Arrhenius plots for reaction Cl +
C3H8 → CH3CHCH3 + HCl (filled circles and solid line),
Cl + C3H8 → CH3CH2CH2 + HCl (filled squares and inter-
rupted line), and modified Arrhenius plots for both reactions
Figure
3 Simple Arrhenius plots for the reaction
Cl + isobutane → CH3CCH3CH3 + HCl (filled circles and
solid line), Cl + isobutane → CH3CHCH3CH2 + HCl (filled
squares and interrupted line), and modified Arrhenius plots
for both reactions (dotted line).
(dotted line).
There is also a very good agreement between our
individual carbon atoms in propane molecule. Tyndall
and co-workers [13] using FTIR spectroscopy deter-
mined the relative rates of Cl atoms attack on propane
at 298 K. The yields of 1- and 2-chloropropane forma-
tion are (43 ± 3)% and (57 ± 3)%, respectively. These
values are in excellent agreement with corresponding
results obtained in this work, (45 ± 2)% and (55 ± 2)%.
A comparison of room temperature values of rate
coefficient for Cl + propane reaction is shown in
Table III.
rate coefficient and the newest value of (1.33 ± 0.08) ×
−
10
3
−1 −1
1
0
cm molecule s at298KreportedbyHitsuda
2
et al. [17] for the reaction of Cl( P3/2) with C3H8 mea-
sured by direct experimental technique.
To our best knowledge, there is only one report
concerning the rates of hydrogen abstraction from
Reaction of Cl with n-Butane
Analogously to propane, the primary and secondary
hydrogen abstraction from n-butane leading to the
formation of n-butyl and s-butyl radical are both ex-
−1
−1
othermic at 298 K by 7.0 kJ mol and 18.9 kJ mol ,
respectively [26]. The greater stability of a secondary
radical is confirmed by experimental data. The attack of
chlorine atom is faster and activation energy lower in
the case of ꢀ-hydrogen than for ꢁ-hydrogen abstrac-
prim
tion (Table II). Relative rate constants ratios, k2
/k4
determined for reactions system Cl + n-butane/ethane
appeared to be nearly the same (≈1) as in the case
of previously discussed system Cl + propane/ethane
(
Table I). This finding leads to the conclusion that
Figure 2 Simple Arrhenius plots for the reaction Cl + n-
butane → CH3CHCH2CH3 + HCl (filled circles and solid
line), Cl + n-butane → CH3CH2CH2CH2 + HCl (filled
squares and the interrupted line), and modified Arrhenius
plots for both reactions (dotted line).
the attack of chlorine atom on the hydrogen attached
to primary carbon in propane and butane molecule is
equally fast. The simple Arrhenius expressions show
very weak temperature dependence with the positive

656
SARZYNSKI AND SZTUBA
Table III Overall Room Temperature Rate Coefficients
for the Reaction of Cl Atoms with Propane, n-Butane,
and Isobutane Obtained in This Work and Comparison
to Recent Literature Data
hydrogen abstraction from primary carbons and Ea
close to zero for the hydrogen abstraction from
secondary carbons.
Lewis et al. [9] published the first experimental de-
termination of absolute rate coefficients of reaction
Cl + n-butane at temperatures 298, 422, and 598 K.
Their Arrhenius expression
−
10
3
k (10
molecule
cm
Pressure
(Torr)
−
1
−1 a
Methodb
s
)
Reference
Cl + C3H8
1
1
1
1
1
1
1
1
.51(± 0.06)
.38(± 0.05)
.23(± 0.10)
.31(± 0.03)
.34(± 0.05)
.27(± 0.02)
.60(± 0.04)
.33(± 0.08)
1–3
10
1
FFD/RF
LP/cwIRLPA
FFD/RF
LP/RF
RR
[9]
[8]
[15]
[10]
[11]
[12]
[16]
[17]
k = (2.15 ± 0.10) × 10−10
3
−1 −1
× exp[(12 ± 26)/T ] cm molecule
s
60
760
760
760
1.5
exhibits weak negative temperature dependence of the
RR
RR
LP/LIF
overall rate coefficient. The room temperature rate
−
10
3
−1
coefficient k = (2.25 ± 0.1) × 10
cm molecule
−
1
s
is slightly higher than our value of (1.99 ± 0.13) ×
1
1
.37
.40
[18] (IUPAC)
[19] (JPL)
This work
−10
3
−1 −1
10
cm molecule s calculated as the sum of
the rate coefficients for hydrogen abstraction from pri-
mary and secondary carbons in n-butane molecule.
Our simple Arrhenius equation for overall rate co-
1
.30(± 0.04)
100
RR
Cl + n-Butane
2
.25(± 0.10)
1–3
760
740
760
1
FFD/RF
RR
RR
[9]
[11]
[14]
[12]
[15]
[10]
[13]
[13]
[20]
−
10
3
efficient (1.7 ± 0.2) × 10
exp[(56 ± 53)/T ] cm
−1
s exhibits stronger negative temperature
1
1
1
.97
.94
.94
−
1
molecule
dependence than that determined by Lewis and co-
workers [9].
RR
2
2
2
2
1
.11(± 0.18)
.25(± 0.06)
.20(± 0.30)
.15(± 0.15)
.91(± 0.20)
FFD/RF
LP/RF
RR
LP/RF
LP/IR
60
There is a very good agreement between our room
temperature rate coefficient and the values of 1.97 ×
700
45, 48
60
−
3
10
3
−1 −1
−10
1
0
cm molecule
s
[11] and 1.94 × 10
−1 −1
cm molecule
s
[14] derived by Atkinson and
2
.18
[18] (IUPAC)
This work
Aschmann by carrying out a least-squares fit of ab-
solute rate coefficients for Cl atoms reactions with the
C2 C4 alkanes to their rate constant ratios determined
for the alkane/n-butane pairs. The latter value of the
rate coefficient is identical with the value estimated
with a standard deviation of 11% by Hooshiyar et al.
[12].
1
.99(± 0.13)
100
RR
Cl + Isobutane
1
1
1
1
1
.46(± 0.06)
.37(± 0.02)
.51(± 0.09)
.30(± 0.01)
.40(± 0.08)
1–3
760
740
760
760
FFD/RF
RR
RR
RR
FFD/RF
[9]
[11]
[16]
[12]
[15]
1
.43
[18] (IUPAC)
This work
Tyndall and co-workers [13] have reported the rel-
ative yields of the 1- and 2-chlorobutane formation,
which reflect the extent of initial attack at each site
of n-butane. They obtained the yields of 1- and 2-
chlorobutane formation (29 ± 2)% and (71 ± 3)%, re-
spectively, which are in excellent agreement with our
corresponding values of (29 ± 2)% and (71 ± 4)%.
Summary of the overall room temperature rate co-
efficients of the Cl atoms reaction with n-butane in-
cluding the value recommended by Atkinson [18] is
presented in Table III. These results are shown to be
consistentwiththeuncertaintybetterthan15%withour
value of absolute room temperature rate constant mea-
sured as the sum of both hydrogen abstractions from
primary and secondary carbons in n-butane molecule.
Analogous trends in temperature dependence were
observed, slightly positive temperature dependence for
abstraction of hydrogen attached to primary carbon and
slightly negative for hydrogen abstraction from sec-
ondary carbon.
1
.43(± 0.05)
100
RR
a
The indicated errors are uncertainties (± 2σ precision).
b
FFD/RF: Fast flow discharge–resonance fluorescence;
LP/cwIRLPA: Laser photolysis-continous wave infrared long-path
absorption; LP/RF: Laser photolysis–resonance fluorescence;
RR: Relative rate method; LP/LIF: Laser photolysis–laser in-
duced fluorescence; LP/IR: Laser photolysis–infrared diode laser
absorption.
value of activation energy for hydrogen abstraction
from primary carbon of n-butane and negative one
for hydrogen abstraction from secondary carbon
(
Table II and Fig. 2). Fitting the results to a modified
prim
Arrhenius expression yields k2
1
= (3.53 ± 7.57) ×
−10
(−0.229± 0.310)
0
× T
exp[−(147.05 ± 114.55)/T ]
sec
−10
(−0.228± 2.946)
−1 −1
s ,
and k2 = (7.28 ± 148.43) × 10
× T
3
exp[(1.314 ± 1081.56)/T ] (k in cm molecule
errors are ± 2σ precision). Their logarithmic plots
shown in Fig. 2 reveal slightly positive Ea for the

GAS-PHASE REACTIONS OF Cl ATOMS WITH PROPANE, n-BUTANE, AND ISOBUTANE
657
Reaction of Cl with Isobutane
constant predicted by Atkinson and Aschmann [14].
The same values of k2 were found by us both for
propane and n-butane in agreement with an estimated
prim
Calculated from the thermochemical data [26] the
enthalpy of the primary and tertiary hydrogen ab-
straction from isobutane by chlorine atoms are both
−11
3
−1 −1
value of 2.62 × 10
cm molecule s [14] for
primary group rate constant related to the Cl atom re-
actions with C3 and C4 alkanes. The rate coefficient
for primary hydrogen atoms abstraction in molecule of
−
1
exothermic at 298 K by 9.4 kJ mol and 31.4 kJ
−
1
mol , respectively. The difference in the enthalpy of
two pathways of hydrogen abstraction is higher than
in the case of propane and n-butane. As can be seen in
Table I, isobutane was found to react with Cl slower
than n-butane did. The overall rate constant of the Cl
atoms reaction with isobutane determined in this work
−11
3
−1 −1
isobutane equal to 3.10 × 10
cm molecule s
at 298 K is only slightly higher than analogous value
for propane and n-butane. Thus, the influence of ter-
tiary carbon atom on the value of hydrogen abstrac-
tion rate constant seems to be of no significance. Sim-
ilar values of secondary group rate constants (within
the experimental uncertainties range) were found in
the case of hydrogen atoms abstraction from propane
(calculated as the sum of the rate coefficients for hydro-
gen abstraction from primary and tertiary carbons in
isobutane) exhibits negligible temperature dependence
whereas the very weak opposite temperature influence
was observed for the individual hydrogen abstraction
reactions. The corresponding simple Arrhenius equa-
tions derived from kinetic measurements have been
plotted in Fig. 3 and Arrhenius parameters are pre-
sented in Table II. The kinetic results were also fitted
to a modified Arrhenius form and the following temp-
erature dependence of the rate constants were de-
−11
3
−1 −1
and n-butane, 7.20 × 10
cm molecule s
and
−1 −1
cm molecule s , respectively. In the
−11
3
7.05 × 10
case of tertiary hydrogen atom abstraction in isobutane,
our experimental rate coefficient is 18% lower than an
estimated value.
At higher temperatures Cl atom reaction rate con-
stants show similar trends with the number of carbon
atoms in the alkane.
prim
−10
(0.038± 2.281)
rived: k2
= (1.00 ± 31.60) × 10
× T
sec
exp[−(90.63 ± 1692.07)/T ] and k2 = (3.03 ±
−
11
(−0.002± 0.117)
2
.39) × 10 × T
exp[(151.31 ± 94.50)/
CONCLUSIONS
3
−1
T ]. The units of the rate constants are cm molecule
−
1
s
and the quoted uncertainties are 2σ. The mod-
We determined the contribution of competing abstrac-
tion of primary, secondary, and/or tertiary hydrogen
atoms from propane, butane, and isobutane by Cl atom
attack using the relative kinetic method with product
formation analysis.
ified Arrhenius expressions plotted in Fig. 3 show
slightly positive activation energy for the hydrogen
abstraction from primary carbons and slightly neg-
ative one for the hydrogen abstraction from tertiary
carbon.
Attempted fitting of the experimental results to a
modified Arrhenius expression leads to the conclu-
sion that the temperature dependence of each reaction
studied is described by a simple Arrhenius equation.
As expected, these dependencies are very weak; how-
ever, interesting differences in their direction have been
observed. First, the abstraction of primary hydrogen
atoms from alkane molecule undergoes with the very
low positive values of activation energy. In the case of
secondary hydrogen atoms abstraction from propane
and n-butane, a simple Arrhenius expression gives
slightly negative value of activation energy, whereas
a modified Arrhenius form gives Ea practically equal
to zero. Slightly negative value of activation energy
was determined from both Arrhenius forms for the
abstraction of tertiary hydrogen atoms in isobutane
molecule.
A comparison of the rate coefficient determined in
this work to other literature data presented in Table III
shows good agreement within 10%.
Comparison of Results Obtained
Experimentally and by an Estimation
Method
The overall rate constants of Cl atoms with propane,
n-butane, and isobutane at room temperature deter-
mined in this work (given in Table III) are also in good
agreement with the results obtained by an estimation
method developed by Atkinson and Aschmann [14].
This method allowed room temperature rate constants
for the reactions of Cl atoms with alkanes to be calcu-
lated. The calculations were based on the estimation
of primary, secondary, and tertiary C H group rate
constants, which depend on the identity of the alkyl
substituents around these groups. The lowest value of
Onthebasisofourexperimentalresultswepostulate
the formation of very weakly bounded complex during
the abstraction of secondary or tertiary hydrogen atom
by Cl atom, but further work is needed to explain fully
this experimental finding.
−
11
3
−1 −1
s at 298 K, resulting
2
.9 × 10
cm molecule
from data in Table I, corresponds to the CH3 group rate

658
SARZYNSKI AND SZTUBA
We thank Dr. Andrzej Drys for his help in statistical analysis
of experimental data.
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6. Wallington, T. J.; Skewes, L. M.; Siegl, W. O.; Wu, C.
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7. Hitsuda, K.; Takahashi, K.; Matsumi, Y.; Wallington, T.
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