Kinetics and mechanism of the gas-phase reaction of Cl atoms with benzene

Article abstract of DOI:10.1021/jp9828080

The gas-phase reaction of Cl atoms with benzene has been studied using both experimental and computational methods. The bulk of the kinetic data were obtained using steady-state photolysis of mixtures containing Cl₂, C₆H₆, and a reference compound in 120-700 Torr of N₂ diluent at 296 K. Reaction of Cl atoms with C₆H₆ proceeds via two pathways; (a) H-atom abstraction and (b) adduct formation. At 296 K the rate constant for the abstraction channel is k_1a = (1.3 ± 1.0) × 10^-16 cm³ molecule^-1 s^-1. Phenyl radicals produced via H-atom abstraction from C₆H₆ react with Cl₂ to give chlorobenzene. The main fate of the C₆H₆-Cl adduct is decomposition to reform C₆H₆ and Cl atoms. A small fraction of the C₆H₆-Cl adduct undergoes reaction with Cl atoms via a mechanism which does not lead to the production of C₆H₅Cl, or the reformation of C₆H₆. As the steady-state Cl atom concentration is increased, the fraction of the C₆H₆-Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for benzene removal and a decrease in the chlorobenzene yield. Thermodynamic calculations show that a rapid equilibrium is established between Cl atoms, C₆H₆, and the C₆H₆-Cl adduct, and it is estimated that at 296 K the equilibrium constant is K_c,1b = [C₆H₆-Cl]/[C₆H₆][Cl] and lies in the range (1-2) × 10^-18 cm³ molecule.¹ Flash photolysis experiments conducted using C₆H₆/Cl₂ mixtures in 760 Torr of either N₂ or O₂ diluent at 296 K did not reveal any significant transient UV absorption; this is entirely consistent with results from the steady-state experiments and the thermodynamic calculations. The C₆H₆-Cl adduct reacts slowly (if at all) with O₂ and an upper limit of k(C₆H₆-Cl + O₂) < 8 × 10^-17 cm³ molecule^-1 s^-1 was established. As part of this work a value of k(Cl + CF₂ClH) = (1.7 ± 0.1) × 10^-15 cm³ molecule^-1 s^-1 was measured. These results are discussed with respect to the available literature concerning the reaction of Cl atoms with benzene.

Full text of DOI:10.1021/jp9828080

J. Phys. Chem. A 1998, 102, 10671-10681
10671
Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms with Benzene
†
,‡
O. Sokolov, M. D. Hurley, T. J. Wallington,* and E. W. Kaiser*
Ford Motor Company, P.O. Box 2053, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053
J. Platz and O. J. Nielsen§
Atmospheric Chemistry, Plant Biology and Biogeochemistry Department, Risø National Laboratory,
DK-4000, Roskilde, Denmark
F. Berho, M.-T. Rayez, and R. Lesclaux*,
|
Laboratoire de Physico-Chimie Mol e´ culaire, UMR 5803, CNRS, UniVersit e´ Bordeaux I,
3
3405 Talence - Cedex - France
ReceiVed: June 30, 1998; In Final Form: October 1, 1998
The gas-phase reaction of Cl atoms with benzene has been studied using both experimental and computational
methods. The bulk of the kinetic data were obtained using steady-state photolysis of mixtures containing Cl
, and a reference compound in 120-700 Torr of N diluent at 296 K. Reaction of Cl atoms with C
proceeds via two pathways; (a) H-atom abstraction and (b) adduct formation. At 296 K the rate constant for
2
,
C
H
6 6
2
6 6
H
-16
3
-1 -1
the abstraction channel is k1a ) (1.3 ( 1.0) × 10 cm molecule s . Phenyl radicals produced via H-atom
abstraction from C react with Cl to give chlorobenzene. The main fate of the C -Cl adduct is
decomposition to reform C and Cl atoms. A small fraction of the C -Cl adduct undergoes reaction
with Cl atoms via a mechanism which does not lead to the production of C Cl, or the reformation of C
As the steady-state Cl atom concentration is increased, the fraction of the C -Cl adduct undergoing reaction
H
6 6
2
6 6
H
H
6 6
6 6
H
H
6 5
6 6
H .
6 6
H
with Cl atoms increases causing an increase in the effective rate constant for benzene removal and a decrease
in the chlorobenzene yield. Thermodynamic calculations show that a rapid equilibrium is established between
Cl atoms, C
[C -Cl]/[C
conducted using C
transient UV absorption; this is entirely consistent with results from the steady-state experiments and the
thermodynamic calculations. The C -Cl adduct reacts slowly (if at all) with O and an upper limit of
6
H
6
, and the C
][Cl] and lies in the range (1-2) × 10 cm molecule. Flash photolysis experiments
/Cl mixtures in 760 Torr of either N or O diluent at 296 K did not reveal any significant
6 6
H -Cl adduct, and it is estimated that at 296 K the equilibrium constant is Kc,1b
-
18
3
1
)
H
6 6
6 6
H
6
H
6
2
2
2
6
H
6
2
-
17
3
-1 -1
k(C
6
H
6
-Cl + O
2
) < 8 × 10 cm molecule
s
was established. As part of this work a value of k(Cl +
-
15
3
-1
CF
2
ClH) ) (1.7 ( 0.1) × 10 cm molecule s-1 was measured. These results are discussed with respect
to the available literature concerning the reaction of Cl atoms with benzene.
Introduction
an unusual mechanism in which the active chain carrier is not
the free Cl atom but is instead a weakly bound C6H6-Cl
The reaction of Cl atoms with benzene has a long chemical
history. In 1903 it was established that 1,2,3,4,5,6-hexachloro-
cyclohexane (HCH) was the final product of the sequence of
chlorination reactions initiated by photolysis of Cl2 in liquid
5
-7
adduct.
The bonding in the C6H6-Cl adduct reduces its
reactivity and increases its selectivity when compared to free
8
,9
Cl atoms. In the liquid phase, the reaction of Cl atoms with
benzene proceeds via addition with a rate constant of k1 ) 1.0
1
benzene. The first quantitative studies of the gas phase reaction
-
11
3
-1 -1 10
×
10
cm molecule s .
2
,3
were performed by Noyes and co-workers in the early 1930s
using UV irradiation of C6H6/Cl2 mixtures with the course of
reaction monitored via changes in total pressure. It was deduced
that the reaction of chlorine atoms with benzene proceeds mainly
via addition, with chlorination occurring via a short-chain
mechanism (chain length of 20-50) giving HCH as the major
Cl + C H f products
(1)
6
6
Our interest in reaction 1 stems from a desire to quantify the
atmospheric chemistry and hence environmental impact of
aromatic compounds released into the atmosphere by motor
vehicles. In smog chamber studies of the atmospheric degrada-
tion mechanism of organic compounds it is often convenient to
use Cl atoms to initiate the sequence of photooxidation reactions.
To facilitate design and interpretation of such experiments,
kinetic and mechanistic data concerning the reaction of Cl atoms
with the aromatic compounds are needed.
2,3
product and chlorobenzene as a minor product. In the late
950s Russell and co-workers4 showed that the photochlori-
-7
1
nation of organic compounds in benzene solvent proceeds via
*
Authors to whom correspondence should be addressed.
Present address: Institute of Chemical Physics, Russian Academy of
†
Sciences, 4 Kosygin St., Moscow 117334, Russia.
‡
E-mail: twalling@ford.com.
E-mail: ole.john.nielsen@risoe.dk.
E-mail: lesclaux@cribx1.u-bordeaux.fr.
§
There have been four published measurements of k1 in the
gas phase, all of which used relative rate techniques at ambient
|
1
0.1021/jp9828080 CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/04/1998

10672 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Sokolov et al.
temperature in 700-740 Torr of air, or N2, diluents. Atkinson
and Aschmann monitored the loss of benzene relative to ethane
following UV irradiation of C6H6/C2H6/Cl2/air mixtures and
2.1. Flash Photolysis System at Bordeaux. The apparatus
consisted of a cylindrical reaction cell (4 cm i.d., 70 cm length).
Radicals were generated by the photolysis of molecular chlorine
using an argon flash lamp, surrounded by a Pyrex tube to filter
out flash radiation of λ < 290 nm, preventing photodissociation
of organic precursors. Molecular chlorine concentrations, 2-5
-11
3
-1 -1 11
derived k1 ) (1.5 ( 0.9) × 10 cm molecule s . Relative
rate studies conducted at Ford Motor Company using C6H6/
CH4/Cl2/N2 and C6H6/CD4/Cl2/N2 mixtures did not reveal any
evidence for the reaction of benzene with Cl atoms and upper
1
6
-3
× 10 molecule cm , were measured by UV absorption at
-
12 12
-16
3
-1
-19
2
-1 16
-3
limits of k1 < 4 × 10
and k1 < 5 × 10 cm molecule
330 nm (σ ) 2.56 × 10
cm molecule . ) Initial radical
-
1 13
13
s
were reported. In the most recent investigation, Shi and
concentrations were (5-10) × 10 molecule cm , as calibrated
Bernhard irradiated C6H6/CF2ClH/Cl2/air mixtures and from the
by photolysis of Cl in the presence of excess NO and measuring
2
-
18
2
relative loss rates of benzene and CF2ClH derived k1 ) (1.3 (
the absorption of the ClNO formed (σ ) 8.96 × 10
cm
-
15
3
-1 -1 14
-1
16
0
.3) × 10
cm molecule s .
molecule at 220 nm. The analyzing light, provided by a
deuterium lamp, passed through the reaction cell and was
detected by a monochromator/photomultiplier unit with 2 nm
band-pass. The signal was digitized by an oscilloscope and
recorded and averaged with a computer for further data analyses.
Kinetic parameters were derived from simulations of the
complete reaction mechanism fitted to the experimental traces
using nonlinear least-squares analysis. Gas mixtures were
prepared using calibrated flow controllers with total replenish-
ment of the gas mixture in the cell between each flash to avoid
secondary reactions involving reaction products.
Nozi e` re et al.13 studied the loss of benzene and methane
following UV irradiation of C6H6/CH4/Cl2/air mixtures and
reported that plots of the decay of benzene versus methane were
curved, with decreasing benzene loss observed at longer
irradiation times. Such behavior was attributed to complications
associated with the formation of OH radicals in the system. The
mechanism by which OH radicals are formed is thought to
involve reaction of Cl atoms with hydroperoxides.13 The Cl atom
initiated oxidation of methane in air produces CH3OOH which
can then react with Cl atoms to give OH radicals:
O2, N2, synthetic air, Cl2 (Messer, 5% in N2, purity > 99.9%),
NO (AGA Gaz Sp e´ ciaux, 0.96% in N2, purity > 99.9%), and
CCl4 (Prolabo, purity >99.8%) were all used without further
purification. Chloroform (Acros Chimica, purity 99+%, stabi-
lized with approximately 0.75% ethanol) was purified by
distillation and storage over molecular sieves. Great care was
taken in the purification of benzene (Aldrich, 99.9+ %, glass-
distilled, HPLC grade).
Cl + CH OOH f CH OOH + HCl
(2)
(3)
3
2
CH OOH f HCHO + OH
2
OH radicals react more rapidly with benzene than with
methane and so will preferentially attack benzene, enhancing
its decay rate. As the irradiation proceeds, oxidation products
build up and compete for the available OH radicals, causing
the apparent decrease in the benzene reactivity observed by
Nozi e` re et al. It seems likely that a similar explanation can
account for the anomalously high value of k1 reported by
Atkinson and Aschmann.11 To avoid OH radical formation
Purification of Benzene. The degree of purification of
benzene is a critical point in the determination of the rate
constant k(Cl + C6H6) in the flash photolysis experiments
because the reaction was found to be very slow and the method
used was equivalent to measuring the depletion of chlorine
atoms (and hence is sensitive to the presence of reactive
impurities). The nature of impurities was determined by GC-
MS. Hydrocarbons exhibiting high reactivity with chlorine
14
during oxidation of the reference compound, Shi and Bernhard
employed CF2ClH as the reference compound. However, as
discussed in the Conclusions section, it is possible that OH
radicals are formed during the oxidation of benzene in air.
As noted above, it is well established that the UV irradiation
of C6H6/Cl2 mixtures results in a series of chlorination reactions
leading to hexachlorocyclohexane (HCH). The γ-isomer of HCH
is an insecticide known as Lindane, and the UV irradiation of
C6H6/Cl2 mixtures has been used as a commercial synthesis for
this compound. In light of its commercial importance and long
history of academic interest, it is very surprising that the kinetics
of the reaction of Cl atoms with benzene are so poorly
understood. To provide insight into this reaction we have
conducted flash photolysis, relative rate, and computational
studies of the kinetics and mechanism of reaction 1. As shown
herein, the effective rate constant for reaction of Cl atoms with
benzene in the gas phase is very small and the reaction leads to
several different products. Reaction 1 is not a promising
candidate for initiating the oxidation of benzene in smog
chamber studies of its atmospheric oxidation mechanism.
-10
3
atoms, e.g., cyclohexane [k(Cl + C6H12) ) 2 × 10
cm
-1 -1 17
molecule s , ) were detected in amounts smaller than 0.15%.
Simple distillation was inadequate to remove such trace impuri-
ties and a new purification method was developed. Cl2 was
dissolved in deoxygenated liquid benzene by bubbling gaseous
Cl2 (5% in nitrogen) through the sample. Chlorine atoms were
generated by photolyzing Cl2 using the radiation provided by a
high-pressure mercury lamp filtered by the Pyrex vessel
containing the solution. They reacted first with reactive impuri-
ties and then with benzene. This procedure converts reactive
impurities and a small fraction of the benzene into chlorinated
compounds which were then eliminated by distillation under a
nitrogen atmosphere. Tetrachlorocyclohexene and hexachloro-
cyclohexane were identified among the chlorinated products,
18,19
in agreement with previous studies in liquid benzene.
These
two compounds were the only products produced by the above
photochemical reaction, using the already purified benzene,
thereby proving that they are formed by the reaction of chlorine
atoms with benzene. This purification procedure proved to be
very efficient; no remaining impurities (<0.01%) could be
detected by GC-MS analyses.
2
. Experimental Section
Relative rate and product studies were performed at Ford
using FTIR-smog chamber and photochemical GC-FID systems.
Absolute kinetic studies were performed at Bordeaux using a
2.2. FTIR-Smog Chamber System at Ford. A 140-liter
15
flash photolysis system described previously. All quoted errors
are 2 standard deviations from least-squares fits; conventional
error techniques were used to propagate errors where appropri-
ate.
Pyrex smog chamber interfaced to a Mattson Sirus 100 FTIR
2
0
spectrometer was used to study the kinetics and products of
reaction 1. The reactor was surrounded by 22 fluorescent
blacklamps (GE F15T8-BL). Chlorine atoms were generated

Gas-Phase Reaction of Cl Atoms with Benzene
J. Phys. Chem. A, Vol. 102, No. 52, 1998 10673
by the photolysis of molecular chlorine in 700 Torr total pressure
of N2 diluent.
reaction of chlorine atoms with benzene: phenyl C6H5 (reaction
1a), chlorocyclohexadienyl C6H6-Cl (reaction 1b), and cyclo-
hexadienyl C6H7, resulting from channel 1c and subsequent
reaction of H atoms with benzene.
Cl + hν f 2 Cl
(4)
2
Cl + hν f 2 Cl
(4)
(1a)
(1b)
(1c)
2
The concentrations of compounds were monitored by Fourier
transform infrared spectroscopy using their characteristic features
over the wavenumber ranges 600-700 (C6H6), 2180-2340
Cl + C H f C H + HCl
6
6
6
5
(
CD4), 1290-1330 (CF2ClH), and 720-760 cm-1 (C6H5Cl).
Cl + C H f C H -Cl
6
6
6
6
Infrared spectra were derived from 32 co-added interferograms
(
2
the data acquisition time was 1.5 min) using a path length of
8 m and a resolution of 0.25 cm . Reference spectra were
Cl + C H f C H Cl + H
6
6
6
5
-
1
-
1
21
acquired by expanding known volumes of reference materials
into the chamber. Experimental conditions for the kinetic study
of reaction 1 were, 2.2-35.5 mTorr of C6H6, 1.1-16 mTorr of
reference (either CF2ClH or CD4), and 14.8-2200 mTorr of
Cl2 in 700 Torr of N2 diluent.
Reaction enthalpies are in (units of kJ mol ): +33, -30
2
2,23
(this work, see section 3.8), and +69
1a, 1b, and 1c, respectively. Only the C H -Cl adduct, resulting
for reaction channels
6
6
from channel 1b, has a chance to be detected in flash photolysis
experiments at room temperature, the other two channels being
too slow as a result of their endothermicity.
Four sets of experiments were performed using the FTIR-
smog chamber. First, the kinetics of the reaction of Cl atoms
with CF2ClH were studied relative to CD4. Second, the kinetics
of the reaction of Cl atoms with benzene were investigated
relative to the reaction of Cl atoms with CF2ClH and CD4. CD4
and CF2ClH were selected as references because their reactivities
toward Cl atoms are comparable to that of benzene and because
one of them, CF2ClH, was used previously as a reference by
1
6
Gas mixtures, [Cl ] ) (2-5) × 10 and [C H ] ) (2-10)
2
6
6
1
7
-3
× 10 cm in N (760 Torr), were flash-photolyzed, resulting
2
13
in initial chlorine atom concentrations of (5-10) 10 molecule
-
3
cm . The temperature was varied from 250 to 298 K, and the
spectral region 220-330 nm was investigated to identify any
transient absorption corresponding to the above radicals. The
-17
phenyl radical absorbs significantly at 250 nm (σ ) 2.8 × 10
1
4
2
-1 24
Shi and Bernhard and so facilitates comparison of the results
from the present work with those reported by Shi and Bernhard.
Third, the rate of reaction of Cl atoms with chlorobenzene was
determined. Fourth, the products formed following the reaction
of Cl atoms with C6H6 in N2 diluent were investigated. All
reagents used at Ford were obtained from commercial sources
at research grade quality and were used without further
purification.
cm molecule ) while cyclohexadienyl radicals absorb strongly
-
17
2
-1 25
around 300 nm (σ ) 2.6 × 10
cm molecule ).
No significant transient signals could be detected whatever
the wavelength. Very weak absorptions were observed at 220
nm and between 260 and 280 nm, but they were too low (optical
densities < 0.001) to be significant and probably result from
some remaining impurities in benzene. Neither cyclohexadienyl
nor phenyl radicals were detected, indicating that reactions 1a
and 1c are too slow to be detected. In the presence of small
2
.3. Photochemical Reactor with Gas Chromatography
3
16
-3
Analysis at Ford. The 80 cm Pyrex reactor was cylindrical,
0 cm in length, and 2.5 cm in diameter and was irradiated by
amounts of NO (3 × 10 molecule cm ) the well-known
-18
2
absorption of ClNO was observed at 220 nm (σ ) 8.96 × 10
2 -1
a single Sylvania F6T5 BLB fluorescent lamp (1.2 cm diameter),
positioned 3.3 cm from the center of the reactor. The reactants,
containing CD4, C6H6, Cl2, and N2, were premixed in a separate
flask, and the reactor was filled to 120 Torr. The mixture was
irradiated for a predetermined time after which the contents of
the reactor were analyzed by gas chromatography using flame-
ionization detection (GC-FID). The percentage consumption of
C6H6 and CD4 was varied by a factor of 3-6 during these
experiments with no discernible impact on the measured rate
constant ratios. The initial mixture contained 90 mTorr C6H6,
cm molecule ); the addition of benzene in concentrations of
1
7
-3
up to 5 × 10 molecule cm had no discernible impact (<10%)
on the measured ClNO absorption, showing that Cl atoms react
preferentially with NO rather than with benzene. Experiments
performed using CCl in place of Cl as the chlorine atom
4
2
precursor gave the same result.
To check whether the conditions were really suitable to form
cyclohexadienyl-type radicals, Cl /C H mixtures were photo-
2
6
6
lyzed using H (700 Torr) as the carrier gas. The chemical
system, forming the cyclohexadienyl radical C H , was:
2
6
7
1
7-21 mTorr CD4, and 0.6-17 Torr Cl2, with the balance N2.
Cl + hν f 2 Cl
(4)
(5)
(6)
(7)
(1)
cm3
The initial Cl2 density was varied to obtain different steady-
state Cl atom densities in the reactor while using a constant
radiative flux. An additional set of experiments using the above
mixture with Cl2 fixed at 3.1 Torr was performed in which the
distance between the center of the lamp and reactor center was
varied from 3.3 to 16.5 cm, causing the steady-state Cl atom
density to change while keeping the mixture composition
constant. As the reactor is irradiated from one side only, the
steady-state Cl atom concentration will vary somewhat across
the diameter of the reactor, particularly when the lamp is close,
and only average Cl atom densities can be calculated in these
experiments.
2
Cl + H f HCl + H
2
H + Cl f HCl + Cl
2
H + C H f C H
7
6
6
6
Cl + C H f products
6
6
-
14
Reaction 5 occurs with a rate constant of 2 × 10
-
1
-1 26
molecule s . In the absence of benzene, a chain reaction
consuming Cl2 is initiated as observed at 330 nm by the
depletion of the Cl2 absorption. In the presence of benzene, a
strong absorption was observed around 300 nm (Figure 1),
consistent with the published UV spectrum of the cyclohexa-
3
. Results
3
.1. Flash Photolysis Study of the Cl + C6H6 Reaction in
2
7
the Absence of O2. The aim of the experiments performed at
Bordeaux was to identify the possible radical products of the
dienyl radical. The experimental signals could be simulated
by only taking into account the absorption corresponding to

10674 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Sokolov et al.
Figure 1. Typical decay trace recorded at 302 nm following flash
photolysis of Cl /C /H mixtures at room temperature and atmo-
2
6
H
6
2
1
7
spheric pressure. Experimental conditions: [C
6
H
6
] ) 3.25 × 10 , [H
2
]
1
9
16
13
)
2.5 × 10 , [Cl
2
] ) 1 × 10 , leading to [C
6
H
7
]
0
) 2.5 × 10 (units
-
3
of molecule cm ); the solid line is a simulation of the data. Negative
values of ln(I
text for details). The insert shows the UV absorption spectrum of the
radical.
o t 2
/I ) at longer reaction times arise from Cl depletion (see
6 7
C H
Figure 2. Decay of CF
2
ClH versus CD
4
when mixtures containing
these compounds were exposed to Cl atoms in 700 Torr total pressure
of N at 296 K. Circles were obtained using mixtures of [Cl ] ) 46
mTorr, [CF ClH] ) 7 mTorr, and [CD ] ) 18 mTorr. Triangles were
obtained using mixtures of [Cl ] ) 2.07 Torr, [CF ClH] ) 7 mTorr,
[CD ] ) 16 mTorr, and [C ] ) 3.7 mTorr.
-
17
2
-1
cyclohexadienyl radicals (with σ ) 2.6 × 10 cm molecule
2
2
25
at 302 nm ) without considering the reaction of Cl atoms with
benzene.
2
4
2
2
From the flash photolysis experiments, it can be concluded
that the reaction Cl + C6H6 is either very slow, which was
expected for channels 1a and 1c or equilibrated (equilibrium
largely shifted toward the reactants) so that chlorine atoms
remain available to react irreversibly with NO and H2. This is
certainly the case for the addition pathway (reaction 1b), as the
rate constant for addition is expected to be large. Theoretical
calculations failed to find any barrier on the addition pathway
4
6 6
H
-
15
3
-1 -1
gives k8 ) (1.7 ( 0.1) × 10 cm molecule s . This result
is consistent with previous measurements of k8 ) 2.0 × 10
and k8 ) 1.7 × 10 cm molecule s , providing
confidence in the experimental procedures used herein.
.3. Relative Rate Study of the Cl + C6H6 Reaction in
00 Torr of N2. Relative rate studies were performed using
-15 30
-
15
3
-1
-1 31
3
7
-
12
the FTIR-smog chamber and photochemical reactor GC-FID
systems at Ford. The loss of C6H6 was monitored relative to
that of CF2ClH or CD4 following the UV irradiation of C6H6/
CF2ClH/Cl2/N2 and C6H6/CD4/Cl2/N2 mixtures. The experimen-
tal conditions used are given in sections 2.2 and 2.3. Control
experiments were performed to check for complications caused
by photolysis, heterogeneous loss, and reaction of Cl2 with C6H6,
CF2ClH, and CD4; no evidence for such complications was
observed. Typical data showing the observed loss of C6H6 versus
CD4 following UV irradiation of C6H6/CD4/Cl2/N2 mixtures in
the FTIR-smog chamber are given in the insert in Figure 3B.
Variation of the initial molecular chlorine concentration, UV
irradiation intensity, and initial reactant concentrations had no
systematic effect on the values of k1/k8 and k1/k9 obtained
(
see section 3.8), and thus a rate constant value between 10
3
-1 -1
13
-11
cm3
cm molecule
s
(as for OH + benzene ) and 10
-
1
-1
10
molecule s , as observed in the liquid phase, is expected.
Assuming that the C6H6-Cl radical behaves like other cyclo-
hexadienyl radicals and absorbs around 300 nm with an
-17
2
-1 28
absorption section of σ ≈ 10 cm molecule , we can derive
an order of magnitude estimate for the upper limit of Kc,1b <
-
18
3
-1
1
0
cm molecule from the flash photolysis experiments.
.2. Relative Rate Study of k(Cl + CF2ClH). Prior to the
FTIR study of reaction 1, experiments were performed to
measure the reactivity of Cl atoms toward CF2ClH. A relative
rate method was used to measure k8 relative to k9 using the 140
liter FTIR-smog chamber at Ford. The experimental techniques
3
29
are described elsewhere.
(providing [Cl]ss does not change). Interestingly, however, the
measured values of k1/k8 and k1/k9 varied in a systematic fashion
with changes in the Cl atom steady-state concentration in the
individual experiments. The steady-state Cl atom concentration,
Cl + CF ClH f products
(8)
(9)
2
Cl + CD f products
4
[Cl]ss, was calculated from the observed rate of decay of the
reference compounds (CF2ClH or CD4) using the expression:
Control experiments were performed to check for complica-
tions caused by photolysis, heterogeneous loss, and reaction of
Cl2 with CF2ClH and CD4; no evidence for such complications
was observed. The experimental conditions used to measure k8/
k9 were [Cl2] ) 46 mTorr, [CF2ClH] ) 7 mTorr, [CD4] ) 18
mTorr, in 700 Torr of N2 diluent at 296 K. The observed loss
of CF2ClH versus that of CD4 following UV irradiation of
CF2ClH/CD4/Cl2/N2 mixtures is shown in Figure 2. The triangles
in Figure 2 are data obtained in the presence of benzene (see
section 3.3 for details). Linear least-squares analysis of the data
in Figure 2 gives k8/k9 ) 0.28 ( 0.02. Using k9 ) 6.1 × 10-15 29
[
Reference]to
1
[
Cl] )
Ln
ss
(
)
(
)
t × kref
[Reference]t
where t is the time of irradiation time in seconds, kref is the
reference rate constant (k8 or k9), and [Reference]to and
[Reference]t are the concentrations of the reference compound
before and after the irradiation. The assumptions inherent in
the use of this expression are the following: (i) the reference
compounds are lost solely via reaction with Cl atoms and are

Gas-Phase Reaction of Cl Atoms with Benzene
J. Phys. Chem. A, Vol. 102, No. 52, 1998 10675
CF2ClH/CD4/Cl2/N2 mixture with [C6H6] ) 3.70 mTorr, [Cl2]
)
2.07 Torr, [CF2ClH] ) 7.55 mTorr, and [CD4] ) 15.5 mTorr,
in 700 Torr of N2 diluent at 296 K. As shown by the triangles
in Figure 2, the relative decay rates of CF2ClH and CD4 were
not impacted by the presence of benzene or its reaction products.
As a test for possible complications caused by reaction of
benzene with products from reaction 8, experiments were
performed using comparable [Cl]ss values with [CF2ClH] varied
by a factor of 7. There was no discernible effect of [CF2ClH]
on the measured value of k1/k8. We conclude that the first
assumption is valid.
Second, the results presented in section 3.2 suggest that the
values used for kref are accurate. Third, fluorescent blacklamps
were used as the UV source; typical periods of UV illumination
were 1-15 min. Prior to their use the UV lamps were switched
on for 10 min to “warm”. It seems reasonable to assume that
the output of the lamps does not change greatly during the course
of an experiment. Fourth, for the concentrations of reactants
used in the present work it can be calculated that the Cl atom
lifetime is at least 2 orders of magnitude less than the UV
irradiation times. Fifth, the measured [Cl]ss did not change
appreciably (<10%) with the amount of the reactants consumed.
All assumptions inherent in the calculation of [Cl]ss seem
reasonable.
Figure 3A,B shows the measured values of k1/k8 and k1/k9
plotted versus [Cl]ss for both FTIR and GC-FID experiments.
As seen from Figure 3 the apparent reactivity of Cl atoms toward
benzene increases linearly with the steady-state Cl atom
concentration. The results from both the GC and FTIR experi-
ments fall on the same line even though the initial reactant
concentrations for these two techniques are very different (FTIR
conditions were: [CD4]o ) 5.2-11.4 mTorr, [C6H6] ) 2.2-
3
)
.7 mTorr, [Cl2] ) 0.044-2.0 Torr, GC conditions were: [CD4]o
17-21 mTorr, [C6H6] ) 90 mTorr, [Cl2] ) 0.6-17 Torr).
This provides strong support for ascribing this effect to the Cl
atom concentration rather than to any of the initial reactant
concentrations such as Cl2.
The plots in Figure 3A,B have positive y-axis intercepts. The
simplest explanation for the behavior shown in Figure 3 is to
postulate that reaction of Cl atoms with benzene proceeds via
two channels: H-atom abstraction and addition to give a C6H6-
Cl adduct which can either undergo decomposition back to
reactants, or react further with Cl atoms in a manner that does
not regenerate benzene.
Figure 3. Effective rate constant for the reaction of Cl atoms with
C
6
H
6
measured relative to the rate constants for reaction of Cl atoms
with CF ClH (A) and CD (B) plotted versus the steady-state Cl atom
concentration in experiments in 700 Torr total pressure of N at 296
2
4
2
K. Open symbols were obtained using the GC-FID apparatus at 120
Torr, closed symbols with the FTIR system at 700 Torr. The diamonds
show results obtained using a constant light intensity with varying [Cl
open circles show results obtained at a constant [Cl ] with the light
intensity varied. The insert in panel B shows the observed loss of C
versus that of CD following the UV irradiation of a mixtures of 3.7
mTorr of C , 11.1 mTorr of CD , and 204 mTorr of Cl (triangles)
and 3.8 mTorr of C , 10.9 mTorr of CD , and 2 Torr of Cl (inverted
triangles) in 700 Torr of N diluent in the FTIR-smog chamber.
2
];
2
6 6
H
4
Cl + C H f C H + HCl
(1a)
(1b)
6
6
6
5
6
H
6
4
2
H
6 6
4
2
Cl + C H f C H -Cl
2
6
6
6
6
not reformed in any process, (ii) the values used for kref are
accurate, (iii) the UV light source provides a constant illumina-
tion of the reaction mixtures, (iv) the lifetime of Cl atoms is
short compared to the irradiation time, and (V) the [Cl]ss is
constant throughout the irradiation period. Let us consider these
assumptions in turn.
First it is not difficult to conceive of reactions by which
benzene or the reference compounds are lost which do not
involve Cl atoms, e.g.,
C H -Cl f Cl + C H
6
(-1b)
(12a)
6
6
6
C H -Cl + Cl f products
6
6
Because of the substantial endothermicity of reaction 1c, it
is ignored in the above mechanism. It is possible, even likely,
that some fraction of the reaction of Cl atoms with the C6H6-
Cl adduct proceeds to regenerate C6H6 and Cl2 which then
constitutes benzene-catalyzed recombination of Cl atoms.
C H -Cl + Cl f C H + Cl
2
(12b)
C H + CF ClH f C H + CF Cl
(10)
(11)
6
6
6
6
6
5
2
6
6
2
The relevant differential equations are
d[C H ]/dt ) k [Cl][C H ] + k [Cl][C H ] -
CF Cl + C H f products
2
6
6
-
6
6
1a
6
6
1b
6
6
To test for loss of CF2ClH or CD4 via reactions with products
from reaction 1 experiments were performed using a C6H6/
k-1b[C H -Cl] - k [C H -Cl][Cl]
6
6
12b
6
6

10676 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Sokolov et al.
d[C H -Cl]/dt ) k [Cl][C H ] - k [C H -Cl] -
6
6
1b
6
6
-1b
6
6
k [C H -Cl][Cl]
12
6
6
where k12 ) (k12a + k12b).
Application of the steady-state approximation for the [C6H6-
Cl] adduct gives
-
d[C H ]/dt ) k [Cl] [C H ] +
6 6 1a ss 6 6
2
ss
k k [Cl] [C H ]/(k +k [Cl] )
1b
12a
6
6
-1b
12
ss
In our experiments we measure an effective rate constant for
reaction 1 given by
-
d[C H ]/dt ) k [Cl] [C H ]
6 6 eff ss 6 6
hence keff ) k1a + k1bk12a[Cl]ss/(k-1b + k12[Cl]ss).
We can consider two extreme behaviors depending on the
relative magnitudes of k-1b and k12[Cl]ss. When k12[Cl]ss . k-1b
the expression reduces to keff ) k1a + k1bk12a/k12 and no
dependence on the Cl atom steady-state concentration will be
observed. When k12[Cl]ss , k-1b the effective rate constant will
have a linear dependence on [Cl] and keff is given by
Figure 4. Effective rate constant for the reaction of Cl atoms with
k ) k + (k k [Cl] )/k
-1b
eff
1a
1b 12a
ss
C
6
H
5
Cl measured relative to the reaction of Cl atoms with CF
plotted versus the steady-state Cl atom concentration in experiments
in 700 Torr total pressure of N at 296 K.
2
ClH
As seen from Figure 3, the experimental observations show
a linear dependence of keff on the Cl atom concentration which
is consistent with the simple model above with decomposition
via reaction -1b being the dominant fate of the C6H6-Cl
adduct. The data in Figure 3B using CD4 reference were
obtained from experiments conducted in both the 140 liter FTIR-
2
ingly, we choose to quote final values of k1a ) (1.3 ( 1.0) ×
cm molecule based upon the CF2ClH data and
k1bk12a)/(k-1b) ) (4.5 ( 1.1) × 10
average from CF2ClH and CD4 experiments).
We can use the measured value of (k1bk12a)/(k-1b) to provide
-16
3
-1 -1
1
0
s
-28
6
-2 -1
(
(
cm molecule
s
3
smog chamber facility (filled symbols) and the 80 cm photo-
chemical reactor - GC-FID system (opened symbols). Indis-
tinguishable results were obtained in the two systems. In both
experimental systems variation of [Cl]ss was achieved by either
maintaining a given UV intensity and varying [Cl2] or by
keeping [Cl2] constant and varying the UV intensity (see sections
an estimate of the equilibrium constant Kc,1b ) k1b/k-1b. By
32
comparison with reactions of other alkyl radicals with Cl atoms
it seems likely that reaction 12a proceeds with a rate constant
which is close to the gas kinetic limit, i.e., k12a ) (2-4) ×
-10
3
-1 -1
1
)
0
cm molecule s . Hence, we estimate Kc,1b ) k1b/k-1b
2.2 and 2.3 for details). This is illustrated by the GC-FID data
-18
3
-1
(1-2) × 10 cm molecule . This result is consistent with
in Figure 3B. This figure shows rate constant ratios obtained
with the same initial reaction mixture but with different light
intensity (open circles) as described in section 2.3. This changes
the [Cl]ss as expected. Also shown are data at constant light
intensity but differing [Cl2] (open diamonds). The method by
which the [Cl]ss was varied had no discernible effect on the
measured value of keff.
the conclusion from the flash photolysis experiments (section
-18
3
-1
3
.1) that Kc,1b is of the order of 1 × 10
cm molecule or
less.
-1 21
Reaction 1a is endothermic by 33 kJ mol . The calculations
presented in section 3.8 show that this reaction proceeds with
an activation barrier close to the reaction endothermicity. We
can use the value of k1a measured above to arrive at a
Linear least-squares analysis of the data in Figure 3A gives
an intercept ) k1a/k8 ) 0.0737 ( 0.0560 and a slope ) (k1bk12a)/
-11
3
-1 -1
preexponential A factor of 6 × 10 cm molecule s . This
result is very reasonable for hydrogen abstraction by Cl atoms
-
12
3
-1
(
)
1
k-1bk8) ) (0.231 ( 0.023) × 10
cm molecule , using k8
-11
3
-1
which typically have A factors of 1-10 × 10 cm molecule
-
15
3
-1 -1
(1.7 ( 0.1) × 10
cm molecule
s
gives k1a ) (1.3 (
-1 16
s .
-
16
3
-1 -1
.0) × 10
cm molecule
s
and (k1bk12a)/(k-1b) ) (3.9 (
3
.4. Relative Rate Study of the Cl + C6H5Cl Reaction in
-
28
6
-2 -1
0
.5) × 10
cm molecule s . Similar analysis of the data
7
00 Torr of N2. Prior to a product study of reaction 1,
in Figure 3B gives an intercept ) k1a/k9 ) 0.0688 ( 0.0402
experiments were performed using the FTIR-smog chamber
system to measure the kinetics of reaction 13 relative to reaction
-
12
and a slope ) (k1bk12a)/(k-1b k9) ) (0.0813 ( 0.0080) × 10
cm molecule . Using k9 ) 6.1 × 10 cm molecule
gives k1a ) (4.2 ( 2.4) × 10 cm molecule
k1bk12a)/(k-1b) ) (4.9 ( 0.5) × 10
3
-1
-15
3
-1 -1 29
s
8
.
-
16
3
-1 -1
s
and
-
28
6
-2 -1
(
cm molecule s .
Cl + C H Cl f products
(13)
(8)
6
5
Within the admittedly large uncertainties (particularly for the
intercept), consistent data are obtained from experiments using
the two different reference compounds. As seen in Figure 3 the
data obtained using CD4 as reference are more scattered than
those obtained using CF2ClH. This extra scatter reflects the
inherent difficulties associated with measurement of rate
constant ratios which deviate markedly from unity. Data were
collected using CF2ClH as reference at lower values of [Cl]ss
which provides a more accurate determination of k1a. Accord-
CF ClH + Cl f products
2
As with the reaction of Cl atoms with benzene, the effective
rate constant for reaction of Cl atoms with chlorobenzene
increased with the steady-state chlorine atom concentration.
Figure 4 shows a plot of the measured values of k13/k8 versus
[Cl]ss. As discussed in the previous section, the simplest
chemical mechanism that can explain the observed behavior is

Gas-Phase Reaction of Cl Atoms with Benzene
J. Phys. Chem. A, Vol. 102, No. 52, 1998 10677
Cl + C H Cl f C H Cl + HCl
(13a)
(13b)
(14a)
(14b)
6
5
6
4
Cl + C H Cl f C H Cl-Cl
6
5
6
5
C H Cl-Cl + Cl f products
6
5
C H Cl-Cl + Cl f C H Cl + Cl
2
6
5
6
5
Linear least-squares analysis of the data in Figure 4 gives an
intercept ) k13a/k8 ) 0.015 ( 0.120 and a slope ) (k13bk14a)/
-
12
-1
3
(
(
1
×
k-13bk8) ) (0.31 ( 0.03) × 10
molecule cm , using k8 )
-
15
3
-1 -1
1.7 ( 0.1) × 10
cm molecule
s
gives k13a < 2.5 ×
-
16
3
-1 -1
0
cm molecule
s
and (k13bk14a)/(k-13b) ) (5.3 ( 0.6)
10- cm molecule s . Within the experimental uncer-
28
6
-2 -1
tainties, there is no evidence for a direct bimolecular component
of the reaction of Cl atoms with chlorobenzene.
3
.5. Study of the Products Formed Following the UV
Irradiation of Cl2/C6H6/N2 Mixtures. To provide insight into
the mechanism of the reaction of Cl atoms with benzene,
experiments were performed to determine the products following
UV irradiation of Cl2/C6H6/N2 mixtures using the FTIR-smog
chamber facility. Figure 5 shows typical IR spectra obtained
before (A) and after (B) a 2 min irradiation of a mixture of 2.6
Torr of Cl2 and 36.4 mTorr of C6H6 in 700 Torr of N2 at 296
K. Comparison of the features in panel B with the reference
spectrum of chlorobenzene given in panel C clearly shows the
formation of chlorobenzene. Chlorobenzene was observed as a
product in all experiments. The insert in Figure 6 shows the
formation of chlorobenzene versus loss of benzene following
irradiation of a mixture containing 38.5 mTorr of Cl2, 30.6
mTorr of C6H6, and 1.18 mTorr of CF2ClH in 700 Torr of N2.
Subtraction of IR features attributable to benzene and chloro-
benzene from panel B gives the residual IR features at 743,
Figure 5. IR spectra obtained before (A) and after (B) a 2 min
irradiation of a mixture of 2.6 Torr of Cl
00 Torr of N at 296 K. Panel C is a reference spectrum of
chlorobenzene. Unidentified product features obtained by subtraction
2 6 6
and 36.4 mTorr of C H in
7
2
6 6
of features attributable to C H and chlorobenzene from panel B are
shown in panel D.
-
1
7
94, 827, 894, 997, and 1020 cm shown in panel D. We are
unable to identify the product(s) responsible for these residual
features. Likely candidates for the unknown product(s) include
various isomers of dichlorocyclohexadiene, tetrachlorocyclo-
hexene, and hexachlorocyclohexane. In the absence of authentic
samples of these compounds which we can introduce into the
chamber we are unable to establish their yields. Small correc-
tions were applied to account for loss of chlorobenzene via
reaction 13. As seen from Figure 6, the yield of chlorobenzene
varied inversely with the Cl atom steady-state concentration.
The following simple model was used to rationalize the kinetic
data in section 3.3.
Cl + C H f C H + HCl
(1a)
(1b)
6
6
6
5
Cl + C H f C H -Cl
6
6
6
6
C H -Cl f Cl + C H
6
(-1b)
6
6
6
C H -Cl + Cl f products
(12)
6
6
Figure 6. Yield of chlorobenzene following UV irradiation of C
Cl /N mixtures plotted versus the steady-state Cl atom concentration.
The insert shows the observed increase in chlorobenzene versus loss
of C following UV irradiation of a mixture of 30.6 mTorr of C
1.18 mTorr of CF ClH, and 38.5 mTorr of Cl , in 700 Torr of N diluent,
the steady-state Cl atom concentration calculated for this experiment
6 6
H /
The phenyl radicals produced in reaction 1a are expected to
react rapidly with Cl2 to give chlorobenzene.
2
2
6
H
6
6 6
H ,
C H + Cl f C H Cl + Cl
(15)
6
5
2
6
5
2
2
2
12
-3
If we assume that all phenyl radicals react via reaction 15
and that there are no other sources of chlorobenzene, then its
molar yield should be given by
was 0.44 × 10 cm
.
Consistent with the experimental observations shown in
Figure 6, the above expression predicts that the yield of
chlorobenzene will vary inversely with the Cl atom steady-state
concentration. The simple model given above predicts that as
k /k ) k /{k + (k k12a[Cl])/k-1b}
1
a
eff
1a
1a
1b

10678 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Sokolov et al.
[
Cl]ss increases, the yield of chlorobenzene should tend toward
cannot be assigned to the C6H5O2 radical since no phenyl
radicals could be observed in the absence of oxygen). These
observations might be consistent with a reaction of the C6H6-
zero which is consistent with the data given in Figure 6. Also
the model predicts that as [Cl]ss approaches zero the chloroben-
zene yield should approach unity. Experimentally we observe
that the chlorobenzene yield increases with decreasing [Cl]ss
but does not appear to quite reach unity. The largest chloroben-
zene yield was 78%. There are three possible explanations for
the apparent 22% shortfall in the chlorobenzene yield at the
lowest values of [Cl]ss. First, the simple model above could be
deficient in a small loss of the C6H6-Cl adduct which is not
dependent on Cl atoms and which does not lead to chloroben-
zene product. Second, the model could be deficient in a loss of
phenyl radicals other than reaction 15. Third, there could be a
systematic calibration error in our measurement of the chloro-
benzene yield or a systematic underestimation of [Cl]ss. With
regard to the second point, on the basis of the database for other
alkyl radicals32 it seems reasonable to assume that reaction 15
Cl adduct with O , alternatively the observed transient signals
2
might be due to the reaction of chlorine atoms with remaining
impurities in the benzene sample (0.01% would be enough).
Because of the equilibrium of the addition pathway (reactions
1
b, -1b), it is difficult to measure the rate constant of the
addition reaction of Cl atoms with C6H6 (k1b). In principle, this
should be possible provided the adduct can be readily eliminated
by reaction with a radical scavenger such as O2. We have
attempted to measure the rate constant k1b in the presence of a
large excess of oxygen, using a relative method, the reference
reaction being Cl + CHCl3. It is well-known that chloroform
33
is oxidized into phosgene via a chain reaction. The reaction
of Cl atoms with benzene was used as a termination of the chain
reaction and the rate constant was derived from simulations of
the time-resolved phosgene build-up signals at 240 nm, for
different benzene concentrations. The result was k1 ) (9.8 (
-
11
3
proceeds with a rate constant of the order of 10
molecule
cm
-
1 -1
s
and in the presence of 0.015-2.0 Torr of Cl2 it
is difficult to imagine any competing loss mechanisms for
phenyl radicals. We estimate that possible systematic errors
associated with calibrations of the reference spectra of benzene
and chlorobenzene together contribute a 15% uncertainty in
measurement of the chlorobenzene yield. After subtraction of
the chlorobenzene features from the product spectra obtained
in experiments using high values of [Cl]ss, residual IR features
attributed to one or more unknown product(s) were observed.
These residual features decrease progressively for experiments
with lower values of [Cl]ss. At the very lowest [Cl]ss there are
no discernible residual features after subtraction of those
belonging to benzene and chlorobenzene. We cannot exclude
the possibility that an undetected product is formed at the 10-
-15
3
-1 -1
4
(
.1) × 10 cm molecule s , using k(Cl + CHCl3) ) (1.1
-13
3
-1 -1
17
0.4) × 10
cm molecule
s
at 298 K. However, the
rate constant value was found unchanged when the oxygen
17
concentration was decreased to values as low as 10 molecule
cm , which is totally inconsistent with the value of the
-3
equilibrium constant estimated in sections 3.1 and 3.3. For the
lowest O2 concentrations, it would require the rate constant value
for the reaction of the adduct with O2 to be larger than the gas
kinetic limit. We conclude that the measured rate constant cannot
be that of the addition channel. Because of the possible presence
of remaining impurities in benzene it rather corresponds to an
upper limit of the overall rate constant, in agreement with other
results reported in this paper. This result implies that even in
the presence of 700 Torr of O2, any reaction between the C6H6-
Cl adduct and O2 is slow.
2
0% yield level in experiments conducted at the lowest [Cl]ss
conditions. However, we do not have any evidence for such a
complication and for simplicity we will proceed on the
assumption that the apparent shortfall in the chlorobenzene
reflects a calibration error. Rearranging the expression given
above for k1a/keff and including an arbitrary calibration factor
3
.7. Relative Rate Study of the Cl + C6H6 Reaction in
7
00 Torr of Air. To provide insight into the reactions occurring
during the Cl-atom-initiated oxidation of benzene in air and to
provide a direct comparison with the previous study of Shi and
“
CF” we can derive:
14
Bernhard, relative rate experiments were performed to measure
k1/k8 in 700 Torr of air diluent at 296 K. Two reaction mixtures
were used with initial concentration ratios [C6H6]o/[CF2ClH]o
chosen to be comparable to those employed by Shi and
chlorobenzene yield )
CF × 1/{ 1 + (k k12a[Cl])/(k-1bk )}
1b
1a
1
4
Bernhard. Figure 7 shows the observed loss of C6H6 versus
that of CF2ClH following UV irradiation of C6H6/CF2ClH/Cl2/
air mixtures (for initial conditions see figure caption). As seen
from Figure 7, variation of [C6H6]o/[CF2ClH]o by a factor of 6
had no discernible effect on the relative decay rates of C6H6
and CF2ClH. The relative rate plot in Figure 7 shows modest
but significant curvature. The line in Figure 7 is a linear fit to
the first 6 data points which gives a slope of 0.71 which is
similar to the value of k1/k8 ) 0.757 ( 0.024 reported by Shi
The smooth curve in Figure 6 shows a fit of the above
expression to the data which gives CF ) 0.76 ( 0.10 and
(
result can be combined with the value of (k1bk12a)/(k-1b) ) (4.5
(
-
12
3
-1
k1bk12a)/(k-1bk1a) ) (2.2 ( 0.9) × 10
cm molecule . This
-
28
6
-2 -1
1.1) × 10
cm molecule
s
from section 3.3 to give
-
16
3
-1 -1
k1a ) (2.0 ( 1.0) × 10
cm molecule
s
which is
-
16
3
consistent with the value of k1a ) (1.3 ( 1.0) × 10
cm
-
1
-1
molecule derived from the intercept in Figure 3A. It is
s
gratifying to note that the simple model of the Cl-atom-initiated
chlorination of benzene presented in section 3.3 provides a self-
consistent qualitative and quantitative explanation of both the
observed kinetic behavior and product distribution of the system.
1
4
and Bernhard. The curvature in Figure 7 indicates that either
reactions 1 and 8 are not the sole loss mechanisms for C6H6
and CF2ClH, or that there are processes that regenerate C6H6
or CF2ClH. In the presence of 700 Torr of air it is difficult to
imagine reactions that regenerate C6H6 or CF2ClH. We believe
that the most likely explanation for the observed curvature lies
in the generation of OH radicals during the Cl-atom-initiated
oxidation of C6H6. OH radicals react 300 times faster with
3
.6. Flash Photolysis Study of the Cl + C6H6 Reaction in
the Presence of O2. In the presence of excess oxygen, transient
signals could be observed at wavelengths between 220 and 290
nm. However, these signals were very low in intensity and were
at the detection limit of the system (approximately 0.2%
absorption), and the corresponding absorption spectrum did not
exhibit any well-characterized maximum. Despite some similar-
ity of the spectrum with those generally observed for peroxy
radicals, no information about the nature of the products of the
reaction could be derived from these absorption signals (they
1
6,34
benzene than with CF2ClH,
and their generation will lead
to an enhanced rate of benzene loss. As reaction products build
up there will be a competition for the available OH radicals,
resulting in decreased benzene loss and curvature in the relative
rate plot. From the rate of CF2ClH loss the [Cl]ss is calculated

Gas-Phase Reaction of Cl Atoms with Benzene
J. Phys. Chem. A, Vol. 102, No. 52, 1998 10679
The effect of reaction 17 is to augment the loss of benzene,
and this reaction may contribute to the “enhanced” loss of
benzene observed in experiments conducted in air diluent. The
yield of chlorobenzene was small (6-8%) and any contribution
by reaction 17a is correspondingly small. It can be argued that
the reaction of the C6H6-Cl adduct with O2 proceeds via more
than one reaction channel and so makes a larger contribution
to the “enhanced” benzene loss. While this may be true there
are two points to bear in mind when discussing the potential
importance of reaction 17. First, the O2 concentration is constant
and any enhanced rate of benzene loss would be constant during
the experiment. The experimental results shown in Figure 7
show a decrease in the rate of benzene loss for longer
irradiations; thus reaction of the C6H6-Cl adduct with O2 cannot
solely explain the experimental observations. Second, in absolute
terms, any “enhancement” caused by reaction of the C6H6-Cl
adduct with O2 is modest. An upper limit for k17, (k17a + k17b),
can be derived by applying the following logic. As discussed
above, the initial rate of benzene loss in 700 Torr air is 2.5
times that expected for the “pure” Cl atom reactions. In the
experiments described above in 700 Torr of air the concentration
1
8
11
6
of O2 was 4.8 × 10 /8 × 10 ) 5.9 × 10 times greater than
that of Cl atoms. Ascribing the entire enhanced benzene loss to
reaction of the adduct with O2, it follows that a factor of 5.9 ×
Figure 7. Decay of C
these compounds were exposed to Cl atoms in 700 Torr total pressure
of air at 296 K. Open symbols were obtained using mixtures of [Cl
370 mTorr, [CF ClH] ) 7.4 mTorr, and [C ] ) 4.4 mTorr. Filled
symbols were obtained using mixtures of [Cl ] ) 370 mTorr, [CF ClH]
4 mTorr, and [C ] ) 15 mTorr.
6 6 2
H versus CF ClH when mixtures containing
6
2
]
10 excess of O2 over Cl atoms increases the benzene loss by
)
2
6
H
6
an additional 150%. Reaction 12a cannot proceed faster than
-10
-17
2
2
the gas kinetic limit, k12a < 3 × 10 , hence k17 < 8 × 10
)
H
6 6
3
-1 -1
cm molecule s . It is clear that reaction of the C6H6-Cl
adduct with O2 occurs slowly (if at all).
11
-3
to be 8 × 10 cm , by reference to Figure 3A it can be seen
To provide insight into the dependence of the C6H5Cl yield
on the O2 concentration, six experiments were performed using
mixtures of 50 mTorr of C6H6, 2 mTorr of CF2ClH, 100 or 500
mTorr of Cl2, and 10, 147, or 700 Torr of O2 in 700 Torr total
pressure made up with N2 diluent as appropriate. The results
are given in Table 1. The molar yields of C6H5Cl were small.
The [Cl]ss concentrations were calculated from the observed loss
rate of the CF2ClH tracer. The increase in the C6H5Cl yield
with increasing [O2] for the experiments conducted using [Cl2]
that at this steady-state Cl atom concentration a value of k1/k8
)
0.3 is expected from the “pure” Cl atom reactions. The initial
rate of C6H6 loss observed in experiments conducted in 700
Torr of air is 2.5 times that expected on the basis of the results
obtained in N2 diluent. It is worth noting that the slope of the
relative rate plot in Figure 7 at high consumptions of CF2ClH
and C6H6 tends toward the value of k1/k8 ) 0.3 expected from
the “pure” Cl atom reactions. This behavior can be rationalized
in terms of loss of OH radicals via reaction with benzene
oxidation products.
)
100 mTorr, although small, was significant and since [O2]
was the only parameter that was changed significantly it seems
reasonable to speculate that reaction 17a makes a small
contribution. It should be stressed that the magnitude of the
sensitivity of the C6H5Cl yield to a 70-fold change in [O2] is
very small and that even in the presence of 700 Torr of O2, any
role played by reaction 17a is minor. The reactions occurring
during the Cl-initiated oxidation of benzene in the presence of
O2 are complex and poorly characterized. The aim of the present
work was to elucidate the kinetics of the reaction of Cl atoms
with benzene, not the detailed chemistry associated with the
subsequent reactions in air diluent. Experiments in N2/O2 diluent
were not pursued further.
Chlorobenzene was observed as a minor product (molar yield
of 6-8%) in the relative rate experiments described above. The
origin of the chlorobenzene product observed in experiments
conducted in N2 diluent (see section 3.4) is reaction 1a followed
by reaction 15:
Cl + C H f C H + HCl
(1a)
(1b)
(15)
6
6
6
5
Cl + C H f C H -Cl
6
6
6
6
C H + Cl f C H Cl + Cl
6
5
2
6
5
In the presence of O2 the phenyl radicals produced in reaction
a will be efficiently scavenged via reaction 16.35
Unlike our data shown in Figure 7, Shi and Bernhard14
1
observed linear behavior in their relative rate plots. Furthermore,
in apparent contrast to our results, Shi and Bernhard14 report
that “no significant C6H5Cl was observed in the C6H6/Cl2/air
system despite extensive efforts for its identification by FTIR
spectroscopy”. The origin of the different behavior observed in
the two studies is unclear.
C H + O + M f C H O + M
(16)
6
5
2
6
5
2
The mechanism by which chlorobenzene is formed in the
experiments conducted in air is different from that operative in
N2 diluent. One possible explanation is that O2 reacts with the
C6H6-Cl adduct:
Theoretical Calculations. The value of the equilibrium
constant, Kc,1b, of the addition channel is a critical parameter
for interpreting the present results. Experimental results can
provide rough estimates of the Kc,1b value, but we have found
it important to assess the consistency of these estimated values
with the thermochemical parameters that can be derived from
C H -Cl + O f C H Cl + HO
(17a)
6
6
2
6
5
2
C H -Cl + O f products other than C H Cl + HO
6
6
2
6
5
2
(17b)

10680 J. Phys. Chem. A, Vol. 102, No. 52, 1998
Sokolov et al.
TABLE 1: Molar Yields of C
6
H
5
Cl Observed Following the UV Irradiation of Mixtures of 50 mTorr of C
6 6
H , 2 mTorr of
CF ClH (added as a Cl atom tracer), and 100 or 500 mTorr of Cl
2
2
in N /O Diluent at 700 Torr Total Pressure and 296 K
2
2
O
2
(Torr)
147
Cl
2
(mTorr)
10
700
1
00
Y(C
[
Y(C
6
H
5
Cl) ) 3.9%
Y(C
6
H
5
Cl) ) 5.2%
Y(C
6
H
5
Cl) ) 6.3%
Cl]ss ) 3.6 × 10 cm-3
1
1
[Cl]ss ) 3.1 × 10 cm-3
11
[Cl]ss ) 3.0 × 10 cm-3
11
5
00
H
6 5
Cl) ) 6.3%
Y(C
6
H
5
Cl) ) 6.4%
6 5
Y(C H Cl) ) 9.8%
11
Cl]ss ) 1.0 × 10 cm-3
1
2
[Cl]ss ) 8.0 × 10 cm-3
11
[Cl]ss ) 6.0 × 10 cm-3
[
TABLE 2: Calculated and Experimental Thermochemical
in Table 1 lies in the range:
6 6
Parameters of the Cl + C H Reaction; the Parameters for
6 6
the Addition Channel of H and OH to C H Are Included
-17
-21
3
-1
Kc,1b ) 10 - 10 cm molecule at 298 K
for Comparison
calculated (BAC-MP4)
experimental
confirming that the equilibrium is largely shifted toward the
reactants at room temperature. The flash photolysis experiments
described in section 3.1 suggest that Kc,1b is of the order of 10
∆
S°298
-
1
∆
H°298
(J mol
∆H°298
kJ mol-1)
K
-1
)
(kJ mol-1)
-18
(
3
-1
+33b
cm molecule , or less. The FTIR experiments described in
Cl + C
Cl + C
Cl + C
6
6
6
H
H
H
6
6
6
f HCl + C
f C -Cl (1b)
f H + C Cl (1c)
6
H
5
(1a)
+48
-30
+60
-79
-74
a
-
18
section 3.3 suggest that Kc,1b lies in the range (1-2) × 10
6
H
6
-92
a
b
3
-1
6
H
5
a
+69c
cm molecule . It is gratifying that the experimental and
computational results are consistent. Based upon the FTIR
H + C
6
H
6
f C
6
H
7
-89
-112
-92
OH + C
a
6
H
6
f C
6
H
6
-OH
-69d
-18
results we conclude that Kc,1b lies in the range (1-2) × 10
Not determined. b Refs 21 and 22. c Ref 28. d Ref 40.
3
-1
cm molecule .
Calculations were also performed to search for a possible
barrier for the addition pathway. As the Cl atom approaches
the benzene molecule the energy of the system was always lower
than the energy of the separated reactants. The same result was
obtained using other ab initio (DFT) or semiempirical (AM1)
methods; there is no barrier on the addition pathway (channel
quantum calculations. For comparison calculations were also
performed to characterize the adducts of H atoms and OH
radicals with benzene.
The heats of formation of the different compounds involved
in this study were calculated using the Melius BAC-MP4
method. Stationary points were fully optimized at the HF/6-
-11
1
b). This finding is consistent with the high rate constant (10
3
-1 -1
cm molecule s ) found for this reaction channel in liquid
31G(d) level of theory using the GAUSSIAN94 package of
1
0
benzene. Similar calculations were performed to search for a
barrier on the hydrogen abstraction pathway (1a); none was
found.
programs. Vibrational frequencies were systematically scaled
down by a factor of 0.89, and thermochemical parameters were
calculated. Electronic correlation was taken into account by
performing MP4(SDTQ) single-point calculations using the
As shown in Table 2, the chlorocyclohexadienyl radical is
much less stable than cyclohexadienyl and hydroxycyclohexa-
dienyl radicals, all values being calculated using the same
method. The values for the cyclohexadienyl and hydroxycyclo-
hexadienyl radicals given in Table 2 are consistent with the
6
-31G(d,p) basis set. In a second step, the Melius BAC
36-38
program
improves the energetical results by empirical
corrections using bond-additivity approximations accounting
for the systematic errors resulting mainly from basis set
truncation. Additional corrections for electron-spin contamina-
tion, related to open-shell and/or unsaturated systems, are also
included.
2
5,40
experimentally measured parameters,
providing confidence
that the thermochemical data reported in Table 2 are reliable.
4
. Conclusions
For the species involved in channels 1a and 1c, the values of
the heats of formation, calculated using the same method, were
A large and self-consistent body of experimental data is
36-38
directly taken from Melius’ results.
The reaction enthalpies
presented which suggests that the gas-phase reaction of Cl atoms
with benzene proceeds via two channels: H atom abstraction
and C6H6-Cl adduct formation. The rate of the abstraction
calculated for each reaction pathway 1a, 1b, and 1c are reported
in Table 2 along with the corresponding experimental values
2
2,23
for channels 1a and 1c, taken from thermochemical tables.
-16
3
-1 -1
channel, k1a ) (1.3 ( 1.0) × 10
cm molecule s , is
We were unable to characterize any stable π-adduct between
Cl atoms and C6H6; the bonding in such a π-adduct is less than
consistent with that expected on the basis of thermochemical
arguments assuming the activation energy is equal to the reaction
endothermicity (see section 3.8). In the liquid-phase reaction,
it has been established that addition of Cl atoms to benzene to
give the C6H6-Cl adduct proceeds rapidly with a rate constant
-
1
1
2 kJ mol . The π-adduct cannot survive more than a few
picoseconds at ambient temperature and is too short-lived to
play any role in the chemical system. The thermodynamic results
shown for reaction 1b in Table 2 are for the σ-adduct.
-11
3
-1 -1 10
of 1.0 × 10 cm molecule s . In view of the size of the
adduct and the presence of 700 Torr of N2 diluent in the present
experiments it seems likely that the addition channel (1b) will
be close to the high-pressure limit and k1b will be comparable
to that measured in the liquid-phase studies. Combining an
-
1
Despite a difference of 15 kJ mol between the calculated
and the experimental values for the hydrogen abstraction
channel, the calculations are consistent with the measured
endothermicity of channels 1a and 1c. This is consistent with
the low rate constant measured for the hydrogen abstraction
reaction. Given the substantial endothermicity of channel 1c
this channel can be ignored. Channel 1b is calculated to be
-
11
3
-1 -1
estimate of k1b ) 1.0 × 10 cm molecule
s
with the fact
that the effective overall rate constant keff for reaction 1 observed
-16
-15
3
-1 -1
here is of the order of 10 -10 cm molecule s , it must
be concluded that the overwhelming fate of the C6H6-Cl adduct
under the present experimental conditions is decomposition back
to reactants. As discussed in section 3.2, the linearity of the
data in Figure 3A,B is also consistent with k-1b . k12[Cl]ss.
-
1
exothermic by 30 kJ mol with an estimated uncertainty of
-
1
(
10 kJ mol ; this result is consistent with the value of ∆H°298
)
-
1
39
-33 kJ mol reported by Ritter et al. The equilibrium
constant, derived from the thermochemical parameters given

Gas-Phase Reaction of Cl Atoms with Benzene
J. Phys. Chem. A, Vol. 102, No. 52, 1998 10681
References and Notes
We show here that the C6H6-Cl adduct reacts with Cl atoms.
From the fact that the chlorobenzene yield tends to zero at high
(1) Slator, A. Z. Physik. Chem. 1903, 45, 540.
[Cl]ss (see Figure 6), we conclude that this reaction does not
(2) Lane, C. E., Jr.; Noyes, W. A., Jr. J. Am. Chem. Soc. 1932, 54,
produce C6H5Cl. At this point the germane question is “What
is the likely nature of the reaction of Cl atoms with the adduct?”
Two products are possible: dichlorocyclohexadiene, or a di-
adduct in which two Cl atoms are bound to the same benzene
molecule. Chemical intuition suggests that the former is more
likely.
1
61.
(
3) Smith, H. P.; Noyes, W. A., Jr.; Hart, E. J. J. Am. Chem. Soc.
1933, 55, 4444.
(
(
(
(
(
4) Russell, G. A.; Brown, H. C. J. Am. Chem. Soc. 1955, 77, 4031.
5) Russell, G. A. J. Am. Chem. Soc. 1957, 79, 2977.
6) Russell, G. A. J. Am. Chem. Soc. 1958, 80, 4987.
7) Russell, G. A. J. Am. Chem. Soc. 1958, 80, 4997.
8) Skell, P. S.; Baxter, H. N., III; Taylor, C. K. J. Am. Chem. Soc.
At room temperature there is a rapid equilibrium between Cl
atoms and the C6H6-Cl adduct, the equilibrium being largely
shifted toward the reactants. Results from experiments and
calculations provide a consistent picture of the instability of
1
983, 105, 120.
(9) Benson, S. W. J. Am. Chem. Soc. 1993, 115, 6969.
(10) Bunce, N. J.; Ingold, K. U.; Landers, J. P.; Lusztyk, J.; Scaiano, J.
-1
C. J. Am. Chem. Soc. 1985, 107, 5464.
the chlorocyclohexadienyl radical, ∆Hdecomp ) 30-33 kJ mol ,
7
-1
(11) Atkinson, R.; Aschmann, S. M. Int. J. Chem. Kinet. 1985, 17, 33.
(
corresponding to k-1b ) 10 s at room temperature. From a
12) Wallington, T. J.; Skewes, L. M.; Siegl, W. O. J. Photochem.
Photobiol. A: Chem. 1988, 45, 167.
13) Nozi e` re, B.; Lesclaux, R.; Hurley, M. D.; Dearth, M. A.; Wall-
combination of experimental results and quantum calculations,
-
18
3
we estimate that Kc,1b ) k1b/k-1b ≈ (1-2) × 10
cm
(
-
1
molecule .
ington, T. J. J. Phys. Chem. 1994, 98, 2864.
The low value of the equilibrium constant indicates that, even
with the high benzene concentrations used in flash photolysis
experiments, the equilibrium is shifted toward the reactants to
a sufficient extent for preventing any transient absorption
corresponding to the chlorocyclohexadienyl radical from being
detected. As far as FTIR experiments are concerned, the low
value of both the equilibrium constant and rate constant for
hydrogen abstraction results in fairly high steady-state concen-
trations of Cl atoms, thus allowing a fraction of the C6H6-Cl
radicals to be scavenged through the fast reaction with Cl atoms.
This results in the determination of effective rate constants and
product yields which both depend on experimental conditions.
This partly explains the discrepancies observed in the literature
concerning this reaction (see Introduction).
(14) Shi, J.; Bernhard, M. J. Int. J. Chem. Kinet. 1997, 29, 349.
(15) Lightfoot, P. D.; Lesclaux, R.; Veyret, B. J. Phys. Chem. 1990,
4, 700.
9
(16) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. Jet Propulsion Laboratory Publication 97-4, Pasadena, CA, 1997.
(17) Rowley, D. M.; Lesclaux, R.; Lightfoot, P. D.; Noziere, B.;
Wallington, T. J.; Hurley, M. D. J. Phys. Chem. 1992, 96, 4889.
(18) Methods in free radical chemistry, Vol 1; Poutsma, M. L., Huyser
E. S., Eds.; Marcel Dekker: New York, 1969.
(19) Walling C. Free radicals in solution; Wiley: New York, 1957.
(20) Wallington, T. J.; Japar, S. M. J. Atmos. Chem. 1989, 9, 399.
(21) Stein, S. E.; Rukkers, J. M.; Brown, R. L. NIST Standard Reference
Database 25, Gaithersburg, MD, 1991.
(22) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.;
McDonald, R. A.; Syveruid, A. N. J. Phys. Chem. Ref. Data 1985, 14,
7
18, 743, and 1211.
23) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin,
R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, 225, 226, and
31.
24) Wallington, T. J.; Egsgaard, H.; Nielsen, O. J.; Platz, J.; Sehested,
J.; Stein, T. Chem. Phys. Lett. 1998, 290, 363.
25) Berho, F.; Rayez, M.-T.; Lesclaux, R. J. Phys. Chem A., submitted
for publication.
26) Ackermann, L.; Hippler, H.; Pagsberg, P.; Reihs, C.; Troe, J. J.
In the presence of oxygen, the effective rate constant has also
been found to be very slow. In this case, discrepancies may
also arise from the formation of the OH radical in the oxidation
process, as already mentioned in the Introduction. It was initially
thought that the C6H6-Cl adduct could be scavenged by the
addition of a large excess of oxygen. It is shown in this work
that this is not the case. Reaction between the adduct and oxygen
proceeds slowly (if at all) and an upper limit of k(C6H6-Cl +
(
2
(
(
(
Phys. Chem. 1990, 94, 5247.
(27) Sauer, M. C., Jr.; Ward, B. J. Phys. Chem. 1967, 71, 3971.
-
17
3
-1 -1
O2) < 8 × 10
cm molecule
s
was established (see
(
(
(
28) Sauer, M.; Mani, I. J. Phys. Chem. 1970, 74, 59.
29) Wallington, T. J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437.
30) Tuazon, E. C.; Atkinson, R.; Corchnoy, S. B. Int. J. Chem. Kinet.
section 3.7).
Low reactivity of the C6H6-Cl adduct with O2 has been
observed in the liquid phase by Skell et al.41 These authors report
a much lower reactivity of the C6H6-Cl radical with O2, than
that of the C6H7 radical, which corroborates the trend observed
1
992, 24, 639.
(31) Sawerysyn, J. P.; Talhaoui, A.; Meriaux, B.; Devolder, P. Chem.
Phys. Lett. 1992, 198, 197.
(32) Westley, F.; Frizzell, D. H.; Herron, J. T.; Hampson, R. F.; Mallard,
W. G. NIST Chemical Kinetics Database Version 6.01, Gaithersburg, MD,
-
14
3
here in the gas phase, since a rate constant of 4 × 10
cm
-
1
-1
25
molecule has been determined recently for the C6H7 +
s
1
994.
-
16
3
O2 reaction. In addition, a low rate constant, <2 × 10
cm
(
(
(
(
33) Catoire, V.; Lesclaux, R. J. Phys. Chem. 1996, 100, 14356.
34) Atkinson, R. J. Phys. Chem. Ref. Data 1989, Monograph No. 1.
35) Yu, T.; Lin, M. C. J. Am. Chem. Soc. 1994, 116, 9571.
-
1 -1
molecule s , has also been reported for the C6H6-OH + O2
reaction.42 The low reactivity of cyclohexadienyl-type radicals
presumably reflects their high resonance stabilization energy
36) Melius, C. F.; Binkley, J. S. 20th Symposium (Intl) on Combustion;
-
1
43
(
100 kJ mol for C6H7) .
The Combustion Institute, Pittsburgh, 575, 1984.
37) Melius, C. F. In Chemistry and Physics of Energetic Materials,
NATO SAI 309, Bulusu, S. N., Ed.; Kluwer Academic: The Netherlands,
990; p 21.
38) Ho, P.; Melius, C. F. J. Phys. Chem. 1990, 94, 5120.
(39) Ritter, E. R.; Bozzelli, J. W.; Dean, A. M. J. Phys. Chem. 1990,
94, 2493.
(
The effective rate constant for reaction of Cl atoms with
benzene in the gas phase is very small and the reaction leads to
several different products. Reaction 1 is not a promising
candidate for initiating the oxidation of benzene in smog
chamber studies of its atmospheric oxidation mechanism.
1
(
(
(
40) Witte, F.; Urbanik, E.; Zetzsch, C. J. Phys. Chem. 1986, 90, 3251.
41) Skell, P. S.; Baxter, H. N., III; Tanko, J. M.; Chebolu, V. J. Am.
Acknowledgment. F.B., M.T.R., and R.L. thank Daniel
Liotard (Universit e´ Bordeaux I) for performing semiempirical
calculations and the French Ministry of Environment for
financial support.
Chem. Soc. 1986, 108, 6300.
(42) Knispel, R.; Koch, R.; Siese, M.; Zetzsch, C. Ber. Bunsen-Ges.
Phys. Chem. 1990, 94, 1375.
(43) Egger, K. W.; Benson, S. W. J. Am. Chem. Soc. 1966, 88, 241.