Rate coefficients and Arrhenius parameters for the reactions between
chloroethenes and the nitrate radical
Ingse M. W. Noremsaune,a* Sarka Langer,b Evert LjungstroŽ mb and Claus J. Nielsena*
a Department of Chemistry, University of Oslo, P. O. Box 1033 Blindern, N-0315 Oslo, Norway
b Department of Inorganic Chemistry, University of GoŽ teborg and Chalmers University of
T echnology, S-412 96 GoŽ teborg, Sweden
Rate coefficients for the reactions between NO and the chloroethenes have been determined by the fast-Ñow-discharge technique
3
and by the relative rate method in a static reactor employing FTIR detection. The relative rate experiments were performed at
298 ^ 2 K and 1013 ^ 3 mbar in a nitrogen atmosphere with excess ethane as a chlorine scavenger. The Ñow tube experiments
were carried out under pseudo-Ðrst-order conditions in NO . The temperature dependence of the NO reactions with chloroe3
3
thene and trichloroethene was investigated over a temperature range of ca. 100 K, and their rate coefficients were Ðtted to
Arrhenius expressions. A comparison between our measured rate coefficient data for the NO reaction with chloroethenes reveals
3
that the relative rate results are up to 20% lower than those from the fast-Ñow-discharge experiments. Possible secondary
reactions in the Ñow tube are discussed.
The nitrate radical and its night-time chemistry in the atmosphere have received much attention during the past two
decades. Indeed, the nitrate radical is able to oxidize a wide
variety of organic compounds, thereby providing a chemical
scavenging mechanism for atmospheric constituents of both
natural and antropogenic origins. However, these oxidation
reactions can lead to the formation of reactive species that
may inÑuence the local environment. In other cases long-lived
reaction products may have an impact on atmospheric processes by long-range transport. Consequently, it is relevant to
know reaction rates and oxidation mechanisms, and extensive
studies have been performed in many laboratories to gain this
knowledge.1,2
The chloroethenes are widely used as solvents and intermediates in the manufacture of polymers. They are toxic and
volatile and may be of major concern in local industrial areas.
The chloroethenes react with the nitrate radical, and the
mechanism for these oxidation reactions has recently been
investigated.3 The major products in these reactions are substituted acid chlorides, formaldehyde, formyl chloride, phosgene, dinitrates and in addition, sometimes chlorine atoms.
The only long-lived reaction product is phosgene, which may
be transported to the stratosphere where photolysis will lead
to the release of both chlorine atoms.4
The room-temperature rate coefficients for the reactions
between the nitrate radical and the chloroethenes have previously been presented by Atkinson et al.5 who used a relative
rate method with ethene as the reference compound. The
published rate coefficients may be subject to signiÐcant uncertainties due to the low reactivity and the potential for production of chlorine atoms from the more chlorinated
haloalkenes.1 Anderson and LjungstroŽm have determined the
rate coefficient for chloroethene (vinyl chloride) by measuring
the enhanced decay of N O in the presence of vinyl chlo2 5
ride.6 Their value for the rate coefficient is rather uncertain
due to the N O ¢ NO ] NO equilibrium constant which
2 5
3
2
is strongly temperature dependent and has an estimated
uncertainty of 1.3 at 296 K.7 Recently, the discharge-Ñow
technique was employed to determine the Arrhenius parameters for the reactions of the nitrate radical with 1,1-dichloroethene and (E)-1,2-dichloroethene.8 The agreement was good
between rate coefficients obtained by the discharge Ñow and
relative rate methods at room temperature. This was inter-
preted as an indication of Cl-atom elimination being unimportant in these reactions.
The present work is the Ðrst investigation of the temperature dependence of the nitrate radical reactions with vinyl
chloride and trichloroethene. The work also includes determination of the rate coefficients for all the chloroethenes by both
an absolute rate and a relative rate method at room temperature. For both methods the problems connected with possible secondary reactions are addressed.
Experimental
The absolute rate coefficients and their temperature dependence were measured under pseudo-Ðrst-order conditions
using a fast-Ñow-discharge (FFD) technique where NO was
3
detected by optical absorption in a multi-path cell (GoŽteborg).
This conventional FFD system was discussed in detail in a
previous publication.9 Nitrate radicals were produced according to reaction (1) by mixing nitric acid with Ñuorine atoms,
generated by passing F in He through a microwave dis2
charge. The dissociation of F was close to 100%, and a
2
massive excess of HNO was immediately added to remove all
3
atomic Ñuorine.
F ] HNO ] NO ] HF
3
3
(1)
The nitrate radicals were added at the upstream end of the
Ñow tube with helium as the carrier gas. The radicals were
detected by optical absorption at 662 nm using a multi-pass
cuvette with a total optical path length of 72 cm. The practical
nitrate radical detection limit was ca. 5 ] 1011 molecules
cm~3. All the chloroethenes, except vinyl chloride, are liquids
at STP, and they were introduced as gases from a low pressure distillation set-up. Vinyl chloride was taken directly from
the lecture bottle. The gases were introduced into the Ñow
tube through a sliding injector, the position of which is proportional to the contact time. The contact time could also be
changed by a throttling stopcock between the pump and the
Ñow tube, and contact times of 10 to 400 ms were achieved.
The Ñow tube could be temperature regulated from 266 to 367
K by passing a waterÈethylene glycol mixture through the
Ñow tube jacket.
J. Chem. Soc., Faraday T rans., 1997, 93(4), 525È531
525
The relative rate (RR) experiments were carried out in a
250 l stainless-steel reactor (Oslo). The reactor was equipped
with a multiple reÑection White type mirror system with an
optical base path of 2 m and adjusted to give a total optical
path of 120 m. The optical system was connected to a Bruker
IFS 88 FTIR spectrometer, allowing in situ analysis of intermediates and end products. Spectra were generally recorded
from the time of mixing and to a maximum of 2 h by coadding 50 scans (collection time ca. 45 s) at 0.5 cm~1 instrumental resolution.
The RR experiments were performed at 298 ^ 2 K and
1013 ^ 3 mbar in a nitrogen atmosphere. Nitrate radicals
were generated by thermal dissociation of N O , that was
2 5
added to the chamber by evaporation of solid N O . Mixing
2 5
ratios of the organic reactants were in the range 12È30 ppm in
the presence of ca. 15 ppm N O . The decrease in reactant
2 5
concentration during the reactions was either determined by
subtraction of reference spectra of the pure compounds or by
subtraction employing the Ðrst recorded spectrum as a reference. A linear relationship between absorption and concentration was conÐrmed by separate calibrations of each reactant.
Ethane was sometimes added to the reaction chamber in sufficient excess (300È450 ppm) to scavenge any Cl atoms generated and thereby counteract the e†ect a possible elimination
of Cl atoms might have on the total loss of organics.3 Additional experiments were carried out at low pressure and an
oxygen content of less than 100 ppmv to mimic the Ñow tube
conditions where the oxygen content is estimated to be
around 5 ppmv or less.
All compounds employed in the FFD and RR experiments
had a stated purity of 98% or better and were used without
further puriÐcation : vinyl chloride (Norsk Hydro), ethene, 1,1dichloroethene, trichloroethene, (Z)-1,2-dichloroethene and
tetrachloroethane (Fluka), (E)-1,2-dichloroethene (Aldrich), N
2
bath gas (grade 2.6, AGA). Helium (grade 5.5, AGA) was
passed through a liquid-N trap with 4 Ó molecular sieves to
2
remove traces of oxygen. Fluorine, (5% in He, MesserGriesheim) was used as supplied. Anhydrous, gaseous HNO
3
in a He carrier was prepared by bubbling He through anhydrous HNO , that was distilled from a 2 : 1 mixture of conc.
3
H SO and HNO .
2 4
3
valid :
) \ k@t
(I)
/*
NO3 NO3`org
The Ðrst-order rate coefficients, k@, for the loss of NO were
3
obtained from least-squares adjusted straight lines according
) vs.
to eqn. (I). Fig. 1(a) and (b) show plots of ln(* /*
NO3 NO3`org
time for various concentrations of vinyl chloride and trichloroethene, respectively. The plots are linear but with non-zero
intercept since it is not easy (nor necessary) to deÐne time
zero. However, the chloroethene reactions are fairly slow, and
a small concentration of a fast reacting impurity will also give
intercepts without altering the slope.
The slopes from unit-weighted least-squares plots of k@ vs.
[chloroethene] yield the apparent bimolecular rate coefficients, k, for the reactions :
ln(*
k products
NO ] CClX1 \ CX2X3 ÈÈÕ
3
X1h3 \ H or Cl
(2)
Examples of second-order plots are shown in Fig. 2(a) and (b)
and Table 1 gives a survey of the determined rate coefficients
at room temperature together with some additional experi-
Results
Fast Ñow discharge measurements
The reaction rates between the nitrate radical and the chloroethenes were investigated under pseudo-Ðrst-order conditions, that is, with the organic reactant being in sufficient
excess. The experiments consisted of measuring values for
*
and *
as a function of contact time. Here, *
is
NO3`org
NO3
NO3
the change in transmittance signal between the case when
NO is produced in the system and the case when no NO is
3
3
produced. Similarly, *
is the change in signal between
NO3`org
the case when both NO and the organic substance are added
3
as compared to the case when no NO is present. For small
3
absorbances and pseudo-Ðrst-order conditions eqn. (I) is
Fig. 1 (a) Pseudo-Ðrst-order plots for the reaction of NO with
3 1015,
various concentrations of vinyl chloride at 295 K. (…) 1.20 ]
(L) 2.91 ] 1015, (=) 4.78 ] 1015, (K) 7.62 ] 1015. (b) Pseudo-Ðrstorder plots for the reaction of NO with various concentrations of
3 ] 1014, (L) 1.46 ] 1015, (=)
trichloroethene at 295 K. (…) 5.48
2.68 ] 1015, (K) 4.90 ] 1015.
Table 1 Experimental conditions employed in measuring the rate coefficients for the reactions between chloroethenes and NO by fast Ñow
3
discharge at 295 K ; [NO ] O 1.5 ] 1013 molecule cm~3
3 0
compound
p/mbar
[org]/1015
molecule cm~3
v/cm s~1
vinyl chloride
1,1-dichloroethene
(Z)-1,2-dichloroethene
(E)-1,2-dichloroethene
trichloroethene
tetrachloroethene
5
3.6È5
1.2È8.0
0.84È7.3
547È630
615È972
6
5
8
1.2È28
0.55È4.9
0.64È2.1
262È390
514È652
280È308
a Errors quoted with 95% conÐdence in the slope of the plots k@ vs. [org]. b Not determined. c Determined at 292 K.
526
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
k /10~16 cm3
org
molecule~1
s~1 a
4.26 ^ 0.39
18.8 ^ 1.2
b
0.90 ^ 0.14c
4.43 ^ 0.32
0.96 ^ 0.81
Fig. 3 Second-order plots for the reaction of NO with tetra3
chloroethene at 295 K. The error bars show the 95% conÐdence in the
) vs. time.
slope of the plots ln(* /*
NO3 NO3`org
Fig. 2 (a) Second-order plots for the reaction of NO with vinyl
3 295 K, (=)
chloride at various temperatures. (…) T \ 280 K, (L) T \
T \ 325 K, (K) T \ 367 K. (b) Second-order plots for the reaction of
NO with trichloroethene at various temperatures. (…) T \ 295 K,
(L) 3T \ 328 K, (=) T \ 343 K, (K) T \ 367 K.
mental information. The rate coefficient for (Z)-1,2-dichloroethene was not studied for costÈbeneÐt reasons. The positive
intercepts that were observed in all the second-order plots, are
not easily explained. Mathematically, the rate coefficient for
wall loss does not appear in eqn. (I), because the loss of NO
3
with reactant is normalized to that without reactant. Thus the
intercept of the second-order plots cannot be interpreted as
wall loss of NO . (The wall loss rate coefficient of NO in the
3
3
Ñow tube was measured to be 0.5 s~1 or less.10)
The requirements for a process causing the intercepts in the
second-order plots are quite demanding. The process has to
be quite insensitive to the concentration of organic compound
over the range where measurements are made. However, it has
to be initiated by the addition of organics and cannot be
active when only NO is observed. The process must also be
3
time dependent so that more NO is lost at longer contact
3
times. In fact, it has to be close to Ðrst order in NO .
3
One possible process causing an intercept could be adsorption of the organic compound on the wall, followed by NO
3
reaction. The surface concentration must not change with the
gas-phase concentration of the organic compound, and thus
the available sites have to be saturated even at the lowest concentration of organic compound used. The limited axial di†usion would restrict the surface covered approximately to the
surface downstream of the injector. This would fulÐl the
requirements of a contact time dependent process. From our
available information, we cannot say whether such an
adsorption/surface reaction actually takes place. It should be
noted, however, that such a process would not a†ect the slope
of the second-order plot.
The rate coefficient for the tetrachloroetheneÈNO reaction
3
was quite difficult to determine, as is clearly displayed in Fig.
3. The reaction is slow, and the rate could not be increased the
normal way by increasing the concentration of tetrachloroethene due to its low vapour pressure. The pressure in
the Ñow tube was increased to lengthen the contact time,
Table 1, but it was still difficult to get reliable results. In addition, the uncertainty in measuring the organic concentration
was much higher, about 10%, compared with 1È2% for
the other chloroethenes. The unit-weighted plot of k@ vs.
tetrachloroethene, Fig. 3, yielded a rate coefficient of
(9.6 ^ 8.1) ] 10~17 cm3 molecule~1 s~1.
The temperature dependence of the reactions with vinyl
chloride and trichloroethene were investigated over a temperature range of about 100 K. Both reactions are clearly temperature dependent as the detailed results listed in Table 2
and as Fig. 4(a) and (b) show. The following Arrhenius expressions were obtained from a non-linear, unit-weighted leastsquares Ðtting procedure :
(T ) \ 1.8 ] 10~13
(vinyl chloride)
] exp([1770/T ) cm3 molecule~1 s~1
k
k
(T ) \ 4.0 ] 10~13
(trichloroethene)
] exp([2030/T ) cm3 molecule~1 s~1
as compared to the parameter values 1.8 ] 10~13/1780 for
vinyl chloride and 4.3 ] 10~13/2060 for trichloroethene when
the Arrhenius expressions are obtained by linear Ðtting of the
usual lnk vs. 1/T .
Table 2 Arrhenius data for the reaction of NO with vinyl chloride
3
and trichloroethene
k/10~16 cm3
molecule~1 s~1 a
temperature/K
vinyl chlorideb
2.43 ^ 0.57
3.57 ^ 0.60
4.26 ^ 0.39
7.52 ^ 1.2
12.1 ^ 1.5
14.5 ^ 1.5
266
280
295
325
348
367
trichloroethenec
278
295
313c
328
343.5
353
367.5
2.75 ^ 0.67
4.43 ^ 0.32
6.14 ^ 2.14
7.36 ^ 0.40
10.4 ^ 1.0
13.9 ^ 1.9
14.9 ^ 2.3
a Errors quoted with 95% conÐdence in the slope of the plots k@ vs.
[org]. b E \ 14.7 ^ 0.9 kJ mol~1, A \ (1.84 ^ 0.55) ] 10~13 cm3
molecule~1a s~1 ; error quoted at 1p. c E \ 16.9 ^ 1.8 kJ mol~1,
A \ (4.0 ^ 2.5) ] 10~13/cm3 molecule~1 s~1a ; error quoted at 1p.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
527
Fig. 4 (a) Arrhenius plot of the reaction between NO and vinyl
chloride. X denotes the rate coefficient determined by the 3relative rate
method. (b) Arrhenius plot of the reaction between NO and trichlo3 relative rate
roethene. Z denotes the rate coefficient determined by the
method.
Attempts to observe any temperature dependence of the
NO reaction with tetrachloroethene were futile.
3
Relative rate measurements
The rate coefficients for the reactions between the chloroethenes and the nitrate radical were determined by the relative
rate technique where the organic of interest competes with a
reference compound for the available nitrate radicals.11 The
degradation of both compounds are measured simultaneously,
and the relative rate coefficient, k /k , is determined from
org ref
the slope of straight lines according to the following expression :
k
[reference]
[organic]
0
0 \ org ] ln
(II)
k
[reference]
[organic]
ref
t
t
The lines were constrained through the origin. Ethene, with a
recommended literature rate coefficient of k(NO ) \ (2.01
3
^ 0.30) ] 10~16 cm3 molecule~1 s~1 at 298 K, was employed
as the reference compound for all the chloroethenes.1 Fig. 5(a)
and (b) show plots of the data for the individual chloroethene
reactions, except for tetrachloroethene, where it was not possible to get an accurate measure of the change in concentration at all times during the experiment. The derived relative
rate coefficients for the individual tetrachloroethene experiments were determined as 0.29, 0.38 and 0.50, but the uncertainty in each value is quite high. The relative rate data are
summarized in Table 3.
The rate coefficients for the chloroethenes span about two
orders of magnitude, Table 3, and ethene was chosen as the
reference compound because it has a rate coefficient that lies
approximately in the middle of this span. The ethene and
chloroethene reactions are fairly slow and 10% or less of the
organic reactant had normally reacted during the time of the
experiment, except for 1,1-dichloroethene where about one
ln
528
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
Fig. 5 (a) Plots of eqn. (II) for 1,1-dichloroethene (A) and trichloroethene (B) with ethene as the reference compound. For each compound the di†erent symbols represent di†erent experiments. In
particular, Ðlled symbols denote experiments with ethane as the chlorine scavenger. (b) Plots of eqn. (II) for vinyl chloride (C), (Z)-1,2dichloroethene (D) and (E)-1,2-dichloroethene (E) with ethene as the
reference compound. For each compound the di†erent symbols represent di†erent experiments.
third had reacted. Due to the low amounts consumed of the
reactants and the problem of Ðnding suitable absorption
bands to measure the small changes in concentration, the
uncertainties in the rate coefficients are fairly large, Table 3.
The rate coefficient for the 1,1-dichloroethene reaction with
NO is almost a factor 10 faster than the analogue reaction
3
with ethene, and the rate coefficient for 1,1-dichloroethene was
also measured with acetaldehyde as the reference compound,
see Table 3.
Additional RR experiments with vinyl chloride as the reference compound were performed, and the resulting rate coefficient ratios, k /k
, are displayed in Table 3. Again,
org vinyl chloride
the rate coefficient for the reaction with tetrachloroethene was
very difficult to determine.
Ethane was sometimes added as a Cl-atom scavenger, but
the rate coefficients derived in these experiments seemed unaffected, see Fig. 5(a), as was also reported in a previous study.5
In addition, we did not observe HCl, which should be a
product in the reaction. However, we do have indications that
Cl atoms were eliminated, since we observe nitrooxyacetyl
Table 3 Data from relative rate measurements of the chloroethenes
compound
vinyl chloride
1,1-dichloroethene
(Z)-1,2-dichloroethene
(E)-1,2-dichloroethene
trichloroethene
tetrachloroethene
temperature/K
k /k
a
org ethene
296
296
299
299
298
298
299
1.78 ^ 0.27
8.72 ^ 0.47
101 ^ 0.16
0.46 ^ 0.13
1.79 ^ 0.18
0.39 ^ 0.11
k /k
b
org vinyl chloride
4.24 ^ 0.14
0.530 ^ 0.028a,c
0.47 ^ 0.03
0.29 ^ 0.01
1.3 ^ 0.04
0.1
a Two standard errors of the average of individual experiments. b Errors quoted
with 95% conÐdence. c Relative to acetaldehyde.
chloride (NAC, O NOwCH wCOCl), which is formed by
2
2
elimination of Cl-atoms from the nitrooxy alkoxy radical
intermediate in the degradation of 1,1-dichloroethene.3 We
also observe ClNO, but we do not know if this is an elimination product or an eventual result of the released Cl atoms.
pseudo-Ðrst-order rate coefficients were varied until agreement
was reached between the NO concentrations simulated with
3
and without secondary reactions at di†erent concentrations of
the organics. The following two simple models were
employed :
Discussion
Model 1
A summary of the rate coefficients determined in this work
and by previous investigators is listed in Table 4. As can be
seen, there is some disagreement between the obtained rate
coefficients. If one compares the rate coefficients determined
by the FFD and RR methods in this work, the FFD rate coefÐcients are as much as 20% higher for the faster reacting chloroethenes. This could indicate that secondary reactions are
signiÐcant in the Ñow tube, but it may also just reÑect the
general uncertainties in the FFD and RR methods. Since the
reactions were measured to be faster at low pressure, the chloroethene reactions are probably not pressure dependent
within the pressure range applied. This is also in agreement
with previous investigations where no pressure dependence
was noted.5,8
Secondary reactions in the fast Ñow system
With the FFD system we have measured changes in the NO
3
concentration when the chloroethenes were being added in
sufficient excess. It is not possible to investigate in situ whether
the nitrate radical reacts with the chloroethenes only or also
with one or several products from the initial reaction. Consequently, the measured rate coefficients must be considered as
upper limits. The same also applies to the subsequently
derived apparent bimolecular rate coefficients.
Several products that are able to initiate secondary reactions may be formed in the Ñow tube. As mentioned before, it
is possible that Cl atoms are eliminated, and these Cl atoms
may react with either of the initial reactants.3 Furthermore,
oxiranes which have been recognized as major products in
reactions with unsaturated hydrocarbons and NO at low
3
pressure,12h14 may also be formed here. Oxirane formation in
Ñow tube experiments with NO will a†ect the rate coefficient
3
determination because the released NO enters a pressure
2
dependent equilibrium with NO forming N O . The e†ect
3
2 5
that a possible elimination of Cl-atoms and NO may have on
2
the rate coefficients was tested by separate simulations with
the computer program FACSIMILE.15 In the simulations the
NO ] chloroethene ] product ] Cl
3
Cl ] chloroethene ] product
(4a)
(Cl ] chloroethene ] product ] Cl)
(4b)
(3)
Cl ] NO ] ClO ] NO
3
2
(5)
Model 2
NO ] chloroethene ] oxirane ] NO
(6)
3
2
NO ] NO ] N O
(7)
2
3
2 5
N O ] NO ] NO
(8)
2 5
2
3
In model 1 the Cl-atom reactions with the chloroethenes are
pressure dependent (ref. 16 and references therein), and the
rate coefficients at 5 mbar total pressure are not known. We
roughly estimate the ClÈchloroethene rate coefficients to be
the same as that of the ClÈethene reaction,8 ca. 2 ] 10~12 cm3
molecule~1 s~1 at 5 mbar total pressure.17 Employing eqn.
(3), (4a) and (5) and a rate coefficient of 4.5 ] 10~11 cm3
molecule~1 s~1 for eqn. (5),2 then the simulations show that
elimination of Cl atoms will have no signiÐcant e†ect on the
rate coefficients determined under our experimental conditions. However, previous investigations show that the Cl
atoms initiate a chain reaction because Cl atoms are also released from the Cl atom adduct. If this chain reaction is
included in the model of the Ñow tube reactions [eqn. (4b)
instead of eqn. (4a)], the simulations indicate that the apparent bimolecular rate coefficients could be as much as a factor
of two larger than the true rate coefficients. It is unlikely that
the total yield of Cl atoms will be unity. In fact, the simulations show that the yield of Cl atoms in both eqn. (3) and (4b)
must be greater than 95 and 80% for 1,1-dichloroethene and
trichloroethene, respectively, in order to have any e†ect on the
measured rate coefficients. Thus, Cl atom elimination is not
considered to be a problem in the FFD experiments.
Table 4 Room-temperature rate coefficients for the reactions of chloroethenes with NO from this work compared with literature values
3
k/10~16 cm3 molecule~1 s~1
this work
compound
vinyl chloride
1,1-dichloroethene
FFD
RRa
RRb
other
4.3 ^ 0.4
19 ^ 1
3.6 ^ 0.8c
18 ^ 3c
16 ^ 4f
15 ^ 1g
2.0 ^ 0.4c
1.8 ^ 0.4f
0.9 ^ 0.3c
1.1 ^ 0.2f
3.6 ^ 0.7c
4.7 ^ 1.1f
0.8 ^ 0.3c
0.4f
4.2 ^ 0.7
12 ^ 2
2.2 ^ 1.1d
12 ^ 3e
(Z)1,2-dichloroethene
n.d.
(E)-1,2-dichloroethene
1.0 ^ 0.2h
trichloroethene
4.4 ^ 0.3
tetrachloroethene
lit. values
1 ^ 0.8
1.4 ^ 0.2
1.0 ^ 0.2
1.2 ^ 0.5e
2.8 ^ 0.4
\0.5
a Errors quoted combining the errors in k /k and k . b Presented relative to k
\ (2.01 ^ 0.30) ] 10~16 cm3 molecule s~1, ref. 5. c k
ethene`NO3ref. 7. e Determined by FFD, ref. 8. f Presented relative
ref
as in footnote b, ref. 1. d Presented relativeorgto ref
K (N Oref) \ 2.90 ] 10~11 cm3 molecule~1,
eq
2
5
\ (2.74 ^ 0.07) ] 10~15
to k
(298) \ (3.73 ^ 0.82) ] 10~16 cm3 molecule~1 s~1, this work. g Presented relative to k
3
acetaldehyde`NO3
chloride`NOs~1,
cm3 vinyl
molecule~1
ref. 12. h Corrected to 295 K, ref. 8.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
529
Oxirane formation has been proposed as a major product
in the chloroethene reactions in Ñow-tube systems.8 To investigate this, the chloroetheneÈNO reactions were studied in
3
the static reactor applying the same total pressure as in the
Ñow tube. Although not immediately comparable due to differences in oxygen and reactant concentrations and in the
surface to volume ratio, such experiments may give an indication as to what could take place in the Ñow tube. Generally,
the same products were observed at low pressure and at atmospheric pressure. However, for 1,1-dichloroethene new bands
that were not observed at normal pressures, appeared at 974
cm~1 and in the region 1040È1070 cm~1. These bands may
originate from 1,1-dichlorooxirane, and infrared spectra of
other halogenated oxiranes show that some of the characteristic absorption bands lie in these regions.18 Unfortunately,
there is no published data on 1,1-dichlorooxirane to conÐrm
or rule out the formation of this compound. Relying on IR
literature data on oxiranes,18 we could not observe these in
the reactions of NO with (Z)- and (E)-1,2-dichloroethene and
3
with tetrachloroethene. On the other hand, oxiranes are
known to have weak absorption bands, and, in addition, the
concentration of the oxiranes should be low due to the low
conversion of the reactants.
There are many uncertainties concerning oxirane formation
in the chloroethene reactions. We have investigated the possible e†ect NO may have on the measured rate coefficients
2
assuming that oxirane and NO are formed with unit yield,
2
model 2. When simulating the reaction system, the forward
and reverse reactions in the equilibrium between NO , NO
2
3
and N O were included [eqn. (7) and (8)]2 together with eqn.
2 5
(6). The model shows that the resulting bimolecular rate coefficients for the faster reacting compounds are less inÑuenced by
the formation of NO . Accordingly, the rate coefficient for
2
(E)-1,2-dichloroethene is a†ected the most (ca. 25%), which is
contrary to the experimental RR and FFD results, Table 4.
However, as mentioned before, the uncertainty in the RR rate
coefficients for (E)-1,2-dichloroethene is fairly large. For vinyl
chloride, 1,1-dichloroethene and trichloroethene the discrepancy in the rate coefficients determined by the FFD and RR
methods may very well be explained by the formation of
NO .
2
Arrhenius parameters
Recently, a theoretical study of the reactivity of the NO
3
radical towards a series of alkenes and halogenoalkenes was
presented.19 In Table 5 the measured and predicted19 Arrhenius parameters are compared for the chloroalkeneÈNO
3
reactions. As can be seen, the agreement is quite good between
the measured and predicted activation energies, and the preexponential factors di†er by only a factor of ca. two. For vinyl
chloride, however, the predicted activation energy is ca. 25%
higher than the measured value, and the pre-exponential
factors di†er by ca. a factor two the opposite way as compared to the other chloroethenes. We o†er at present no
explanation for this.
Relative rate
The validity of the FFD results may be checked against
results from the RR method, although secondary reactions
may also inÑuence the RR coefficient determination. This may
happen if one or several of the intermediates or products react
with the initial organic reactants at di†erent rates than with
the standard compounds. Previous studies show that this
should neither be a problem for vinyl chloride, nor for (Z)and (E)-1,2-dichloroethene.3 Tetrachloroethene reacts so
slowly that it is difficult to observe any reaction at all, and we
have no evidence for secondary reactions here. In contrast, it
is evident that Cl atoms are eliminated during the NO initi3
ated oxidation of 1,1-dichloroethene and trichloroethene,3 and
Cl atoms react fast with their organic precursors as well as
with the reference compound ethene.16 However, as is clearly
displayed in Fig. 5(a), the addition of excess ethane as a Cl
atom scavenger in experiments with 1,1-dichloroethene and
trichloroethene had little or no e†ect on the loss of the other
reactants. This could suggest that the Cl atoms react with the
chloroethenes and ethene with the same relative rate coefficients as in the NO radical reactions. However, the measured
3
rate coefficients for the chloroetheneÈCl atom reactions show
that this is not so.16 Accordingly, the RR rate coefficients
seem to be una†ected by secondary reactions. As mentioned
before, the same was observed by Atkinson et al. in their study
of the reactivity of the chloroethenes.5
The experiments with excess ethane added are somewhat
puzzling ; we did not observe any HCl formation but we did
observe nitrooxyacetyl chloride (NAC), which can only be
formed by elimination of Cl atoms.3 The absence of HCl may
in part be explained by a low yield of NAC. Unfortunately, we
cannot quantify the yield, but it appears to be low because the
characteristic CxO stretching band at 1825 cm~1 is weak.
The absence of HCl may also be explained by the Cl atoms
having other loss channels, of which we can only guess.
Frontier orbital theory has been used to rationalize the
reactivity of the chloroethenes towards NO .19,20 In
3
summary, this theoretical approach predicts the following
order of reactivity : 1,1-dichloroethene [ vinyl chloride [
tetrachloroethene[trichloroethene[(Z)-1,2-dichloroethene[
(E)-1,2-dichloroethene. In view of the approximations
involved in this theoretical approach, the agreement with the
experimental results is quite remarkable, except for tetrachloroethene, Table 4. Our experimental results show that the
reactivity of the chloroethenes falls into three groups : (i) 1,1dichloroethene [ (ii) vinyl chloride and trichloroethene [
(iii) (Z)-1,2-dichloroethene, (E)-1,2-dichloroethene and tetrachloroethene. The objectives of the RR experiments with vinyl
chloride as the reference compound were to conÐrm the order
of reactivity within group (ii) where the ratio found in the
present work di†ers from that of the previous investigation,
Table 4,5 and group (iii) where the compounds react so slowly
that it is difficult to obtain reliable data. The rather superÐcial
analysis of these RR experiments results in trichloroethene
Table 5 Arrhenius parameters
measured
compound
vinyl chloride
1,1-dichloroethene
(Z)-1,2-dichloroethene
(E)-1,2-dichloroethene
trichloroethene
tetrachloroethene
E /kJ
a
mol~1
A/10~13 cm3
molecule~1 s~1
E /kJ
a
mol~1
A/10~13 cm3
molecule~1 s~1
15 ^ 1b
15 ^ 2c
1.8 ^ 0.6b
5 ^ 2c
20.0 ^ 0.3c
17 ^ 2b
4.5 ^ 0.5c
4 ^ 2b
18.7
14.0
18.9
19.4
17.5
15.4
4.7
3.9
1.9
2.0
1.4
0.90
a Ref. 19. b This work. c Ref. 8.
530
predicteda
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
reacting faster, not slower, than vinyl chloride. This observation, combined with the other RR experiments and the FFD
temperature studies, strongly supports the statement that the
rate coefficients for vinyl chloride and trichloroethene are
approximately equal at room temperature. From the amount
consumed of each reactant in group (iii), we must conclude
that the NO reaction with tetrachloroethene deÐnitely is
3
slower than the (E)-1,2-dichloroethene reaction, thus supporting the data in Table 4.
Conclusion
The FFD and RR rate coefficients determined in this work are
in fair agreement with di†erences of 20% or less, Table 4.
These di†erences may be explained by uncertainties in the two
methods employed, but simulations show that they may also
be explained by secondary reactions involving NO , although
2
this has not been conÐrmed experimentally. Secondary reactions due to Cl atom elimination are not likely to a†ect the
FFD rate coefficients, because simulations indicate that the Cl
atoms then must be formed with a total yield near unity. No
secondary reactions were observed to signiÐcantly interfere
with the RR rate coefficient determinations.
Several of the previously reported rate coefficients lie about
20 to 30% below the RR rate cofficients determined in this
work, Table 4. Except for 1,1-dichloroethene and (Z)-1,2dichloroethene most of the previous data are still within the
uncertainty limits of the RR data in this work. For (Z)-1,2dichloroethene this discrepancy can readily be explained by
the small difficulty of Ðnding a suitable absorption band for
measuring changes in the reactant concentration. In contrast,
we cannot explain the di†ering values for 1,1-dichloroethene,
but Ðnd the results puzzling since essentially the same
methods and the same reference compound have been
employed. Another point to note about the previously
published rate coefficients is that all our experimental data
indicate that the rate coefficient for vinyl chloride is approximately equal to the rate coefficient for trichloroethene at
room temperature, in contrast to the previous result.5
Financial support from the Norwegian Research Council and
Norsk Hydro is gratefully acknowledged. Part of this work
was carried out with the support from NorFA enabling
IMWN to spend a training and research period at the Uni-
versity of GoŽteborg and Chalmers University of Technology.
The authors also wish to thank Anastasia Pagou, an IAESTE
trainee, for her assistance in the laboratory.
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Paper 6/06008K ; Received 30th August, 1996
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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