Kinetics of the thermal isomerization of 1,1,2-trimethylcyclopropane

Article abstract of DOI:10.1002/kin.20184

The Arrhenius parameters for the gas phase, unimolecular structural isomerizations of 1,1,2-trimethylcyclopropane to three isomeric methylpentenes and two dimethyl-butenes have been determined over a wide range of temperatures, 688-1124 K, using both static and shock tube reactors. For the overall loss of reactant. E_a =63.7 (±0.5) kcal/mol and log₁₀ A= 15.28 (±0.12). These values are higher by 2.6 kcal/mol and 0.7-0.8 than previously reported from experimental work or predicted from thermochemical calculations. E_a for the formation of trans-4-methyl-2-pentene is 1.5 kcal/mol higher than E_a for the formation of the cis isomer, which is identical to the E_a difference previously reported for the formation of trans- and cis-2-butene from methylcyclopropane. Substitution of methyl groups for hydrogen atoms on the cyclopropane ring is expected to weaken the C - C ring bonds, and it has been reported previously that activation energies for structural isomerizations of methylcyclo-propanes do decrease substantially over the series cyclopropane > methylcyclopropane > 1, 1-or 1,2-dimethylcyclopropane. However, the present study shows that the trend does not continue beyond dimethylcyclopropane isomerization. Besides reductions in C - C bond energy, steric interactions may be increasingly important in determining the energy surface and conformational restrictions near the transition state in isomerizations of the more highly substituted methylcyclopropanes.

Full text of DOI:10.1002/kin.20184

Kinetics of the Thermal
Isomerization of 1,1,2-
Trimethylcyclopropane
DAVID K. LEWIS,1 STEVEN V. HUGHES,1 JUSTINE D. MILLER,1 JESSICA SCHLIER,1
KEVIN A. WILKINSON,1 SARA R. WILKINSON,1 BANSI L. KALRA2
1Department of Chemistry, Connecticut College, New London, CT 06320
2Department of Chemistry, Hollins University, Roanoke, VA 24020
Received 1 July 2003; revised 7 September 2005; accepted 27 January 2006
DOI 10.1002/kin.20184
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The Arrhenius parameters for the gas phase, unimolecular structural isomeriza-
tions of 1,1,2-trimethylcyclopropane to three isomeric methylpentenes and two dimethyl-
butenes have been determined over a wide range of temperatures, 688–1124 K, using both
static and shock tube reactors. For the overall loss of reactant, Ea = 63.7 ( 0.5) kcal/mol and
log10 A = 15.28 ( 0.12). These values are higher by 2.6 kcal/mol and 0.7–0.8 than previously
reported from experimental work or predicted from thermochemical calculations. Ea for the
formation of trans-4-methyl-2-pentene is 1.5 kcal/mol higher than Ea for the formation of the
cis isomer, which is identical to the Ea difference previously reported for the formation of
trans- and cis-2-butene from methylcyclopropane. Substitution of methyl groups for hydro-
gen atoms on the cyclopropane ring is expected to weaken the C C ring bonds, and it has
been reported previously that activation energies for structural isomerizations of methylcyclo-
propanes do decrease substantially over the series cyclopropane > methylcyclopropane > 1,1-
or 1,2-dimethylcyclopropane. However, the present study shows that the trend does not con-
tinue beyond dimethylcyclopropane isomerization. Besides reductions in C C bond energy,
steric interactions may be increasingly important in determining the energy surface and confor-
mational restrictions near the transition state in isomerizations of the more highly substituted
methylcyclopropanes. ꢀ 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 475–482, 2006
C
been underway in this laboratory on a series of alkyl-
substituted cyclopropanes, both to confirm the results
of earlier kinetic work done in static reactors and to ex-
tend the range of the studies to the much higher temper-
atures that can be reached in shock tubes. The kinetic
data obtained over the much wider range of tempera-
tures achieved by combining the static reactor and the
shock tube results allow evaluation of the Arrhenius
parameters of competing isomerization channels with
high precision (assuming Arrhenius behavior and no
change of mechanism). In a recent study of methyl-
cyclopropane [1], for example, we confirmed the ex-
perimental activation parameters previously reported
INTRODUCTION
To better understand the mechanisms and energetics
of cyclopropane isomerizations, kinetic studies have
Correspondence to: David K. Lewis; e-mail: david.lewis@
conncoll.edu.
Jessica Schlier was a visiting student from Wesleyan University,
Middletown, CT 06459.
Contract grant sponsor: National Science Foundation.
Contract grants numbers: CHE-9714356 and CHE-0213374.
Contract grant sponsor: Research Corporation.
Contract grant sponsor: Connecticut College.
2006 Wiley Periodicals, Inc.
c
ꢀ

476
LEWIS ET AL.
for its isomerization to 1-butene and 2-methylpropene,
and we found that there is a small but definite differ-
ence in activation energies for the formation of cis- and
trans-2-butenes (trans > cis by 1.5 kcal/mol) [1]. This
latter finding differed not only from an earlier report
of an experimental study which found a much larger
difference in activation energies for the production of
the two 2-butenes [2], but also from a prediction based
on thermochemical calculations that there should be no
difference since the two 2-butenes were expected to be
formed via the same transition state [3].
From a substantial number of earlier experimen-
tal kinetic studies, it has been well established
that the activation energies for structural isomeriza-
tions of methyl-substituted cyclopropanes decrease
with increasing methyl substitution through the se-
ries cyclopropane [4] > methylcyclopropane [1] >
1,1-dimethylcyclopropane [5] and 1,2-dimethylcycl-
opropane [6] (see Table I). This is consistent with
the expectation that increasing methyl-for-hydrogen
substitution should weaken the C C bonds of the
cyclopropane ring and facilitate (stabilize) the for-
mation of a diradical intermediate [7]. But there is
experimental evidence that this trend does not continue
with further methyl substitution. A report by O’Neal
and Henfling [8] of a static reactor study of 1,1,2-
trimethylcyclopropane (tri-MCP) structural isomeriza-
tion presented an Ea value similar to that reported
for 1,1- and 1,2-dimethylcyclopropane isomerizations
(see Table I). Furthermore, a quadruply substituted cy-
clopropane, 1,1,2,2-tetramethylcyclopropane, appears
to deviate from this pattern even more strongly.
Workers in other laboratories using either a static
reactor [9] or a single-pulse shock tube [10] de-
duced Ea values that are significantly higher than the
Ea reported for tri-MCP structural isomerization [8]
(Table I). Also, a recent combined shock tube and
static-reactor study in this laboratory covering the tem-
perature range 656–1120 K [11], yielded an activa-
tion energy of 64.7 ( 0.5) kcal/mol for isomerization
of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-
pentene over the full temperature range of the study,
a value very close to that for isomerization of unsub-
stituted cyclopropane and more than 5 kcal/mol higher
than predicted by the early thermochemical calcula-
tions [7].
In light of the previous studies of the isomeriza-
tions of methylcyclopropanes summarized above, fur-
ther study of the isomerization of tri-MCP seems war-
ranted. One issue to be clarified is the trend in activation
energies. Does the activation energy lie at or below that
for 1,1- and 1,2-dimethylcyclopropane structural iso-
merizations, leaving 1,1,2,2-tetramethylcyclopropane
the curious deviant from the group trend? Or does ad-
dition of the third methyl group produce a turnaround
in the trend of decreasing activation energies and fill
the gap between Ea values for dimethyl- and tetram-
ethylcyclopropanes? A second issue is whether there
is any trend in the pre-exponential A factors in the
rate constants for this family of reactions. From the
data previously reported, no trend can be discerned; but
this may be due to experimental uncertainties resulting
from the relatively small temperature ranges covered
in the earlier studies that made use of a single type
of reactor. A third issue is the relative activation ener-
gies for the formation of isomerization products. Five
are expected from tri-MCP, including cis- and trans-
4-methyl-2-pentene. Will the 1.5 kcal/mol difference
previously noted for the production of cis- and trans-
2-butene from methylcyclopropane be reproduced with
tri-MCP? The present study was carried out to answer
these questions.
In the present study of tri-MCP, we have ex-
tended the kinetic experiments to a much larger range
Table I Arrhenius Parameters for Structural Isomerizations of Cyclopropane and Methylcyclopropanes, as Reported
from Experimental Studies
Reaction
Ea (kcal/mol)
log (A, s−1
)
Reference
Cyclopropane → propene
65.0
64.4
61.8
61.6
61.1
63.7
64.7
64.0
62.2
64.7
15.2
[4]
[1]
[5d]
[6]
[8]
Methylcyclopropane → alkene isomers
1,1-Dimethylcyclopropane → alkene isomers
cis-1,2-Dimethylcyclopropane → alkene isomersa
1,1,2-Trimethylcyclopropane → alkene isomers
15.37
15.04
14.56
14.47
15.28
15.55
15.27
14.82
15.47
This study
[9a]
1,1,2,2-Tetramethylcyclopropane → dimethyl-2-pentene
[9b]
[10]
[11]
a Calculated from the parameters for parallel reactions in [6].

KINETICS OF THE THERMAL ISOMERIZATION OF 1,1,2-TRIMETHYLCYCLOPROPANE
477
of temperatures (688–1124 K) than was covered in
Sample Preparation and Kinetic Runs
earlier work, by using both a static reactor and a
single-pulse shock tube. In both the lower and the
higher temperature regions employed in this study,
the isomerization of tri-MCP leads irreversibly to the
formation of five principal isomeric products: cis-
and trans-4-methyl-2-pentene (4M2P), 2-methyl-2-
pentene (2M2P), 2,3-dimethyl-2-butene (DM2B), and
2,3-dimethyl-1-butene (DM1B), by apparently parallel
mechanisms.
All reagents were degassed through freeze–pump–
thaw cycles in a greaseless, metal vacuum line, trans-
ferred to 1- or 2-L glass storage flasks with greaseless
stopcocks to which a few 5 mm diameter smooth
glass beads had been added beforehand, then di-
luted with argon. Cyclopropane (CP) was added to
most mixtures as an internal thermometer to deter-
mine the reaction temperature (see below). The tri-
MCP/CP/argon mixtures were well mixed before use
by applying a circular motion to the flasks. This caused
the glass beads inside to orbit the inner surface of
the flasks, creating turbulence to achieve the desired
mixing.
EXPERIMENTAL SECTION
Materials
The following mixture compositions (tri-
MCP%:CP%) were used to measure the rates of
tri-MCP isomerization:
Tri-MCP (99.88 0.03%)∗ was obtained from the
American Petroleum Institute’s Standard Reference
MaterialsprojectatCarnegieMellonUniversity. Allthe
other reagents, 4M2P (99.8%), 2M2P (99.9%), DM2B
(96%), and DM1B (96%), were obtained from Wi-
ley Organics Co. (Now ChemSampCo). These reagents
weredilutedwithMathesonresearchgrade(99.9999%)
argon to prepare calibration and reactant samples.
Matheson CP-grade (99%) cyclopropane (CP) was
added to reactant mixtures as an internal thermome-
ter. It was found by GC analysis to be 98.3% CP, 1.7%
propene, with no other detectable impurities. In the
shock tube runs, HP grade helium was used as the
driver gas.
In the static reactor: (a) 0.20:0.40; (b) 2.00:4.75,
(c) 2.50:7.00, and (d) 2.00:0.00.
In the shock tube: (e) 0.42:0.83; (f) 2.00:4.00, (g)
2.50:5.00; (h) 1.50:0.00;
and (i) 2.00:0.00.
Reactant mixture samples ranging from 25 to
90 Torr were heated in the shock tube for
8.0( 0.5) × 10−4 s, producing reaction temperatures
of 1054–1124 K and total gas pressures of 2–3 bar. In
the static reactor experiments, samples of the reactant
mixture ranging from 99 to 400 Torr were placed into
one of the three reaction cells at room temperature.
These cells were inserted into the reactor oven, which
had been previously stabilized at a temperature in the
range 688–745 K, for times ranging from 30 min to
5.0 h; total gas pressure during reaction was 230–
1000 Torr. The reaction cells were then withdrawn
from the reactor and rapidly cooled. Reaction products
frombothheatingdeviceswereanalyzedgaschromato-
graphically.
Mixtures were also made of individual alkenes (the
expected tri-MCP unimolecular isomerization prod-
ucts), with each mixture containing 2.0% of one alkene
plus 2.0% CP in argon, to determine whether these
products would interconvert or further react under the
conditions(T, p)oftheseexperiments. Samplesofthese
mixtures were heated to 710–714 K for 50–60 min in
the static reactor and to 1074–1116 K for 8.0 × 10−4 s
in the shock tube. Toluene was added to some runs on
each reactant as a radical scavenger.
Apparatus
The high-temperature runs were carried out in a
2.5-cm diameter single-pulse shock tube. For the lower
temperature runs, the samples were heated in one of
three well-aged and conditioned 100 cm3 glass cells
inserted into a constant temperature furnace. Both the
shock tube and the static reactors and their opera-
tion have been described earlier [12]. A Varian model
1440-20 isothermal gas chromatograph with hydrogen
flame detector, and a 3-m column of 20% polypropy-
lene glycol saturated with AgNO3 on 80–100 mesh
Chromosorb W at room temperature was used to an-
alyze the reactant and product samples. A Shimadzu
GC-14 gas chromatograph with hydrogen flame detec-
tor, equipped with a 50 m × 0.35 mm Petrocol capillary
column, was used to separate and quantify tri-MCP and
all five principal isomerization products.
Calculations
The relative amounts of CP and propene, and of tri-
MCP and its isomerization products, were determined
∗Purity levels are as indicated by the supplier, unless otherwise
stated.

478
LEWIS ET AL.
from GC peak heights corrected for sensitivity dif-
ferences determined from calibration runs on known
samples. Rate constants for CP → propene, and for
tri-MCP → isomerization products were calculated as-
suming that both CP and tri-MCP underwent homo-
geneous first-order irreversible reactions. For runs on
mixtures containing CP, the reaction temperature was
calculatedfromtherateconstantforCP → propeneiso-
merization, using the well-established Arrhenius pa-
rameters Ea = 65.0 kcal/mol and log10(A, s−1) = 15.2
[4]. For runs on samples that did not contain CP,
the temperature was calculated from the total extent
of tri-MCP isomerization to all products, using the
Arrhenius parameters deduced from the runs that
did contain CP (see below): Ea = 63.7 kcal/mol and
log10 (A, s−1) = 15.28.
sent the statistical uncertainty range at the 95% confi-
dence level. The data points from the lowest percent-
age mixtures are well intermingled with data points
from mixtures which had initial total percentages of
tri-MCP plus CP that were larger by factors of six to
10, providing experimental evidence that the results are
not significantly affected by unimolecular falloff of ei-
ther reaction. This point is discussed in greater detail
below.
The presence of all five isomeric alkene products
of tri-MCP isomerization in the product samples from
even the lowest percent conversion experiments run in
both the static and shock tube reactors indicates that
these products are formed via parallel rather than se-
quential pathways. However, all were found to undergo
isomerization to other isomers to a small but significant
extent (2–6% consumption of reactant) when each was
heated individually to the higher end of the temper-
ature range covered in each reactor (>710 K for the
static reactor and >1074 K in the shock tube). Toluene
did not significantly change the extent of isomeriza-
tion of the alkenes in the static reactor experiments,
but it did appear to slightly increase the isomeriza-
tion rates in the shock tube experiments. Baldwin and
Shukla [13], studying 1,1-dimethylcyclopropane iso-
merization in a static reactor, noted isomerization of
product alkenes and attributed this to surface-triggered
formation of cationic species, followed by hydrogen
or methyl shifts. They cited additional evidence for
such processes from earlier isotope-labeling experi-
ments and NMR-spectroscopic studies. Such processes
could account for the slow isomerization of alkenes
in the static reactor experiments of the present study,
and the negligible effect of added toluene. The ap-
parent interconversion of alkenes in the shock tube
experiments, however, is an indication of the pres-
ence of some radical species in the gas phase in the
higher temperature (and larger extent of tri-MCP de-
pletion) runs, since the walls of the shock tube re-
mained at room temperature during each shock tube
run and therefore should not have catalyzed radical
processes.
RESULTS AND DISCUSSION
At temperatures obtained in both the static reactor and
the shock tube runs, five products of tri-MCP isomer-
ization were identified: cis- and trans-4M2P, 2M2P,
DM1B, and DM2B. An Arrhenius plot for the total
isomerization of tri-MCP to all alkene isomers in the
runs that contained CP for temperature determination
is shown in Fig. 1. A linear least-squares regression of
the data from 688 to 1124 K gave the activation param-
eters:
log10 (A, s−1) = 15.28 ( 0.12)
Ea = 63.7 ( 0.5) kcal/mol
The specified uncertainties are one standard devia-
tion as calculated from the linear least squares regres-
sion. Doubling these listed uncertainties would repre-
To obtain the Arrhenius parameters for the forma-
tion of the individual alkenes from tri-MCP, static re-
actor and shock tube runs were carried out on tri-MCP
samples that did not contain CP, in the lower half of
the useful temperature range of each reactor (<715
and <1090 K, respectively) and with short reaction
times in the static reactor, to minimize further reaction
of newly formed alkenes. From the product ratios for
the isomerizations we obtained the Arrhenius plots as
shown in Fig. 2. Linear regression of the individual data
sets gives the following:
Figure 1 Arrhenius plot for 1,1,2-trimethylcyclopropane
total isomerization to all alkene products.

KINETICS OF THE THERMAL ISOMERIZATION OF 1,1,2-TRIMETHYLCYCLOPROPANE
479
carbon atoms would not be expected to lead to a ma-
jor product as there is no H atom to migrate from the
central carbon of the resulting diradical.
Product
log10(A, s−1
)
Ea(kcal/mol)
cis-4M2P
trans-4M2P
2M2P
DM2B
DM1B
14.25 0.11
14.77 0.07
15.31 0.17
14.74 0.14
14.03 0.23
62.5 0.4
64.0 0.3
66.6 0.7
65.9 0.6
61.0 0.9
From the observed distribution of products from
each run, the total amounts of the three methylpentenes
range from 2–4 times that of the two dimethylbutenes.
This echos observations made by O’Neal and Henfling
in their study [8] and is consistent with the expectation
that the C C bond between the most highly substituted
carbon atoms (1 and 2) would require the least energy
to break and therefore would break most frequently.
Once a diradical is formed from cleavage of either the
C1 C2 or the C1 C3 ring bond, the transfer of an
H atom to the doubly substituted C1 carbon is favored
over an H atom transfer to the singly substituted C2 or
the unsubstituted C3 carbons: the ratio [4M2P]/[2M2P]
is 3.8 at 702 K but drops to 1.0 at 1122 K; and the ratio
[DM1B]/[DM2B] is 6.1 at 702 K and drops to 0.8 at
1122 K. Thus, the preference for H atom migration to
the more highly substituted carbon of the diradical is
clearly observed in the lower temperature static reactor
data, but is not significant at the much higher temper-
atures reached in the shock tube.
As mentioned in the introduction, the isomeriza-
tions of CP and the variously substituted methylcy-
clopropanes follow a pattern of decreasing activation
energies with increasing substitution of Hs by
CH3 groups (Table I), through the dimethylcyclo-
propanes. The Ea values descend from a high of
65.0 kcal/mol for structural isomerization of CP to a
low of 61.8 kcal/mol for 1,1-dimethylcyclopropane and
61.2–62.3 kcal/mol for cis-1,2-dimethylcyclopropane.
The GC analyses of tri-MCP and its isomerization
products achieved good but not perfect peak separation,
which produced some uncertainty in the estimation of
baselines. The result could have been a slight overesti-
mation of DM1B and both cis-4M2P and trans-4M2P,
which would have lowered their apparent activation
energies of formation and raised the corresponding
activation energies for 2M2P and DM2B formation.
Estimating the largest reasonable baseline errors and
recalculating changed the above Ea values by no more
than 1 kcal/mol.
Of particular interest is the difference in activation
energies for the production of 4M2P isomers: Ea for
the formation of the trans form exceeds Ea for the for-
mation of the cis form by 1.5 kcal/mol. This is identical
to what was found earlier for the production of cis- and
trans-2-butene from MCP [1].
Assuming that cyclopropane isomerization is initi-
ated by the breaking of a C C bond in the cyclopropane
ring to form a diradical transition state or intermediate,
the formation of the five principal alkene products can
be understood from Fig. 3. Breakage of the C C bond
between the singly substituted and the unsubstituted
Figure 2 Arrhenius plots for the formation of different products from the thermal isomerization of tri-MCP. The plots for the
different products have been offset by different Y-axis increments for clarity: n = 6 for DM1B, 8 for DM2B, 10 for 2M2P, 12
for cis-4M2P, and 14 for trans-4M2P.

480
LEWIS ET AL.
through linear least-squares data regressions; in other
cases, they represented the result of a thoughtful as-
sessment by the authors of the probable error limits
of their experiments. Considering the different bases
for error estimation, we have not included uncertainty
estimates with the entries in Table I. The key consider-
ation, however, is whether the value of Ea reported here
for tri-MCP isomerization is really significantly differ-
ent from that reported earlier [8], beyond experimental
uncertainties. We believe that the following analysis
establishes that it is.
In the experiments on tri-MCP reported earlier [8]
and in the present study, the dominant source of ex-
perimental error is probably the measurement of the
reaction temperature. In the earlier study, the accu-
racy of the temperature measurements depended on
the calibration of a mechanical or electrical device
and the maintaining of total thermal equilibrium at
that temperature throughout the reaction cell. In the
experiments reported here, the accuracy of the mea-
surement of reaction temperature depended upon the
accuracy of the activation parameters for the CP-to-
propene reaction and the assumption that the reaction
was proceeding at the high-pressure limit under our ex-
perimental conditions. The Arrhenius parameters de-
duced in the two laboratories give virtually identical
rate constants in the temperature range of the static
reactor experiments; where log10(k, s−1) = −4.00,
the temperature is expected to be 722 K accord-
ing to the O’Neal and Henfling parameters [8], and
723 K from the parameters deduced in this study.
But contrast the situation at the higher temperatures
of the shock tube experiments: log10(k, s−1) = 3.00
is expected to occur at 1164 K from the O’Neal and
Henfling parameters [8] and 1134 K from the parame-
ters of this study, a substantial difference of 30 K. Could
the present shock tube study be that much in error?
It has been well established [14] that gas-phase
reactions are more likely to be affected by unimolec-
ular falloff as temperature increases, assuming the
reaction rates are compared at constant pressure. Also,
the smaller the molecule, the higher the pressure
below which falloff becomes important, because of
the smaller number of active vibrations. Thus, one
would expect that if either the tri-MCP or the CP
isomerization was significantly slowed by falloff, the
reduction in reaction rate would be greater for CP
isomerization. The temperature calculated from extent
of CP isomerization would then be too low, and the
resulting Arrhenius plot for tri-MCP isomerization
would be too steep, giving erroneously large values of
Ea and A. But to give a temperature error of 30 K, the
CP reaction would have to be proceeding at only half
the high-pressure rate. Thus, if the previously reported
Figure 3 Formation of likely diradical intermediates and
the observed product alkenes from tri-MCP.
This trend might lead one to expect an overall activa-
tion energy for the structural isomerization of 1,1,2-
trimethylcyclopropane in the range 59–61 kcal/mol.
Indeed, earlier experimental evidence supported by
the theory seem to support that expectation: O’Neal
and Henfling’s 61.1 kcal/mol Ea and the values they
estimated from transition state calculations for the for-
mation of methylpentenes and dimethylbutenes, 59.4
and 61.6 kcal/mol, respectively [8].
The activation energy of 63.7( 0.5) kcal/mol deter-
mined for the unimolecular isomerization of tri-MCP
found in the present study is more than two stan-
dard deviations higher than the previously measured
and predicted values [8]. The value determined for
log10 (A, s−1) in the present study, 15.28 ( 0.12), is
also substantially higher than the value 14.47 previ-
ously reported [8]. However, if actual rate constants
from the two studies are compared, our measured
rate constants at 700–755 K are larger than those of
O’Neal and Henfling [8] by only a factor of 1.06–1.11,
which represents full agreement within experimental
uncertainties. The observed differences in Arrhenius
parameters may be due to small systematic errors in
the earlier study which covered a much narrower tem-
perature range than the present work.
In examining the trends evident in Table I, it is
important to consider whether the differences in Ea
are statistically significant. Each of the earlier pub-
lished reports of the activation parameters of a methyl-
cyclopropane isomerization (those summarized in
Table I) included uncertainty estimates. In some cases,
these were the statistical standard deviations obtained

KINETICS OF THE THERMAL ISOMERIZATION OF 1,1,2-TRIMETHYLCYCLOPROPANE
481
Arrhenius parameters for tri-MCP isomerization were
correct, then for the present study to have produced
the parameters we obtained would have required
that both reactions proceeded at the high-pressure
limit in the static reactor, but that the rate of the CP
reaction dropped to half its high-pressure value in the
shock tube while the tri-MCP reaction proceeded at
its high-pressure limit. This is an implausible set of
circumstances, especially since the partial pressure of
organic species (tri-MCP plus CP) in the shock tube ex-
periments was varied by a factor of 6 with no resulting
In contrast, the pre-exponential factors for overall
structural isomerization of all the methylcyclopropanes
listed in Table I appear to group (within margins
of error) at log10 (A, s−1) = 15.2( 0.3), showing no
particular trend with increasing substitution. This
indicates that methyl-for-hydrogen substitution, except
for changing the height of the enthalpy barrier, does
not cause significant changes in the structure of the
transition state or the entropy of activation in these
isomerizations.
Activation energies for the formation of the trans
and cis isomers of 4-methyl-2-pentene differ by
1.5 kcal/mol (trans > cis). This difference is identical
to the one found earlier for the production of cis- and
trans-2-butene from methylcyclopropane.
evidence of changes in the ratio of ktri-MCP : kCP
.
CONCLUSIONS
Isomerizations of cyclopropane and its alkyl-
substituted derivatives serve as important test reactions
for the development of computational methods for es-
timating transition state enthalpies, and for assessing
probable trajectories for cyclopropane structural iso-
merizations and stereomutations [15]. The reversal in
the decline of Ea with increasing methyl-for-hydrogen
substitution reported here provides an important oppor-
tunity for testing and further improving the predictive
accuracy of the computations. Also, tables of thermo-
chemical data that have been used to analyze transition
states and predict reaction rates [16] should be updated
to correctly account for the trends and differences in
activation energies noted here.
The identities and the relative amounts of five
principal thermal isomerization products of 1,1,2-
trimethylcyclopropane have been determined over a
wide range of temperatures, 688–1124 K. The prod-
ucts are likely formed from two diradical intermedi-
ates produced by competitive breaking of two of the
C C bonds of the cyclopropane ring (Fig. 3). The five
alkene products can interconvert, albeit slowly, under
conditions covered in this study, complicating absolute
determination of their individual rates of production.
However, this complication was minimized by consid-
ering product production ratios in only the lower tem-
perature runs in each reactor, runs in which extents of
tri-MCP depletion were low. In this study, estimates of
the activation parameters of individual isomerization
channels made from the lowest percentage conversion
experiments are consistent with expectations based on
the relative ease of rupture of the differently substituted
C C bonds of the tri-MCP ring (see earlier discussion).
From the rate constants at different temperatures for
the structural isomerization of tri-MCP → alkenes, ki-
netic parameters at the 95% confidence level (twice the
standard deviation) been determined to be Ea = 63.7
BLK thanks Hollins University for partial support.
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