Temperature dependence of the reactions of phenyl radicals with 1,1-diphenylethylene, carbon tetrachloride, and cyclohexene

Article abstract of DOI:10.1021/jo961008n

The reaction of phenyl radicals with 1,1-diphenylethylene (DPE), carbon tetrachloride, and cyclohexene has been examined over a range of temperatures using laser flash photolysis techniques. The activation energies are 0.71 ± 0.04, 3.53 ± 0.11 and 2.32 ± 0.33 kcal/mol, and the preexponential factors, expressed as log (A/M^-1s^-1), are 9.19 ± 0.03, 8.96 ± 0.09, and 9.02 ± 0.26, for DPE, CCl₄, and c-C₆H₁₀, respectively. In particular the CCl₄ data provide a much needed reference for competitive studies.

Full text of DOI:10.1021/jo961008n

8544
J . Org. Chem. 1996, 61, 8544-8546
Tem p er a tu r e Dep en d en ce of th e Rea ction s of P h en yl Ra d ica ls
w ith 1,1-Dip h en yleth ylen e, Ca r bon Tetr a ch lor id e, a n d
Cycloh exen e†
Dean Weldon, Stacey Holland, and J . C. Scaiano*
Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada, K1N 6N5
Received May 30, 1996X
The reaction of phenyl radicals with 1,1-diphenylethylene (DPE), carbon tetrachloride, and
cyclohexene has been examined over a range of temperatures using laser flash photolysis techniques.
The activation energies are 0.71 ( 0.04, 3.53 ( 0.11 and 2.32 ( 0.33 kcal/mol, and the preexponential
factors, expressed as log (A/M-1s-1), are 9.19 ( 0.03, 8.96 ( 0.09, and 9.02 ( 0.26, for DPE, CCl4,
and c-C6H10, respectively. In particular the CCl4 data provide a much needed reference for
competitive studies.
Several absolute rate constants for the reactions of
using laser and ESR techniques, showed that decarbox-
ylation of benzoyloxyl radicals is not as fast as assumed
in our earlier report.15-19 We were fortunate in our
earlier phenyl radical study that most rate constants
were verified with iodobenzene (which obviously cannot
incorporate benzoyloxyl-related errors) as a radical pre-
cursor. A worrisome exception was the case of cyclohex-
ene, a system where the rate constant was not verified
with iodobenzene as a phenyl radical precursor;1 further,
the presence of allylic hydrogens makes it a good candi-
date for high reactivity toward oxygen-centered radi-
cals.20 Our choice of cyclohexene as a second substrate
was a reflection of the concerns mentioned above. Our
use of 1,1,2-trichloro-trifluoroethane (Freon-113) as sol-
vent follows on our earlier observation that it is a
relatively unreactive solvent toward phenyl radicals.
phenyl radicals in solution have been reported during the
last 15 years.1-4 Most of those involving unsubstituted
phenyl radicals in solution derive from our 1983 article.1
In addition, a few values have been reported in the gas
phase,5-7 and in aqueous solution for p-carboxyphenyl
radicals.3,8-10 In fact, an early article by Madhavan,
Schuler, and Fessenden8 reported the first absolute
values in solution. The same radical has also been
examined in recent work.10 To the best of our knowledge,
the temperature dependence of phenyl radical reactions
has not been examined in solution, and only a very few
systems have been examined in the gas phase.5-7
Our choice of substrates deserves some comment.
Carbon tetrachloride has been a frequent choice as
reference substrate by those carrying out competitive
studies.11-14 Absolute values of the rate constants and
the corresponding activation parameters facilitate the
conversion of data from competitive studies into absolute
values. The reasons are somewhat different in the case
of cyclohexene. In our earlier work,1 our sources of
phenyl radicals were the photodecompositions of iodo-
benzene and of benzoyl peroxide. The latter decomposes
to benzoyloxyl radicals, that at the time were believed
to decarboxylate very rapidly to yield phenyl radicals. In
the late 1980s, work by Ingold’s and Tokumaru’s groups,
The kinetic studies reported herein were carried out
with laser flash photolysis techniques (see Experimental
Section). Phenyl radicals are not readily detectable in
the spectral region (near UV and visible) where typical
phenyl radical precursors (such as iodobenzene or benzoyl
peroxide) are transparent. We thus resorted to the
“probe technique”, a method that we developed about 20
years ago and that has proven useful in the study of
numerous free radicals and other reaction intermedi-
ates.21 In the probe technique, one needs to identify a
substrate that upon reaction with the intermediate of
interest yields a product with a convenient absorption
in the spectral region accessible. The “product” may be
a transient that is long-lived in the time scale of interest.
In our earlier work on phenyl radicals,1 we used di-
phenylmethanol (yielding diphenyl ketyl radicals) or
â-methylstyrene (yielding readily detectable benzylic
† Dedicated to Professor A. L. J . Beckwith, a friend who knows how
to enjoy a good rate constant, on the occasion of his retirement from
the Australian National University.
X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Scaiano, J . C.; Stewart, L. C. J . Am. Chem. Soc. 1983, 105, 3609.
(2) Mertens, R.; von Sonntag, C. Angew. Chem., Int. Ed. Engl. 1994,
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265.
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Soc. 1978, 100, 888.
(9) Sommeling, P. M.; Mulder, P.; Louw, R.; Avila, D. V.; Lusztyk,
J .; Ingold, K. U. J . Phys. Chem. 1993, 97, 8362.
(10) Garden, S. J .; Avila, D. V.; Beckwith, A. L. J .; Bowry, V. W.;
Ingold, K. U.; Lusztyk, J . J . Org. Chem. 1996, 61, 805.
(11) Russell, G. A.; Bridger, R. F. J . Am. Chem. Soc. 1963, 85, 3765.
(12) Migita, T.; Takayama, K.; Abe, Y.; Kosugi, M. J . Chem. Soc.,
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1988, 110, 2886.
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1988, 110, 2877.
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K. Chem. Lett. 1992, 563.
(20) Paul, H.; Small, R. D., J r.; Scaiano, J . C. J . Am. Chem. Soc.
1978, 100, 4520.
(21) This approach has been used in
a number of cases. E.g.:
(13) Pryor, W. A.; Echols, J . T., J r.; Smith, K. J . Am. Chem. Soc.
1966, 88, 1189.
(14) Kopinke, F. D.; Zimmermann, G.; Anders, K. J . Org. Chem.
1989, 54, 3571.
Malatesta, V.; Scaiano, J . C. J . Org. Chem. 1982, 47, 1455. Griller, D.;
Howard, J . A.; Marriott, P. R.; Scaiano, J . C. J . Am. Chem. Soc. 1981,
103, 619. Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1982, 104, 5123.
S0022-3263(96)01008-0 CCC: $12.00 © 1996 American Chemical Society

Temperature Dependence of the Reactions of Phenyl Radicals
J . Org. Chem., Vol. 61, No. 24, 1996 8545
F igu r e 1. Concentration dependence of the growth rate
constant, kgrowth, for the reaction of phenyl radical with 1,1-
diphenylethylene in freon-113 at three different temperatures,
41 °C (0), -1 °C (4), and -33 °C (O). Monitored at 330 nm.
F igu r e 2. Concentration dependence of the growth rate
constant, kgrowth, for the reaction of phenyl radical with CCl4
in freon-113 at three different temperatures, 40 °C (9), 0 °C
(2), and -33 °C (b). Probed with 2 mM 1,1-diphenylethylene
and monitored at 330 nm.
radicals) as probes for the phenyl radical. In the present
study, we have used 1,1-diphenylethylene as probe.
Thus, reactions 1 and 2 yield a strongly absorbing free
radical (I) with λmax ∼330 nm.
PhI
9
hν8 Ph• + I•
(1)
(2)
•
PhCH -C˙ Ph
Ph + CH2dCPh2 f
2
I
2
The growth of I following laser excitation follows
pseudo first order kinetics (kgrowth) according to eq 322
kgrowth ) k0 + k2[Ph2CdCH2]
(3)
where k0 is the rate constant for radical decay in the
absence of scavenger, and in this case, may be dominated
by reaction with the solvent. Figure 1 shows plots from
1,1-diphenylethylene according to eq 3 at three different
temperatures. From the slopes one obtains k2’s of (3.54
( 0.13) × 108 M-1 s-1, (4.16 ( 0.15) × 108 M-1 s-1, (5.05
( 0.11) × 108 M-1 s-1 at -33 °C, -1 °C, and 41 °C,
respectively.
Addition to the system of a substrate that does not
yield a signal upon reaction with phenyl radicals (e.g.
CCl4, see reaction 4) leads to growth kinetics for I that
can be expressed by eq 5.
F igu r e 3. Arrhenius plots for the temperature dependence
of the reactions of 1,1-diphenylethylene (9), cyclohexene (2),
and carbon tetrachloride (b) with phenyl radicals in freon-
113. The phenyl radicals were produced from 308 nm laser
flash photolysis of iodobenzene. In the cases of cyclohexene
and carbon tetrachloride the reactions were probed with 2 mM
1,1-diphenylethylene.
Ta ble 1. Kin etic Da ta fr om th e Va r ia ble Tem p er a tu r e
Stu d ies
Ea,
kcal/mol
log
(A/M-1 s-1
k(M-1 s-1
)
at 25 °C
substrate
)
1,1-diphenylethylene 0.71 ( 0.07 9.19 ( 0.03 (4.66 ( 0.04) × 108
CCl4
cyclohexene
3.53 ( 0.11 8.96 ( 0.09 (2.33 ( 0.06) × 106
2.32 ( 0.33 9.02 ( 0.26 (2.07 ( 0.14) × 107
Ph• + CCl4 f PhCl + Cl3C•
(4)
(5)
mination are available as supporting information. Analy-
sis of the slopes and intercepts in Figure 3 give the data
of Table 1.
kgrowth ) k0 + k2[Ph2CdCH2] + k4[CCl4]
Thus, if the concentration of 1,1-diphenylethylene is
kept constant, and that of the substrate (carbon tetra-
chloride in the example above) is changed, eq 5 allows
the determination of the rate constant for reaction 4, even
if this reaction involves reactants and products, all of
which are invisible to the technique employed. A similar
analysis allows the determination of rate constants for
cyclohexene where the dominant reaction is presumed
to be abstraction of allylic hydrogens. Typical quenching
plots are shown in Figure 2. The experiments can be
repeated at various temperatures, thus leading to the
Arrhenius plots of Figure 3. Full details of each deter-
For CCl4, the values compare well with gas phase data
which gives log (A/M-1 s-1) of 9.10 ( 0.08 and an
activation energy of 2.8 ( 0.2 kcal/mol.5,7 The room
temperature rate constants are in line with those re-
ported earlier by Lorand23 and by our group.1 For
cyclohexene, the rate constants at room temperature are
about 1 order of magnitude lower than reported earlier,
suggesting that benzoyloxyl radicals must have intro-
duced errors in our earlier determination of this value,
where the relatively modest reactivity of phenyl and high
reactivity of benzoyloxyl must have compounded to lead
to the high rate constant reported earlier.1
(22) The fate of the iodine atoms has been discussed in an earlier
publication (see ref 1). They do not interfere with the determination
(23) Kryger, R. G.; Lorand, J . P.; Stevens, N. R.; Herron, N. R. J .
Am. Chem. Soc. 1977, 99, 7589.
of kgrowth
.

8546 J . Org. Chem., Vol. 61, No. 24, 1996
Weldon et al.
to 4.0 mL with the PhI solution. The solution, capped with
a white rubber septum, was then purged of oxygen by bubbling
nitrogen into the solution through a syringe needle for 10 min.
The septum was fitted with a second needle to allow the gases
to escape. To minimize solvent evaporation, the cuvette was
immersed in an ice bath for the duration of the deoxygenating.
After 10 min, the cuvette was removed from the nitrogen line,
and the septum was wrapped with parafilm. Once in the laser
flash photolysis system, the samples were each allowed 10 min
to thermally equilibrate with the particular temperature of
the system. The kinetic traces are an average of each sample
being exposed 10 times by the system. Ten exposures gives
excellent averaged signals, while minimizing product build-
up. A fresh sample was prepared for each measurement.
Cycloh exen e Qu en ch in g. Because the products of the
reaction of Ph• and cyclohexene are invisible, DPE was added
to the solutions as a probe. A 2 mM solution of DPE was added
to a 36 mM solution of PhI in freon-113. The samples were
then treated identically as above except that the aliquots of
pure cyclohexene were injected only after the 10 min oxygen
purge. Once injected with cyclohexene, the samples were then
purged for an additional 2 min.
Ca r bon Tetr a ch lor id e Qu en ch in g. As for cyclohexene,
DPE was used as a probe for the reaction of CCl4 with Ph•.
Unlike cyclohexene, the CCl4 was added before the 10 min
purge. Because the reaction of CCl4 with Ph• is much slower,
larger volumes of CCl4 were required. This required the
addition of both PhI and DPE to the CCl4 in order to match
the 36 mM and 2 mM concentrations in freon-113.
Kin etic Mea su r em en ts. The laser system employed in
these experiments uses a Lumonics EX-530 for 308 nm
excitation. Pulse duration is ca. 6 ns, and typical pulse
energies are between 5 and 50 mJ . The signals from the
monochromator/photomultiplier system were initially captured
by a Tektronix 2440 digitizer and transferred to a Power-
Macintosh computer that controlled the experiment with
software developed in the LabVIEW 3.1.1 environment from
National Instruments. Other aspects of the system are similar
to those described earlier.24,25
In particular, the CCl4 data should provide a much
needed reference for competitive studies in solution.
While data for other substrates has recently become
available in aqueous solution, it should be noted that
reactivities by phenyl and p-carboxyphenyl appear to be
somewhat different, at least in the case of oxygen.2-4
Similarly with other substrates there are significant
differences (occasionally exceeding a factor of 2) reported
between phenyl and substituted derivatives;3 in any
event, it is clear that the fact that phenyl is a σ-radical
still makes these differences smaller than they would be
otherwise. Further, studies with CCl4, the “standard”
reference substrate, are limited to organic media, where
CCl4 is soluble.
We believe that for other substrates in our earlier
report,1 there is no need to revise the values for phenyl
radical reactivity. Among the few substrates that were
not examined with iodobenzene as a phenyl precursor,
carbon tetrachloride is unreactive toward benzoyloxyl,
and our current results agree well with the earlier
report.1 Further, under the conditions of our earlier work
benzoyloxyl can decarboxylate photochemically in a two-
photon process16 (thus adding an instantaneous source
of phenyl), and unsubstituted benzoyloxyl has a lifetime
of ca. 500 ns at infinite dilution and shorter under most
experimental conditions.15,16
Given the essentially identical values of the three
preexponential factors determined here, as well as the
gas phase value for CCl4, we suggest that when the
temperature dependence is not available, it may be a
reasonable approximation to assume a value of log (A/
M-1 s-1) ) 9.1. While not perfect, such an approximation
will allow the estimation of rate constants at other
temperatures.
Exp er im en ta l Section
Ack n ow led gm en t. This work has been supported
by the Natural Sciences and Engineering Research
Council of Canada. J .C.S. is the recipient of a Killam
Fellowship awarded by the Canada Council.
1,1,2-Trichlorotriflouroethane (freon-113) (99+%, spectro-
photometric grade) and cyclohexene (99+%) were purchased
from Aldrich and used as received. Carbon tetrachloride
(99.9%, HPLC grade) was purchased from Sigma-Aldrich and
was also used as received. 1,1-Diphenylethylene (97%, Ald-
rich) and iodobenzene (98%, Acros) were first passed through
a small plug of silica gel prior to usage.
1,1-Dip h en yleth ylen e Qu en ch in g. A solution of iodo-
benzene (PhI) in freon-113 was prepared such that its absorb-
tion was ∼0.3 at the excitation wavelength (308 nm). A 0.556
M stock solution of 1,1-diphenylethylene (DPE) in freon-113
was also prepared (1:10 dilution). For each sample, a specific
volume of the DPE solution was injected via syringe into a 7
mm × 7 mm square quartz cuvette. Typical volumes injected
were 20, 40, 60, 80, or 100 µL. The sample was then diluted
Su p p or tin g In for m a tion Ava ila ble: Tables S1-S19 of
rate constants and quenching plots (22 pages). This material
is contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS, see any current masthead page for
ordering information.
J O961008N
(24) Scaiano, J . C. J . Am. Chem. Soc. 1980, 102, 7747.
(25) Scaiano, J . C.; Tanner, M.; Weir, D. J . Am. Chem. Soc. 1985,
107, 4396.