Kinetics of the Thermal Isomerization of Methylcyclopropane

Article abstract of DOI:10.1021/jp982991x

Kinetic parameters for the unimolecular thermal isomerizations of methylcyclopropane to four isomeric butenes have been determined from rate constant measurements over a wide range of temperatures, 695-1154 K, using a single-pulse shock tube and a static reactor. For the overall reaction, E^a = 64.4 ± 0.3 and log(A, s_-1) = 15.37 ± 0.07, values consistent with earlier studies at lower temperatures. The measured rate constants for the isomerization to cis- and trans-2-butene give E_a = 63.3 ± 0.3 and 64.9 ± 0.3 kcal/mol, and log(A, s^-1) = 14.60 ± 0.07 and 14.75 ± 0.06, respectively, values in good agreement with the thermochemically based predictions of O'Neal and Benson. The similarity of kinetic parameters for the two reactions is consistent with the formation of the two 2-butenes through a common diradical intermediate. Kinetic data for 1-butene and 2-methylpropene formation are well represented by E_a = 64.5 ± 0.5 and 66.4 ± 0.2 kcal/mol and 1og(A, s^-1) = 15.02 ± 0.11 and 14.81 ± 0.05. The higher E_a for 2-methylpropene formation compared with E_a for the production of 1-butene, and of cis- and trans-2-butene, is due to the higher energy required to break the C(2) - C(3) bond compared with the methyl-substituted C(1) - C(2) bond.

Full text of DOI:10.1021/jp982991x

362
J. Phys. Chem. A 1999, 103, 362-364
Kinetics of the Thermal Isomerization of Methylcyclopropane†
Bansi L. Kalra* and Janet Y. Cho
Department of Chemistry, Hollins UniVersity, Roanoke, Virginia 24020
David K. Lewis*
Department of Chemistry, Connecticut College, New London, Connecticut 06320
ReceiVed: July 13, 1998; In Final Form: NoVember 6, 1998
Kinetic parameters for the unimolecular thermal isomerizations of methylcyclopropane to four isomeric butenes
have been determined from rate constant measurements over a wide range of temperatures, 695-1154 K,
using a single-pulse shock tube and a static reactor. For the overall reaction, Ea ) 64.4 ( 0.3 and log(A, s-1)
) 15.37 ( 0.07, values consistent with earlier studies at lower temperatures. The measured rate constants for
the isomerization to cis- and trans-2-butene give Ea ) 63.3 ( 0.3 and 64.9 ( 0.3 kcal/mol, and log(A, s-1)
) 14.60 ( 0.07 and 14.75 ( 0.06, respectively, values in good agreement with the thermochemically based
predictions of O’Neal and Benson. The similarity of kinetic parameters for the two reactions is consistent
with the formation of the two 2-butenes through a common diradical intermediate. Kinetic data for 1-butene
and 2-methylpropene formation are well represented by Ea ) 64.5 ( 0.5 and 66.4 ( 0.2 kcal/mol and log(A,
s-1) ) 15.02 ( 0.11 and 14.81 ( 0.05. The higher Ea for 2-methylpropene formation compared with Ea for
the production of 1-butene, and of cis- and trans-2-butene, is due to the higher energy required to break the
C(2)-C(3) bond compared with the methyl-substituted C(1)-C(2) bond.
Introduction
Rabinovitch1a calculated an activation energy of 68.1 kcal/mol
for the formation of trans isomer, a value higher than that for
the cis isomer by 4.2 kcal/mol. Work at 653-693 K by Setser
and Rabinovitch2 gave activation energies of 61.9 and 64.4 kcal/
mol for the formation of the cis- and trans-2-butene, respec-
tively. From their thermochemical calculations, O’Neal and
Benson3 predicted values of 63.0 kcal/mol for production of
both isomers. By extending the temperature range by about 400
°C over that of the earlier work, we have tried to reconcile the
differences between the experimental and the predicted values
for the Arrhenius parameters.
Kinetics of the thermal isomerizations of methylcyclopropane
at relatively low temperatures attainable in a static reactor have
been studied by a number of workers.1,2 Chesick1b investigated
the reaction at 713-763 K over a wide range of pressures. In
the high-pressure range, the overall reaction was shown to be
homogeneous with methylcyclopropane isomerizing to 1-butene,
cis-2-butene, trans-2-butene, and 2-methylpropene in parallel
first-order reactions. There was little cis-trans isomerization
of 2-butenes. Ea and log A values deduced for the overall
reaction were 65.0 kcal/mol and 15.45, respectively. In studies
of the geometric and structural isomerization of methylcyclo-
propane and 1,2-dideuterio-3-methylcyclopropane carried out
by Setser and Rabinovitch2 at 653-693 K, the corresponding
values for these parameters were 62.4 ( 0.6 kcal/mol and 14.61
( 0.19, respectively. Assuming that this isomerization proceeds
via a diradical intermediate, and using thermodynamic estimates
of the transition state, O’Neal and Benson3 predicted an
activation energy of 64.2 kcal/mol. The difference in the two
experimental Ea values from each other, and from the predicted
value, could be due to the small temperature ranges of these
studies.
Experimental Section
Materials. Methylcyclopropane (99.95%) was obtained from
API Standard Reference Materials. Matheson CP grade cyclo-
propane was added to reactant mixtures as an internal ther-
mometer. 1-Butene (99.0%), cis-2-butene (95%), trans-2-butene
(95%), and 2-methylpropene (99%), all Matheson CP grade
obtained in lecture bottles, were used to prepare calibration
samples for the products. The reagents were degassed through
multiple freeze-pump-thaw cycles before use. Linde helium
was used as the driver gas in the shock-tube runs.
Sample Preparation. For the shock tube runs methylcyclo-
propane was mixed with cyclopropane in different proportions
and diluted with Matheson Research grade (99.9999%) argon.
The mixtures were prepared, in 1 or 2 L glass storage flasks
with greaseless stopcocks, from degassed samples of reagents
and allowed to equilibrate before use for at least 3 days. The
addition of cyclopropane was necessitated for the determination
of the temperature attained during a shock run (see below).
For the shock-tube runs the methylcyclopropane/cyclopropane
mixture compositions were 1.0%/1.0%, 2.0%/1.0%, and 1.3%/
4.3% respectively. Mixtures for the static reactor runs had
compositions of 2.0%/1.0% and 1.3%/4.3% respectively.
In the present study, a shock-tube and a static reactor have
been used to investigate the isomerization of methylcyclopro-
pane over a much larger temperature range, 695-1154 K, for
the accurate determination of these parameters. An even more
significant aspect of this investigation was to determine reliable
values of the Arrhenius parameters for the conversion of
methylcyclopropane to cis- and trans-2-butenes. While Chesick1b
had reported only the activation parameters for the overall
reaction, from his published rate constants Placzek and
† Experimental work carried out at Department of Chemistry, Colgate
University, Hamilton, NY 13346.
10.1021/jp982991x CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/05/1999

Thermal Isomerization of Methylcyclopropane
J. Phys. Chem. A, Vol. 103, No. 3, 1999 363
Apparatus. The high-temperature experiments in this study
were run in a 2.54 cm diameter single-pulse shock tube; the
static reactor used for the lower temperature runs consisted of
a 100 cm3 glass cell surrounded by an aluminum furnace.
Descriptions and operating procedures for these devices have
been reported earlier.4,5 Analyses of reactant and product
samples were carried out on a Varian Model 1440-20 isothermal
gas chromatograph with FID, using a 1.5 m 5% n-octane on
Porasil C column at 43 °C in tandem with a 3 m column of
polypropylene glycol saturated with AgNO3 on 80-100 mesh
Chromosorb W at room temperature. The polypropylene glycol
column was also used alone for the separation of cyclopropane
from propene, needed for the determination of the reaction
temperature.
At the higher temperatures obtained in the shock tube,
however, the fractions of 2-methylpropene and trans-2-butene
increased. The product ratios at the highest temperature were
1.00:0.60:0.54:0.28. Whereas the cis-2-butene to trans-2-butene
ratio was 2.0 or higher at the lower temperatures in both the
previous studies1b,2 and the present investigation, at 1154 K the
amount of trans-2-butene produced approached that of cis-2-
butene, the cis to trans ratio becoming 1.1.
Arrhenius Parameters. Figure 1 shows the Arrhenius plots
for the overall reaction methylcyclopropane f products, and
for isomerization to the individual butenes from 18 shock-tube
experiments and two static reactor runs. Linear least-squares
fits for the data gave the following activation parameters:
for methylcyclopropane f all products:
Kinetic Runs. For the shock-tube runs, 45-75 Torr samples
of the methylcyclopropane-cyclopropane mixtures in argon
were placed in the driven section of the tube, and the samples
were heated for 800 ( 50 µs. The temperatures achieved in
these runs ranged from 990 to 1154 K, as determined from the
extent of conversion of cyclopropane to propene. At the end of
the reaction the product samples were collected in 25 mL glass
bulbs and analyzed chromatographically.
log(A, s-1) ) 15.37 ( 0.07
for methylcyclopropane f 1-butene:
Ea ) 64.4 ( 0.3 kcal/mol
log(A, s-1) ) 15.02 ( 0.11
Ea ) 64.5 ( 0.5 kcal/mol
for methylcyclopropane f cis-2-butene:
For the runs in the static reactor, 343 and 212 Torr sample
mixtures were heated at 695-698 K. The heating times for these
runs were 75 and 85 min, respectively. The temperatures were
measured both directly using an Omega Engineering Model 650-
J-D digital thermometer, and calculated from the extent of
conversion of cyclopropane to propene. Two samples of the
reaction products were collected simultaneously and chromato-
graphed.
log(A, s-1) ) 14.60 ( 0.07
Ea ) 63.3 ( 0.3 kcal/mol
for methylcyclopropane f trans-2-butene:
log(A, s-1) ) 14.75 ( 0.06
Ea ) 64.9 ( 0.3 kcal/mol
for methylcyclopropane f 2-methylpropene:
Calculations. Extents of conversion of methylcyclopropane
to each of the four products, and of cyclopropane to propene,
were determined from the GC peak heights, corrected for
measured sensitivity differences of the various hydrocarbons.
Rate constants were calculated assuming that both methylcy-
clopropane and cyclopropane underwent homogeneous first-
order irreversible reactions.
log(A, s-1) ) 14.81 ( 0.05
Ea ) 66.4 ( 0.2 kcal/mol
The activation energy of 64.4 kcal/mol and the log A value of
15.37 for the overall reaction agree well with values of 65.0
kcal/mol and 15.45 determined by Placzek and Rabinovitch1a
from Chesick’s data at 713-763 K;1b these values are somewhat
higher than those reported by Setser and Rabinovitch,2 62.4 kcal/
mol and 14.61, from their work at 693-748 K.
The reaction temperatures were determined from the calcu-
lated rate constants for the cyclopropane isomerization to
propene and the well-established Arrhenius parameters for this
reaction: Ea ) 65.0 kcal/mol and log(A, s-1) ) 15.2.6
For each experiment, the rate constant for total consumption
of methylcyclopropane was apportioned to rate constants for
isomerization to 1-butene, cis-2-butene, trans-2-butene, and
2-methylpropene by multiplying the overall rate constant by the
fraction of the corresponding component in the product mixture,
following the evidence that these isomerizations are parallel,
first-order reactions.1b
Assuming that most three-membered ring compounds undergo
isomerization via the formation of a diradical intermediate,
Benson and O’Neal have made a priori estimates of the
Arrhenius parameters for these reactions.3 Our experimental
values are in excellent agreement with their estimates of 64.2
kcal/mol for Ea and 15.2 for log A for methylcyclopropane
isomerization to all products. The activation energy of 65.0 kcal/
mol for the cyclopropane to propene isomerization via a diradical
is now well established.6 Substitution of a methyl group for an
H atom on one of the cyclopropane carbons is expected to lower
the strength of the adjacent carbon-carbon bond7 and should
lead to an activation energy slightly lower than 65.0 kcal/mol.
The present work extending the earlier low-temperature study
upward to 1154 K shows that the Arrhenius parameters for the
overall reaction are essentially constant over a wide range of
temperature.
The activation energy for the isomerization of methylcyclo-
propane to trans-2-butene in the present study appears to be
higher by 1.6 ( 0.4 kcal/mol than Ea for the conversion to cis-
2-butene. This difference is significantly smaller than the 4.2
kcal/mol, calculated from Chesick’s data,1b and smaller but
comparable to 2.5 ( 0.8 kcal/mol obtained by Setser and
Rabinovitch,2 in studies done over narrow temperature ranges
of 50 and 40 K. From their thermochemical calculations, O’Neal
and Benson3 predicted an activation energy of 63.0 kcal/mol
Results and Discussion
Product Distribution. In both the shock-tube and the static-
reactor runs, all four isomeric butenes (1-butene, cis-2-butene,
trans-2-butene, and 2-methylpropene) were detected as major
products, with 1-butene most abundant (over 40%) and 2-me-
thylpropene least abundant (about 11%) in the majority of the
runs. At static reactor temperatures the distribution of the
products is quite consistent with those reported by Chesick1b
and by Setser and Rabinovitch.2 For example, their relative rate
constants at 740 K for 1-butene:cis-2-butene:trans-2-butene:2-
methylpropene were 1.00:0.63:0.28:0.161b and 1.00:0.71:0.30:
0.18,2 respectively, while in the present study the relative
amounts of these products at 698 K were found to be
approximately 1.0:0.8:0.4:0.2.

364 J. Phys. Chem. A, Vol. 103, No. 3, 1999
Kalra et al.
the values calculated from Chesick’s data and from O’Neal and
Benson’s calculations, 66.0 and 65.8 kcal/mol, respectively. This
results from breaking the C-C bond â to the methyl substitution,
in contrast to the other three products which come from an R
cleavage.
Conclusions
The Arrhenius parameters for the thermal isomerization of
methylcyclopropane to different butenes have been redetermined
over a wide range of temperatures. The activation energies and
the preexponential factors for the overall reaction, and for the
isomerization to 2-methylpropene are consistent with the earlier
values determined by Chesick1b and predicted from the ther-
mochemical calculations of O’Neal and Benson.3 This work has
shown that the these values are reliable even at much higher
temperatures than previously explored.
The difference in activation energies, ∆Ea, for the isomer-
ization to cis- and to trans-2-butene is revised downward to
1.6 ( 0.4 kcal/mol, as determined from the present data covering
temperatures from 696 to 1154 K. This value is much smaller
than the 4.2 kcal/mol calculated from Chesick’s data. The source
of this former large value was the seemingly high Ea of 68.1
kcal/mol for the methylcyclopropane f trans-2-butene isomer-
ization. In comparison, ∆Ea determined by Setser and Rabino-
vitch is 2.5 kcal/mol. It is reasonable to assume that one diradical
precursor leads to these two isomeric products by following
two different exit channels.
Figure 1. Rate constants k for the isomerization of methylcyclopropane
to various butenes: (0) all products (x ) 10); (O) 1-butene (x ) 9);
(4) 2-methylpropene (x ) 8); ([) trans-2-butene (x ) 7); (]) cis-2-
butene (x ) 6).
Acknowledgment. This work was supported at Colgate
University by NSF grant CHE 9320501 and at Connecticut
College by NSF grant CHE 9714356. B.L.K. also thanks Hollins
University for partial support.
for formation of both these isomers. If these 2-butenes come
from the same diradical intermediate produced by breaking the
C(1)-C(2) bond, but form products via different rotational exit
channels, a slight difference in the apparent activation energies
would be expected if the proportion exiting via each channel is
temperature dependent.
References and Notes
(1) (a) Placzek, D. W.; Rabinovitch, B. S. J. Phys. Chem. 1965, 69,
2141-2145. (b) Chesick, J. P. J. Am. Chem. Soc. 1960, 82, 3277-3285.
(2) Setser, D. W.; Rabinovitch, B. S. J. Am. Chem. Soc. 1964, 86, 564-
569.
(3) (a) O’Neal, H. E.; Benson, S. W. J. Phys. Chem. 1968, 72, 1866-
1887. (b) Benson, S. W.; O’Neal, H. E. “Kinetic Data on Gas-Phase
Unimolecular Reactions”; NSRDS NBS 21, U.S. Government Printing
Office, Washington, DC, 1970; p 225.
That the reaction channel involved in the formation of
1-butene is different from the one leading to the 2-butene
isomers is indicated by the fact that over almost the entire
temperature range, the ratio of the 1-butene concentration to
the sum of the 2-butenes in the product mixture is about 0.9
and not 1.0. A least-squares fit of the data for total 2-butene
(4) Lewis, D. K.; Giesler, S. E.; Brown, M. S. Int. J. Chem. Kinet.
1978, 10, 277-294.
formation leads to values of Ea ) 63.9 kcal/mol and log(A, s-1
)
) 14.95, values consistent with the Benson and O’Neal
predictions.
(5) Lewis, D. K.; Bergman, J.; Manjoney, R.; Paddock, R.; Kalra, B.
L. J. Phys. Chem. 1984, 88, 4112-4116.
(6) (a) Bradley, J. N.; Frend, M. A. Trans. Faraday Soc. 1971, 67, 72.
(b) Jeffers, P. M.; Lewis, D. K.; Sarr, M. S. J. Phys. Chem. 1973, 77, 3037-
3041.
The activation energy of 66.4 ( 0.2 kcal/mol for the
formation of 2-methylpropene via the diradical formed by
breaking the C(2)-C(3) bond is in excellent agreement with
(7) Rossi, A. R. J. Phys. Chem. 1979, 83, 2554-2556.