Kinetics of thermal gas-phase decomposition of 2-bromopropene using static system

Article abstract of DOI:10.1002/kin.20207

The gas phase elimination kinetics of 2-bromopropene was studied over the temperature range of 571-654 K and pressure range of 12-46 Torr using the seasoned static reaction system. Propyne was the only olefinic product formed and accounted for >98% of the reaction. This product was formed by homogeneous, unimolecular pathways with high-pressure first-order rate constant k_∞ given by the equation k_∞ = 10 ^13.47±0.6 exp^{-208.2±6.7(kj mol-1)/RT}. The error limits are 95% certainty limits. The observed Arrhenius parameters are consistent with the four centered activated complex. The presence of methyl group on α-carbon lowers the activation energy by 41 kj mol^-1.

Full text of DOI:10.1002/kin.20207

Kinetics of Thermal
Gas-Phase Decomposition
of 2-Bromopropene Using
Static System
JAN NISAR, IFTIKHAR A. AWAN
National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan
Received 26 April 2006; revised 26 June 2006; accepted 5 July 2006
DOI 10.1002/kin.20207
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The gas phase elimination kinetics of 2-bromopropene was studied over the tem-
perature range of 571–654 K and pressure range of 12–46 Torr using the seasoned static reaction
system. Propyne was the only olefinic product formed and accounted for >98% of the reaction.
This product was formed by homogeneous, unimolecular pathways with high-pre−s1sure first-
0.6
6.7(k J mol
order rate constant k given by the equation k∞ = 1013.47
exp−208.2
)/RT . The
∞
error limits are 95% certainty limits. The observed Arrhenius parameters are consistent with
the four centered activated complex. The presence of methyl group on α-carbon lowers the
activation energy by 41 kJ mol−1
.
2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 1–5, 2007
ꢀ
C
under static system conditions using seasoned reactors.
The study extends the range of temperature for the
INTRODUCTION
unimolecular decomposition of 2-C3H5Br.
Although there have been numerous studies of the gas
phase thermal dehydrohalogenation of alkyl halides
and have been reviewed [1–3], there have been few
examples in these studies of vinylic halides. The only
experimental studies on hydrogen halide elimination
from unsaturated compounds are those for HCl and
HBr elimination from vinyl chloride [4,5] and vinyl
bromide [6], and a recently reported study by Roy
et al. [7] of the decomposition of 2-bromopropene and
2-chloropropene over a temperature range from 1100
to 1250 K using single pulse shock tube techniques.
The purpose of this paper is to present the kinetics
of the thermal gas phase dehydrobromination reaction
of 2-bromopropene at lower temperatures attainable
EXPERIMENTAL
Materials
2-Bromopropene (freezing point 4◦C and boiling point
47–49◦C) was obtained commercially (Acros, 99+%)
and tested for impurities by gas chromatography.
Mesitylene (Acros) was used as a radical scavenger.
Kinetic Experiments
All thermal kinetic studies were carried out in a con-
ventional “static” system using Pyrex reaction vessels
[8]. Two reaction vessels were employed; one was
packed with short lengths of Pyrex tubing to give a
surface to volume ratio (S/V ) of ca. 12 cm−1, and the
Correspondence to: Iftikhar A. Awan; e-mail: iftikhar.awan@
nist.gov.
c
ꢀ
2006 Wiley Periodicals, Inc.

2
NISAR AND AWAN
other was of similar external dimensions but not packed
(S/V ca. 1 cm−1, volume ca. 150 cm3). Young’s grease-
less stopcocks were used in all parts of the vacuum
system in contact with pyrolyzed material. The reac-
tions vessels were kept at constant temperature in a
salt bath constructed out of stainless steel can 30 cm in
depth, 21 cm in diameter; this was logged and ternary
eutectic enclosed in 42 × 42 × 44 cm steel box. The
salts used were sodium nitrite (7.1 mol), sodium nitrate
(1 mol), and potassium nitrate (6.4 mol). The bath was
heated by a heater that was made up of an 8-m length
of stainless steel heating cable. The temperature of the
bath was controlled by Rotatherm ADP 15 temperature
controller. The reaction temperature was measured us-
ing the PT-100 platinum resistance thermometer with
an accuracy of 0.1◦C. Reactions vessels were sea-
soned by pyrolysis of ca. 30 Torr 2-bromopropene at
500◦C for 48 h. The reaction mixtures were analyzed
by gas chromatography using Shimadzu GC 7AG gas
chromatograph with a flame ionization detector. Anal-
yses were carried out using stainless steel 6 ft × 1/8 in.
prepacked Porapak Q. Typical chromatographic con-
ditions were column oven temperature = 70–170◦C,
temperature-programming rate = 32◦C min−1, carrier
Figure
1 First-order rate plot for the loss of 2-
bromopropene at 611.5 K. [Color figure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]
the packed reaction vessel. Kinetic runs were carried
out in both packed and unpacked reaction flasks with a
pressure of 20 Torr and pyrolysis time of 15 min. It was
observed that the contribution of surface is maximum at
lower temperature and minimum at high temperature.
The surface to volume ratio ranges from 0.22 to 0.63%
in the temperature range (576.2–643.2 K) studied.
The effect of change in pressure on the reaction rate
was studied at 611.5 K. The effect of pressure on the
percent product is graphically represented in Fig. 3.
It is shown that pressure has no effect on the rate of
reaction, and the observed Arrhenius parameters are
close to high pressure limit.
To reduce the amount of secondary radical reac-
tions taking place in the system, mesitylene was added
as a radical trap. In the presence of 10% mesitylene,
the rate decreases only by 3.89%, which is within the
experimental error.
The decomposition of 2-bromopropene to propyne
is the result of first-order, homogeneous, nonradical,
and unimolecular reaction. The observed activation pa-
rameters log A = 13.47 s−1 and Ea = 208.2 kJ mol−1
gas = nitrogen, flow rate of carrier gas = 60 mL min−1
,
hydrogen pressure = 1 kg cm−2, air pressure = 0.5 kg
cm−2, and injection port temperature = 170◦C. The ar-
eas under the peaks were determined using the Spectra
Physics model SP-4600 data jet integrator. The identi-
fication of products was carried out by comparison of
the retention times of authentic samples with those of
unknown.
RESULTS AND DISCUSSION
Over the temperature range 571–654 K, 2-
bromopropene decomposed to give propyne as a re-
action product. The rates of formation of this product
mirrored the rate of disappearance of 2-bromopropene.
Table I Rate Constant for 2-Bromopropene
Decomposition at 20 Torr Initial Pressure
CH3 C(Br) CH2 → CH3 C CH + HBr
Temperature (K)
k
(s−1
)
(total)
First-order rate plots for the loss of 2-bromopropene
using 20 Torr initial pressure were linear as shown in
Fig. 1. The rate constants based on the loss of ini-
tial reactant were obtained at 11 temperatures and are
listed in Table I. The Arrhenius dependence of the rate
constants is given by
570.9
576.4
583.4
593.7
603.4
611.5
621.2
633.3
643.1
649.2
654.2
2.30E-06
4.80E-06
9.6E-06
1.15E-05
3.07E-05
3.84E-05
8.96E-05
2.03E-04
4.99E-04
5.34E-04
6.58E-04
6.7(kJ mol−1)/RT
k(total) = 1013.47 0.6 exp−208.2
The plot is given in Fig. 2.
The effect of surface on the reaction was investi-
gated at high, middle, and lower temperatures using
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THERMAL GAS-PHASE DECOMPOSITION OF 2-BROMOPROPENE USING STATIC SYSTEM
3
Figure 3 Pressure dependence study for the decomposition
of 2-bromopropene at 611.5 K (reaction time 10 min). [Color
figure can be viewed in the online issue, which is available
at www.interscience.wiley.com.]
Figure 2 Arrhenius plot for 2-bromopropene. [Color fig-
ure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
energy. The observed reduction by 41 kJ mol−1 for
2-bromopropene compared to vinyl bromide can be
attributed to the stabilization of transition state due
to the presence of methyl group at α-carbon atom. A
similar trend has been observed for 2-bromopropane
compared with bromoethane.
are consistent with semi-ionic four centered transition
state for the elimination of HBr.
The first-order dissociation rate constants have
been theoretically modeled with a distinct Rice,
Ramsperger, Kassel, Marcus (RRKM) calculation us-
ing the Chemrate program. In Table III, the input data
for RRKM calculation are presented. Vibrational fre-
quencies of 2-C3H5Br molecule and those of its com-
plex were obtained from the results of Lee and Kim
report [12]. The calculations of the moment of inertia
for the molecule and activated complex are also based
on the same report. ꢀfH◦ of 2-C3H5Br molecule was
obtained from NIST Kinetics Database [13] and that
A
comparison of the rate constants of
2-bromopropene with vinyl bromide at 600 K
shows that the former reaction is four orders of
magnitude faster (see Table II). Assuming similar
transition state in both reactions, this increase in rate
can be assigned entirely to the change in the activation
Table II High Pressure Arrhenius Parameters for Decomposition of Alkyl Bromide
Reactant
E (kJ mol−1
)
log A (s−1
)
log k at 600 K
Reference
a
C2H3Br
2-C3H5Br
C2H5Br
249.4
208.2 6.7
225.5
13.33
13.47 0.6
13.3
−8.38
−4.64
−7.2
[9]
This work
[10]
2-C3H7Br
200
13.6
−3.67
[11]
Table III Input Parameters used in RRKM Calculation for 2-Bromopropene Decompositiona
Species
CH3 C(Br) CH2
69.9b
[CH3 C(Br) CH2]#
ꢀHf 298 K (kJ mol−1
)
270c
Moment of inertia (10−19 kg m2)
Ix = 6.8145, Iy = 6.8145,
Ix# = 6.5693, Iy# = 6.5693,
Iz = 6.8145
Iz# = 6.5693
Frequencies
3242, 3146, 3106, 3031,
1696, 1488, 1435, 1411,
1178, 1015, 928, 543, 354,
301, 3089, 1470, 1072, 926,
700, 429, 209
3258, 3144, 3137, 3013, 1512, 1477,
1384, 1351, 1205, 1006, 936, 606, 359,
324, 3057, 1433, 1024, 962, 444, 330
a After [12].
b After [13].
c Adjusted to get best fit.
International Journal of Chemical Kinetics DOI 10.1002/kin

4
NISAR AND AWAN
in values of activation energy and pre-exponential fac-
tor. The activation energy and A factor in the case
of experimental results are about 2.23% and 3.5%
higher than the RRKM results, respectively. This very
small difference in the Arrhenius parameters is prob-
ably within the combined experimental errors of all
determinations.
To interpret the falloff behavior of our sample,
RRKM calculations were adopted and the pressure-
dependent rate constants were calculated at collision
efficiency of 0.4. The falloff behavior is represented
in Fig. 5. On the basis of the pressure-dependence
studies and RRKM calculations, we can reasonably
conclude that the Arrhenius parameters obtained
in this work are the high-pressure limiting values.
Thus, minimal errors are introduced by the extrap-
olation procedure. Whatever, the uncertainty is due
to the fact that no purely vibrator rotor transition
state for bond breaking can fit experimental results
over the entire temperature range for bond-breaking
Figure 4 Comparison of calculated and experimental rate
constants for thermal decomposition of 2-bromopropene.
Filled squares represent experimental results, and open
squares denote calculated data. [Color figure can be viewed
in the online issue, which is available at www.interscience.
wiley.com.]
of its complex was assumed to have the value, which
give similar Arrhenius A factor to the experimental
results.
RRKM-treated data exactly reproduced the experi-
mental rate constants over the entire temperature range
studied. This is represented in Fig. 4. The agreement
is fairly satisfactory. There is a very small difference
Figure 5 Comparison of calculated and experimental falloff curves for 2-bromopropene at 611.5 K. Filled squares represent
experimental results, and open squares represent calculated data. [Color figure can be viewed in the online issue, which is
available at www.interscience.wiley.com.]
10
0
2
4
6
8
10
12
14
16
18
20
y = −1.4334x + 14.93
R 2= 0.9955
y = −1.0873x + 13.479
R2 = 0.9905
5
0
−5
y = −1.0644x + 13.029
2 = 1
R
−10
−15
1/T *10000 K
Figure 6 Arrhenius plot showing comparison of static, shock tube and RRKM-treated data for thermal decomposition of 2-
bromopropene. Triangles denote shock tube results; circles represent RRKM-treated data, and diamonds represent static system
investigations.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THERMAL GAS-PHASE DECOMPOSITION OF 2-BROMOPROPENE USING STATIC SYSTEM
5
Table IV Comparison of Arrhenius Parameters Determined by Various Methods for Thermal Decomposition of
2-Bromopropene
Method
Temperature (K)
E (kJ mol−1
)
log A (s−1
)
r
Reference
a
Shock tube (at 100 Torr)
Static system
RRKM
1078.4–1221
570.9–654.2
720–840
274.4
208.2
203.8
14.93
13.47
13.03
0.9977
0.9952
1
[7]
This work
This work
r = Correlation coefficient.
reactions. Tsang, while comparing the experimental
results with the calculated results for decomposition
pressure, can be represented by
0.6
6.7(kJ mol−1)/RT
k(total) = 1013.47
exp−208.2
of 3-C3H5Br, observed that a satisfactory A factor is
∝
not obtained unless the moment of inertia of the two-
dimensional rotor is lowered from 9 × 10−39 g cm2 to
0.54 × 10−39 g cm2 [14]. For this, he followed the Ben-
son’s prescription of a “hindered rotor” transition state
[15].
The pressure dependence study and RRKM calcu-
lation indicate that the Arrhenius parameters obtained
in this work are the high pressure limiting values.
The RRKM-treated data exactly reproduce the
experimental rate constants over the entire tempera-
ture and pressure range studied.
An exact fit can be made when we use a slightly
lower value of β (collision efficiency) 0.2–0.6. In
the present investigations, we use β = 0.4. The same
conclusion has also been drawn by Tsang in his report
on 3-C3H5Br decomposition.
BIBLIOGRAPHY
A combined Arrhenius plot of the present static,
shock tube (at 100 Torr) and RRKM-treated data (in
the temperature range of 720–840 K) is represented
in Fig. 6, showing a good agreement between static
and RRKM-treated data, but no satisfactory agreement
of shock tube investigations with static and RRKM-
treated data is observed. A comparison of Arrhenius
parameters determined by all the three methods is given
in Table IV. It can be seen from the table that the
Arrhenius parameters deduced from the static system
and RRKM-treated data are in consonant with each
other but deviate considerably from the shock tube
determinations.
1. Maccoll, A. Adv Phys Org Chem 1965, 91, 3.
2. Maccoll, A. Chem Rev 1969, 69, 33.
3. Benson, S. W.; O’Neal, H. E. Kinetic Data on Gas Phase
Unimolecular Reactions, NSRDS-NBS 21, 1970.
4. Manion, J. A.; Louw, R. J Chem Soc, Perkin Trans 1988,
2, 1547.
5. Zabel, F. Int J Chem Kinet 1977, 9, 6551.
6. Shilov, A. E.; Sbirova, R. D. Kinet Catal 1964, 5, 32.
7. Roy, K.; Awan, I. A.; Manion, J. A.; Tsang, W. Phys
Chem Chem Phys 2003, 5, 1806.
8. Awan, I. A.; Mahmood, T. J Chem Soc Pak 1999, 21(2),
87.
9. Laws, P. A.; Hayley, B. D.; Anthony, L. M.; Roscoe,
J. M. J Phys Chem A 2001, 105, 1830.
10. Thomas, P. J. J Chem Soc 1959, 1192.
11. Maccoll, A.; Thomas, P. J. Chem Soc 1955, 969.
12. Lee, M.; Kim, M. S. J Chem Phys 2003, 119(23), 12351.
13. NIST Chemistry Webbook, NIST Standard Reference
Data, no. 69, National Institute of Standards and Tech-
nology: Gaithersburg, MD, 2005.
14. Tsang, W. J Phys Chem 1984, 88, 2812.
15. Benson, S. W. Thermochemical Kinetics; Wiley:
New York, 1976.
CONCLUSION
2-Bromopropene decomposes in the temperature range
570.9–654.2 K to give propyne and hydrogen bromide.
The decomposition proceeds by a unimolecular mech-
anism. The decomposition obeys first-order kinetics,
and the rate constant, which is independent of initial
International Journal of Chemical Kinetics DOI 10.1002/kin