Absolute rate constants for the gas-phase reactions of O(3P) atoms with CF2=CFC1 and (E/Z)-CFC1=CFC1 at 298 K. Reactivity trends of the halogen-substituted ethenes series

Article abstract of DOI:10.1002/kin.20028

Absolute rate coefficients for the gas-phase reactions of CF₂=CFC1 and (E/Z)-CFC1=CFC1 with O(3P) atoms have been measured at 298 K using a discharge flow tube coupled to a chemiluminescence detection system. The observed rate constant values are (4.5 ± 0.4) × 10^-13 and (1.5 ± 0.3 × 10^-13 cm³ molecule-¹ s^-1, respectively. The experiments were carried out under pseudo-first-order conditions with [O(³P)]₀ ? [alkene]₀. These results are compared to those of O atom reactions with other chlorine- and fluorine-substituted ethenes. Different factors that affect the rate of addition to the double bond are considered. The O(³P)/chloroethenes reactions do not obey the reactivity trend with the ionization potential, as is the case in the alkene and methyl-substituted alkene reactions.

Full text of DOI:10.1002/kin.20028

Absolute Rate Constants for
the Gas-Phase Reactions of
O(3P) Atoms with CF2 CFCl
and (E/Z)-CFCl CFCl at
298 K. Reactivity Trends of
the Halogen-Substituted
Ethenes Series
MARIANO A. TERUEL, RAUL A. TACCONE, SILVIA I. LANE
´
´
´
Instituto de Investigaciones en Fisicoquımica de Cordoba (I.N.F.I.Q.C.), Departamento de Fisicoquımica, Facultad de Ciencias
´
´
´
Quımicas, Universidad Nacional de Cordoba, Ciudad Universitaria, 5000 Cordoba, Argentina
Received 15 October 2003; revised 23 December 2003; accepted 12 May 2004
DOI 10.1002/kin.20028
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Absolute rate coefficients for the gas-phase reactions of CF2 CFCl and (E/Z)-
CFCl CFCl with O(3P) atoms have been measured at 298 K using a discharge flow tube
coupled to a chemiluminescence detection system. The observed rate constant values are
(4.5 0.4) × 10−13 and (1.5 0.3) × 10−13 cm3 molecule−1
s
−1, respectively. The experiments
were carried out under pseudo-first-order conditions with [O(3P)]0 ꢀ [alkene]0. These results
are compared to those of O atom reactions with other chlorine- and fluorine-substituted
ethenes. Different factors that affect the rate of addition to the double bond are considered. The
O(3P)/chloroethenes reactions do not obey the reactivity trend with tꢁhe ionization potential, as
C
is the case in the alkene and methyl-substituted alkene reactions. 2004 Wiley Periodicals,
Inc. Int J Chem Kinet 36: 525–533, 2004
INTRODUCTION
The gas-phase reactions of ground-state oxygen atoms,
Correspondence to: Silvia I. Lane; e-mail: s-lane@mail.fcq.
O(3P), with alkenes are of both fundamental and prac-
unc.edu.ar.
Contract grant sponsor: CONICET (Argentina).
tical importance. While there are many studies of O
atom reactions with a variety of unsaturated hydrocar-
bons, only limited information is available for halo-
genated alkenes, most of which has been reviewed by
Contract grant sponsor: ANPCyT (Pre´stamo BID 1201/OC-AR).
Contract grant sponsor: Agencia Co´rdoba Ciencia.
Contract grant sponsor: SECyT-UNC (Co´rdoba, Argentina).
2004 Wiley Periodicals, Inc.
c
ꢁ

526
TERUEL, TACCONE, AND LANE
Cvetanovic[1]. Thedatabaseforperhaloalkenesiseven
more scarce [2–7], and fluoroalkene reactivity studies
toward O atoms are still less [8–11]. Most of these
studies have been relative measurements of the rate
coefficients.
to the C C bond, we report in this work absolute rate
constants for the reactions of O(3P) with the two title
perhaloalkenes, measured in a conventional discharge
flow tube apparatus at 298 K, as an extension of our ear-
lier measurements of the rate coefficients for the reac-
tions of O(3P) with CF2 CCl2 and CF3CF CF2 [10].
In addition, and in order to clarify whether CF2O
is, in fact, formed in the reaction with CF2 CFCl in
view of the discrepancy between Mitchell and Simons’
and Tyerman’s observations, we conducted several ex-
periments to determine the products of this reaction in
an attempt to shed light into its mechanism, i.e., if O
atom attack on CF2 CFCl occurs exclusively at the
CF2 group.
Reliable rate constant values for these reactions are
of interest for the role of chemical oxidative processes
inatmosphericandcombustionchemistry. Inparticular,
the reactions of O(3P) atoms with fluorinated alkenes
have attracted considerable attention because of their
general usefulness in the chemical etching processes of
the semiconductor industry. The presence of electron-
withdrawing substituents in the alkene is expected to
modify the electron distribution considerably, and this
should be reflected in the reactivity of the alkene toward
the O atoms, thus providing an opportunity to examine
halogen substituent effects as a function of extent and
positioning about the double bond.
Mitchell and Simons used the long wavelength flash
photolysis of NO2 as the O(3P) atom source and stud-
ied its reactions with CF2 CF2 and a series of 1,1-
difluoroethylenes [4]. The reactants were subjected to
continuous photolysis and the final products analyzed
by IR spectroscopy. In the reaction with CF2 CF2, a
strong signal from the CF2 radical in its ground state
(X1A1) was observed by its ultraviolet absorption in the
region of 250 nm. CF2O was the predominant product.
Conversely, in the reaction with CF2 CFCl, CF2O was
also the predominant product, but the CF2 absorption
signal was very weak, implying that Cl atom substitu-
tion inhibits the formation of CF2 but enhances that of
CF2O. Mitchell and Simons did not report the reaction
rate constants. Tyerman [5], using the same experimen-
tal approach, also did not observe the absorption sig-
nal for CF2 in the CF2 CFCl and CF2 CCl2 reations
with O(3P). In addition, and contrary to Mitchell and
Simons’ observations, Tyerman did not find CF2O as
a reaction product. However, this author was able to
measure the rates of disappearance of these alkenes in
reaction with O atoms using a competitive method.
Huie, Herron, and Davis [6], using a discharge flow
system coupled to a mass spectrometer, studied the re-
actions of atomic oxygen with several haloethylenes.
The main product in the reactions with CF2 CCl2
and CF2 CFCl was CF2O, and CFClO in the reaction
with CFCl CFCl. The corresponding rate constants,
however, were not reported.
EXPERIMENTAL
The experiments were conducted in a discharge flow
system using the air afterglow chemiluminescence sig-
nal to follow the oxygen atom concentration. The
schematicrepresentationofthedischargeflowtubeem-
ployed in this work is shown in Fig. 1. Briefly, the reac-
As part of a systematic study of O(3P) atom reac-
tivity and a prototype system for the addition reactions
Figure 1 Schematic diagram of the discharge flow chemiluminescence system used in O(3P) atom reaction studies. PM: pho-
tomultiplier tube; MFC: mass flow controller; MFM: mass flow meter; MW: microwave discharge; CM: capacitance manometer.

REACTIVITY TRENDS OF THE HALOGEN-SUBSTITUTED ETHENES SERIES
527
tor consisted of a Pyrex tube (1.00 m long and 2.50 cm
i.d.) coupled with an axially sliding injector (0.25 cm
i.d.) used to introduce the olefin into the reactor.
Oxygen atoms, O(3P), were generated by an elec-
trodeless microwave discharge (30 W, 2.5 GHz) on
a 2% O2/He mixture and were flowed into the tube
through a fixed side-arm port. Initial concentrations of
The signal, S, was proportional to the light intensity
of chemiluminescence. In all experiments, the back-
ground signal obtained when [NO] = [alkene] = 0,
arising from scattered light in the reaction tube, was
subtracted from S before further analysis. Concentra-
tions of O(3P) atoms in the kinetic experiments were
estimated by the fast reaction with NO2 under second-
order conditions [15].
O atoms ranged from 4 × 1012 to 5 × 1012 atom cm−3
.
Reaction times could be varied by adjusting the dis-
tance of the sliding injector with respect to the fixed
position of the photomultiplier tube. The system was
evacuated by a rotary pump (90 m3 h−1) and a capaci-
tance manometer (MKS Baratron, 0–10 torr) was used
to measure the pressure in the tube. Electronic mass
flow controllers (MKS 179 A, 1259 C), previously cal-
ibrated for each gas mixture, were used to control and
measure the gas flows in the reaction tube. The total lin-
ear flow velocity for the reaction with CF2 CFCl was
between 1503 and 1610 cm s−1 and the total pressure
was 3.8 torr; for the reaction with (E/Z)-CFCl CFCl
the total linear flow velocity was between 1056 and
1609 cm s−1 and the total pressure was varied between
1.86 and 5.50 torr. The experiments were conducted at
298 K and helium was used as the carrier gas because
of its inertness and excellent diffusion coefficient, thus
ensuring rapid radial mixing of all reactants with the
bulk of the flow. Under our experimental conditions,
the Reynolds number is <50; therefore the flow can be
considered laminar. When the flow is laminar and the
radial diffusion of the gases occurs very quickly such
that there is no radial concentration gradient or radial
velocity gradient, the condition known as “plug flow”
[12,13] is satisfied.
Several experiments were carried out measuring
the chemiluminescence signal in the absence of added
alkene, introducing O atoms at different contact times,
in order to assess wall losses of O atoms. The measured
values of kw were always <15 s−1
.
In order to determine the products of the reaction
of O(3P) atoms with CF2 CFCl, several experiments
were performed at ambient temperature under static
conditions, generating the oxygen atoms, O(3P), by the
mercury-photosensitized decomposition of N2O [16].
The cylindrical Pyrex cell of 250-cm3 volume, sealed
with quartz windows, was illuminated by two low-
pressure mercury lamps. In each experiment, a new
drop of Hg was admitted into the cell, after which
the CF2 CFCl and N2O were measured manometri-
cally and introduced into the reaction cell. Typically,
the pressure of N2O was about 100 torr, and that of
CF2 CFCl about 10 torr.
The analytical techniques employed for qualitative
identification of the products formed after irradiation
was terminated were infrared spectroscopy using a
Buck Scientific model 500 infrared spectrometer and
GC-mass spectrometry on a Shimadzu GC-MS QP
5050 spectrometer equipped with a 30 m by 0.12 mm
DB-5 MS column.
The pressure drop along the tube was estimated,
consideringtheviscosityofHe, asnegligible. Theplug-
flow conditions in the flow tube allowed us to follow
the reaction kinetics of the O(3P) atoms in a known
excess of alkene in order to assume the pseudo-first-
order approximation.
The position of the injector could be changed from
9 to 50 cm from the center of the detection zone, giving
contact times of the O(3P) atoms with the alkene in the
range of 6–20 ms.
Calculations of the carbon atomic charges of the
alkenes were carried out using the semiempirical
method PM3 and ab initio HF and B3LYP/NBO lev-
els of theory with the 6-31G** basis set, using the
Gaussian-98 suite of programs [17]. All geometries
were fully optimized by minimizing the energy with
respect to all geometrical variables. The stationary
pointswerecharacterizedastheminimun-energy struc-
tures when the number of imaginary frequencies was
zero by calculations of the corresponding Hessian ma-
trix. The ionization potential of the alkenes was taken
from the literature [18]. The corresponding values
for CFCl CCl2 and (E/Z)-CFCl CFCl that were not
available in the literature were calculated in this work
using the PM3 semiempirical method.
The concentration of O(3P) atoms was monitored by
measuring the chemiluminescence from the air after-
glow reaction in which O(3P) atoms react with NO to
produceelectronicallyexcitednitrogendioxide(NO2*)
[14]. A constant flow of NO was added 8.5 cm before
the photomultiplier tube detector (Hamamatsu R636)
sothatNOconcentrationwasca. (5–9)×1014 molecule
cm−3 and the chemiluminescence from NO2* passed
through a wide band pass filter (ꢀ < 500 nm) before
reaching the photomultiplier tube, the output signal of
which was amplified and displayed on an oscilloscope.
Materials
The commercial gases used in this study had
the following stated minimum purity: He (AGA
99.999%), O2(AGA 99.999%), NO (AGA 99.5%),

528
TERUEL, TACCONE, AND LANE
N2O (AGA 99.5%), CF2 CFCl (PCR 99%), and
(E/Z)-CFCl CFCl (PCR 97%). Nitric oxide was pu-
rified by passing it through a trap held at 140 K to
remove NO2. Helium was flowed through traps con-
taining silica gel and molecular sieves (BDH Type 4
A) at 77 K, to remove water. The alkenes were purified
by successive trap-to-trap distillations under vacuum,
and oxygen was used as supplied. Nitrous oxide was
used without further purification.
The first-order kinetic analysis is based on the ra-
tio S0/St , which is applicable to our system. S0 is the
chemiluminescence signal without a reactant and St is
the signal after addition of a reactant at a fixed time of
contact. The O(3P) loss between the NO entrance and
the detection zone for the reaction with the NO was
calculated as less than 4%.
Taking into account the above considerations and
Eq. (4), it is possible to write
ln(S0/St ) = k[alkene]0t
(7)
Results
The pseudo-first-order decay rate coefficient, kꢂ, was
computed from the slope of a plot of ln(S0/St ) vs time
of contact, which was linear for approximately three
1/e lifetimes. The second-order rate coefficient for re-
actions (1) and (2) , k1 and k2, were obtained from the
slope of the line of a plot of kꢂ vs. [alkene]0 (Figs. 2 and
3). Alinearleast-squares fittothedatapointsyieldsval-
ues of k1 = (4.5 0.5) × 10−13 cm3 molecule−1s−1
Using the above-mentioned technique the rate con-
stants, k, for the following reactions at 298 K were
obtained:
O(3P) + CF2 CFCl → Products
(1)
O(3P) + (E/Z)-CFCl CFCl → Products (2)
and k2 = (1.5 0.3) × 10−13 cm3 molecule−1 s−1
.
All experiments were carried out under pseudo-
first-order conditions with a ratio, [alkene]0/[O(3P)]0,
always greater than 40.
Therefore, intheabsenceofsecondaryreactionsthat
significantly deplete the transient O(3P) atoms, O(3P)
concentration varied with time according to the follow-
ing expression:
Thus, the resultant uncertainties in the values of k1 and
k2 are about 11 and 20%, respectively. The error lim-
its are one standard deviation from the least-squares
analysis. The pseudo-first-order rate coefficients were
corrected for axial and radial diffusion [12], using the
following expression:
kcꢂorr = keꢂxp(1 + keꢂxp D/v2 + keꢂxp R2/48D) (8)
[O(3P)]t = [O(3P)]0 exp[−(k[alkene]0 + kx )t] (3)
where D (cm2 s−1) is the diffusion coefficient of O(3P)
in He, v (cm s−1) is the linear flow velocity, and R (cm)
is tube radius. The values of D calculated for our exper-
imental conditions were in the range of 130–380 cm2
ln{[O(3P)]0/[O(3P)]t } = kꢂt
(4)
where [O(3P)]0 is the concentration in the absence of
olefin, [O(3P)]t is the concentration after reaction with
the olefin over time t, kꢂ is the measured pseudo-first-
order rate coefficient, and kx is the first-order rate coef-
ficient for O(3P) atom disappearance by diffusion out
of the detection zone, reaction with background impu-
rities or wall losses.
s
−1. This procedure resulted in <5% upward correction
of the kꢂ values, which means that the second and third
InordertomonitortheO(3P)concentration, aknown
excess of NO was added to produce electronically
excited NO2*, the chemiluminescence of which was
checked downstream of the NO inlet port, following
the air afterglow reaction:
O(3P) + NO + M → NO2∗ + M
NO2 → NO2 + hꢁ
(5)
(6)
∗
It has been shown that the intensity, S, of the air
afterglow is proportional to the product of the NO and
O concentrations [14]. Since NO concentration is con-
stant, the intensity of the air afterglow at selected points
down the tube is a relative measure of O(3P) concen-
tration.
Figure 2 Second-order plot for the O(3P) + CF2 CFCl
reaction at 298 K.

REACTIVITY TRENDS OF THE HALOGEN-SUBSTITUTED ETHENES SERIES
529
in the literature for comparison are scarce. Tyerman [5]
studiedthereactionwithCF2 CFCl. Huie, Herron, and
Davis [6] reported the product carbonyl halide of the
reaction of O + CFCl CFCl, but the corresponding
rate coefficient was not reported. Tyerman obtained a
value of 0.51 for the ratio of the rates of reaction of
oxygen atoms with CF2 CFCl relative to CF2 CF2 at
298 K [5]. If the value recommended by Cvetanovic
[1] for the reference reaction rate constant, O(3P) +
CF2 CF2, k298 = 9.76 × 10−13 cm3 molecule−1 s−1
,
is used, the rate constant for the reaction of interest
is 4.98 × 10−13 cm3 molecule−1
s
−1, based on the
value of 0.51 for the relative rates. Thus, our value
of k1 = (4.5 0.5) × 10−13 cm3 molecule−1 s−1 is in
good agreement with the previous one obtained from
relative measurements.
Figure 3 Second-order plot for the O(3P) + (E/Z)-
More recently, Yi et al. [11], using the chemilumi-
nescence imaging technique, determined the rate coef-
ficients at room temperature for O(3P) + CF2 CFCl
CFCl CFCl reaction at 298 K.
terms in Eq. (8), representing the corrections for axial
and radial diffusion, respectively, are negligible.
The linearity of the data points, especially in the low
alkene concentration range in our plots (Figs. 2 and 3),
suggests that the contribution to the decay of O(3P)
atoms due to secondary reactions with the products of
the title reactions is negligible. Also, the fact that the
plots show practically no intercepts is consistent with
a negligible loss of oxygen atoms by wall reactions
(kw = 9–15 s−1). Therefore no corrections for the ve-
locity profile effect were required [12]. The pressure of
the reactor for the O(3P) + (E/Z)-CFCl CFCl experi-
ments was changed from 1.9 to 5.5 torr and no variation
in the global rate constant was observed.
and O(3P) + CFCl CFCl as (5.04
0.14) × 10−13
and (2.67 0.07) × 10−13 cm3 molecule−1
s
−1, re-
spectively. Again, our value of k1 for CF2 CFCl is in
good agreement, within the combined uncertainties of
both investigations, whereas for CFCl CFCl our value
of k2 = (1.5 0.3) × 10−13 cm3 molecule−1 s−1 is al-
most a factor of two lower.
The room-temperature rate constants for the reac-
tions of the ethenes studied in this work and some
other fluorine- and chlorine-substituted ethenes with
the O atom are listed in Table I. Whenever hydrogen is
replaced by fluorine or chlorine at any position in the
molecule, the reactivity of the substituted ethene to O
atoms is less than that of the unsubstituted ethene, the
only exception being that CF2 CF2 is more reactive
than CH2 CH2.
For the sake of clarity and order of this discus-
sion, we will first analyze the fluoroethenes, then the
chloroethenes, and finally the perhalogenated ethenes.
The rate constant values for the reactions of the fluo-
roethenes show that (a) The magnitude of the effect of
hydrogen substitution by fluorine varies considerably
with the position of substitution, and (b) as the number
of fluorine atoms attached to the carbon atoms of the
double bond increases, the reactivity of the ethene does
not continue to decrease but appears to pass through
a minimum. Asymmetric fluorination of the olefinic
carbon atom seems to lessen reactivity, whereas fur-
ther fluorination (more symmetric substitution) com-
pensates for the original deactivation, so much so that
CF2 CF2 has a rate coefficient greater than that of
CH2 CH2. This suggests that alkene reactivity toward
O atoms depends on the geometrical localization of the
ꢂ-electron density. The ꢂ-electron density localization
in the center of the C C bond (CH2 CH2, CF2 CF2)
In order to check the performance of our flow sys-
tem in obtaining reliable results, we also measured
the overall rate constant for the reaction of CH≡CH
with O(3P), obtaining a value of (1.4 0.3) × 10−13
cm3 molecule−1 −1, which is in excellent agreement
s
with the literature value of (1.4 0.4) × 10−13 cm3
molecule−1 s−1 [19].
The products identified by IR spectroscopy and GC-
mass spectrometry from experiments of the reaction of
O(3P) + CF2 CFCl, generating the O atoms by the
Hg-photosensitizeddecompositionofN2OwereCF2O,
CFCl CFCl, and c-C3F4Cl2. The formation of these
products suggests that the O atom attaches to the CF2
group of the alkene, producing CF2O and leaving the
CFCl radicals to recombine and form CFCl CFCl and
c-C3F4Cl2 by reaction with the reactant CF2 CFCl.
DISCUSSION
The rate constant values at 298 K for the reactions of O
atoms with CF2 CFCl and (E/Z)-CFCl CFCl found

530
TERUEL, TACCONE, AND LANE
Table I Rate Constants for the Reactions of Haloalkenes with O(3P) at 298 K, Total Atomic Charges, and Ionization
Potential (IP) for the Corresponding Haloalkenes
Alkene
C(1) C(2)
Total Atomic Charges
k(298 K) (cm3 molecule−1 s−1
O(3P) + alkene
)
Method of
Calculation
C(1)
C(2)
Ref. IPa (eV)
CH2 CH2
PM3
RHF
−0.3196
−0.2544
−0.3196
−0.2544
−0.4347
−0.0797
0.2977
0.2272
0.1681
0.7647
0.7969
7.31 × 10−13
7.40 × 10−13
7.38 × 10−13
3.25 × 10−13
[1]
[20]
[11]
[1]
10.51
10.36
10.29
10.18
10.15
10.14
9.99
B3LYP/NBO −0.4347
CH2 CHF
PM3
RHF
−0.3841
−0.3610
B3LYP/NBO −0.5517
CH2 CF2
PM3
RHF
−0.4074
−0.4382
2.34 × 10−13
4.48 × 10−13
[1]
[1]
B3LYP/NBO −0.6392
(E/Z)-CHF CHF
CHF CF2
PM3
RHF
−0.1311
−0.1311
0.2038
0.2038
B3LYP/NBO
PM3
RHF
B3LYP/NBO
PM3
RHF
B3LYP/NBO
PM3
0.1307
0.1307
0.1252
0.6933
0.7087
0.1247
0.6366
0.6336
−0.3314
−0.2103
−0.2474
−0.2355
−0.2054
−0.1176
−0.2928
−0.2009
−0.2810
−0.2673
−0.2023
−0.1439
−0.2785
−0.1935
−0.1643
−0.1102
0.1522
−0.1394
5.73 × 10−13
4.05 × 10−13
[1]
[8]
0.1321
0.0466
0.1247
0.6366
0.6336
CF2 CF2
9.76 × 10−13
[1]
CH2 CHCl
CH2 CCl2
−0.3314
5.94 × 10−13
5.19 × 10−13
[1]
[21]
RHF
−0.2445
B3LYP/NBO −0.4540
PM3
RHF
−0.3270
−0.2257
4.88 × 10−13
[21]
[1]
9.81
9.79 × 10−13
B3LYP/NBO −0.4629
(E)-CHCl CHCl
CHCl CCl2
CCl2 CCl2
CF2 CFCl
PM3
RHF
−0.2928
−0.2009
2.20 × 10−13
9.58 × 10−14
[22]
[23]
9.64
B3LYP/NBO −0.2810
PM3
RHF
−0.2979
−0.1916
1.40 × 10−13
[22]
[21]
9.45
1.07 × 10−13
B3LYP/NBO −0.2950
PM3
RHF
−0.2785
−0.1935
9.33
B3LYP/NBO −0.1643
b
PM3
RHF
0.1450
0.7291
0.7099
0.1795
0.8536
0.7801
−0.0443
0.3774
0.3241
−0.0848
0.2478
0.2578
4.5 × 10−13
4.98 × 10−13
5.04 × 10−13
3.10 × 10−13
6.47 × 10−13
9.81
[1]
[11]
[10]
[1]
B3LYP/NBO
PM3
RHF
B3LYP/NBO
PM3
RHF
B3LYP/NBO
PM3
RHF
B3LYP/NBO
0.1887
CF2 CCl2
−0.3538
−0.4149
−0.2904
−0.3223
−0.3218
−0.2247
−0.0848
0.2478
9.65
CFCl CCl2
(E/Z)-CFCl CFCl
1.36 × 10−13
[11]
9.49c
9.77c
1.5 × 10−13
b
2.67 × 10−13
[11]
0.2578
a Ionization potentials from Ref. [18] except where marked otherwise.
b This work.
c Calculated by PM3 method (see text).

REACTIVITY TRENDS OF THE HALOGEN-SUBSTITUTED ETHENES SERIES
531
promotes O atom addition, whereas the shift of electron
density toward the carbon atom due to the high elec-
tron affinity of the F atom obstructs it. This suggests
the importance of a geometrical factor, as well as the
electron affinity of the substituent on the delocalization
of the ꢂ electrons.
effect rather than the electronic effect dominates when
F is successively replaced by Cl. Thus, kCF =CF
>
2
2
k
CF =CFCl > kCF =CCl > k(E/Z)-CFCl=CFCl > kCFCl=CCl
.
2
2
2
2
Ab initio calculations carried out in this work of
the total atomic charges for the ethenes presented in
Table I, reveal that the reactivity of the ethenes does
not seem to correlate with the charge density on the
olefinic carbon atoms. It is rather the charge density
between the two carbon atoms that controls reactivity,
thus suggesting the attachment of the electrophilic O
atom to the double bond as pointed out by Cvetanovic
[16]. This line of reasoning is consistent with the en-
hancement of the O addition reaction observed with
the fluoroethenes for which more symmetric substitu-
tion of H by F offsets the original deactivation so that
CF2 CF2 is more reactive than CH2 CH2.
Recent work by Washida et al. [27] on the mecha-
nism of the O + halogenated ethene reactions through
observationoftheradicalsproducedintheadditionpro-
cess, concludes that the O atom attaches to the fluorine-
substituted carbon in the fluoroethenes. In the case of
chloroethenes it is not clear whether O atoms add to
either side or only to the chlorine-substituted side of
the double bond. This evidence is opposed to the ex-
pected orientation of addition considering the charge
densities on the olefinic carbon atoms. This would con-
firm Cvetanovic’s [16] statement that the factors which
control the reactivity of the alkenes are not the same
as those which control the orientation of the addition
reaction.
An analysis of the rate coefficients presented in
Table I for the hydrochloroethenes shows that the ad-
dition of a single chlorine to ethene leads to a small
decrease in the rate constant, while addition of a sec-
ond chlorine on the same carbon results in an additional
small decrease in reactivity. However, if the two chlo-
rine atoms are located on different carbon atoms, there
is a considerable reduction in the rate constant. The ad-
dition of the third chlorine leads to a further reduction
in the rate constant, and a still further decrease would
be expected for tetrachloroethene taking into account
what is observed for OH and NO3radicals reacting with
CCl2 CCl2 [24]. We must point out that to the best of
our knowledge, the reaction of O + CCl2 CCl2 was
studied only by Sanhueza and Heicklen [25] using a
competitive method, but the corresponding rate con-
stant was not reported.
Again, the presence of Cl atoms in the olefin pro-
duces a strong electron-withdrawing effect, which re-
duces the charge density on the carbon atoms next to
the double bond as well as the polarizability of the
ꢂ electrons, leading to a decrease of the rate constants.
Substitution of F by Cl, however, would favor an in-
crease of the ꢂ electron density in the double bond
through contributions of the Cl atom lone-pair, thus
leading to a smaller decrease of the rate coefficients as
F atoms are substituted by Cl (kCH =CHF < kCH =CHCl
In our study of the products of the reaction of O
atoms with CF2 CFCl, we found mainly CF2O and
traces of CFCl CFCl and c-C3F4Cl2. Neither CFClO
nor C2F4 was detected, which would indicate that of
the two possible reaction channels
2
2
and kCH =CF < kCH =CCl ).
2
2
2
2
Within the O/chloroethenes series, progressive sub-
stitution of H by Cl decreases the reaction rate, contrary
to fluorine substitution in the fluoroethenes. This indi-
cates that the reactivity change of chlorinated ethenes
cannot be induced only by the electronic effect. Steric
hindrance by the Cl atom appears to be a dominant fac-
tor for the change of the rate of addition. The bulky
Cl atom offers significant electron density above the
molecular plane compared to the F atom, inhibiting
the O atom attack on the ꢂ orbital of the alkene. The
cross-section of the Cl and F atoms has been esti-
mated to be 7.2 and 3.8 A2, respectively [26]. In 1,2-
dichloroethene, addition is hindered as compared to
1,2-difluoroethene.
CF2 CFCl + O(3P) → CF2O + CFCl
CF2 CFCl + O(3P) → CFClO + CF2
the first is largely favored, suggesting that the O atom
attaches to the carbon atom most substituted by flu-
orine. Thus, our findings support the observations of
Mitchell and Simons [4] that attack occurs mainly at
the CF2 group.
The reactivity of O atom addition to ethene and
methyl-substituted ethenes has been found to corre-
late with the ionization potentials (IP) of the alkenes
[28] . The energy required to remove a ꢂ electron, the
ionization potential, is lowered in the alkenes by sub-
stitution of an H atom by an alkyl group, while the
electron density of the double bond is increased by the
substitution leading to an increase in the reaction rates.
It is evident from Fig. 4 that the observed correlation
between rate constant and IP does not hold for the
The reactivity of the chloro-fluoro-ethenes toward
O atom addition is suppressed with respect to ethene
itself because of electronic effects introduced by halo-
gen substitution, but here again steric hindrance by
increased Cl substitution seems to play a major role in
the reaction rate change. It can be inferred that the steric

532
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chlorofluoroethenes.
ꢀ
•
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same behavior as for the hydrogenated ethenes is ob-
served for the fluoroethenes, whereas for the chlorine-
substituted ethenes the opposite trend is observed, i.e.,
the reaction rate constants decrease as the ionization
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chloroethenes, the highest occupied molecular orbital
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O atom thus experiences greater nonbonding interac-
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