Kinetics and mechanisms of gas phase elimination of ethyl 1-piperidine carboxylate, ethyl pipecolinate, and (revisited) ethyl 1-methyl pipecolinate

Article abstract of DOI:10.1002/kin.20084

The kinetics of the gas-phase elimination of the title compounds has been determined in a static reaction system over the temperature range of 340-420°C and pressure range of 45-96 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first-order rate law. The estimated rate coefficients are represented by the following Arrhenius expressions: Ethyl 1-piperidine carboxylate log k₁ (S^-1) = (12.61 ± 0.11) - (191.1 ± 1.4) kJ mol^-1 (2.303 RT)^-1, r = 0.9999 Ethyl pipecolinate log k₁ (S^-1) = (12.87 ± 0.16) - (204.3 ± 2.1) kJ mol^-1 (2.303 RT)^-1, r = 0.9998 Ethyl 1-methyl pipecolinate log k₁ (S^-1) = (13.34 ± 0.32) - (209.4 ± 4.0) kJmol^-1 (2.303 RT) ^-1, r = 0.9992 The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of this elimination reactions is suggested on the basis of the kinetic and thermodynamic parameters.

Full text of DOI:10.1002/kin.20084

Kinetics and Mechanisms
of Gas Phase Elimination
of Ethyl 1-piperidine
Carboxylate, Ethyl
Pipecolinate, and
(
Revisited) Ethyl 1-methyl
Pipecolinate
1
2
3
3
´
FELIX ROSAS, ANGIEBELK MONSALVE, MARIA TOSTA, ARMANDO HERIZE,
ROSA M. DOMINGUEZ, DORIS BRUSCO, GABRIEL CHUCHANI
3
1
3
´
1
Departamento de Qu ı´ mica, Universidad de Oriente, Cumana, Sucre, Venezuela
Departamento de Qu ı´ mica, Universidad de Carabobo, Valencia, Venezuela
Centro de Qu ı´ mica, Instituto Venezolano de Investigaciones Cient ı´ ficas (I.V.I.C.), Apartado 21827,
2
3
Caracas 1020-A, Venezuela
Received 1 October 2004; accepted 28 January 2005
DOI 10.1002/kin.20084
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the gas-phase elimination of the title compounds has been deter-
◦
mined in a static reaction system over the temperature range of 340–420 C and pressure range
of 45–96 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first-order
rate law. The estimated rate coefficients are represented by the following Arrhenius expressions:
Ethyl 1-piperidine carboxylate
log k (s ) = (12.61 ± 0.11) − (191.1 ± 1.4) kJ mol−1(2.303 RT)−1, r = 0.9999
−1
1
Ethyl pipecolinate
log k (s ) = (12.87 ± 0.16) − (204.3 ± 2.1) kJ mol−1(2.303 RT)−1, r = 0.9998
−1
1
Correspondence to: Gabriel Chuchani; e-mail: chuchani@ivic.
ve.
ꢀ
c 2005 Wiley Periodicals, Inc.

384
ROSAS ET AL.
Ethyl 1-methyl pipecolinate
log k (s ) = (13.34 ± 0.32) − (209.4 ± 4.0) kJmol−1(2.303 RT)−1, r = 0.9992
−1
1
The first step of decomposition of these esters is the formation of the corresponding carboxylic
acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The
mechanism of this elimination reactions is suggested on the basis of the kinetic and thermo-
dynamic parameters. ꢀC 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 383–389, 2005
INTRODUCTION
causing a lowering in the energy. Therefore, a very
rapid decomposition may be expected. The fact that
the neutral amino acids decompose rapidly in the gas
phase is supported from the experimental results on
the elimination kinetics of N,N-dimethylglycine [1],
picolinic acid [2], and N-phenylglycine [3], when com-
pared as product intermediates of their corresponding
ethyl ester pyrolysis [1–3]. These results suggested the
elimination kinetics of neutral amino acids in the gas
phase may occur. An interesting fact is that when the N
atom is adjacent to the acid side of carboxylic acid, that
is carbamic acids, they are unstable molecules even at
room temperature. Attempts to determine the kinetic
and thermodynamic parameters from gas phase elimi-
nation of simple types of carbamic acid substrates are
difficult. These organic acids have been described to
be as intermediates of carbamates pyrolysis [11–20],
according to the following general mechanistic con-
sideration [reaction (3)].
Low molecular weight amino acids are solids, and they
are difficult to examine in gas phase elimination re-
actions. Moreover, these compounds on heating sinter
or decompose into amorphous materials, and also are
insoluble in most organic solvents. Their high solubil-
ity in water, forming zwitterions species, restricts their
study as neutral molecules in the gas phase. In spite of
these limitations, few recent works have reported the
homogeneous, unimolecular gas phase elimination of
N,N-dimethylglycine [1], picolinic acid [2], and N-
phenylglycine [3]. These substrates as 2-substituted
amino carboxylic acids undergo decarboxylation as
shown in reaction (1).
(1)
The mechanism described in reaction (1) differs
from the already reported both experimentally and the-
oretically the gas phase elimination of several types
of 2-substituted carboxylic acids. The latter com-
pounds are found to decarbonylate [4–10] as depicted
in reaction (2)
(3)
In spite of the limitations to study the elimination of
carbamic and ꢀ-amino acids in the gas phase, it is in-
tended, if possible, to determine the kinetic and thermo-
dynamic parameters of decarboxylation of these acid
intermediates through a consecutive reaction from the
corresponding ester decomposition. In addition to this
fact, it was thought interesting to examine the extent
to which nitrogen atom may influence decarboxylation
process at the 1- and 2-position of the carboxylic acid
intermediates. Consequently, this work aimed at ex-
amining the gas phase pyrolysis of ethyl 1-piperidine
carboxylate, ethyl pipecolinate, and to revise ethyl 1-
methylpipecolinate.
(
2)
The gas phase elimination of above-mentioned neu-
tral amino acids [1–3] showed to be very reactive
species in the gas phase. This fact was rationalized in
terms of the N atom becoming more polarized when
approaching to the acidic H of the COOH group, thus

KINETICS AND MECHANISMS OF GAS PHASE ELIMINATION OF TITLE COMPOUNDS
385
EXPERIMENTAL
carbon dioxide [reaction (4)].
The substrates ethyl 1-piperidine carboxylate
Aldrich), ethyl pipecolinate (Aldrich), and ethyl
-methylpipecolinate (Aldrich) were distilled several
times, and the fraction over 98.6% purity was used
GC-MS : Saturn 2000, Varian, with a DB-5MS
capillary column 30 m × 0.53 mm. i.d., 0.53 ꢁm film
thickness). The quantitative chromatographic analysis
of ethylene was determined by using a gas chromato-
graph HP 5710A with a Porapak Q (80-100 mesh).
The identifications of the products piperidine and
(
1
(
(4)
Theexperimentalstoichiometryforthepyrolysisofthis
substrate [reaction (4)] requires Pf/P0 = 3.0, where
Pf and P0 are the final and initial pressure, re-
spectively. The average experimental Pf/P0 at four
different temperatures and 10 half-lives is 3.0 (Ta-
ble I). Additional confirmation of stoichiometry (4)
was obtained by comparing the pressure measurements
with the quantitative analyses of ethylene formation
1-methylpiperidine were made in a GC-MS (Saturn
2000, Varian with a DB-5MS capillary column 30 m ×
0.25 mm. i.d., 0.25 ꢁm).
Kinetics
The kinetic determinations were carried out in a
static reaction system as described before [21–23].
At each temperature, six to nine runs are carried out
in our experiments. The rate coefficients were cal-
culated from the pressure increase manometrically
and/or by formation of ethylene product. The temper-
(
Table II).
The reaction can be said to be homogeneous since
no significant effects on the rates were obtained on
using both clean Pyrex and seasoned Pyrex vessels
with a surface-to-volume ratio of 6.0 relative to the
normal clean and seasoned vessels in these experi-
ments (Table III). The presence of different propor-
tions of toluene, a free radical suppressor, had no ef-
fect on the rates and no induction period was obtained
Table IV). The rates are reproducible with a relative
standard deviation of not greater that 5% at a given
temperature.
The rate coefficients for elimination, calculated
from k1 = (2.303/t) log[2P0/(3P0 − Pt )], are invari-
able to initial pressures (Table V), and the first-order
plots of log(3P0 − Pt )] against time t gave a good
straight line for up to 60% decomposition. The vari-
ation of the rate coefficients with temperature is shown
in Table VI. The experimental data were fitted to the
Arrhenius equation shown in Table VI (Fig. 1), where
90% confidence limits from a least-squares procedure
are given.
◦
ature was maintained within ± 0.2 C through control
with a Shinko DIC-PS 23TR resistance thermometer
and was measured with a calibrated iron-Constantan
type k thermocouple. No temperature gradient was ob-
served along the reaction vessel. The starting materi-
als were all injected directly into the reaction vessel
with a syringe through a silicone rubber septum. The
amount of substrate used for each reaction was ∼0.05–
(
0
.1 mL.
RESULTS AND DISCUSSION
Ethyl 1-piperidine Carboxylate
The product of gas phase elimination of ethyl piperi-
dine carboxylate are mainly piperidine, ethylene, and
Table I Ratio of Final (P ) to Initial (P ) Pressure
f
0
◦
Compound
Temperature ( C)
P0 (Torr)
Pf (Torr)
Pf/P0
Aver.
3.0
Ethyl 1-piperidine carboxylate
Ethyl pipecolinate
360.4
54.5
59.5
60
163.5
178.5
174.0
177.5
3.0
3.0
2.9
3.0
3
3
3
70.5
80.8
91.0
59
400.2
56
64
59
64
165
184
173
203
3.0
2.9
2.9
3.0
3.0
4
4
4
10.4
20.0
30.5

386
ROSAS ET AL.
Table II Stoichiometry of the Reaction
◦
Compound
Temperatures ( C)
370.5
Parameter
Value
Ethyl 1-piperidine carboxylate
Time (min)
3
5
7
9
12
Reaction (%) (pressure)
Ethylene (%) (GC)
18.3
17.8
29.3
29.4
39.7
40.0
46.0
45.8
58.0
57.7
Ethyl pipecolinate
410.0
Time (min)
3
5
7
9
12
Reaction (%) (pressure)
Ethylene (%) (GC)
25.7
25.3
39.2
39.9
50.3
49.5
61.8
60.8
71.8
70.6
Table III Homogeneity of Pyrolysis Reactions
−
1 a
4
−1 b
4
−1 c
Compound
S/V (cm
)
10 k1 (s
)
10 k1 (s
)
◦
Ethyl 1-piperidine carboxylate at 370.5 C
1
6
12.5 (± 0.1)
12.5 (± 0.2)
12.8 (± 0.1)
12.4 (± 0.1)
◦
Ethyl pipecolinate at 420.3 C
1
6
30.2 (± 0.1)
30.9 (± 0.1)
29.6 (± 0.2)
31.7 (± 0.1)
a
S = Surface area; V = volume.
b
Clean Pyrex vessel.
c
Vessel seasoned with allyl bromide.
Table IV Effect of Free Radical Inhibitor Toluene on the Reactions
◦
4
−1
)
Substrate
Temperature ( C)
370.5
Ps (Torr)
Pi (Torr)
Pi/Ps
10 k1 (s
Ethyl 1-piperidine carboxylate
90
–
117
49
–
11.5
12.6
12.6
12.4
6
3
3
6
9
3
1.8
1.3
1.6
51
Ethyl pipecolinate
420.0
64.5
–
117
190.5
163
–
30.1
30.1
30.3
30.2
7
9
5
4.5
4
9
1.6
2.0
2.8
P
s
= pressure substrate. P = pressure inhibitor.
i
Table V Invariability of the Rate Coefficients with Initial Pressure
◦
Compound
Temp. ( C)
370.5
Parameters
Ethyl 1-piperidine carboxylate
P0 (Torr)
49.5
12.4
45
64
12.6
59
30.2
71
12.6
68
85
95.5
12.5
94.0
30.3
4
−1
1
1
0 k1 (s
P0 (Torr)
)
)
12.6
74.5
30.1
Ethyl pipecolinate
420.0
4
−1
0 k1 (s
30.1
30.3
Table VI Variation of the Rate Coefficients with Temperatures
Substrate
Parameters
Values
◦
Ethyl 1-piperidine
carboxylate
Temp. ( C)
341.0
2.33
350.9
4.06
360.4
7.13
370.5
12.5
380.8
22.1
391.0
38.8
4
−1
10 k1 (s
)
−1
−1
−1
Rate equation log k1 (s ) = (12.61 ± 0.11) − (191.1 ± 1.4) kJ mol (2.303 RT ) , r = 0.9999
◦
Ethyl pipecolinate
Temp. ( C)
380.2
3.51
390.1
5.98
400.2
10.2
410.4
17.7
420.3
30.2
430.5
51.6
4
−1
)
1
0 k1 (s
−1
−1
−1
Rate equation log k1 (s ) = (12.87 ± 0.16) − (204.3 ± 2.1) kJ mol (2.303 RT ) , r = 0.9998
◦
Ethyl 1-methyl
pipecolinate
Temp. ( C)
361.8
1.35
371.2
2.36
380.5
4.05
390.5
6.90
398.5
12.6
410.5
21.6
419.7
38.0
4
−1
)
10 k1 (s
−1
−1
−1
Rate equation log k1 (s ) = (13.34 ± 0.32) − (209.4 ± 4.0) kJ mol (2.303 RT ) , r = 0.9992

KINETICS AND MECHANISMS OF GAS PHASE ELIMINATION OF TITLE COMPOUNDS
387
Figure 1 Graphic representation of the Arrhenius plot for the gas phase elimination of the ethyl-1-piperidine carboxylate.
Ethyl Pipecolinate
fect of the surface area in the rate of elimination was ex-
aminedbyemployingavesselwithasurface-to-volume
ratio of six times greater than that of the normal vessel.
The rate coefficient for product formation was unaf-
fected on using both clean Pyrex and seasoned Pyrex
vessels in these experiments (Table III). The reaction
can be said to be homogeneous. Toluene inhibitor does
not appear to have effect on the rates (Table IV), and
no induction period was observed. The rates are repro-
ducible with a relative standard deviation of not greater
than 5% at a given temperature.
The elimination products of ethyl pipecolinate are
piperidine, ethylene, and carbon dioxide [reaction (5)].
The first-order rate coefficients for ethyl pipecol-
inate, calculated from k1 = (2.303/t) log [2P0/
(
3P0 − Pt )], were found to be independent of the initial
(5)
pressure (Table V). The k-value, at a given temperature,
is measured at each reaction time, and the average k-
value is estimated within ± 5% standard deviation. A
plot of log(3P0 − Pt ) against time t gave a good straight
line up to 70% decomposition. The temperature de-
pendence of the rate coefficients and the correspond-
ing Arrhenius equation are given in Table VI (Fig. 2)
(90% confidence coefficients from a least-squares pro-
cedure).
The theoretical stoichiometry (5) demands Pf/
P0 = 3.0. The average experimental Pf/P0 obtained
at four different temperatures and 10 half-lives is 3.0
(Table I). To verify stoichiometry (5), up to 70% de-
composition, it was made by comparing the pressure
measurements with those obtained by the quantitative
GC analysis of the product ethylene (Table II). The ef-
Figure 2 Graphic representation of the Arrhenius plot for the gas phase elimination of the ethyl pipecolinate.

388
ROSAS ET AL.
Figure 3 Graphic representation of the Arrhenius plot for the gas phase elimination of the ethyl-1-methylpipecolinate.
Ethyl 1-methylpipecolinate
According to Table VII, the electron-withdrawing
effect of the nitrogen atom at the 1-position of ethyl
piperidine carboxylate shows a faster rate of ethylene
formationthanwhentheNatomisatthe2-positionasin
ethyl pipecolinate. It is rather unfortunate that under the
workingreactiontemperature, thekineticparametersof
the corresponding acids intermediates could not be es-
timated. It is well known that carbamic acids are more
unstable and decarboxylate more rapidly than ꢀ-amino
acids in the gas phase. The rate of the gas phase pyrol-
ysis of ethyl 1-methylpipecolinate [2] was found to be
a little faster in rate than ethyl pipecolinate. Because of
this, the former substrate was revisited in order to ver-
ify their previous reported kinetic and thermodynamic
data. The reexamination of ethyl 1-methylpipecolinate
gave similar parameters reported before [2]. The appar-
ent very small difference in rate between ethyl pipecol-
inate and ethyl 1-methylpipecolinate implies, from ring
structure, steric acceleration of the methyl group of the
latter compound. From products elimination and the
results given in Table VII, ethyl piperidine carboxy-
late appears to proceed via a mechanism similar to
carboxylic esters in the gas phase [24,25], while the
carbamic acid intermediate decarboxylates through a
four-membered cyclic transition state [reaction (7)].
Ethyl pipecolinate and ethyl 1-methylpipecolinate de-
compose, as expected, to ethylene and the correspond-
ing neutral amino acid derivatives. These acid interme-
diates are unstable under the experimental condition
and proceed to decarboxylate through a five-membered
Inviewofthesmalldifferencesinratesofethylpipecol-
inate elimination (Table VI) when compared to the al-
ready reported data of ethyl 1-methylpipecolinate [2],
led to us to reexamine the elimination kinetics of the
latter compound. This verification is to assure an ade-
quate argument relative to both substrates.
(6)
The stoichiometry for the reaction (6), the effect of the
inhibitor, the examination of homogeneity of the reac-
tion, and the invariability of the k-values with initial
pressure were carried out in the same manner as de-
scribed for the above-mentioned esters. However, the
experimental variation of the rate coefficients with tem-
perature and the corresponding Arrhenius equation are
reported and given in Table VI (Fig. 3) (90% confidence
limits from a least-squares procedure). The actual re-
sult was found to be quite similar to that described in
the previous investigation [2].
◦
Table VII Kinetic and Thermodynamic Parameters at 390 C
4
−1
−1
ꢁ= ꢁ=
ꢁ=
Compound
10 k1 (s
)
Ea (kJ/mol) log A (s
)
ꢀS (J/mol K) ꢀH (kJ/mol) ꢀG (kJ/mol)
Ethyl 1-piperidine carboxylate
Ethyl pipecolinate
Ethyl 1-methylpipecolinate
Ethyl 1-methylpipecolinateb
35.85
5.95
7.16
6.96
191.1 ± 1.4 12.61 ± 0.11
204.3 ± 2.1 12.87 ± 0.16
209.5 ± 3.9 13.36 ± 0.31
209.4 ± 4.0 13.34 ± 0.32
−18.4
−13.4
−4.1
185.6
198.8
204.0
203.9
197.8
207.7
206.7
206.8
a
−4.4
a
After [2].
This work.
b

KINETICS AND MECHANISMS OF GAS PHASE ELIMINATION OF TITLE COMPOUNDS
389
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