Kinetics and mechanism of the gas phase reaction of Cl atoms with iodobenzene

Article abstract of DOI:10.1016/S0009-2614(01)01318-5

Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms with iodobenzene (C₆H₅I) in 20-700 Torr of N₂, air, or O₂ diluent at 296 K. The reaction proceeds with a rate constant k(Cl+C₆H₅I)=(3.3±0.7)×10^-11cm ³molecule^-1s^-1 to give chlorobenzene (C₆H₅Cl) in a yield which is indistinguishable from 100%. The title reaction proceeds via a displacement mechanism (probably addition followed by elimination).

Full text of DOI:10.1016/S0009-2614(01)01318-5

2
8 December 2001
Chemical Physics Letters 350 ꢀ2001) 423±426
www.elsevier.com/locate/cplett
Kinetics and mechanism of the gas phase reaction of
Cl atoms with iodobenzene
a
b
c
M.P. Sulbaek Andersen , D.A. Ponomarev , O.J. Nielsen ,
M.D. Hurley *, T.J. Wallington d
d,
a
Department of Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
b
St.Petersburg Forest Technical Academy, 194021 St.Petersburg, Russia
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Ford Research Laboratory, SRL-3083, Ford Motor Company, P.O. Box 2053, Dearborn, MI 48121-2053, USA
c
d
Received 20 August 2001; in ®nal form 11 October 2001
Abstract
Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms with
iodobenzene ꢀC I) in 20±700 Torr of N , air, or O diluent at 296 K. The reaction proceeds with a rate constant
6
H
5
2
2
À11
3
À1 À1
kꢀCl ‡ C
6
H
5
I† ˆ ꢀ3:3 Æ 0:7†  10
cm molecule s to give chlorobenzene ꢀC
guishable from 100%. The title reaction proceeds via a displacement mechanism ꢀprobably addition followed by
6 5
H Cl) in a yield which is indistin-
elimination). Ó 2001 Elsevier Science B.V. All rights reserved.
1
. Introduction
studies of the spectroscopy and kinetics of radical
intermediates formed during the oxidation of ar-
omatic compounds it is often convenient to pro-
duce the radicals via reaction of Cl atoms with
suitable organic precursors. Kinetic and mecha-
nistic data concerning the reaction of Cl atoms
with aromatic compounds are needed to facilitate
the design and interpretation of smog chamber
and ¯ash photolysis studies. We report here the
results of the ®rst kinetic and mechanistic study of
the reaction of Cl atoms with iodobenzene.
A detailed understanding of the atmospheric
chemistry of aromatic compounds is needed for an
accurate assessment of their environmental impact
following release into the atmosphere. Unfortu-
nately, substantial uncertainties exist in our un-
derstanding of the atmospheric oxidation
mechanisms of aromatic species [1]. In smog
chamber studies of the atmospheric degradation
mechanisms of organic compounds it is often
convenient to use Cl atoms to initiate the sequence
of photooxidation reactions. In ¯ash photolysis
2. Experimental
*
Experiments were performed in a 140 l Pyrex
reactor interfaced to a Mattson Sirus 100 FTIR
Corresponding author. Fax: +1-313-594-2923.
E-mail address: mhurley3@ford.com ꢀM.D. Hurley).
0
009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S0009-2614ꢀ01)01318-5

424
M.P. Sulbaek Andersen et al. / Chemical Physics Letters 350 ꢀ2001) 423±426
spectrometer [2]. The optical path length of the
infrared beam was 27 m. The reactor was sur-
rounded by 22 ¯uorescent blacklamps ꢀGE
F40BLB), which were used to generate Cl atoms
Cl ‡ C
2
H
5
Cl ! products
ꢀ2†
ꢀ3†
ꢀ4†
Cl ‡ C H ! products
2
6
by photolysis of Cl
2
. Reactant and product con-
Cl ‡ C
2
H
4
! products
centrations were monitored by Fourier transform
infrared spectroscopy using characteristic absorp-
tion features in the wavenumber range 700±1850
Initial reactant concentrations were 5±10 mTorr of
C
of the three references in 20, 100, or 700 Torr of
6
H
5
I, 100 mTorr of Cl , and 5±10 mTorr of one
2
À1
cm . IR spectra were derived from 32 coadded
interferograms with a spectral resolution of 0.25
either N , or air, diluent. The observed loss of
2
C H I versus that of the reference compounds in
6
5
À1
cm . Reference spectra were acquired by ex-
panding known volumes of reference material into
the chamber. Experiments were performed at 296
the presence of Cl atoms is shown in Fig. 1. As
seen from Fig. 1, there was no observable e€ect of
total pressure ꢀ20±700 Torr) or nature of the dil-
2 2
K in 20±700 Torr of N , air, or O diluent. All
uent gas ꢀN
Linear least-squares analysis of the data in Fig. 1
gives k =k =k ˆ 0:56 Æ 0:05, and
ˆ 3:77 Æ 0:25, k
2
or air) on the kinetics of reaction ꢀ1).
reactants were obtained from commercial sources
at purities >99%. Ultrahigh purity nitrogen and air
diluent gases were used as received. The samples of
1
2
1
3
k =k ˆ 0:39 Æ 0:03, quoted uncertainties are two
1
4
6 5 6 5
C H I and C H Cl were subjected to repeated
standard deviations from the linear regressions.
À12
freeze-pump-thaw cycles before use. In smog
chamber experiments unwanted loss of reactants
and products via photolysis and heterogeneous
reactions have to be considered. Control experi-
ments were performed in which product mixtures
obtained after UV irradiation of C H I=Cl /air
Using k ˆ ꢀ8:04 Æ 0:57†  10
[3], k ˆ ꢀ5:75Æ
2
3
À11
À11
0
cm molecule
:45†  10 [4], and k ˆ ꢀ9:29 Æ 0:51†  10 [5]
4
3
À1 À1
s
we derive k
1
ˆ ꢀ3:03 Æ 0:29† Â
À11
À11
10
, ꢀ3:22 Æ 0:38†  10 , and ꢀ3:62 Æ 0:34† Â
6
5
2
mixtures were allowed to stand in the dark in the
chamber for 15 min. There was no observable
ꢀ
<2%) loss of reactants or products, showing that
heterogeneous reactions or products are not a
signi®cant complication over the time scale of the
present experiments. The IR features used for
À1
analysis were: C H I ꢀ731 cm †, C H Cl ꢀ741
6
5
À1
6
5
À1
À1
cm †, C H ꢀ822 cm †, C H ꢀ949 cm †, C H
2
6
2
4
2
5
À1
Cl ꢀ677; 1288 cm †. Analysis of the IR spectra
was achieved through a process of spectral strip-
ping in which small fractions of the reference
spectrum were subtracted incrementally from the
sample spectrum.
3
. Results and discussion
3
.1. Relative rate study of the Cl ‡ C H I reaction
6
5
in 20, 100 and 700 Torr of N or air
2
Fig. 1. Decay of C
Cl ꢀcircles), C
6
H
5
I versus the reference compounds
ꢀdiamonds) or C ꢀtriangles) in the
C
2
H
5
2
H
6
2 4
H
The kinetics of reaction ꢀ1) were measured rel-
ative to reactions ꢀ2)±ꢀ4):
presence of Cl atoms in 700 ꢀopen symbols), 100 ꢀshaded
symbols) or 20 ꢀsolid symbols) Torr of either air ꢀcross-hair
marked symbols) or N
2
ꢀunmarked symbols) diluent. For
Cl ‡ C H I ! products
ꢀ1†
clari®cation, the initial data range is included as an insert.
6
5

M.P. Sulbaek Andersen et al. / Chemical Physics Letters 350 ꢀ2001) 423±426
425
À11
3
À1 À1
1
®
0
cm molecule s . We choose to cite a
formation of this compound. The feature at 685
À1
nal value for k which is the average of
1
cm is present in spectra for both C H Cl and
5
6
those determined using the three di€erent ref-
erence compounds together with error limits
which encompass the extremes of the indi-
6 5
C H I. A small unidenti®ed product feature was
À1
observed at 749 cm . C H Cl was the only car-
6
5
bon-containing product identi®ed.
Fig. 3 shows the observed formation of C H Cl
vidual determinations. Hence, k ˆ ꢀ3:3 Æ 0:7† Â
1
6
5
À11
3
À1 À1
1
0
cm molecule s .
versus the loss of C H I following UV irradiation
6 5
of C H I=Cl mixtures in 700 Torr of either O or
2
6
5
2
3
.2. Product study Cl ‡ C
6
H
5
I in 700 Torr N
2
or O
2
N diluent. As seen from Fig. 3, there was no
2
discernable di€erence between the results obtained
in O and N diluent. The straight line in Fig. 3 is a
To investigate the products of the reaction of Cl
2
2
atoms with C I, mixtures of 5.4±15.2 mTorr
I and 25±100 mTorr Cl in 700 Torr of either
N , or O , diluent were introduced into the reac-
6
H
5
linear least-squares ®t which gives a ꢀ101 Æ 7†%
molar yield of chlorobenzene in 700 Torr of either
N and O . Quoted errors are two standard devi-
C
6
H
5
2
2
2
2
2
tion chamber and irradiated using the UV black-
lamps. Consumptions of C H I were in the range
ations from the regression analysis. We estimate
that possible systematic errors associated with
uncertainties in the calibration of the reference
spectra for C H I and C H Cl combine to give an
6
5
3
±90%. Fig. 2 shows spectra acquired before ꢀA)
and after ꢀB) a 110 s irradiation of a mixture of 8.1
mTorr C I and 25 mTorr Cl in 700 Torr of O
The consumption of C I was 33%. Comparison
6
5
6
5
6
H
5
2
2
.
6 5
additional 10% uncertainty in the C H Cl product
6
H
5
yield. Within the experimental uncertainties, the
observed formation of C H Cl accounts for 100%
of the loss of C H I.
of the IR features in panel B with the reference
spectrum of C H Cl given in panel C shows the
6
5
6
5
6
5
The reaction of Cl atoms with iodobenzene can
proceed either via abstraction or displacement.
Abstraction would give a phenyl radical and ICl
as initial products. Displacement would give
Fig. 2. IR spectra obtained before ꢀA) and after ꢀB) 110 s of
irradiation of a mixture of 8.1 mTorr C I and 25 mTorr Cl
in 700 Torr of O . Panel C is reference spectrum of C Cl.
6
H
5
2
Fig. 3. Yield of C
6
H
5
Cl versus loss of C
6 5
H I in 700 Torr of
2
6
H
5
either N ꢀ®lled symbols) or O
2
2
ꢀopen symbols).

426
M.P. Sulbaek Andersen et al. / Chemical Physics Letters 350 ꢀ2001) 423±426
À11
3
À1 À1
s at 296 K in
chlorobenzene and an I atom. In 700 Torr of N
diluent the fate of the phenyl radical will be the
reaction with molecular chlorine to give chloro-
2
ꢀ3:3 Æ 0:7†  10
cm molecule
diluent. The mechanism
20±700 Torr of air or N
2
of the reaction is rather unusual for a gas-phase
reaction involving Cl atoms. The reaction proceeds
benzene and a chlorine atom. In 700 Torr of O
diluent the phenyl radical will add O rapidly to
2
2
via a displacement mechanism to give C
the sole carbon-containing compound.
6 5
H Cl as
give a phenyl peroxy radical [6] which, in turn, will
react to give 4-phenoxyphenol via formation of a
phenoxy radical [7]. From the fact that chloro-
benzene is observed as a product in essentially
Acknowledgements
1
conclude that the reaction of Cl atoms with C
00% yield in the absence and presence of O
2
we
I
6
H
5
We thank Christian Lohse ꢀUniversity of
Southern Denmark) for helpful discussions.
occurs via a displacement mechanism in which the
incoming Cl atom displaces the I atom. In light of
the known propensity of Cl atoms to form short-
lived adducts with aromatic compounds [8,9] it
seems reasonable to speculate that the reaction
mechanism probably proceeds via the formation
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Oxford University Press, Oxford, 2001.
of the C
6
H
5
I±Cl adduct which then decomposes to
give C H
6
5
Cl ‡ I.
[
[
[
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Wahner and Zetzsch [11] have speculated that a
similar mechanism may play a role in the reaction
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venient source of phenyl radicals for laboratory
study.
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[
[
1
1328.
11] A. Wahner, C. Zetzsch, J. Phys. Chem. 87 ꢀ1983)
945.
3
.3. Conclusions
4
It is shown here that the reaction of Cl atoms
with C H I occurs with a rate constant of
6
5