The gas-phase reaction of the CF3 radical with thiophene

Article abstract of DOI:10.1139/v03-155

The reaction of CF₃ radicals, generated by photolysis of CF ₃I or hexafluoroacetone with thiophene, was studied in the gas phase at 25°C. At conversion of thiophene less than 20%, monosubstituted CF ₃-thiophenes were found as the main reaction products, in addition to CF₃H, C₂F₆, and monosubstituted dihydro-CF₃-thiophene, the latter in very low proportion. An isomeric mixture of 2- and 3-CF₃-thiophene was obtained in a ratio of about 16, independent of the radical source used (CF₃I or hexafluoroacetone) to produce the CF₃ radicals. A plausible mechanism that accounts for the observed products is proposed, and the reactivity of thiophene toward the CF₃ radical at 25°C was determined as k_add/k_c^1/2 = 106 ± 4 cm^3/2 mol^-1/2 s^-1/2.

Full text of DOI:10.1139/v03-155

1
477
The gas-phase reaction of the CF radical with
3
thiophene
Olga S. Herrera, Jorge D. Nieto, Silvia I. Lane, and Elena V. Oexler
Abstract: The reaction of CF radicals, generated by photolysis of CF I or hexafluoroacetone with thiophene, was
3
3
studied in the gas phase at 25 °C. At conversion of thiophene less than 20%, monosubstituted CF -thiophenes were
3
found as the main reaction products, in addition to CF H, C F , and monosubstituted dihydro-CF -thiophene, the latter
3
2
6
3
in very low proportion. An isomeric mixture of 2- and 3-CF -thiophene was obtained in a ratio of about 16, independ-
3
ent of the radical source used (CF I or hexafluoroacetone) to produce the CF radicals. A plausible mechanism that ac-
3
3
counts for the observed products is proposed, and the reactivity of thiophene toward the CF radical at 25 °C was
3
1
/2
3/2
–1/2 –1/2
determined as kadd/kc = 106 ± ± cm mol
s
.
Key words: thiophene, trifluoromethyl radical, reaction mechanism, reactivity.
Résumé : Opérant en phase gazeuse, à 25 °C, on a étudié la réaction du thiophène avec des radicaux CF obtenus par
3
photolyse du CF I ou de l’hexafluoroacétone. À des taux de conversion du thiophène inférieurs à 20%, les produits
3
principaux obtenus en plus du CF H et du C F sont les thiophènes monosubstitués par du CF et les dihydrothiophè-
3
2
6
3
nes monosubstitués par du CF qui ne sont présent que dans de très faibles proportions. On obtient un mélange de 2-
3
et de 3-trifluorométhylthiophènes présents dans un rapport d’environ 16 quelle que soit la source utilisée, CF I ou
3
hexafluoroacétone, pour produire les radicaux. On propose un mécanisme plausible qui permet d’expliquer la formation
des produits obtenus et on a déterminé que la réactivité du thiophène par rapport au radical CF , à 25 °C, est égale à
3
1
/2
3/2
–1/2 –1/2
kadd/kc = 106 ± ± cm mol
Mots clés : thiophène, radical trifluorométhyle, mécanisme réactionnel, réactivité.
Traduit par la Rédaction] Herrera et al. 1481
s
.
[
Introduction
was an addition product, monosubstituted dihydro-CF3-
thiophene, also reported.
In the gas phase, however, no studies have been under-
The reactions of CF radicals with unsaturated organic
3
compounds such as olefins and aromatics in the gas phase
have been the subject of a number of investigations (1). Ki-
netic parameters and mechanisms were, in most cases, ratio-
nalized via two major pathways, addition and abstraction.
To the best of our knowledge, however, very few studies
have so far been undertaken of the reaction between CF3
radicals and heteroaromatic compounds (2). In particular,
with thiophene, the trifluoromethylation reaction was studied
in the condensed phase using bis(trifluoroacetyl) peroxide
taken of the reaction between CF radicals and thiophene. To
3
gain some insight into the distribution of the products, the
reaction mechanism, and the reactivity of thiophene toward
the CF radical, we became interested in the study of this re-
3
action, using the photolysis of CF I or HFA as CF radical
3
3
sources.
Results and discussion
(
3), CF I, and Te(CF ) (±). In all cases the product yields
3 3 2
were reported, monosubstituted CF -thiophenes being the
Experiments were carried out at 25 °C; the CF radicals
3
3
main products obtained in good yields. Only with Te(CF3)2
were generated by the photolysis of CF I or HFA at λ >
3
2
75 nm. Thiophene vapor has a weak absorption band start-
ing at about 260 nm and becomes very strong below 2±0 nm
(5). Therefore, under our experimental conditions, thiophene
should not absorb. However, to ensure that there was no
photolysis of thiophene, the spectral curve for the Corning
glass filter employed was checked experimentally, and con-
trol photolysis experiments with neat thiophene were per-
formed. Decomposition of thiophene was not observed.
The initial concentration of thiophene was varied between
Received 15 November 2002. Published on the NRC
Research Press Web site at http://canjchem.nrc.ca on
2
9 October 2003.
O.S. Herrera. Depto. de Química, Fac. de Ciencias
Naturales, Universidad Nacional de La Patagonia S.J.B.,
C.C. 786. 9000 Comodoro Rivadavia, Argentina.
1
J.D. Nieto, S.I. Lane, and E.V. Oexler. Instituto de
–
7
–3
Investigaciones en Fisicoquímica de Córdoba (INFIQC),
Depto. de Fisicoquímica, Fac. de Ciencias Químicas,
Universidad Nacional de Córdoba. Pab. Argentina. Ciudad
Universitaria. 5000 Córdoba, Argentina.
2
.±2 and 15.1 × 10 mol cm , while for the CF radical
3
–7
–3
sources, initial concentrations of 8.0 to 23.0 × 10 mol cm
and 5.09 to 10.8 × 10 mol cm were used for CF I and
–7
–3
3
HFA, respectively. Thus the molar ratio of thiophene:CF I or
3
1Corresponding author (e-mail:
oexler@fisquim.fcq.unc.edu.ar).
thiophene:HFA was always less than 2. Most of the experi-
ments were performed at a conversion of thiophene lower
Can. J. Chem. 81: 1±77–1±81 (2003)
doi: 10.1139/V03-155
© 2003 NRC Canada

1
478
Can. J. Chem. Vol. 81, 2003
than 20%. Products analysis was performed after separation
of the total content of the reaction cell by fractional conden-
sation at –±0 °C.
Moreover, results in Table 2 also show that with HFA as
the source of CF radicals, the total yields of monosub-
3
stituted CF -thiophenes seem higher than with CF I (for the
3
3
In the series of experiments carried out with CF I as the
same thiophene concentration and photolysis time). This
variation in measured yields would reflect a difference in ef-
ficiency of CF I or HFA as sources of CF radicals.
3
source of CF radicals, the only products observed in the
3
volatile fraction at –±0 °C were CF H and C F , in addition
3
2
6
3
3
to unreacted CF3I.
In the heavier fraction analyzed by GC–MS, in addition to
the unreacted thiophene, two isomers of monosubstituted
One of the features worth noting in Tables 1 and 2 is the
high ratio (~16) of 2-CF3-thiophene : 3-CF3-thiophene, re-
gardless of the radical source. This evidence would support
the well-known fact that the preferential position for electro-
philic attack on thiophene is C2 (8), also confirmed by com-
puter charge density data of thiophene (9), which determined
that the C2(C5) positions are richer in electron density than
C3(C±).
CF -thiophene were separated and identified as 2- and 3-
3
CF -thiophene. Monosubstituted dihydro-CF -thiophene was
3
3
also found in very small proportion, as well as traces of 2-
iodothiophene in the high-conversion runs. However, no
other products, as from the addition of CF I to the double
3
bond, were observed, in spite of an exhaustive search by
GC–MS, even at high conversions. The colour of the heavier
fraction, when it was condensed, showed clearly that iodine
was also produced, but again no products from its addition
reaction or the addition of I atoms to thiophene were ob-
served. This was also confirmed experimentally by
It can also be observed in Tables 1 and 2 that in each run,
the yield of CF3H is practically equal to the total yield of
(2+3)-CF3-thiophene. While the yields of CF3H increase
with increasing concentrations of thiophene, the C2F6 yields
decrease.
1/2
Figure 1 shows that a plot of RCF H /R (R denotes rate
C F
2 6
3
photolysis of I in the presence of thiophene over periods
much longer than those used in the actual experiments.
of formation) against [thiophene], of the data points up to
20% conversion, is a straight line with a zero intercept and
2
3
/2
–1/2 –1/2
With HFA as the CF radical source, the same products
with a slope equal to 106 ± ± cm mol
s
, irrespective
3
mentioned above — CF H, C F , monosubstituted CF -
of the source of CF3 radicals (CF3I or HFA) and also at dif-
ferent concentrations of these compounds.
3
2
6
3
thiophenes, and monosubstituted dihydro-CF -thiophene —
3
were found. We could not exclude that the CF CO radical is
On account of these results, the following could be con-
sidered to be the main steps of the reaction mechanism for
the reaction of the CF3 radicals produced by the photolysis
of CF3I or HFA with thiophene:
3
formed in the primary process when HFA is photolyzed, but
several studies have shown that this radical is fairly unstable
and rapidly decomposes by CO elimination (6, 7). In the
present work, CO was detected by gas chromatography on a
Molecular Sieves column, although its yield was not mea-
k
add
[
[
1]
2]
CF + thiophene   → CF -thiophene
3
3
(add)
sured. However, even if CF CO radicals are formed, the data
k
H
3
CF + thiophene → CF H + thienyl radical
3
3
seem to indicate that they do not affect the reported mea-
surements. Traces of a product, tentatively identified as
[
3]
CF + CF -thiophene
→ CF H
3
3
(add)
3
disubstituted dihydro-CF -thiophene, were also observed in
3
+
CF -thiophene (monosubstituted (monosubs.))
3
experiments at very high conversion but were practically ab-
sent in the series of experiments carried out with CF3I.
The effect of variations of concentration of thiophene,
[
±]
2 CF -thiophene
→
(add)
3
CF I, or HFA and photolysis time on the yields of CF -
3
3
dihydro-CF -thiophene (monosubs.)
3
thiophenes and the other products mentioned above are
shown in Tables 1 and 2, respectively. Most of the runs were
performed in triplicate to ensure that the separation proce-
dure, previous to analysis, did not affect the precision of the
results. Hence, the reported values of the yields are average
values. Furthermore, a second series of runs carried out un-
der the same experimental conditions but without fractional
condensation at –±0 °C was analyzed chromatographically
on a Porapak T column, and the yields of CF H and C F re-
+
CF -thiophene (monosubs.)
3
k
c
[
5]
2 CF → C F
3 2 6
Other reactions, such as the self-combination of CF3-
thiophene(add) or CF + CF -thiophene to give disubsti-
tuted dihydro-CF -thiophene, could also be included in this
mechanism for the purpose of completeness. However, the
products of these combination reactions were not found;
only trace amounts of disubstituted dihydro-CF -thiophene
3
3
(add)
3
3
2 6
mained essentially unchanged within experimental error.
3
The total yield of (2+3)-CF -thiophenes shows a gradual
were detected when HFA was used as the CF3 radical
source, as mentioned previously.
3
increase with increasing concentration of thiophene em-
ployed. However, concentrations of thiophene higher than
According to the reaction scheme, the formation of CF H
3
–
7
–3
5
.0 × 10 mol cm were not used because a light brown de-
could take place by the H-abstraction reaction, eq. [2], and
by the pseudo-H-abstraction reaction, eq. [3], the latter
equivalent to that proposed in the gas-phase reactions of CF3
radicals with benzene (10) and substituted benzenes (11, 12)
for the formation of CF H and CF -benzenes. In these, as
posit, probably of a polymeric material, was observed on the
wall of the reaction cell after several experiments with con-
centrations higher than 15.0 × 10–7 mol cm had been car-
ried out at high conversion. On the other hand, the
separation of the reaction mixture into its components by
GC on the SE 30 column was unsuccessful, largely because
of the small difference in the retention times of 2- and 3-
–3
3
3
well as in the present study, the concentrations of the sub-
strates used were low, thus ensuring that the concentration of
CF radicals is large enough for the reaction described by
3
CF -thiophene with respect to thiophene.
eq. [3] to occur, at variance with what has been argued for
3
©
2003 NRC Canada

Herrera et al.
1479
Table 1. Product yields for the reaction of CF radicals with thiophene at 25 °C; CF I as the source of CF radicals.
3
3
3
b
Ratio 2-CF3-
thioph. : 3-CF3-
thioph.
Ratio dihydro-
7
[CF I]×107
(mol cm )
[
(
thioph.]×10
mol cm )
Photolysis
time (s)
(2+3)-CF -thioph.
(mol×10 )
CF -thioph. :
(2+3)-CF -thioph.
CF H
(mol×10 )
C2F6
(mol×10 )
3
3
3
3
–
3 a
–3 a
5
5
8
3
6
5
8
8
7
7
7
7
8
1
1
1
1
1
1
1
1
1
1
.50
.00
.07
.08
.75
.75
.95
.96
.50
0.3
0.8
0.7
0.8
0.8
1.6
3.5
5.1
±.±
±.6
9.60
11.7
10.1
22.9
10.±
8.00
9.80
10.9
10.±
10.3
9.15
23.1
22.9
23.0
10.6
22.9
10.5
11.0
11.1
1 800
5 100
3 600
3 600
7 200
7 200
1 800
10 800
10 800
1 800
3 600
1 800
3 600
5 100
5 100
3 600
1 800
3 600
5 100
0.5±
1.1±
1.3±
1.±6
2.07
2.05
0.6±
3.03
3.13
0.86
1.67
0.90
1.80
2.3±
2.±2
2.23
1.05
2.08
2.97
16.8
18.7
16.9
15.9
17.1
16.8
1±.9
16.3
16.5
15.1
17.3
15.9
16.1
16.0
15.2
16.1
16.6
1±.3
13.9
0.9
1.1
0.8
0.9
1.3
0.7
0.9
1.1
1.5
1.0
1.±
0.9
1.2
1.1
1.2
1.0
0.8
1.1
1.2
0.56
1.19
1.33
1.±7
2.09
2.0±
0.65
3.06
3.11
0.8±
1.69
0.92
1.78
2.36
2.±1
2.25
1.07
2.10
2.96
3.76
11.6
7.69
9.53
1±.6
13.9
3.2±
18.8
18.6
3.22
6.36
3.53
7.32
10.0
8.00
7.05
2.61
5.26
7.56
Note: thioph. = thiophene.
a
b
3
Vcell = 109 cm .
The values given are equal to 100 × dihydro-CF -thiophene : (2+3)-CF -thiophene.
3
3
Table 2. Product yields for the reaction of CF radicals with thiophene at 25 °C; HFA as the source of CF radicals.
3
3
b
Ratio 2-CF3-
thioph. : 3-
Ratio dihydro-
7
[HFA]×107
(mol cm )
[
(
thioph.]×10
mol cm )
Photolysis
time (s)
(2+3)-CF -thioph.
(mol×10 )
CF -thioph. :
CF H
(mol×10 )
C2F6
(mol×10 )
3
3
3
–
3 a
–3 a
5
5
7
CF -thioph.
(2+3)-CF -thioph.
3
3
2
±
5
5
5
7
7
7
8
1
1
1
1
.±2
.30
.30
.60
.92
.±0
.26
.85
.70
0.±
0.6
2.±
2.5
a
10.8
8.01
7.20
6.80
6.72
7.60
7.53
10.5
10.2
11.8
10.8
11.3
12.1
3600
1800
1800
3600
7200
900
1800
3600
3600
900
0.92
0.97
1.16
2.00
3.57
0.73
1.±1
2.71
3.06
1.02
2.0±
3.92
6.62
±.5
1±.5
15.0
1±.9
16.3
17.2
17.1
15.3
15.8
16.5
16.2
15.6
15.9
1.±
1.3
1.1
1.3
1.2
1.0
0.9
1.3
1.±
1.1
1.3
0.9
1.±
0.90
0.96
1.15
2.02
3.55
0.76
1.±3
2.73
3.08
1.0±
2.02
3.90
6.60
5.89
2.87
2.59
±.92
9.50
1.12
2.29
±.2±
±.20
1.03
1.96
3.32
6.±7
1800
3600
7200
3
Vcell = 109 cm .
b
The values given are equal to 100 × dihydro-CF -thiophene : (2+3)-CF -thiophene.
3
3
these reactions in the liquid phase, where pseudo-abstraction
is unlikely to proceed (13).
eq. [±], would be an unimportant channel for the formation
of monosubstituted CF -thiophenes. Therefore, this reaction,
3
In the absence of any extra production of CF H by reac-
eq. [±], which also consumes the adduct formed in the addi-
tion step in eq. [1], could be considered negligible, so that
3
tions other than those described in eqs. [2] and [3], the rate
of formation of CF H would be:
3
[
7]
k [CF ][CF -thiophene(add)] ≈
3 3 3
[
6]
R
= k [CF ][thiophene]
CF H
H 3
3
kadd[CF ][thiophene]
3
+
k [CF ][CF -thiophene(add)]
3 3 3
and the rate of formation of CF H can be made approxi-
3
Taking into account that monosubstituted dihydro-CF3-
thiophene was produced, to the extent of about 1% of the
mately equal to
[
8]
R
≈ (kH + kadd) [CF ][thiophene]
CF H
3
(
2+3)-CF -thiophenes, the disproportionation reaction,
3
3
©
2003 NRC Canada

1
480
Can. J. Chem. Vol. 81, 2003
1
C2F6
/2
Fig. 1. Plot of RCF 3H /R
as a function of [thiophene] at 25 °C,
The following expression for k was obtained:
H
of the data points up to 20% conversion. ٗ CF I, ~10 ×
3
[11] kH = 5.5 × 1011 exp[–(5150) K/T] (cm mol s )
3
–1 –1
–
7
–3
–7
–3
1
1
0
0
mol cm ; ᭿ CF I, ~23 × 10 mol cm ; ᭺ HFA, ~10 ×
3
–
7
–3
mol cm .
The rate constant for CF recombination, k , has been mea-
3
c
sured near room temperature by various workers using dif-
ferent techniques, and the reported values (1) show more
than an order of magnitude variation, ranging from about
1
2
13
3
–1 –1
1
.3 × 10 to 2.± × 10 cm mol s . Thus, the rate
constant at 25 °C for the H-abstraction reaction from
1
/2
thiophene relative to k
0 –10 lower than kadd/kc . Therefore, the dispropor-
would be approximately a factor of
c
2
3
1/2
1
tionation reaction, eq. [3], could be considered the main step
for the production of CF H, and so the value of 106 ±
3
3
/2
–1/2 –1/2
1/2
±
cm mol
s
could be assigned to kadd/kc .
As far as we know, insufficient data are available in the
gas phase for the reactions of CF radicals with aromatic and
3
heteroaromatic compounds to allow any detailed compari-
sons. Moreover, the reported data are relative values of the
rate constants. Absolute rate constants have only been re-
ported for reactions of CF radicals with alkenes in the gas
3
phase (15) and in solution (16). However, we could compare
the reactivity of thiophene toward CF radicals with other
3
nonsaturated compounds, alkenes such as ethene and buta-
diene, and aromatics such as benzene.
1
/2
The values of kadd/kc for the addition reactions of CF to
3
the foregoing compounds are displayed in the first columm
of Table 3 for comparison purposes. The data obtained in
this work indicate that the reactivity of thiophene towards
Taking into account that
1
/2
= kc1/2[CF3]
C F
6
[
9]
R
2
CF radical addition is intermediate between the olefins and
3
benzene. A similar sequence in reactivity is observed in the
reactions of these compounds with the electrophilic OH
radical. This trend may reflect the effects of the resonance
stabilization energies (resonance stabilization energies
a simple expression can be found for the relative rate of for-
mation of CF H
3
1
/2
2
[
10]
R
/R
≈ {(kH + kadd)/kc1/2}[thiophene]
CF H
C F
3
6
–1
–1
are ~138–151 kJ mol for benzene, ~92 kJ mol for
–
1
1/2
thiophene, and ~15 kJ mol for 1,3-butadiene (20)), indicat-
ing that resonance stabilization decreases the reactivity of
A plot of R
/R
as a function of [thiophene], shown in
C F
2 6
CF H
3
Fig. 1, gives a straight line with zero intercept, which is in-
the double bonds toward CF radical addition, as has been
1
/2
3
dicative that eq. [10] holds, and thus, (k + kadd)/kc may be
H
pointed out previously by Atkinson et al. (21) for the OH ad-
dition to the same compounds.
3
/2
–1/2 –1/2
related to the observed value 106 ± ± cm mol
tained from the slope of Fig. 1 at 25 °C.
s
ob-
We conclude that, for the reaction of CF radicals and
3
To evaluate the contribution made by the endothermic H-
abstraction reaction, eq. [2],
thiophene in the gas phase at 25 °C, the formation of the
monosubstituted products, 2- and 3-CF -thiophene, can be
3
k
explained through a similar mechanism to that proposed for
other aromatic compounds (benzenes and halobenzenes) in-
H
CF + thiophene → CF H + thienyl radical
3
3
toward the formation of CF H, we calculated the activation
volving the addition of the CF radical to thiophene followed
3
3
energy and the pre-exponential factor for the H-abstraction
reaction, eq. [2], applying the bond-strength-bond-length
preferentially by the subsequent disproportonation reaction.
The addition rate constant is intermediate between that of
the olefins and benzene.
(
BSBL) – transition-state-theory method, developed by
Bérces and Dombi (1±), based on the bond-energy-bond-
order (BEBO) method. With this treatment, good results
have been obtained for the estimation of the kinetic parame-
ters for metathesis reactions involving unsaturated reaction
centers. The BSBL reaction profile for the H-abstraction re-
action was calculated along the reaction path, and the maxi-
mum value of the potential energy — that is, the potential
activation energy and, consequently, the transition state —
occurs in a late stage of the reaction, as expected for an en-
dothermic process. The pre-exponential factor and the acti-
vation energy were estimated by considering that the
activated complex is well represented by a linear five-mass-
point model.
Experimental section
The high-vacuum apparatus employed was similar to that
described in ref. 12. Briefly, the photolysis experiments were
carried out in a cylindrical quartz cell (V = 109 cm ) con-
nected through Teflon valves to a conventional grease-free
vacuum line and storage traps, with the reactants kept at liq-
3
uid N temperature.
2
The light source was an Osram HBO 500 W high-pressure
mercury lamp. A Corning Pyrex Glass Plate 77±0 (2 mm
thick) was used to remove wavelengths lower than 275 nm
(22).
©
2003 NRC Canada

Herrera et al.
1481
Table 3. Addition rate constants for some reactions of CF and
OH radicals at 25 °C.
3
Acknowledgments
The authors thank CONICET (Argentina), AGENCIA
CORDOBA CIENCIA (Córdoba, Argentina), SECyT-U.N.C.
1
/2
a
kadd/k
(CF )
k (OH)
c
3
/2
–1/2
s–1/2
3
–1 –1
Compound
Ethene
(cm3 mol
)
(cm mol s )
(
Córdoba,
Argentina),
and
CIUNPAT-U.N.P.S.J.B.
377b
12
5.1±×10
±.12×10
5.7×10
7.3×10
(Comodoro Rivadavia, Argentina) for partial financial sup-
port.
13
1
,3-Butadiene
~3000c
d
12
Thiophene
Benzene
a
106
e
11
31
References
Reference 17. Recommended high-pressure limit rate constants at room
temperature with an estimated uncertainty at 25 °C of about ±20%.
1
. NIST standard reference data. Chemical kinetics database 17.
U. S. Department of Commerce. Gaithersburg, Md. 1999.
b
Reference 18. Extrapolated value calculated at 25 °C using the reported
activation parameters.
2. L. Pasteris and E.H. Staricco. J. Chem. Soc. Faraday Trans. 2,
2, 1991 (1986), and refs. quoted therein.
c
Reference 19. Estimated from the competitive reaction with ethene at
8
1
73 °C.
3
. N. Sawada, M. Nakayama, M. Yoshida, T. Yoshida, and N.
d
This work.
e
Kamigata. J. Fluorine Chem. 46, ±23 (1990).
Extrapolated value calculated at 25 °C using the activation parameters
±
. D. Naumann and J. Kischkewitz. J. Fluorine Chem. 46, 265
reported in ref. 10.
(
1990).
5
6
. H.A. Wiebe and J. Heicklen. Can. J. Chem. 47, 2965 (1969).
. M.M. Maricq, J.J. Szente, T.S. Dibble, and J.S. Francisco. J.
Phys. Chem. 99, 11 875 (1995).
In a typical experiment, the gaseous reactants were intro-
duced into the evacuated reaction vessel, and pressures were
measured. After a period of irradiation the reaction vessel
contents were immediately collected in a trap cooled in liq-
7
8
9
. A. Thomas, F. Caralp, and R. Lesclaux. Z. Phys. Chem. 214,
1
3±9 (2000).
. G. Vernim, H.J.M. Dou, and J. Metzger. Bull. Soc. Chim. Fr.
17± (1972).
uid N and separated by fractional distillation at –±0 °C. In
2
some experiments the volatile portion at this temperature
was distilled into an evacuated gas chromatograph sample
loop and passed through a 2 m column packed with Porapak
T 80/100 mesh at ±0 °C. Product samples were collected
1
. N. Karunanidhi, V. Kannappan, and U. Ponnambalan. Acta
Ciencia Indica, 17, 279 (1991).
1
0. G.A. Chamberlain and E. Whittle. Trans. Faraday Soc. 67,
downstream for IR and MS identification. CF H and C F
3
2
6
2077 (1971), and refs. quoted therein.
were identified by comparison of their GC retention times
and their IR and MS spectra with authentic samples. Known
concentrations of the appropriate standards were used for
calibration purposes.
11. S.I. Lane, E.V. Oexler, and E.H. Staricco. Int. J. Chem. Kinet.
23, 361 (1991).
12. J.D. Nieto, S.I. Lane, and E.V. Oexler. Ber. Bunsenges. Phys.
Chem. 101, 868 (1997).
13. J.M. Birchall, G.P. Irvin, and R.A. Boyson. J. Chem. Soc.
Perkin 2, ±35 (1975).
The residue remaining at –±0 °C was analysed by GC–MS
using a capillary column (0.32 mm i.d. × 30 m, SE-30). To
build up a sufficient amount of some of the products for
identification, special preparative photolyses were carried
out, generally at higher conversion. Mass spectrometry anal-
ysis (GC–MS) gave the theoretical parent mass for 2- and 3-
CF -thiophene at m/z = 152 (23), for dihydro-CF -thiophene
1
1
1
±. T. Bérces and J. Dombi. Int. J. Chem. Kinet. 12, 183 (1983),
and refs. quoted therein.
5. Z. Linyang, W. Fuss, and K.L. Kompa. Ber. Bunsenges. Phys.
Chem. 102, 821 (1998).
6. D.V. Avila, K.U. Ingold, J. Lusztyk, W.R. Dolbier, Jr., and
3
3
H.Q. Pan. J. Org. Chem. 61, 2027 (1996).
at m/z = 15±, and for 2-iodothiophene at m/z = 210 (2±).
Control experiments at room temperature in the dark
showed that no product formation occurred thermally under
standard reaction conditions, not even in periods 2–3 times
longer than the usual irradiation times.
Infrared spectra were recorded using a Nicolet 55X FT in-
frared spectrometer and mass spectra were obtained with a
Finnigan 3300 instrument and a PerkinElmer QM 910 GC–
MS.
1
1
7. R. Atkinson. Chem. Rev. 86, 69 (1986).
8. H.C. Low, J.M. Tedder, and J.C. Walton. Trans. Faraday Soc.
7
2, 1300 (1976).
9. A. El Soueni, J.M. Tedder, and J.C. Walton. J. Fluorine Chem.
7, 51 (1981).
1
2
2
1
0. S.W. Benson. Thermochemical kinetics. 2nd ed. Wiley, New
York. 1976. p.60.
1. R. Atkinson, S.M. Aschmann, and W.P.L. Carter. Int. J. Chem.
Kinet. 15, 51 (1983).
Thiophene, CF I, and HFA, as well as the reference com-
3
22. J.G. Calvert and J.N. Pitts. Photochemistry. John Wiley and
pounds CF H, C F , and 2-iodothiophene, were obtained
Sons, Inc., New York. 1966.
3
2 6
commercially and were trap-to-trap distilled in a vacuum
line, several times, and thoroughly degassed prior to use.
Their purity was checked by IR spectroscopy and GC–MS.
23. B. Verkoczy, A.G. Sherwood, I. Sofarik, and O.P. Strauz. Can.
J. Chem. 61, 2268 (1983).
2±. B. Akesson and S. Gronowitz. Ark. Kemi. 28, 155 (1967).
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2003 NRC Canada