Kinetics of the gas-phase homogeneous unimolecular elimination of selected ethyl esters of 2-oxo-carboxylic acids

Article abstract of DOI:10.1002/poc.1705

The gas-phase elimination kinetics of selected ethyl esters of 2-oxo-carboxylic acid have been studied over the temperature range of 270-415 °C and pressures of 37-114 Torr. The reactions are homogeneous, unimolecular, and follow a first-order rate law in a seasoned static reaction vessel, with an added free radical suppressor toluene. The observed overall and partial rate coefficients are expressed by the following Arrhenius equations: Ethyl oxalyl chloride log k_overall (s^-1)=(13.22 ± 0.45) - (179.4 ± 4.9) kJ mol^-1 (2.303 RT)^-1 Ethyl piperidineglyoxylate log k_(CO2) (s^-1)=(12.00 ± 0.30) - (191.2 ± 3.9) kJ mol^-1 (2.303 RT)^-1 log k_(CO) (s^-1)=(12.60 ± 0.09) - (210.7 ± 1.2) kJ mol^-1 (2.303 RT)^-1 log k_t(overall) (s ^-1)=(12.22 ± 0.26) - (193.4 ± 3.4) kJ mol^-1 (2.303 RT)^-1 Ethyl benzoyl formate log k_(CO2) (s ^-1)=(12.89 ± 0.72) - (203.8 ± 9.0) kJ mol^-1 (2.303 RT)^-1 log k_(CO) (s^-1)=(13.39 ± 0.31) - (213.3 ± 3.9) kJ mol^-1 (2.303 RT)^-1 log k_t(overall) (s^-1)=(13.24 ± 0.60) - (205.8 ± 7.6) kJ mol^-1 (2.303 RT)^-1 The kinetic and thermodynamic parameters of these reactions, together with those reported in the literature, lead to consider three different mechanistic pathways of elimination.

Full text of DOI:10.1002/poc.1705

Research Article
Received: 14 December 2009,
Revised: 27 January 2010,
Accepted: 2 February 2010,
Published online 20 April 2010 in Wiley Online Library: 2011
(
wileyonlinelibrary.com) DOI 10.1002/poc.1705
Kinetics of the gas-phase homogeneous
unimolecular elimination of selected ethyl
esters of 2-oxo-carboxylic acids
a
a
a
a
Andreina Reyes , Rosa M. Dom ı´ nguez , Maria Tosta , Armando Herize
a
and Gabriel Chuchani *
The gas-phase elimination kinetics of selected ethyl esters of 2-oxo-carboxylic acid have been studied over the
temperature range of 270–415 -C and pressures of 37–114 Torr. The reactions are homogeneous, unimolecular, and
follow a first-order rate law in a seasoned static reaction vessel, with an added free radical suppressor toluene. The
observed overall and partial rate coefficients are expressed by the following Arrhenius equations:
Ethyl oxalyl chloride
S1
S1
S1
log koverall (s ) ¼ (13.22 W 0.45) S (179.4 W 4.9) kJ mol (2.303 RT)
Ethyl piperidineglyoxylate
S1
S1
S1
log k(CO2) (s ) ¼ (12.00 W 0.30) S (191.2 W 3.9) kJ mol (2.303 RT)
S1
S1
S1
log k(CO) (s ) ¼ (12.60 W 0.09) S (210.7 W 1.2) kJ mol (2.303 RT)
S1
S1
S1
S1
log kt(overall) (s ) ¼ (12.22 W 0.26) S (193.4 W 3.4) kJ mol (2.303 RT)
S1
S1
Ethyl benzoyl formatelog k(CO2) (s ) ¼ (12.89 W 0.72) S (203.8 W 9.0) kJ mol (2.303 RT)
S1
S1
S1
log k(CO) (s ) ¼ (13.39 W 0.31) S (213.3 W 3.9) kJ mol (2.303 RT)
S1
S1
S1
log kt(overall) (s ) ¼ (13.24 W 0.60) S (205.8 W 7.6) kJ mol (2.303 RT)
The kinetic and thermodynamic parameters of these reactions, together with those reported in the literature, lead
to consider three different mechanistic pathways of elimination. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: ethyl benzoyl formate; ethyl oxalyl chloride; ethyl piperidineglyoxylate; gas-phase elimination; kinetics;
mechanism
The r value is defined as the nature of the reaction. The plot of
INTRODUCTION
this method between the experimental data (log k/k
H
) and the
The unimolecular gas-phase decomposition of organic carboxylic
esters has been postulated to proceed through a six-membered
cyclic transition state type of mechanism to yield the corres-
ponding carboxylic acid and the olefin, respectively [reaction
three substituent parameters s , s , and s was found to be
approximately linear.
a F R
log k=k ¼ꢀð0:68ꢁ 0:12Þs þð2:57ꢁ0:12Þs ꢀð1:18 ꢁ 0:27Þs
H
a
F
ꢀ
R
[
1,2]
ꢂ
(
1)].
The presence of a C
b
—H bond at the alcoxy side of the
ðr ¼ 0:984; sd ¼ 0:119 at 400 CÞ
ester is necessary for molecular elimination.
The negative value of r ¼ ꢀ0.68 suggests the polarizability
a
or steric effect has a little participation on the rate of elimination
of ethene, while the positive value of rF
the field or electronic effect appears to have the major influence
in the elimination process. Finally, rR
ꢀ
¼ þ2:57 suggests that
ꢀ
¼ ꢀ1:18 suggests an
interaction of the substituent Z with an incipient negative
reaction center, thus favoring the abstraction of the b-hydrogen
of the ethyl ester by the oxygen carbonyl in the transition state.
The kinetic parameters and the comparative rates of
substituents (Z) other than carbon at the acid side of the organic
[
3]
ethyl esters indicate that the substituted compounds still react
via the mechanism described in reaction (1). Substituent effects
on reaction rates are often examined by the Taft–Topsom
[
4]
method of log k/k ¼ s r þ s r þ s r which takes into
H
a
a
F
F
R
R
*
Correspondence to: G. Chuchani, Centro de Qu ´ı mica, Instituto Venezolano de
Investigaciones Cient ´ı ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela.
E-mail: chuchani@ivic.ve
consideration the steric (s
contributions in the quantitative study of structural and
substituent effects on chemical reactivity. The substituent
constant s is defined as the polarizability or steric effect, s
a F
as the field or electronic effect, while s
a
), electronic (s
F
R
), and resonance (s )
a
A. Reyes, R. M. Dom ´ı nguez, M. Tosta, A. Herize, G. Chuchani
Centro de Qu ´ı mica, Instituto Venezolano de Investigaciones Cient ´ı ficas (IVIC),
Apartado, Caracas, Venezuela
[
4]
R
as the resonance effect.
J. Phys. Org. Chem. 2011, 24 74–82
Copyright ß 2010 John Wiley & Sons, Ltd.

GAS-PHASE ELIMINATION KINETICS
Investigation of the gas-phase elimination kinetics of some
compounds having the interposition of a carbonyl between the
substituents Z and the acid side of ethyl ester, that is, ethyl esters
calculations have been found to be in good agreement with
experiments.
Few years ago, the decomposition of some ethyl esters
of 2-oxo-carboxylic acids with a nitrogen atom attached
at the acid side, that is, ethyl oxamate, ethyl N,N-
dimethyloxamate, and ethyl oxanilinate [reactions (6–8)],
[
5]
of 2-oxo-carboxylic acids, has recently been reported. It
seemed interesting to find if some correlation with substituent Z
may be obtained. Otherwise, the migration of Z to the ethoxy-
carbonyl with loss of CO may occur rather than a simple forma-
tion of ethylene in the gas-phase elimination of ethyl esters.
It seemed interesting to inquire the works on the
thermal decomposition of 2-oxoacids and their alkyl esters
in solution and in the gas phase. Thermolyses of oxalic
and oxamic acids in several solvents [reaction (2)] have
[
5]
was described.
[
6–8]
been described.
The polar nature of the solvents was
found to affect the rate of decomposition. The fact that
oxo-acids are being referred is because they may be the
intermediates of the corresponding 2-oxo-esters decompo-
sition, and they are generally found to be unstable at the
working temperatures.
In related works, the mechanism and kinetics of the
gas-phase decomposition of oxalic acid was reported
[
9]
[
reaction (3)]
Experimental results of the decomposition products
of ethyl oxamate [reaction (6a)] and ethyl oxanilinate
[reaction (7a)] indicated decarbonylation as the first step
The thermal decomposition of pyruvic and benzoylformic
[
10,11]
acids
was found to proceed through a concerted
—
of decomposition due to bond polarization of the N—C —O
mechanism with semi-polar type transition states to
give carbon dioxide and the corresponding aldehydes
reaction (4)]
rate determining step, thus favoring migration of the
amino group. The corresponding ethyl ester intermediate
subsequently undergoes parallel eliminations [reactions
[
(
6b, c;7b, c)]. Ethyl N,N-dimethyloxamate decomposition
proceeds via different elimination pathways [reaction (8)].
This result was surprising, since decarbonylation was
expected to be the first step of the reaction. This fact may
well be due to the electron release of the two methyl groups
on the N atom reducing the electronegativity of this
substituent when compared to a Cl atom [reaction (5)].
Several additional works showed that ethyl glyoxylate
A theoretical and experimental study on the gas-phase
[
12]
elimination kinetics of methyl oxalyl chloride
suggested a
undergoes
decomposition as depicted in reaction (9),
-oxo-propanoate and ethyl 3-methyl-2-oxo-butyrate elim-
inate ethylene to give the
a parallel and consecutive homogeneous
concerted semi-polar type transition state [reaction (5)], in
which the Cl atom migrates to the adjacent carbonyl group as
CO gas is eliminated. Semi-empirical PM3 and MP2/6-31G*
[
13]
while ethyl
2
J. Phys. Org. Chem. 2011, 24 74–82
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com

A. REYES ET AL.
of the gas-phase elimination kinetics and the effect of such
compounds containing electron-withdrawing substituents (Z)
such as chlorine Cl, piperidyl C H10N, and phenyl C H , i.e., ethyl
5 6 5
oxalyl chloride, ethyl piperidine glyoxylate, and ethyl benzoyl
formate.
RESULTS AND DISCUSSION
Ethyl oxalyl chloride
The stoichiometry for the decomposition of ethyl oxalyl chloride
via reaction (11) requires P
f
/P
and final pressures, respectively. The average
0
¼ 2, where P
f 0
and P are the initial
corresponding 2-oxocarboxylic acid [reaction (10)]. The oxocar-
boxylic acids are unstable under the conditions of the reaction and
rapidly decarboxylate to produce the corresponding aldehyde.
In view of the several types of decomposition pathways found
experimental P /P at four different temperatures and ten
f 0
half-lives was 1.93 (Table 1). Additional examination of the
above stoichiometry of reaction (11) was made by comparing, up
to 55% decomposition, the pressure measurements with the
results of quantitative analyses of ethyl chloroformate (Table 2).
The comparison is found in reasonable agreement.
with different substituents Z in ZCOCOOCH
2
CH
3
(Z ¼ amino groups,
hydrogen atoms, and alkyls), further research was thought of interest
to examine the gas-phase decomposition of a series of 2-oxo-esters.
Consequently, the present work aimed at studying the mechanisms
f 0
Table 1. Ratio of final (P ) to initial (P ) pressure
Compound
Temperature (8C)
P
0
(Torr)
P
f
(Torr)
P
f
/P
0
Average
1.93
Ethyl oxalyl chloride
279.9
63
78
79
54
42.0
42.5
27.5
60.0
66.5
115
152
154
107
1.83
1.95
1.95
1.98
2.86
3.04
2.91
3.02
1.94
1.90
1.99
3
3
3
00.3
09.5
19.3
Ethyl piperidine glyoxylate
Ethyl benzoyl formate
365.0
120
2.96
1.94
3
3
3
75.0
84.7
95.8
129.0
80.0
181.0
129.0
242.5
175.0
399.6
4
4
14.0
26.5
128.5
88.0
Table 2. Stoichiometry of the reaction
Substrate
Temperature (8C)
Parameters
Time (min)
Value
Ethyl oxalyl chloride
390.0
7
10
12
15
20
Reaction (%) (pressure)
Ethyl chloroformate (%) (GC)
Time (min)
Substrate reaction (%) (GC)
Piperidine formate (%) (GC)
Piperidine (%) (GC)
26.9
26.2
5
34.2
31.0
3.2
34.6
33.5
10
57.1
51.4
5.7
40.4
41.0
15
71.7
64.2
7.5
46.8
46.8
20
87.0
77.5
9.5
56.4
54.8
Ethyl piperidine glyoxylate
Ethyl benzoyl formate
395.9
379.6
Time (min)
6
12
20
25
Substrate reaction (%) (GC)
Benzaldehyde (%) (GC)
Ethyl benzoate (%)
27.1
16.0
11.1
41.5
29.4
12.1
56.3
36.9
19.4
72.9
45.7
27.2
a
a
Ethyl benzoate was estimated by using the equation %Pethyl benzoate ¼ [P ꢀ P
ꢀ Pbenzaldehyde/P ] ꢃ 100 ¼ %P
0
substrate
0
CO
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 74–82

GAS-PHASE ELIMINATION KINETICS
The effect of the surface on the rate of elimination was
tested by carrying out several runs in a vessel with a surface
to volume ratio of 6.2 relative to that of the normal vessel,
which is equal to 1.0 (Table 3). No effect on the reaction
rates was observed in packed and unpacked Pyrex vessels
seasoned with allyl bromide. However, with clean Pyrex
vessels the results with packed and unpacked reactors
indicated a dramatic heterogeneous effect and unreliable
and irreproducible k-values were obtained. The effect of
the free radical inhibitor toluene is shown in Table 4. The
kinetic determinations had to be carried out with at least
two-fold excess of toluene in order to prevent any possible
free radical processes of the substrate and/or products. No
induction period was observed. The rate coefficients were
reproducible with a relative standard deviation of ꢁ5% at
any given temperature.
Ethyl piperidine glyoxylate
The experimental stoichiometry for the gas elimination of ethyl
piperidine glyoxylate in the gas phase [reaction (12)] showed Pf/
P
0
¼ 2.96 (Table 1). However, since different
The first-order rate coefficient of this substrate from
k ¼ ꢀ(2.303/t) log [(2P ꢀP )/P ] was independent of the initial
1
0
t
0
pressure (Table 5). A plot of log (2P
0
ꢀP
t
) against time t gave
a good straight line up to 55% reaction at all working
temperatures. The variation of the rate coefficients with
temperature and the corresponding Arrhenius equation are
given in Table 6 (Fig. 1; 90% confidence coefficient from
least-squares procedure).
proportions of parallel reaction products are obtained,
stoichiometry reaction (12) was checked, up to 70% decompo-
sition, by comparing the quantitative chromatographic analyses
of the substrate with the quantitative chromatographic analyses
Table 3. Homogeneity of the elimination reaction
S/V ¼ 1 cmꢀ
1a
S/V ¼ 6.2 cmꢀ1a
Substrate
Parameter
4
ꢀ1
ꢀ1
ꢀ1
Ethyl oxalyl chloride at 300.2 8C
Ethyl piperidine glyoxylate at 395.6 8C
Ethyl benzoyl formate at 389.8 8C
10 k
T
(s
)
)
)
7.05
13.32
10.79
7.70
4
10 k (s
13.52
T
4
b
T
10 k (s
S ¼ Surface area; V ¼ Volume.
a
In a vessel seasoned with allyl bromide.
Unreliable and irreproducible.
b
Table 4. Effect of the free radical chain inhibitor toluene on rates
4
ꢀ1
)
Substrate
Temperature (8C)
P
s
(Torr)
84
P
i
(Torr)
—
P
i
/P
s
10 k
T
(s
Ethyl oxalyl chloride
300.4
—
1.2
2.0
3.1
7.10
6.97
6.88
6.99
7
7
6
4
0
3
90
137.5
194
Ethyl piperidine glyoxylate
Ethyl benzoyl formate
375.5
398.8
42.0
—
—
1.7
2.2
3.7
—
3.3
3.5
4.1
5.1
5.23
5.05
4.91
3
4
4
4.0
2.0
2.5
59.0
92.0
156.0
—
95.0
91.5
107.0
159.0
4.99
63.0
15.97
16.57
16.63
18.20
18.65
2
2
2
3
9.0
6.5
6.0
1.5
P
s
¼ Pressure of substrate. P
i
¼ Pressure of free radical inhibitor. k
T
¼ total rate coefficient
J. Phys. Org. Chem. 2011, 24 74–82
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com

A. REYES ET AL.
Table 5. Variation of the rate coefficients from initial pressure
Substrate
Temperature (8C)
Parameters
Value
Ethyl oxalyl chloride
290.2
P
0
(Torr)
ꢀ1
37
3.80
44
22.27
19.5
6.73
88
3.71
63
21.82
25
6.57
121
3.94
73
22.16
44
6.19
157
3.86
114
21.37
47
6.27
4
1
0 k (s
)
T
Ethyl piperidine glyoxylate
Ethyl benzoyl formate
405.6
379.6
0
P (Torr)
4
ꢀ1
)
1
1
0 k
T
(s
P (Torr)
0
4
ꢀ1
)
0 k
T
(s
Table 6. Temperature dependence of the rate coefficients
Ethyl oxalyl chloride Temperature (8C)
270.2
1.01
279.9
1.80
290.2
3.82
300.2
7.00
320.1
29.36
4
ꢀ1
1
0 k
1
(s
)
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (s ) ¼ (13.22 ꢁ 0.45) ꢀ (179.5 ꢁ 4.9) kJ mol (2.303 RT) , r ¼ 0.9990
1
Ethyl piperidine glyoxylate
Temperature (8C)
395.6
11.95
1.41
405.8
19.30
2.48
415.8
32.11
4.24
4
ꢀ1
1
1
1
0 k
0 k (s
0 k
a
(s
)
4
ꢀ1
)
)
b
4
ꢀ1
T
(s
13.36
21.78
36.35
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
Rate equation log k
a
(s ) ¼ (12.00 ꢁ 0.30) ꢀ (191.2 ꢁ 3.9) kJ mol (2.303 RT) , r ¼ 0.9998
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (s ) ¼ (12.60 ꢁ 0.09) ꢀ (210.7 ꢁ 1.2) kJ mol (2.303 RT) , r ¼ 0.9999
b
ꢀ1
Rate equation log k
T
(s ) ¼ (12.22 ꢁ 0.26) ꢀ (193.4 ꢁ 3.4) kJ mol (2.303 RT) , r ¼ 0.9999
Ethyl benzoyl formate
Temperature (8C)
369.6
2.25
0.91
379.6
3.83
2.12
5.95
390.7
6.71
4.08
398.8
11.62
6.31
4
ꢀ1
1
1
1
0 k (s
0 k (s
0 k
)
a
4
ꢀ1
)
)
b
4
ꢀ1
T
(s
3.16
10.79
17.93
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
Rate equation log k
a
(s ) ¼ (12.89 ꢁ 0.72) ꢀ (203.8 ꢁ 9.0) kJ mol (2.303 RT) , r ¼ 0.9961
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (s ) ¼ (13.39 ꢁ 0.31) ꢀ (213.3 ꢁ 3.9) kJ mol (2.303 RT) , r ¼ 0.9993
b
ꢀ1
Rate equation log k
T
(s ) ¼ (13.24 ꢁ 0.60) ꢀ (205.8 ꢁ 7.6) kJ mol (2.303 RT) , r ¼ 0.9973
of the products piperidine formate plus piperidine (Table 2). The
product piperidine results from the rapid decomposition of
ethyl piperidine formate, which is unstable at the working
temperature. The sum of quantitative analysis of the products
piperidine and piperidine formate agrees with the quantitative
amount of the substrate ethyl piperidine glyoxylate decompo-
sition.
Table 3 indicates that reaction (12) is homogeneous in
seasoned and clean packed and unpacked Pyrex vessels. The
effect of the free radical suppressor is shown in Table 4. The
kinetic determinations had to be carried out in the presence of
toluene (the concentration of toluene is at least two times the
initial pressure of this substrate) to inhibit any possible chain
reactions of the substrate and/or products. No induction period
was observed. The rate coefficients are reproducible with a
relative standard deviation not greater than ꢁ5% at a given
temperature.
The first-order rate coefficient was found to be independent of
the initial pressure (Table 5). The temperature dependence of the
rate coefficients and the corresponding Arrhenius equation are
given in Table 6 (Fig. 2; 90% confidence coefficient from
least-squares procedure).
Ethyl benzoyl formate
The gas elimination of this substrate proceeds according to
reaction (13).
Figure 1. Arrhenius plot for the elimination kinetics of ethyl oxalyl
chloride
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 74–82

GAS-PHASE ELIMINATION KINETICS
Figure 3. Arrhenius Plot for the elimination kinetics of ethyl benzoyl
formate
Figure 2. Arrhenius Plot for the elimination kinetics of ethyl piperidine
glyoxylate
The average experimental stoichiometry reaction (13) showed
P /P to be 1.94 (Table 1). However, to confirm stoichiometry
f 0
reaction (13) was possible by comparing the quantitative
chromatographic analyses of the reacted amount of substrate
with the sum of products formation benzaldehyde and the
ethene were unreliable, since CO gas impedes the condensa-
tion of the olefin. Benzoic acid was only detected. Some surface
catalysis was found, even for vessels seasoned with allyl
bromide. Rates were only measured in seasoned unpacked
Pyrex vessels in order to obtain reproducible values (Table 3).
The pyrolysis runs of the starting material were carried out
with added toluene, at least five times the amount of the
inhibitor in order to suppress any possible radical chain
processes (Table 4). The rate coefficients are reproducible with
CO gas (PCO gas ¼ P
¼ P ꢀ P
ꢀ Pbenzaldehyde)
ethyl benzoate
0
substrate
(
Table 2). The ethyl benzoate was estimated as the above
equation, since it decomposes partially to ethene and benzoic
acid at the working temperature. The quantitative analyses of
Table 7. Comparative kinetic and thermodynamic parameters at 400 8C
4
6
6
6
k
1
ꢃ 10
DS
DH
DG
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
Substrate
(s
)
E
a
(kJ mol
)
log A (s
)
(J mol K) (kJ mol
)
(kJ mol )
Ref.
13
a
b
c
a
a
Ethyl glyoxalate HCOCOOCH
2
CH
3
8.91
216.1 ꢁ 3.3
213.1 ꢁ 3.3
232.9 ꢁ 7.0
205.1 ꢁ 2.0
198.4 ꢁ 4.1
13.72 ꢁ 0.25
13.43 ꢁ 0.25
14.06 ꢁ 0.54
13.03 ꢁ 0.15
12.58 ꢁ 0.31
2.7
ꢀ2.9
9.2
210.5
208.7
209.7
226.9
206.6
205.7
7
0
.77
.96
207.8
233.1
199.5
192.8
Ethyl 2-oxo-propionate CH
Ethyl 3-methyl-2-oxo-butyrate (CH3)2
CHCOCOOCH CH
3
COCOOCH
2
CH
3
12.92
15.18
ꢀ10.6
13
13
ꢀ19.2
2
3
a
f
f
Ethyl oxalyl chloride ClCOCOOCH
Ethyl piperidine glyoxylate
C H NCOCOOCH CH
2
CH
3
1968.5
14.67
13.10
179.5 ꢁ 4.9
193.4 ꢁ 3.4
191.2 ꢁ 3.9
210.7 ꢁ 1.2
205.8 ꢁ 7.6
203.8 ꢁ 9.0
213.3 ꢁ 3.9
203.7 ꢁ 2.5
208.0 ꢁ 2.8
195.9 ꢁ 2.3
206.8 ꢁ 4.4
13.22 ꢁ 0.45
12.22 ꢁ 0.27
12.00 ꢁ 0.30
12.60 ꢁ 0.09
13.24 ꢁ 0.60
12.89 ꢁ 0.72
13.39 ꢁ 0.31
13.28 ꢁ 0.20
13.44 ꢁ 0.22
12.19 ꢁ 0.18
13.06 ꢁ 0.34
ꢀ6.9
ꢀ26.1
ꢀ30.3
ꢀ18.8
ꢀ6.6
173.8
187.8
185.6
205.1
200.2
198.2
207.7
198.1
202.4
190.3
201.2
178.5
205.3
206.0
245.7
204.6
207.1
216.9
202.0
204.2
208.3
207.9
a
b
c
5
10
2
3
1
.58
a
b
e
f
Ethyl benzoyl formate C
6
H
5
COCOOCH
2
CH
3
18.48
1.80
.83
1
ꢀ13.2
ꢀ3.7
6
29.5
0.0
a
Ethyl oxamate H
2
NCOCOOCH
2
CH
3
ꢀ5.79
ꢀ2.73
ꢀ26.7
ꢀ10.0
12
d
2
e
9
10.2
.77
a
Ethyl N,N-Dimethyl
oxamate(CH ) NCOCOOCH CH
Ethyl oxanilate C H NHCOCOOCH CH
3
12
12
3
2
2
3
a
d
e
57.9
2.0
5.6
207.4 ꢁ 1.5
205.2 ꢁ 2.2
214.1 ꢁ 2.2
13.86 ꢁ 0.12
13.55 ꢁ 0.18
13.81 ꢁ 0.18
5.31
ꢀ0.62
4.36
201.8
199.6
208.5
198.2
200.0
205.6
6
5
2
4
1
a
b
c
d
e
f
Overall rate.
Rate of decarboxylation.
Rate of decarbonylation.
Rate of ethanol formation.
Rate of ethylene formation.
This work.
J. Phys. Org. Chem. 2011, 24 74–82
Copyright ß 2010 John Wiley & Sons, Ltd.
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A. REYES ET AL.
a relative standard deviation not greater than ꢁ5% at a given
temperature.
from the elimination of ethyl 2-oxo-propionate [reaction (14a)],
[
14]
undergoes decomposition in the gas phase into acetaldehyde
and CO in a few seconds at temperatures of 284–334 8C with
The rate coefficients were invariable to the initial pressure of
the substrate chromatographic analyses (Table 5). The variation
of the rate coefficients with temperature is shown in Table 6. The
experimental data were fitted to the Arrhenius equation shown in
Table 6 (Fig. 3), where 90% confidence limits from an unweighted
least-squares procedure are given.
2
ꢀ1
ꢀ1
an Arrhenius expression of log k (s ) ¼ 13.53ꢀ172.4 kJ mol
1
ꢀ
1
(2.303 RT) . These data suggest the pyruvic acid intermediate,
under the condition of the present work, will decarboxylate rapidly.
To rationalize the rate increase caused by electron releasing alkyl
groups CH3 and (CH ) CH of the oxy-esters, one may invoke
3 2
The kinetic and thermodynamic parameters of several ethyl esters
of 2-oxo-acids reported in the literature, together with the present
data, at 400 8C, are collected and shown in Table 7. The information
given in Table 7 suggests, to some extent, that several types of
reaction mechanisms occur in the decomposition of 2-oxo-esters.
The various pathways are depicted in reaction (14). Compounds
nucleophilic assistance of the oxygen carbonyl leading to charge
stabilization of the incipient positive C in the transition state
g
[reaction (15)]. Thus, the greater the electron release of the alkyl
group R, the greater the nucleophilicity of oxygen carbonyl, the
more the stabilization of the positive C
elimination. In contrast to the above, electron-withdrawing
substituents Cl (ethyl oxalyl chloride), and NH (ethyl oxamate),
NH (ethyl oxanilinate), C N (ethyl piperidine glyoxylate),
and C H (ethyl benzoyl formate) lead to decarbonylation
g
, and the faster the rate of
with electron-releasing groups CH
3
(ethyl 2-oxo-propionate) and
2
(
CH CH (3-methyl-2-oxo-butyrate) decompose as organic esters
3
)
2
C H
6 5
5 5
H
[
12,13]
to produce the corresponding carboxylic acid and ethylene (14a),
where the elongation and subsequent polarization of the C—O
via
6
5
(14c). The presence of an H attached to the nitrogen substituent
may lead to subsequent elimination of ethanol (14d) or ethylene
dþ
dꢀ
bond, in the sense Ca ....O , is the rate determining step. The
corresponding 2-oxo-acids and ethylene are formed via six-
membered cyclic transition state [reaction (14a)]. These oxo-acids,
under the reaction conditions, are unstable and rapidly decarboxy-
(14e). Surprisingly, the exception is Z ¼ (CH
3 2
) N in ethyl
N,N-dimethyl oxamate, because no migration of this substituent
to the carboethoxy group takes place [reaction (14)]. Apparently,
some re-examination of the elimination kinetics of these
compounds may be needed to confirm the reported results.
[10,11]
late as reported before
to give the corresponding aldehyde.
The product intermediate 2-oxo-propionic acid or pyruvic acid,
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 74–82

GAS-PHASE ELIMINATION KINETICS
In the case of Z ¼ H (ethyl glyoxalate) gives a parallel
decomposition (14a, 14c), where reaction (14a) is more impor-
tant than reaction (14c). Because of the apparent electro-
n-withdrawing property of H, this atom undergoes a little
migration to the carboethoxy group (14c). This fact finds
Kinetics
The kinetic experiments were performed in a static reaction
[
system as previously reported.
the decomposition product of allyl bromide. The rate coefficients
of ethyl oxalyl chloride were calculated from the pressure
increased manometrically, according to following equation
described below:
16–18]
The reactor was seasoned by
[
15]
support from Taft original substituent effect of H,
s* ¼ þ0.49. Consequently, a small amount of decarbonylation
14c) occurs.
(
CONCLUSIONS
k1 ¼ ꢀð2:303=tÞ log ½ð2P0 ꢀ PtÞ=P0ꢄ
The gas-phase elimination kinetics of ethyl oxalyl chloride, ethyl
piperidineglyoxilate, and ethyl benzoyl formate has been found
to be homogeneous, unimolecular, and follow a first-order rate
law.
or by the quantitative chromatographic analyses of ethyl
chloroformate, thus
k1 ¼ ꢀð2:303=tÞ log ½ðP0 ꢀ Pethyl chloroformareÞ=P0ꢄ
Ethyl oxalyl chloride undergoes only decarboxylation process,
while ethyl piperidineglyoxilate and ethyl benzoyl formate
proceed to a parallel decomposition of decarboxylation and
decarbonylation reactions.
The mechanisms of these decomposition reactions are
described in terms of concerted discrete polar cyclic transition
state structures.
The rate of decomposition of ethyl piperidineglyoxylate is
given by Eqn (1) [reaction (12)]:
ꢀ
ꢁ
1
PSubstrate
P0
kTotal ¼ ꢀ ln
(1)
t
For piperidine formate [reaction (12a)]
dPpiperidine formate
¼
k
aðpiperidine formateÞðPSubstrateÞ
(2)
(3)
dt
EXPERIMENTAL
For piperidine [Eqn (3)]
The substrates ethyl oxalyl chloride (98%) and ethyl piperidine
glyoxalate (99%) were acquired from Acros Organics, while ethyl
benzoyl formate (99%) from Aldrich. The identification and
purity of these substrates were verified by GC-MS: Saturn 2000,
Varian, with a DB-5MS capillary column 30 m ꢃ 0.25 mm. i.d.,
dPpiperidine
dt
¼
k
bðpiperidineÞðPSubstrateÞ
Therefore, the ratio of piperidine formate/piperidine is shown
in Eqn (4)
Ppiperidine formate
Ppiperidine
kaðpiperidine formateÞ
kbðpiperidineÞ
0
.25 mm film thickness. The products ethyl chloroformate,
¼
(4)
piperidine, piperidine formate, benzaldehyde, ethyl benzoate,
ethylene, and carbon dioxide were identified in a GC-MS (Saturn
The values of the rate coefficients of product formation
ka(piperidine formate) and kb(piperidine) were determined by Eqn (5)
2
000, Varian) with a DB-5MS capillary column 30 m ꢃ 0.25 mm.
i.d., 0.25 mm. Analysis of the products ethylene (Matheson) and
CO was carried out by using a Gas Chromatograph Varian
8
2
>
>
>
<
3
600ꢃ with a thermal conductivity detector (capillary column:
kTotal¼ k
þkbðpiperidineÞ
aðpiperidine formateÞ
GS-Q, 30 m long and 0.53 id., Helium gas carrier). Ethyl
chloroformate, ethyl benzoyl formate, and benzaldehyde
were quantitatively analyzed using a chromatograph Hewlett-
Packard Model 5710-A with a flame ionization detector and a
(5)
>
k
aðpiperidine formateÞ
Ppiperidine formate
>
>:
¼
k
Ppiperidine
bðpiperidineÞ
2
3 4
m packed column of 10% SP-1200, 1% H PO Chromosorb
The rate of piperidine formation is given by the following
expression [Eqn (6)]:
WAW 80/100 mesh. Ethyl piperidineglyoxylate, piperidine, and
piperidine formate were analyzed in the same Hewlett-Packard
k
ꢂ
ꢃ
bðpiperidineÞ
kTotal
1 ꢀ eꢀk
Totalt
5
1
710-A, but with a 1.5 m packed column of Chromosorb 103
00/120 mesh.
k
¼ P0
(6)
bðpiperidineÞ
J. Phys. Org. Chem. 2011, 24 74–82
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A. REYES ET AL.
REFERENCES
Substituting Eqn (6) in Eqn (5), we obtain the rate of formation
of piperidine formate [Eqn (7)]
[
[
[
1] R. Taylor, in The Chemistry of Functional Group. Supplementary Volume
B, Acid Derivatives, Chapter 15 (Ed.: S. Patai), Wiley, London, 1979,
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P
kTotal
ðpiperidine formateÞ
k
¼
(7)
aðpiperidine formateÞ
ꢀ
kTotalt
ð1 ꢀ e
Þ
In the case of piperidine formation, the kpiperidine is calculated
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kpiperidine ¼ kTotal ꢀ kpiperidine formate
Because the intermediate ethyl piperidine carboxylate from
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(8)
[
[
(
[
6] L. Clark, J. Phys. Chem. 1961, 65, 659–661.
[
19]
peratures (341–391 8C) into piperidine, ethylene, and CO
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,
, the
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[
parallel reaction (12b) was possible to calculate according to Eqn
8).
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coefficients was made by using the similar equations as in ethyl
piperidineglyoxylate [Eqn (1)–(8)], but substituting in the above
equations the subscript piperidineformate by benzaldehyde, and
piperidine by CO formation.
The temperature was controlled by a Shinko DC-PS resistance
thermometer controller maintained at ꢁ0.2 8C and measured
with a calibrated Iron Constantan thermocouple. No temperature
gradient was observed along the reaction vessel. The starting
materials 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 ꢅ0.05–0.2 ml.
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(
[
[
[
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Chuchani, J. Phys. Org. Chem. 2005, 18, 539–545.
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Chemistry and Biology, Wiley, USA, 1979.
[
[
[
[16] A. Maccoll, J. Chem. Soc. 1955, 965–973.
[
[
17] E. S. Swinbourne, Aust. J. Chem. 1958, 11, 314–330.
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[
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 74–82