Kinetics and mechanism of the reaction of Cl atoms with nitrobenzene

Article abstract of DOI:10.1021/jp002696o

The kinetics and mechanism of the reaction of Cl atoms with nitrobenzene in 10-700 torr of N₂, or air, at 296 K were studied using smog chamber/FTIR methods. The reaction of Cl atoms with nitrobenzene proceeded with a rate constant of 9.3 × 10^-13 cc/molecule-sec to give C₆H₅Cl and NO₂ products in essentially 100% yield. The UV-visible spectrum of nitrobenzene was measured. The dominant atmospheric fate of nitrobenzene was photolysis, which occurred at a rate of 3 × 10^-5/sec for a solar zenith angle of 25° representative of a typical summer day at 40°N.

Full text of DOI:10.1021/jp002696o

11328
J. Phys. Chem. A 2000, 104, 11328-11331
Kinetics and Mechanism of the Reaction of Cl Atoms with Nitrobenzene
L. Frøsig, O. J. Nielsen,† and M. Bilde
UniVersity of Copenhagen, Department of Chemistry, KL5, UniVersitetsparken 5, 2100 KøbenhaVn Ø, Denmark
T. J. Wallington*,‡
Ford Motor Company, 20000 Rotunda DriVe, Mail Drop SRL-3083, Dearborn, Michigan 48121-2053
J. J. Orlando§ and G. S. Tyndall
Atmospheric Chemistry DiVision, National Center for Atmospheric Research, Boulder, Colorado 80303
ReceiVed: July 27, 2000; In Final Form: September 29, 2000
Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms
with nitrobenzene (C6H5NO2) in 10-700 Torr of N2, or air, at 296 K. The reaction proceeds with a rate
constant k(Cl + C6H5NO2) ) (9.3 ( 1.9) × 10-13 cm3 molecule-1 s-1 to give C6H5Cl and NO2 products in
essentially 100% yield. The observed product yields suggest that the reaction proceeds via a displacement
mechanism (probably addition followed by elimination). The UV-visible absorption spectrum of C6H5NO2
was measured. Photolysis of C6H5NO2 is estimated to occur at a rate of (3 ( 2) × 10-5 s-1 for a solar zenith
angle of 25° (representative of a typical summer day at 40°N) and is likely to be the dominant atmospheric
loss mechanism for C6H5NO2.
Introduction
ferograms with a spectral resolution of 0.25 cm-1. Reference
spectra were acquired by expanding known volumes of reference
materials into the chamber. All experiments were performed at
296 K. All reactants were obtained from commercial sources
at purities >99%. The samples of C6H5NO2 and C6H5Cl were
subjected to repeated 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 experiments were performed in which
product mixtures obtained after the UV irradiation of C6H5-
NO2/Cl2/air 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 are
not a significant complication.
A detailed understanding of the atmospheric chemistry of
aromatic compounds is needed for an accurate assessment of
their environmental impact following release into the atmo-
sphere. Unfortunately, substantial uncertainties exist in our
understanding of the atmospheric oxidation mechanisms of
aromatic species.1 In smog chamber studies of the atmospheric
degradation mechanism of organic compounds it is often
convenient to use Cl atoms to initiate the sequence of photo-
oxidation reactions. To facilitate the design and interpretation
of such experiments, kinetic and mechanistic data concerning
the reaction of Cl atoms with aromatic compounds are needed.
We report here the results of a kinetic and mechanistic study
of the reaction of Cl atoms with nitrobenzene in 10-700 Torr
of N2 or air diluent at 296 K. In addition, the UV spectrum and
approximate photolysis quantum yield for nitrobenzene in the
UV are reported, and these data are used to estimate the
atmospheric photolysis rate of this species.
UV-Visible Absorption Spectrum Measurements at
NCAR. The UV absorption spectrum of C6H5NO2 was mea-
sured over the range 210-400 nm, using a diode array
spectrometer system described in detail elsewhere.3 The system
consisted of a 90-cm long Pyrex absorption cell equipped with
quartz windows. The output from a deuterium lamp was
collimated through the absorption cell, focused onto the entrance
slit of a 0.3 m Czerny-Turner spectrograph, and dispersed onto
a 1024-element diode array detector. The spectrograph was
equipped with a 300 grooves mm-1 grating, and provided
spectra with a resolution of 0.6 nm. The wavelength scale of
the spectrometer system was calibrated using emission lines
from a low-pressure Hg Penray lamp. Spectra were obtained
from the summation of 100 exposures of the diode array, each
of 0.15 s duration. Spectra were measured both with the cell
evacuated (Io) and with the cell filled with a gaseous sample
(I) and converted to absorbance (A), where A ) ln(Io/I).
Experimental Section
FTIR Smog Chamber System at Ford. Experiments were
performed in a 140 L Pyrex reactor interfaced to a Mattson Sirus
100 FTIR spectrometer.2 The optical path length of the infrared
beam was 27 m. The reactor was surrounded by 22 fluorescent
blacklamps (GE F15T8-BL), which were used to generate Cl
atoms by photolysis of Cl2. Reactant and product concentrations
were monitored by Fourier transform infrared spectroscopy using
characteristic absorption features in the wavenumber range 700-
1850 cm-1. IR spectra were derived from 32 coadded inter-
* Author to whom correspondence should be addressed.
† E-mail: ojn@kiku.dk.
C6H5NO2 liquid was obtained from Aldrich Chemical Co. at
a stated purity of >99%. Gas-phase C6H5NO2 samples were
obtained by expanding the vapor above a liquid sample into
‡ E-mail: twalling@ford.com.
§ E-mail: orlando@acd.ucar.edu.
10.1021/jp002696o CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/05/2000

Reaction of Cl Atoms with Nitrobenzene
J. Phys. Chem. A, Vol. 104, No. 48, 2000 11329
Figure 1. Decay of C6H5NO2 versus the reference compounds (CH3-
Cl or CH3OCHO) in the presence of Cl atoms in 700 (circles) or 10
Torr (triangles) total pressure of either air (filled symbols) or N2 (open
symbols) diluent.
the absorption cell. Before use, the liquid C6H5NO2 sample was
purified by multiple freeze-pump-thaw cycles. Purified samples
were transferred to the absorption cell, and absorbance spectra
were recorded at room temperature (296 ( 2 K). The absorption
cell was purged with a small flow of N2 between measurements.
Figure 2. IR spectra obtained before (A) and after (B) 80 s of
irradiation of a mixture of 4.7 mTorr C6H5NO2 and 100 mTorr Cl2 in
700 Torr of air. Panels C and D are reference spectra of C6H5Cl and
NO2, respectively.
C6H5NO2 were in the range 5-50%. Figure 2 shows spectra
acquired before (A) and after (B) an 80 s irradiation of a mixture
of 4.7 mTorr C6H5NO2 and 100 mTorr Cl2 in 700 Torr of air.
Comparison of the IR features in panel B with reference spectra
of C6H5Cl and NO2 given in panels C and D shows the
formation of these compounds. The product features in panel
B at 795, 1265, 1315, 1685 (partly hidden at the left side of the
broad feature at 1715 cm-1), and 1715 cm-1 are due to
formation of ClNO2 and ClONO. C6H5Cl was the only carbon-
containing product observed.
Figure 3 shows plots of the observed formation of C6H5Cl
and NO2 versus the loss of C6H5NO2 following UV irradiation
of C6H5Cl/Cl2 mixtures in N2 or air diluent. The straight lines
in Figure 3 are linear least-squares fits to the data. The curved
line through the NO2 data in panel A is a second-order fit to
the data to aid in visual inspection of the data trend. We ascribe
the curved nature of the product plot for NO2 at 700 Torr (Figure
3A) to loss of NO2 via reaction with Cl atoms to give the two
isomers ClNO2 and ClONO:6
Results
Relative Rate Study of the Cl + C6H5NO2 Reaction in 10
and 700 Torr of N2/Air. The kinetics of reaction 1 were
measured relative to reactions 2 and 3:
Cl + C6H5NO2 f products
Cl + CH3Cl f products
(1)
(2)
(3)
Cl + CH3OCHO f products
Initial concentrations were 1.8-3.0 mTorr of C6H5NO2, 100
mTorr of Cl2, and 6-24 mTorr of the two references in 10 or
700 Torr of either N2, or air, diluent. The observed loss of C6H5-
NO2 versus those of the reference compounds in the presence
of Cl atoms is shown in Figure 1. As seen from Figure 1 there
was no observable effect of total pressure over the range 10-
700 Torr, or nature of the diluent gas (air or N2). Linear least-
squares analysis of the data in Figure 1 gives k1/k2 ) 1.81 (
0.17 and k1/k3 ) 0.71 ( 0.06 with the quoted uncertainties being
( two standard deviations. Using k2 ) 4.8 × 10-13 4 and k3 )
1.4 × 10-12 cm3 molecule-1 s-1 5 we derive k1 ) (8.67 ( 0.81)
× 10-13 and (9.95 ( 0.84) × 10-13 cm3 molecule-1 s-1. We
estimate that the potential systematic errors associated with the
uncertainties in the reference rate constants add 10% uncertainty
range for k1. Propagating this additional uncertainty gives k1 )
(8.67 ( 1.19) × 10-13 and (9.95 ( 1.30) × 10-13 cm3
molecule-1 s-1. We choose to cite a final value for k1 which is
the average of those determined using two different reference
compounds together with the error limits which encompass the
extremes of the individual determinations. Hence, k1 ) (9.3 (
Cl + NO2 + M f ClNO2/ClONO + M
(4)
Reaction 4 is an association reaction and has a rate which
increases with total pressure from k4 ) 4 × 10-13 at 10 Torr to
k4 ) 2 × 10-11 cm-3 molecule-1 s-1 at 700 Torr.4 Chlorine
atoms react with ClNO2 with a rate constant of 5.5 × 10-12
cm-3 molecule-1 s-1 (independent of total pressure) to regener-
ate NO2.7 Reaction 4 represents a substantial loss of NO2 for
experiments conducted at high pressure but is much less signif-
icant at lower pressures. Simulations using the Acuchem chem-
ical kinetic program8 confirmed that the curvature of the NO2
yield in Figure 3A is consistent with expectations based upon
the discussion above. The reaction of Cl atoms with C6H5Cl
proceeds extremely slowly and is not important in this work.9
Linear least-squares analysis of the data in Figure 3 gives molar
yields of chlorobenzene at 700 and 10 Torr of 113 ( 6% and
107 ( 8%, respectively, and a 100 ( 9% yield of NO2 at 10
Torr total pressure. Quoted errors are two standard deviations
from the regression analysis. Possible systematic errors associ-
1.9) × 10-13 cm3 molecule-1 s-1
.
Product Study Cl + C6H5NO2 in 10-700 Torr N2 or Air.
To investigate the products and mechanism of the reaction of
Cl atoms with C6H5NO2, mixtures of 4.7-10.0 mTorr C6H5-
NO2 and 100 mTorr Cl2 in either 10 or 700 Torr of either air or
N2 diluent were introduced into the reaction chamber and
irradiated using the UV blacklamps. Typical consumptions of

11330 J. Phys. Chem. A, Vol. 104, No. 48, 2000
Frøsig et al.
Figure 4. UV-visible spectrum (base e) of C6H5NO2 in the gas phase
(thick line, this work) and in liquid hexane (thin line).1 The circles
show the absorption spectrum of Cl2.4
cm2 molecule-1 (base e) at 300-340 nm. The spectrum
measured here is very similar to that reported by Galloway et
al.,15 though our cross sections appear to be 15-20% lower.
The gas-phase UV spectrum of C6H5NO2 is very similar in shape
to that reported in liquid hexane,1 solvent effects result in an
approximate 20 nm red shift of the C6H5NO2 spectrum in
hexane.
To determine a first-order photolysis rate (j) for C6H5NO2 in
the atmosphere, wavelength-dependent cross section, σ(λ),
quantum yield, φ(λ), and actinic flux, I(λ), data are required:
Figure 3. Yield of C6H5Cl (squares) and NO2 (circles) versus loss of
C6H5NO2 in 700 (A) or 10 (B) Torr of either N2 (open symbols) or air
(filled symbols).
j ) σ(λ)φ(λ)I(λ)dλ
∫
λ
To the best of our knowledge, no gas-phase photolysis quantum
yield data are available for C6H5NO2. To provide information
concerning φ(λ), experiments were performed using the atmo-
spheric reaction chamber at NCAR16 in which mixtures of C6H5-
NO2 in either 700 Torr N2 or 250 Torr N2 and 450 Torr O2
were irradiated using the output of a Xe arc lamp with a 7-54
filter allowing transmission of 250-420 nm light. The gas
mixtures were left to stand in the chamber for 2.5 h during which
time the Xe lamp was switched on for 96 min. A small but
discernible loss of C6H5NO2 was observed, corresponding to a
pseudo first-order rate of approximately 1.5 × 10-5 s-1 in the
dark and 3 × 10-5 s-1 with UV irradiation. There was no
observable difference in behavior in experiments in N2 or N2/
O2 diluent. NO2 was observed as a photolysis product with yields
of about 50% relative to the amount of C6H5NO2 that was
photolyzed. No NO2 was observed in the absence of UV
irradiation. Molecular beam studies12 have shown that three
pathways are operative in the near-UV photolysis of C6H5NO2,
producing either NO2, NO, or O atoms. The large but nonunity
NO2 yield observed here is consistent with the molecular beam
findings.
ated with uncertainties in the calibration of the reference spectra
for C6H5NO2, C6H5Cl, and NO2 combine to give an additional
approximately 10% uncertainty in the molar yields for C6H5Cl
and NO2. Within the experimental uncertainties the observed
formation of C6H5Cl and NO2 accounts for 100% of the loss of
C6H5NO2. The simplest interpretation of the observed product
yields is that the reaction of Cl atoms with C6H5NO2 occurs
via a displacement mechanism in which the incoming Cl atom
displaces the NO2 group. In light of the known propensity of
Cl atoms to form short-lived adducts with aromatic com-
pounds9-13 it seems reasonable to speculate that the reaction
mechanism probably proceeds via the formation of the C6H5-
NO2-Cl adduct which then decomposes to give C6H5Cl + NO2:
Wahner and Zetzsch14 have speculated that a similar mechanism
may play a role in the reaction of OH radicals with chlorinated
aromatics with decomposition of the adduct proceeding in part
via loss of a Cl atom.
UV-Visible Absorption Spectrum of C6H5NO2. The
spectrum of C6H5NO2 was recorded at NCAR over the
wavelength range 225-400 nm. As seen in Figure 4, the
spectrum of C6H5NO2 in the region of relevance for tropospheric
chemistry (>295 nm) consists of a broad featureless shoulder
with an absorption cross section of approximately 3.5 × 10-19
Although the slow photolysis rate observed in the presence
of a comparable wall loss rate makes quantitative analysis
difficult, we conclude that the C6H5NO2 photolysis rate in the
chamber is approximately (1.5 ( 1.0) × 10-5 s-1. The circles
in Figure 4 show the literature data for absorption cross sections
of Cl2.4 By chance, there is a reasonable match between the
absorption cross sections of Cl2 and C6H5NO2 in the range 320-
400 nm, while below 320 nm the C6H5NO2 absorption is
substantially greater than that of Cl2. Calculations were per-

Reaction of Cl Atoms with Nitrobenzene
J. Phys. Chem. A, Vol. 104, No. 48, 2000 11331
formed to quantify the relative overlap of the lamp emission
spectrum with the absorption spectra of Cl2 and C6H5NO2. The
ratio of the overlap integrals is 0.5 (i.e., if the photolysis
quantum yield was unity for both species, C6H5NO2 would
photolyze twice as fast as Cl2 in the chamber). The measured
photolysis rate of Cl2 in the chamber is 5 × 10-4 s-1. From
three pieces of information, (i) the ratio of the observed C6H5-
NO2 and Cl2 photolysis rates in the chamber (approximately
0.03), (ii) the calculated ratio of overlap integrals (0.5), and
(iii) the unity photolysis quantum yield of Cl2,4 we estimate
that the photolysis quantum yield for C6H5NO2 in the wave-
length range 250-420 nm is approximately 0.015.
Adopting φ(λ) ≈ 0.015 for all wavelengths and using actinic
flux data from Finlayson-Pitts and Pitts,17 a photolysis rate of
3 × 10-5 s-1 is obtained for C6H5NO2 for a solar zenith angle
of 25° (representative of a typical summer day at 40°N),
corresponding to a photolysis lifetime of about 10 h, with an
uncertainty of about a factor of two to three. C6H5NO2 reacts
slowly with OH radicals and ozone (kOH ) 1.40 × 10-13, kO3
< 7 × 10-21 cm3 molecule-1 s-11), for typical atmospheric
concentrations of [OH] ) 1 × 106 and [O3] ) 1.2 × 1012 cm-3
(50 ppb) the lifetime of C6H5NO2 with respect to reaction with
OH and O3 is 80 days and >4 years, respectively. It seems likely
that photolysis is the dominant atmospheric loss of C6H5NO2.
At this point it should be noted that the photolysis experiments
described here employed a range of UV wavelengths (250-
420 nm) which extend beyond the tropospheric cutoff of 290
nm. In fact, approximately 1/4 of the absorption by C6H5NO2
in the chamber occurs below 290 nm, which may lead to an
overestimate of the quantum yield in the actinic region of the
spectrum and thus an overestimate of the atmospheric photolysis
rate. However, we believe it unlikely that the magnitude of such
an overestimation would be sufficient to change the conclusion
that photolysis dominates the atmospheric chemistry of C6H5-
NO2.
molecule-1 s-1 at 296 K in 10-700 Torr of air or N2 diluent.
The mechanism of the reaction is unusual for a gas-phase
reaction involving Cl atoms. The reaction proceeds essentially
100% via a displacement mechanism to give C6H5Cl and NO2
products. The UV-visible spectrum of C6H5NO2 has been
measured. The dominant atmospheric fate of C6H5NO2 is
photolysis which is estimated to occur at a rate of (3 ( 2) ×
10-5 s-1 for a solar zenith angle of 25° representative of a typical
summer day at 40°N).
Acknowledgment. L.F. thanks the Danish Research Acad-
emy for a grant to support this work. The National Center for
Atmospheric Research is operated by the University Corporation
for Atmospheric Research under the sponsorship of the National
Science Foundation.
References and Notes
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Arey et al.18 measured a photolysis quantum yield of φ(λ) )
3.5 × 10-3 for 2-methyl-1-nitronaphthalene under atmospheric
conditions. Even this lower quantum yield (an order of
magnitude lower than our estimate for nitrobenzene) would be
sufficient to allow photolysis to dominate the loss of nitroben-
zene. As with the other nitroaromatic compounds studied to date
(1- and 2-nitronaphthalene,19,20 and 2-methyl-1-nitronaphtha-
lene),18 photolysis dominates the atmospheric chemistry of C6H5-
NO2.
(16) Shetter, R. E.; Davidson, J. A.; Cantrell, C. A.; Calvert, J. G. ReV.
Sci. Instrum. 1987, 58, 1427.
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Conclusions
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It is shown here that the reaction of Cl atoms with C6H5NO2
occurs with a rate constant of (9.3 ( 1.9) × 10-13 cm3