Kinetics of elimination of several heterocyclic carbamates in the gas phase

Article abstract of DOI:10.1002/poc.525

The kinetics of the gas-phase elimination of several heterocyctic carbamates were determined in a static system over the temperature range 190.0-409.7°C and the pressure range 26.5-125 Torr (1 Torr = 133.3 Pa). The reactions in seasoned vessels, with the free radical inhibitor cyciohexene and/or toluene always present, are homogeneous and unimolecular and obey a first-order rate law. The observed rate coefficients are represented by the following Arrhenius equations: for tert-butyl-1-pyrrolidine carboxylate, log k₁ (s^-1) = (11.36 ± 0.31) - (145.4 ± 3.1) kJ mol^-1 (2.303RT)^-1; for 1-(tert-butoxycarbonyl)-2-pyrrolidinone, log k₁ (s^-1) = (11.54 ± 0.29) - (140.8 ± 2.8) kJ mol^-1 (2.303RT)^-1; for tert-butyl-1-pyrrole carboxylate, log k₁ (s^-1): (12.12 ± 0.05) - (145.2 ± 1.0) kJ mol^-1 (2.303RT)^-1; and for 1-ethylpiperazine carboxylate, log k₁ (s^-1): (12.05 ± 0.19) - (188.2 ± 4.6) kJ mol^-1 (2.303RT)^-1 The saturated heterocyclic carbamates show a decrease in rates of elimination due to electronic factors. Heterocyclic carbamates with a nitrogen atom able to delocalize its electrons with π-bonds present in the ring were found to enhance the rates due to resonance interactions. Copyright

Full text of DOI:10.1002/poc.525

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 796–800
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.525
Kinetics of elimination of several heterocyclic carbamates
†
in the gas phase
1
2
2
2
1
Yannely Brusco, Rosa M. Dom õÂ nguez, Alexandra Rotinov, Armando Herize, Mary Cordova and
2
Gabriel Chuchani *
1
Departamento de Qu õÂ mica, Universidad de Oriente, Cumana, Sucre, Venezuela
Centro de Qu õÂ mica, Instituto Venezolano de Investigaciones Cient Âõ ®cas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
2
Received 18 February 2002; revised 27 March 2002; accepted 29 March 2002
ABSTRACT: The kinetics of the gas-phase elimination of several heterocyclic carbamates were determined in a static
system over the temperature range 190.0–409.7°C and the pressure range 26.5–125 Torr (1 Torr = 133.3 Pa). The
reactions in seasoned vessels, with the free radical inhibitor cyclohexene and/or toluene always present, are
homogeneous and unimolecular and obey a first-order rate law. The observed rate coefficients are represented by the
À1
À1
following Arrhenius equations: for tert-butyl-1-pyrrolidine carboxylate, log k (s ) = (11.36 Æ 0.31) À(145.4 Æ 3.1)
1
À1
À1
À1
À1
kJ mol (2.303RT) ; for 1-(tert-butoxycarbonyl)-2-pyrrolidinone, log k (s ) = (11.54 Æ 0.29) À(140.8 Æ 2.8)
1
À1
À1
À1
kJ mol (2.303RT) ; for tert-butyl-1-pyrrole carboxylate, log k (s ) = (12.12 Æ 0.05) À(145.2 Æ 1.0) kJ mol
1
À1
À1
(
(
2.303RT) ; and for 1-ethylpiperazine carboxylate, log k1 (s ) = (12.05 Æ 0.19) À(188.2 Æ 4.6) kJ mol
À1
2.303RT) . The saturated heterocyclic carbamates show a decrease in rates of elimination due to electronic
factors. Heterocyclic carbamates with a nitrogen atom able to delocalize its electrons with p-bonds present in the ring
were found to enhance the rates due to resonance interactions. Copyright  2002 John Wiley & Sons, Ltd.
KEYWORDS: elimination; kinetics; heterocyclic carbamates
INTRODUCTION
slightly compared with (CH ) N. This difference was
3 2
explained in terms of the greater electron-releasing power
of the CH CH group to the N atom than CH . Along this
The gas-phase pyrolysis kinetics of 2-alkyl-substituted
ethyl N,N-diethylcarbamates have been shown to
enhance the rate of ethylene elimination in the order
tert-butyl > isopropyl > ethyl, which is consistent with
the sequence of the comparative rates for the correspond2-
ing 2-alkyl-substituted ethyl N,N-dimethylcarbamates.
3
2
3
1
line of work, the effect of a phenyl group at the N atom in
3
ethyl N-methyl-N-phenylcarbamate gave a small de-
crease in rate compared with (CH ) NCOOCH CH .
3
2
2
3
Moreover, when the remaining CH at the N atom of
3
ethyl N-methyl-N-phenylcarbamate was replaced by an
additional phenyl group, i.e. ethyl N,N-diphenylcarba-
mate, the rate decreased significantly by a factor of 9.8.
This result suggested that steric factors may well affect
this process of elimination.
These reactions suggested that C —O bond polarization,
a
ꢀ
‡
ꢀÀ
in the direction Ca —O , is the rate-determining step.
The mechanism of these reactions was believed to
proceed according to reaction (1).
Further investigation of the factors which may affect
the rate of elimination of carbamates involves consider-
ing heterocyclic nitrogen subtituents at the acid side of
carbamates, where the nitrogen atom is part of a cyclic
saturated or conjugated p-bond system. Therefore, the
present work was aimed at examining the pyrolysis
kinetics of tert-butyl-1-pyrrolidine carboxylate, 1-(tert-
ꢀ1†
The (CH CH ) N substituent at the acid side of the
butoxycarbonyl)-2-pyrrolidinone,
tert-butyl-1-pyrrole
3
2 2
above-mentioned esters was found to decrease the rate
carboxylate and 1-ethylpiperazine carboxylate in the
gas phase.
*
Correspondence to: G. Chuchani, Centro de Qu ´ı mica, Instituto
Venezolano de Investigaciones Cient ı´ ficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
†
Presented in part at the Sixth Latin American Conference on Physical
RESULTS AND DISCUSSION
Organic Chemistry (CLAFQO-6), held at Isla Margarita, Venezuela,
during December 2001.
Contract/grant sponsor: Fondo Nacional de Ciencia, Tecnolog ı´ ae
Innovaci o´ n (FONACIT); Contract/grant number: S1-97000005.
The molecular elimination of the carbamates described
by reaction (2): in a static system, with vessels seasoned
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 796–800

ELIMINATION OF CARBAMATES
797
with allyl bromide and in the presence of cyclohexene
The rate coefficients of these eliminations were found
to be invariable to initial pressures (Table 5), and the first-
order rate was calculated from k = (2.303/t) log [2P /
and/or toluene inhibitors, demands that P /P = 3.0,
f
0
where P and P0 are the final and initial pressure,
f
1
0
respectively (Table 1). The average experimental results
(3P À P )]. A plot of log(3P À P ) against time t gave a
0
t
0
t
for P /P values at four different temperatures and 10
good straight line up to 60–75% decomposition. The
variation of the rate coefficient with temperature and the
corresponding Arrhenius equations are given in Table 6
(90% confidence coefficient from the least-squares
procedure).
f
0
half-lives were 2.95 for tert-butyl-1-pyrrolidine carboxy-
late, 2.84 for 1-(tert-butoxycarbonyl)-2-pyrrolidinone,
and 2.81 for tert-butyl-1-pyrrole carboxylate.
ꢀ2†
1-Ethylpiperazine carboxylate
The elimination products of reaction (3):
The departure from P = 3P results mainly from a
f
0
small extent of polymerization of an authentic sample of
the olefinic product isobutene. Additional verification of
the stoichiometry (2) was made by comparing the
percentage decomposition of the substrate calculated
from pressure measurements with that obtained from the
chromatographic analyses of the olefin product isobutene
ꢀ3†
(
Table 2).
The homogeneity of these reactions was examined in
the presence of the inhibitors cyclohexene and/or toluene
in a vessel with a surface-to-volume ratio 6.0 times
greater than normal, which is equal to 1.0 (Table 3). The
packed and unpacked clean Pyrex vessels had a
significant effect on the rates. However, when the packed
and unpacked vessels were seasoned with allyl bromide,
except 1-(tert-butoxycarbonyl)-2-pyrrolidinone, no
marked effect on the rate coefficients of these carbamates
occurred. The pyrolysis of these carbamates, in seasoned
vessels, had to be carried out in the presence of at least
twice the amount of the inhibitor cyclohexene and/or
toluene (Table 4). No induction period was obtained. The
rate coefficients are reproducible with a relative standard
deviation not greater than 5% at a given temperature.
suggest a theoretical P /P = 3.0. The average experi-
f
0
mental results for P /P at four temperatures and 10 half-
f
0
lives is 2.77 (Table 1). The small departure from
stoichiometry was found to be due to a small extent of
decomposition of an authentic sample of piperazine.
Further verification of the stoichiometry of reaction (3),
up to 45% decomposition, was obtained by comparing
the pressure increase with the quantitative GLC analysis
of ethylene formation (Table 2). The homogeneity of this
pyrolytic elimination was examined by using a packed
reaction vessel with a surface-to-volume ratio six times
Table 1. Ratio of ®nal (P ) to initial pressure (P ) of the substrate
f
0
Substrate
tert-Butyl-1-pyrrolidine carboxylate
Temperature (°C)
P0 (Torr)
Pf (Torr)
Pf/P0
Average
2.95
230.2
18
21
31
45
27
34
26.5
59
96
81
63
53
90
92
80
67
54.5
62.5
89.5
130.5
76
97.5
75
164.5
281
226
164
145
255
259
217
181
3.03
2.98
2.89
2.90
2.81
2.87
2.83
2.83
2.93
2.79
2.76
2.74
2.83
2.82
2.71
2.70
2
2
2
40.8
50.8
60.9
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone
210.0
2.84
2.81
2.77
2
2
2
19.6
29.3
39.8
tert-Butyl-1-pyrrole carboxylate
213.7
2
2
2
24.4
35.1
45.7
1
-Ethylpiperazine carboxylate
372.0
3
3
3
80.5
89.0
99.0
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 796–800

7
98
Y. BRUSCO ET AL.
Table 2. Stoichiometry of the reaction
Substrate
Temperature (°C)
Parameter
Value
tert-Butyl-1-pyrrolidine carboxylate
260.9
239.8
235.1
372.0
Time (min)
5
6
8
10
12
Reaction (%) (pressure)
Isobutene (%) (GLC)
Time (min)
31.6
29.1
3
39.3
37.9
5
44.7
43.8
7
57.1
55.2
10
64.9
63.9
15
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone
Reaction (%) (pressure)
Isobutene (%) (GLC)
Time (min)
Reaction (%) (pressure)
Isobutene (%) (GLC)
Time (min)
25.6
25.5
1.5
15.5
14.6
1.5
38.3
36.4
3
24.3
24.6
3
47.1
45.1
6
42.0
42.3
7
61.8
63.8
9
53.3
53.9
10
71.0
75.6
11
62.1
61.3
16
tert-Butyl-1-pyrrole carboxylate
1
-Ethylpiperazine carboxylate
Reaction (%) (pressure)
Ethylene (%) (GLC)
8.3
7.7
16.1
15.8
33.3
33.3
40.6
41.0
43.9
45.1
Table 3. Homogeneity of the reactions
Compound
À1 a
4
À1 b
4
À1 c
S/V (cm
)
10 k (s
)
10 k (s
)
1
1
tert-Butyl-1-pyrrolidine carboxylate at 260.9°C
1
6
1
6
1
6
1
6
7.90
12.27
47.05
13.85
13.92
8.15d
44.25
15.16
15.58
4.81
d
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone at 229.3°C
d
50.0
tert-Butyl-1-pyrrole carboxylate at 235.1°C
16.4
20.98
5.21
5.60
1
-Ethylpiperazine carboxylate at 369.9°C
4.83
a
b
c
d
S = surface area; V = volume.
Clean Pyrex vessel.
Vessel seasoned with allyl bromide.
Average k value.
a
Table 4. Effect of the free radical inhibitor on rates
b
c
4
À1
Substrate
Temperature (°C)
Ps (Torr)
Pi (Torr)
Pi/Ps
10 k (s
)
1
tert-Butyl-1-pyrrolidine carboxylate
260.9
229.3
44
—
16
—
0.7
1.6
2.7
3.4
—
0.8
1.9
2.6
3.2
4.2
—
0.7
1.4
2.9
3.5
—
0.5
1.2
2.4
2.7
14.47
13.91
13.68
13.72
13d.74
2
3
4
3
4
7
60
5
122
132
—
8.5
33.5
3
2.5
3
0
6.5
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone
—
4
5
4
4
2
35
7.07
7.97
101.5
109.5
127.5
111
—
8.34
8.03
8.35
tert-Butyl-1-pyrrole carboxylate
235.1
380.5
57
15.02
15.20
15.27
15.29
15.26
13.29
11.60
11.64
11.54
11.69
6
6
5
5
6
4
5
2
50
88
161
182
—
1
-Ethylpiperazine carboxylate
103
44
1
73
9
6
7
8
5
5
114
156
204
a
b
c
d
Cyclohexene or toluene inhibitor.
P
s
, pressure of the substrate.
P
i
, pressure of the inhibitor.
k Value unreliable.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 796–800

ELIMINATION OF CARBAMATES
Table 5. Invariability of the rate coef®cients with initial pressure
799
Substrate
Temperature (°C)
260.9
Parameter
Value
45
13.72
43
8.34
81
7.51
80
11.57
tert-Butyl-1-pyrrolidine carboxylate
P (Torr)
28
38.5
13.64
34.5
8.05
51
7.45
70
11.62
75
0
4
À1
1
0 k (s
)
)
)
)
13.93
26.5
8.35
41
13.99
52.5
7.97
105
7.46
98
11.60
1
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone
tert-Butyl-1-pyrrole carboxylate
-Ethylpiperazine carboxylate
229.3
P (Torr)
61
8.00
121
7.41
108
11.59
0
4
À1
1
0 k (s
1
224.3
P (Torr)
0
4
À1
1
0 k (s
7.32
51
11.54
1
1
380.5
P (Torr)
0
4
À1
1
0 k (s
1
greater than that of the unpacked vessel (Table 3). The
packed and unpacked Pyrex vessels seasoned with allyl
bromide had no effect on rates. However, the packed and
unpacked clean Pyrex vessels gave a small but significant
heterogeneous effect. The effect of addition of different
proportions of toluene inhibitor is shown in Table 4.
According to this result, the pyrolysis experiments had to
be carried out in the presence of at least an equal amount
of toluene in order to prevent any possible radical
reactions. No induction period was observed and the rates
were reproducible with a relative standard deviation not
greater than 5% at a given temperature.
that these carbamates are not spontaneous, unstable and
endergonic is reflected by the free energy of activation
DG given in Table 7.
≠
When the terminal CH of the (CH CH )N group of
3
3
2
The rate coefficients, in seasoned vessels and in the
presence of toluene, were found to be independent of the
initial pressure, and the first-order plots are satisfactorily
linear up to about 45% reaction (Table 5). The
temperature dependence of the rate coefficients, at the
the tert-butyl ester 1 are bound together to form the
heterocyclic substituent of the carbamate, i.e. tert-butyl-
1-pyrrolidine carboxylate 2, the rate of elimination of this
substrate decreased compared with ethyl N,N-diethylcar-
bamate 1 (Table 7). Apparently, the electronic effect of
the 1-pyrrolidine substituent may be operating during the
process of decomposition. Moreover, when the pyrroli-
dine skeleton in 2 is modified by introducing a carbonyl
group such as 3, i.e. 1-(tert-butoxycarbonyl)-2-pyrrolidi-
none, or to a conjugated p-system such as 4, i.e. 1-tert-
butylpyrrole carboxylate, both substrates give a signifi-
cant enhancement in the rate of isobutylene elimination
with respect to 1 (Table 7). Consequently, an electronic
effect seems to be the paramount factor in the decom-
position process of these substrates. Delocalization of the
nitrogen electrons to the carbonyl group in 3, and to the
9
0% confidence level with the least-squares method, is
described in Table 6.
The kinetic and thermodynamic parameters of several
heterocyclics containing the nitrogen atom at the acid
side of esters are given in Table 7. The negative entropy
≠
of activation DS of these eliminations suggests a
symmetrical arrangement and possible planarity of the
≠
transition state, while the enthalpy of activation DH
implies endothermic processes. The values of logA
between 11.36 and 12.87 are reasonable for a six-
membered cyclic transition-state mechanism. The fact
Table 6. Variation of rate coef®cients with temperature
Substrate
Parameter
Value
tert-Butyl-1-pyrrolidine carboxylate
Temperature (°C)
220.6
0.99
230.2
1.82
241.0
3.63
250.8
7.48
260.9
270.0
23.82
280.0
42.88
4
À1
1
0 k (s
)
13.72
1
À1
À1
À1
Rate equation
Log[k (s )] = (11.36 Æ 0.31) À(145.4 Æ 3.1) kJ mol (2.303RT) ; r = 0.9997
1
1
-(tert-Butoxycarbonyl)-2-pyrrolidinone Temperature (°C)
209.5
2.03
219.3
3.85
229.3
8.15
239.8
250.1
260.0
56.00
4
À1
1
0 k (s
)
15.83
29.78
1
À1
À1
À1
Rate equation
tert-Butyl-1-pyrrole carboxylate
Log[k (s )] = (11.54 Æ 0.29) À(140.8 Æ 2.8) kJ mol (2.303RT) ; r = 0.9998
1
Temperature (°C)
193.4
0.72
203.2
1.53
213.8
3.49
224.3
7.39
235.1
245.7
31.23
254.7
56.14
4
À1
À1
1
0 k (s
)
15.15
1
À1
À1
Rate equation
Log[k (s )] = (12.12 Æ 0.05) À(145.2 Æ 1.0) kJ mol (2.303RT) ; r = 0.9999
1
1
-Ethylpiperazine carboxylate
Temperature (°C)
353.4
2.60
369.9
4.81
372.0
6.94
380.5
389.0
399.0
30.02
409.7
50.20
4
À1
À1
1
0 k (s
)
11.58
18.08
1
À1
À1
Rate equation
Log[k (s )] = (12.05 Æ 0.19) À(188.2 Æ 4.6) kJ mol (2.303RT) ; r = 0.9997
1
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 796–800

8
00
Y. BRUSCO ET AL.
Table 7. Kinetic and thermodynamic parameters for pyrolysis of ZCOOC(CH ) at 230°C
3
3
À1
À1
≠
À1 À1
≠
À1
≠
À1
Z
k1 Â 10À4 (sÀ1
)
Ea (kJ mol
)
Log A (s
)
DS (J mol
À52.3
K
)
DH (kJ mol
)
DG (kJ mol
)
Ref.
1
(CH CH ) N
2.52
158.6 Æ 6.2 12.87 Æ 0.62
145.4 Æ 3.1 11.36 Æ 0.31
154.4
141.2
180.7
161.4
3
2 2
1
8
.83
.31
À40.2
This work
140.8 Æ 2.8 11.54 Æ 0.29
145.2 Æ 1.0 12.12 Æ 0.05
À36.8
À25.5
136.6
141.0
155.1
153.8
This work
This work
1
1.03
Table 8. Comparative rates and kinetic parameters for pyrolysis of ZCOOCH CH at 380°C
2
3
k1 Â 10À4 (sÀ1
)
Ea (kJ mol
À1
)
Log A (s
À1
)
DS (J mol
≠
À1 À1
K
) DH (kJ mol
≠
À1
)
DG (kJ mol
≠
À1
)
Ref.
Z
(
CH CH ) N
15.83
9.90
178.4 Æ 3.1 11.47 Æ 0.25
188.2 Æ 4.6 12.05 Æ 0.19
À40.2
À29.1
173.0
182.8
199.2
201.8
1
2
3 2
HNC H N
This work
6
8
conjugated p-system in 4, makes these substituents
and electrical devices. The reaction vessel was seasoned
with the product of decomposition of allyl bromide. The
rate coefficients were determined manometrically. The
temperature was found to be constant within Æ0.2°C
when controlled with a Shinko DIC-PS 23TR resistance
thermometer controller, and measured with a calibrated
platinum/platinum–13% rhodium thermocouple. The
reaction vessel showed no temperature gradient at
different points, and the substrate was injected directly
into the reaction vessel through a silicone-rubber septum.
electron withdrawing in nature, thus favouring the C —
a
O bond breaking, which is rate determining in the
transition state [reaction (1)].
The small but significant decrease in rate of 1-
ethylpiperazine carboxylate compared with ethyl N,N-
diethylcarbamate (Table 8) leads us to believe that the
piperazine substituent at the acid side of the ethyl ester
affects the rate in terms of electronic effects.
EXPERIMENTAL
Acknowledgements
tert-Butyl-1-pyrrolidine carboxylate, 1-(tert-butoxycar-
bonyl)-2-pyrrolidinone, tert-butyl-1-pyrrole carboxylate
and 1-ethylpiperazine carboxylate were acquired from
Aldrich. Quantitative analyses of the substrates were
performed by GLC (Pennwalt 223, KOH, 80–100 mesh).
The olefin products ethylene and isobutylene were
analyzed in a column of Porapak Q (80–100 mesh).
The verification of the substrates and identifications of
the products were carried out by GLC–MS (Saturn 2000,
Varian) with a DB-5MS capillary column, 30 Â
We are grateful for the financial support of FONACIT
Fondo Nacional de Ciencia, Tecnolog ı´ a e Innovaci o´ n) of
Venezuela, Project No. S1-97000005.
(
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Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 796–800