Kinetics of the homogeneous, unimolecular elimination reactions of ethyl oxamate, ethyl N,N-dimethyloxamate and ethyl oxanilate in the gas phase

Article abstract of DOI:10.1002/poc.894

The gas-phase elimination kinetics of the title compounds were determined over the temperature range 350-430°C and pressure range 35-240 Torr (1 Torr = 133.3 Pa). The reactions, which were carried out in a static reactor system, seasoned with allyl bromide and in the presence of a free radical inhibitor, are homogeneous, unimolecular and obey a first-order rate law. The temperature dependences of the overall rate coefficients are given by the following Arrhenius equations: for ethyl oxamate, log [k₁ (s^-1)] = (13.28±0.20)-(203.7±2.5)kJ mol^-1 (2.303RT) ^-1, for ethyl N,N-dimethyloxamate, log [k₁(s ^-1)] = (13.06±0.34)-(206.8±4.4)kJ mol ^-1(2.303RT)^-1, and for ethyl oxanilate, log [k₁ (s^-1)] = (13.86±0.12)-(207.4±1.5)kJ mol ^-1(2.303RT)^-1. The overall rates, the partial rates and the kinetic and thermodynamic parameters of these eliminations are presented and discussed. These reactions appear to proceed through moderately polar cyclic transition states. Copyright

Full text of DOI:10.1002/poc.894

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2005; 18: 539–545
Published online 20 December 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.894
Kinetics of the homogeneous, unimolecular elimination
reactions of ethyl oxamate, ethyl N,N-dimethyloxamate
and ethyl oxanilate in the gas phase
1
2
2
2
1
Esker V. Chacin, Mar ı´ a Tosta, Armando Herize, Rosa M. Dom ı´ nguez, Ysaias Alvarado
2
and Gabriel Chuchani *
1
Departamento de Qu ı´ mica, Facultad Experimental de Ciencias, Departamento de Qu ı´ mica, Universidad del Zulia, Maracaibo, Venezuela
Centro de Qu ı´ mica, Instituto Venezolano de Investigaciones Cient ı´ ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
2
Received 15 June 2004; revised 14 August 2004; accepted 30 October 2004
ABSTRACT: The gas-phase elimination kinetics of the title compounds were determined over the temperature range
ꢀ
3
50–430 C and pressure range 35–240 Torr (1 Torr ¼ 133.3 Pa). The reactions, which were carried out in a static
reactor system, seasoned with allyl bromide and in the presence of a free radical inhibitor, are homogeneous,
unimolecular and obey a first-order rate law. The temperature dependences of the overall rate coefficients are given by
ꢁ1
ꢁ1
the following Arrhenius equations: for ethyl oxamate, log [k (s )] ¼ (13.28 ꢂ 0.20) – (203.7 ꢂ 2.5) kJ mol
ꢁ1
,
1
ꢁ
1
ꢁ1
ꢁ1
(
2.303RT) , for ethyl N,N-dimethyloxamate, log [k (s )] ¼ (13.06 ꢂ 0.34) – (206.8 ꢂ 4.4) kJ mol (2.303RT)
1
ꢁ1
ꢁ1
ꢁ1
and for ethyl oxanilate, log [k (s )] ¼ (13.86 ꢂ 0.12)–(207.4 ꢂ 1.5) kJ mol (2.303RT) . The overall rates, the
1
partial rates and the kinetic and thermodynamic parameters of these eliminations are presented and discussed.
These reactions appear to proceed through moderately polar cyclic transition states. Copyright # 2004 John Wiley &
Sons, Ltd.
KEYWORDS: pyrolysis; elimination; kinetics; ethyl oxamate; ethyl N,N-dimethyloxamate; ethyl oxanilate
INTRODUCTION
A carbamate with a hydrogen atom at the nitrogen and
without a C —H bond at the alkyl side of the ester,
ꢀ
7
such as methyl N-methylcarbamate, yielded elimination
products which differ from those of ethyl N-methyl-N-
Ethyl N-methyl-N-phenylcarbamate was the first carba-
1
mate substrate to be pyrolyzed in the gas phase. This
compound was found to produce N-methylaniline, ethy-
lene and carbon dioxide [reaction (1)].
1
phenylcarbamate [reaction (3)].
CH NHCOOCH ! CH NCO þ CH OH
ð3Þ
3
3
3
3
PhðCH3ÞNCOOCH2CH3 ! PhNHCH3 þ H2C ¼ CH2 þ CO2
ð1Þ
Apparently, the H atom attached to nitrogen was
considered to be responsible for a different mechanistic
pathway, and the decomposition process was rationalized
in terms of a four-membered cyclic transition state,
Additional work on the elimination kinetics of N,N-
dimethylcarbamates led researchers to consider a gener-
ally accepted mechanism of a six-membered cyclic
transition state similar to those described for acetates,
7
shown in Scheme 1.
1–6
carbonates and xanthates [reaction (2)].
Scheme 1
Kwart and Slutsky suggested from ethyl N,N-dimethy-
6,8
lcarbamate pyrolyses that carbamate decomposition
proceeds through an intermediate whose structure lies
between a semi-concerted six-membered cyclic transition
state and an intimate ion-pair type of mechanism. Further
work on the elimination kinetics of 2-alkyl-2-substituted
ethyl N,N-diethylcarbamates revealed a rate enhance-
ment of ethylene elimination in the order tert-butyl >
isopropyl > ethyl, which is consistent with the sequence
ð2Þ
*
Correspondence to: G. Chuchani, Centro de Qu ´ı mica, Instituto
9
Venezolano de Investigaciones Cient ´ı ficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
E-mail: chuchani@quimica.ivic.ve
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545

5
40
E. V. CHACIN ET AL.
of the comparative rates for the corresponding 2-alkyl-
10
2
sults of these eliminations suggested that C —O bond
-substituted-ethyl N,N-dimethylcarbamates. The re-
ꢁ
ꢂþ
ꢂꢁ
polarization, in the direction Cꢁ —O , is impor-
tant and that the mechanism proceeds according to
reaction (2).
ð5Þ
Two additional literature references have been found
useful in the conception of the present research work.
These are the study of the kinetics of CO elimination
from oxamic acid in several solvents and the kinetics
and stoichiometry of oxalic acid decomposition in the
The presence of phenyl and bulky groups at the N atom
of ethyl carbamates was found to decrease the rate of
11
2
elimination due to steric factors. The application of
12,13
various correlations methods
18–20
such as the steric para-
meters Hancock’s E , Taft’s E , Charton’s ꢃ, the inductive
c
s
s
21
gas phase.
ꢄI values and the Taft–Topsom equation for a consider-
able number of 2-substituted-ethyl N,N-dimethylcarba-
1
2
mates, (CH ) NCOOCH CH Z, gave random points
2
3
2
2
RESULTS AND DISCUSSION
Ethyl oxamate
with no meaning for mechanistic interpretation. How-
ever, the plotting of log (k /kCH3) against the original Taft
Z
13
ꢄ* values gave rise at the origin [ꢄ*(CH ) ¼ 0.00] to
3
three good straight lines. This result implied that small
alterations in the polarity of the transition state may be
due to changes in electronic transmission at the reaction
center. This means that a simultaneous effect may be
operating during the process of elimination. The mecha-
nisms were described according to each slope of the
straight lines. Moreover, the point positions of Z ¼ phenyl
The products of decomposition of ethyl oxamate in a
vessel seasoned with allyl bromide are ethanol and
isocyanic acid and, in slightly less amounts, ethylene,
CO and ammonia, by reaction (6).
2
—
C H ) and isopropenyl [CH —C(CH )] substituents
6 5 2 3
(
were found to fall far above the three lines. These
substituents were demonstrated to enhance the rate of
elimination due to the acidity of the benzylic and allylic
14
C —H bond.
ꢀ
ð6Þ
With this background and with the literature de-
scribed below, it was of interest to investigate a related
type of carbamate molecule, such as one with the
The theoretical stoichiometry in reaction (6) for the
molecular elimination of this substrate where pathways A
—
interposition of a carbonyl group (C—O) between the
N atom and the acid side of the carbonyl ester, that is,
R NCOCOOCH CH . Therefore, the substrates to be
examined were ethyl oxamate (H NCOCOOCH CH ),
and B occur equally requires that the final pressure, P
3.5 times the initial pressure, P . The average experi-
mental value of P /P at four different temperatures and
10 half-lives is 3.3 (Table 1). The fact that P /P < 3.5
f
, be
2
2
3
0
2
2
3
f
0
ethyl N,N-dimethyloxamate [(CH ) NCOCOOCH
2
f
0
3
2
CH ] and ethyl oxanilate (PhNHCOCOOCH CH ).
3
can be attributed to the fact that the ethyl carbamate
intermediate of reaction (6) does not decompose equiva-
lently to five products (see Table 6). However, it was
possible to corroborate the stoichiometry of reaction (6),
up to 76% decomposition, by comparing the percentage
decomposition of the substrate from pressure measure-
ments with those obtained from the sum of the gas chro-
matographic (GC) analyses of the products ethanol and
ethylene (Table 2). The reaction appears to be homo-
geneous since no significant effects on rates were ob-
tained on using both clean and seasoned Pyrex vessels
with a surface-to-volume ratio of 6.0 relative to normal,
which is equal to 1.0 (Table 3). The addition of dif-
ferent proportions of propene, an effective free radical
inhibitor, had no effect on the rates and no induction
period was obtained (Table 4). The rates are reproducible
with a standard deviation of <5% at a given temperature.
In view of the stoichiometry of reaction (6), where
P /P is nearly 3, the rate coefficients for elimination,
2
3
The substrates containing at least an H atom attached
to N may proceed in one step via a five-membered cyclic
transition state to yield CO gas, ethanol and the corre-
sponding isocyanate. However, as organic esters, they
are expected to produce ethylene gas and the corre-
sponding 2-oxo acid intermediates. These acid inter-
mediates decarboxylate in a similar to that observed for
1
5,16
pyruvic and benzoyl formic acids [reaction (4)].
Another possible path of elimination can be related to a
recent publication on the pyrolysis kinetics of methyl
1
7
oxalyl chloride [reaction (5)], i.e. decarbonylation is
the first step of oxamate elimination, after which the
resulting carbamate decomposes to isocyanate and
ethanol and/or ethylene gas and the unstable carbamic
acid.
RCOCOOH ! RCHO þ CO2
ð4Þ
f
0
R ¼ Me; C H
6
5
calculated from k ¼ (2.303/t)log[2P /(3P ꢁ P )], are
1
0
0
t
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545

KINETICS OF ELIMINATION REACTIONS OF OXAMATES
Table 1. Ratio of final (P ) to initial (P ) pressure
541
f
0
ꢀ
a
a
Compound
Temperature ( C)
P0 (Torr)
Pf (Torr)
Pf/P0
Average
3.3
Ethyl oxamate
370.3
44.8
40.5
29.0
27.6
108
104
88
141.8
134.5
95.0
93.6
302
288
238
246
91.0
225
3.2
3.3
3.3
3.4
2.8
2.8
2.7
2.8
3.0
3.1
2.7
2.7
3
3
4
80.9
90.4
00.1
Ethyl N,N-dimethyloxamate
400.2
2.8
2.9
4
4
4
08.0
18.7
29.7
88
Ethyl oxanilate
359.2
30.0
76.0
54.0
76.0
3
3
3
70.9
78.9
91.7
147.5
205.0
a
1
Torr ¼ 133.3 Pa.
Table 2. Stoichiometry of pyrolysis reactions
ꢀ
Compound
Temperature ( C)
Parameter
Time (min)
Value
Ethyl oxamate
380.9
5
10
15
20
Reaction (%) (pressure)
Ethylene (%) (GC)
Ethanol (%) (GC)
Sum (%) (GC)
25.5
9.3
17.8
27.1
4.5
43.5
15.1
26.3
41.4
7
55.0
20.1
34.0
54.1
8.5
76.2
26.1
49.4
75.5
20
Ethyl N,N-dimethyloxamate
408.0
378.9
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GC)
Time (min)
38.7
34.0
3
51.2
53.1
5
63.6
64.1
10
81.6
82.3
15
Ethyl oxanilate
Reaction (%) (pressure)
Ethylene (%) (GC)
Ethanol (%) (GC)
Sum (%) (GC)
27.3
4.9
19.5
24.4
42.9
13.1
32.9
46.0
64.3
18.2
44.1
62.3
72.4
19.0
54.3
73.3
found to be invariant to the initial pressures (Table 5), and
agree with the sum of the k values from the ethanol
and ethylene analyses (Table 2).
PT ¼ P0 ꢁ P þ P þ P þ P ¼ P0 þ 2P
P ¼ ðP ꢁ P Þ=2
T
0
The above-mentioned equation is the expression for
the formation of three products, and the rate coefficients
were calculated as follows:
Since
k ¼ ꢁ1=t lnðC=C0Þ
where C ¼ initial concentration and C ¼ concentration at
A ! B þ C þ D
0
time t,
P ꢁ P P
P
P
0
C ¼ P and C ¼ P ꢁ P ¼ ½P ꢁ ðP ꢁ P Þ=2ꢃ
0
0
0
0
T
0
If P ¼ initial pressure, P ¼ pressure at time t and P ¼
0
T
total pressure, then
C ¼ ð2P ꢁ P þ P Þ=2 ¼ ð3P ꢁ P Þ=2
0
T
0
0
T
Table 3. Homogeneity of pyrolysis reactions
ꢁ1 a
)
4
10 k (s
ꢁ1 b
4
10 k (s
ꢁ1 c
Compound
Ethyl oxamate at 390.4 C
S/V (cm
)
)
1
1
ꢀ
1
6
1
6
1
6
16.9 (ꢂ0.4)
17.6 (ꢂ0.3)
10.9 (ꢂ0.4)
10.6 (ꢂ0.3)
10.3 (ꢂ0.3)
10.0 (ꢂ0.2)
16.7 (ꢂ0.8)
17.5 (ꢂ0.3)
10.4 (ꢂ0.3)
10.5 (ꢂ0.4)
10.8 (ꢂ0.4)
10.2 (ꢂ0.5)
ꢀ
Ethyl N,N-dimethyloxamate at 400.2 C
ꢀ
Ethyl oxanilate at 370.4 C
a
S ¼ surface area; V ¼ volume.
b
Clean Pyrex vessel.
Vessel seasoned with allyl bromide.
c
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545

5
42
E. V. CHACIN ET AL.
a
Table 4. Effect of free radical inhibitors on pyrolysis reactions
ꢀ
b
Ps (Torr)
c
i
4
10 k (s )
ꢁ1
Substrate
Temperature ( C)
P (Torr)
Pi/Ps
1
d
Ethyl oxamate
380.9
408.0
370.5
56
—
46
39
33
—
1.0
1.3
1.6
—
1.3
2.0
2.8
—
10.9 (ꢂ0.5)
10.1 (ꢂ0.2)
10.1 (ꢂ0.7)
10.6 (ꢂ0.2)
16.0 (ꢂ0.4)
17.0 (ꢂ0.64)
17.1 (ꢂ0.3)
17.7 (ꢂ0.6)
11.3 (ꢂ0.2)
11.2 (ꢂ0.4)
10.3 (ꢂ0.2)
10.3 (ꢂ0.4)
4
4
5
5
9
1
e
Ethyl N,N-dimethyloxamate
85
—
8
8
8
2
2
8
108
162
250
—
27
28
d
Ethyl oxanilate
76
3
4
3
6
2
8
1.3
1.6
2.2
85
a
Propene or toluene inhibitor.
, pressure of the substrate.
, pressure of the inhibitor.
Propene inhibitor was used.
Toluene inhibitor was used.
b
P
s
c
d
e
P
i
Table 5. Variation of rate coefficients with initial pressure
ꢀ
Compound
Temperature ( C)
Parameter
Value
Ethyl oxamate
380.9
P (Torr)
0
0 k (s
1
37
10.5 (ꢂ0.5)
45
10.1 (ꢂ0.2)
49
10.1 (ꢂ0.7)
51
10.6 (ꢂ0.2)
4
ꢁ1
1
1
1
)
)
)
Ethyl N,N-dimethyloxamate
420.2
378.9
P (Torr)
0
71
27.7 (ꢂ0.8)
35
130
26.6 (ꢂ0.9)
41
169
26.7(ꢂ0.8)
44
240
27.1 (ꢂ1.0)
54
4
ꢁ1
0 k (s
1
Ethyl oxanilate
P (Torr)
0
4
ꢁ1
0 k (s
1
19.1 (ꢂ0.3)
18.0 (ꢂ0.2)
18.8 (ꢂ0.3)
18.5 (ꢂ0.5)
ꢁ
1
Consequently,
Ethyl oxamate : log½kðs Þꢃ ¼ ð13:28 ꢂ 0:20Þ
ꢁ1
ꢁ
1
k ¼ ðꢁ1=tÞ ln½ð3P ꢁ P Þ=2P ꢃ
Changing sign:
0
T
0
ꢁ
ð203:7 ꢂ 2:5ÞkJ mol ð2:303RTÞ ; r ¼ 0:9998
ꢁ
1
Ethanol formation : log½kðs Þꢃ ¼ ð13:44 ꢂ 0:22Þ
k ¼ ð1=tÞ ln½2P =ð3P ꢁ P Þꢃ
0
0
T
ꢁ
1
ꢁ1
ꢁ
ð208:0 ꢂ 2:8ÞkJ mol ð2:303RTÞ ; r ¼ 0:9997
or
k ¼ ð2:303=tÞ log½2P =ð3P ꢁ P Þꢃ
0
0
T
ꢁ1
Ethylene formation : log½kðs Þꢃ ¼ ð12:19 ꢂ 0:18Þ
ꢁ1
ꢁ1
The temperature dependence of the overall and partial
ꢁ ð195:9 ꢂ 2:3ÞkJ mol ð2:303RTÞ ; r ¼ 0:9998
rates of product formation [reaction (6), Table 6] gives,
by the least-squares procedure and with a 90% confidence
limit, the following Arrhenius equations:
Ethyl N,N-dimethyloxamate
The elimination products of ethyl N,N-dimethyloxamate
are dimethylformamide, ethylene and CO gas [reaction
Table 6. Temperature dependence of the rate coefficients
of ethyl oxamate
(7)].
4
0 k (s
ꢁ1
1
)
ꢀ
a
b
c
T ( C)
Chromatography
Ethanol
Ethylene
359.5
370.3
380.9
390.4
400.1
2.91
5.43
10.1
17.2
30.6
1.86 (ꢂ0.1)
3.47 (ꢂ0.1)
6.60 (ꢂ0.2)
11.4 (ꢂ0.3)
20.4 (ꢂ0.3)
1.05 (ꢂ0.1)
1.96 (ꢂ0.1)
3.52 (ꢂ0.2)
5.80 (ꢂ0.2)
10.2 (ꢂ0.2)
a
Sum of the rate coefficients obtained from the quantitative chromato-
graphic analyses of ethanol and ethylene.
Rate from quantitative chromatographic analysis of ethanol formation.
Rate from quantitative chromatographic analysis of ethylene formation.
b
c
ð7Þ
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545

KINETICS OF ELIMINATION REACTIONS OF OXAMATES
543
The theoretical stoichiometry suggests a P /P ratio
f
0
of 3. The average experimental P /P value at four tem-
f
0
peratures and 10 half-lives is 2.8 (Table 1). The small
departure of the stoichiometry of reaction (7) from the
theoretical value may be due to some polymerization
of the substrate or the products. Additional verification
of the stoichiometry of reaction (7), up to 80% decom-
position, was obtained by comparing the pressure mea-
surements with the quantitative GC analysis of ethylene
formation (Table 2). Elimination reaction (7) is homo-
geneous (Table 3), since no significant variation in the
rate coefficients was obtained in these experiments when
using both clean Pyrex and seasoned Pyrex vessels with a
surface-to-volume ratio of 6.0 to both normal clean Pyrex
and seasoned Pyrex vessels, which are equal to 1.0.
The effect of the addition of different proportions of
toluene inhibitor is shown in Table 4. Therefore, the
elimination experiments were carried out in the presence
of at least twice the amount of toluene with respect to the
substrate in order to prevent any possible free radical
chain reactions. The rate coefficients were reproducible
with a standard deviation not greater than 5% at a given
temperature.
ð8Þ
Apparently, the theoretical stochiometry, on the as-
sumption of 1:1 parallel elimination, requires P /P ¼ 3.5.
f
0
However, the average experimental P /P value at four
f
0
temperatures and 10 half-lives was 2.9 (Table 1). The
result is evidently not a 1:1 ratio of parallel decomposi-
tion of the substrate. Therefore, the confirmation of the
stoichiometry of reaction (8), up to 72% decomposition,
was possible by comparing the pressure measurements
with the sum of the quantitative (GC) analyses of the
ethanol and ethylene formation (Table 2). The homoge-
neity of this reaction was examined in a vessel with a
surface-to-volume ratio 6.0 times greater than normal,
and determined as equal to 1.0 (Table 3). The packed and
unpacked clean and seasoned Pyrex vessels showed no
difference in rates. Therefore, reaction (8) may be said to
be homogeneous. The effect of adding different propor-
tions of propene inhibitor is shown in Table 4. According
to this result, the pyrolysis experiments had to be carried
out in the presence of at least twice the amount of
propene with respect to this substrate in order to prevent
any possible radical reactions. No induction period was
observed and the rates were reproducible with a re-
lative standard deviation not greater than 5% at a given
temperature.
The first-order rate coefficients of this substrate calcu-
lated from k ¼ (2.303/t) log 2P /(3P ꢁ P ) was indepen-
1
0
0
t
dent of initial substrate pressure (Table 5). A plot of
log(3P ꢁ P ) against time (t) gave a good straight line for
0
t
up to 80% reaction. The variation of the rate coefficients
with temperature is given in Table 7 (90% confidence
limits from a least-squares procedure) with the following
Arrhenius expression:
ꢁ
1
Ethyl N; N ꢁ dimethyloxamate : log½k ðs Þꢃ
1
The rate coefficient of this elimination was found to be
independent of initial pressures (Table 5), and the first-
ꢁ
1
ꢁ1
¼
ð13:06 ꢂ 0:34Þꢁð206:8 ꢂ 4:4ÞkJ mol ð2:303RTÞ ;
order rate coefficient estimated from k ¼ (2.303/t)
1
r ¼ 0:9991
log[2P /(3P ꢁ P )] agrees with the sum of the k-values
0
0
t
for ethylene and ethanol formation. The variation of the
overall and partial rate coefficients with temperature is
given in Table 8 [reaction (8)], and the corresponding
Arrhenius equations are expressed as follows (90% con-
fidence coefficient from the least-squares procedure):
Ethyl oxanilate
The products from this pyrolysis reaction are phenyl
isocyanate and ethanol, and, in lesser amounts, aniline,
ethylene and carbon dioxide from ethyl oxanilate
[
reaction (8)].
Table 8. Variation of rate coefficient of ethyl oxanilate
4 ꢁ1
0 k (s
1
)
Table 7. Temperature dependence of the rate coefficients
of ethyl N,N-dimethyloxamate
ꢀ
a
b
c
T ( C) Chromatography
Ethanol
Ethylene
3
50.4
359.2
3.08
5.30
10.6
18.3
35.2
2.34 (ꢂ0.1)
3.95 (ꢂ0.2)
8.00 (ꢂ0.2)
13.5 (ꢂ0.2)
26.0 (ꢂ0.5)
0.74 (ꢂ0.0)
1.35 (ꢂ0.0)
2.60 (ꢂ0.1)
4.80 (ꢂ0.1)
9.20 (ꢂ0.3)
ꢀ
4
10 k (s )
ꢁ1
T ( C)
3
3
3
70.4
79.5
91.0
3
3
4
4
4
4
77.8
89.1
00.2
08.0
18.7
29.7
2.93 (ꢂ0.1)
5.69 (ꢂ0.2)
10.4 (ꢂ0.3)
17.4 (ꢂ0.3)
27.0 (ꢂ0.9)
49.7 (ꢂ0.9)
a
Sum of the rate coefficients obtained from the quantitative chromato-
graphic analyses of ethanol and ethylene.
Rate from quantitative chromatographic analysis of ethanol formation.
Rate from quantitative chromatographic analysis of ethylene formation.
b
c
Copyright # 2004 John Wiley & Sons, Ltd.
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5
44
E. V. CHACIN ET AL.
ꢀ
Table 9. Kinetic and thermodynamic parameters of ZCOCOOCH CH at 400 C
2
3
ꢁ
4
6
6
6¼
ꢀG
k ꢄ 10
Ea
(kJ mol
Log
[A (s )]
ꢀS
(J mol K)
ꢀH
(kJ mol
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
(kJ mol )
Z
Reaction
(s
)
)
)
N(CH3)2
NH2
Overall rate
Overall rate
Ethanol formation
Ethylene formation
Overall rate
10.2
29.5
20.0
9.77
57.9
42.0
15.6
206.8 ꢂ 4.4
203.7 ꢂ 2.5
208.0 ꢂ 2.8
195.9 ꢂ 2.3
207.4 ꢂ 1.5
205.2 ꢂ 2.2
214.1 ꢂ 2.2
13.06 ꢂ 0.34
13.28 ꢂ 0.20
13.44 ꢂ 0.22
12.19 ꢂ 0.18
13.86 ꢂ 0.12
13.55 ꢂ 0.18
13.81 ꢂ 0.18
ꢁ10.0
ꢁ5.79
ꢁ2.73
ꢁ26.7
5.31
201.2
198.1
202.4
190.3
201.8
199.6
208.5
207.9
202.0
204.2
208.3
198.2
200.0
205.6
PhNH
Ethanol formation
Ethylene formation
ꢁ0.62
4.36
ꢁ
1
Ethyl oxanilate : log½kðs Þꢃ ¼ ð13:86 ꢂ 0:12Þ
Thus, step 2 leads to the formation of an isocyanate,
perhaps through a concerted four-membered cyclic transi-
tion state, while step 3 affords ethylene and the corre-
sponding carboxylic acid. This acid intermediate could
then decarboxylate by way of a concerted cyclic four-
membered transition state (step 4). The fact that the partial
rate coefficient for the formation of phenyl isocyanate
from ethyl oxanilate is greater than that for the formation
of isocyanic acid from ethyl oxamate (Table 9) may be
due to the more facile abstraction of a hydrogen atom
ꢁ
1
ꢁ1
ꢁ
ð207:4 ꢂ 1:5ÞkJ mol ð2:303RTÞ ; r ¼ 0:9999
ꢁ1
Ethanol formation : log½kðs Þꢃ ¼ ð13:55 ꢂ 0:18Þ
ꢁ
1
ꢁ1
ꢁ
ð205:2 ꢂ 2:2ÞkJ mol ð2:303RTÞ ; r ¼ 0:9998
ꢁ1
Ethylene formation : log½kðs Þꢃ ¼ ð13:81 ꢂ 0:18Þ
ꢁ
1
ꢁ1
ꢁ
ð214:1 ꢂ 2:2ÞkJ mol ð2:303RTÞ ; r ¼ 0:9999
The kinetic and thermodynamic parameters for the
overall and parallel eliminations of the several ethyl
esters of oxamic acid derivatives are given in Table 9.
According to these data and the formation of the products
of ethyl N,N-dimethyloxamate [Table 9, reaction (9)],
which has no H atom bonded to N, the elimination
process of this substrate is similar to that of an alkyl
ester of a carboxylic acid, which proceeds through a
concerted six-membered cyclic transition state type of
mechanism [step 1, reaction (9)]. The 2-oxo acid inter-
mediate at the working temperature is unstable and
decarboxylates, as in reaction (4), very rapidly, possibly
through a four-membered cyclic transition state [step 2,
reaction (9)] to give dimethylformamide.
from the PhNH group than from NH group (step 2).
2
Because of the different elimination pathways in
the N,N-dimethyloxamate pyrolysis, the overall rate
coefficient of this substrate cannot be compared with the
overall k values for ethyl oxamate and ethyl oxani-
late (Table 9). This result is surprising, since decarbonyla-
tion was expected to be the first step of the reaction. Further
studies relative to the influence of the (CH ) N group in the
3 2
ethyl ester of the 2-oxocarboxylic acid may be needed in
order to explain the difference in mechanism.
EXPERIMENTAL
The elimination products of ethyl oxamate [reaction
6)] and ethyl oxanilate [reaction (8)] and the data in
Ethyl oxamate (Aldrich), ethyl N,N-dimethyloxamate
(Aldrich) and ethyl oxanilate (Aldrich) of >98% purity
were employed [GC–MS: Varian Saturn 2000 with a DB-
5MS capillary column (30 m ꢄ 0.53 mm. i.d., 0.53 mm
film thickness)]. The products of ethyl oxamate, which
are hydrocyanic acid, ammonia, carbon dioxide, ethylene
and ethanol, were identified as gases by GC–MS (Varian
Saturn 2000) with a DB-5MS capillary column (30 m ꢄ
(
Table 9 suggest decarbonylation to be the first steps of
decomposition [reaction (10), step 1]. Apparently, the
mechanisms may involve a semi-polar concerted three-
membered cyclic transition state similar to that in
reaction (5). However, the corresponding ethyl ester
intermediate may undergo subsequent fragmentations.
ð9Þ
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545

KINETICS OF ELIMINATION REACTIONS OF OXAMATES
545
ð10Þ
0
.25 mm. i.d., 0.25 mm). The quantitative analysis of the
REFERENCES
product ethylene (Matheson) was carried out by using an
HP 5710A gas chromatograph with a Porapak Q (80–
100 mesh) column. Ethanol (Merck) was quantitatively
analyzed by using a 3 m column of Porapak S (80–
1
2
. Daly NJ, Ziolkowski F. Aust. J. Chem. 1971; 24: 2541–2546.
. Taylor R. In The Chemistry of Functional Groups, Supplementary
Volume B, Acid Derivatives, Patai S (ed). Wiley: Chichester,
1979; 859–914.
3
. Holbrook KA. In The Chemistry of Acids Derivatives,
Volume 2, Vapor and Gas Phase Reactions of Carboxylic Acids
and Their Derivatives, Patai S (ed). Wiley: Chichester, 1992;
703–746.
. Smith GG, Voorkees KI, Kelly FM. J. Chem. Soc., Chem.
Commun. 1971; 789–790.
1
00 mesh).
Phenyl isocyanate and aniline as products of ethyl
oxanilate were identified as 1,3-diphenylurea and some
amount of phenyl isocyanate. 1,3-Diphenylurea results
from condensation of phenyl isocyanate with aniline
when collected in the trap. However, ethanol and ethylene
were collected as gases and quantitatively analyzed as
described above. All these products were identified by
GC–MS (Varian Saturn 2000) with a DB-5MS capillary
column (30 m ꢄ 0.25 mm. i.d., 0.25 mm).
4
5
. Gordon AS, Norris WP. J. Phys. Chem. 1965; 69: 3013–3017.
6. Kwart H, Slutsky J. J. Chem. Soc., Chem. Commun. 1972; 552–
53.
7
5
. Daly NJ, Ziolkowski F. Aust. J. Chem. 1972; 25: 1453–1458.
. Kwart H, Slutsky J. J. Chem. Soc., Chem. Commun. 1972;
1182–1183.
8
9
. Herize A, Dom ´ı nguez RM, Rotinov A, Nu n˜ ez O, Chuchani G.
J. Phys. Org. Chem. 1999: 12: 201–206.
0. Daly NJ, Ziolkowsky F. J. Chem. Soc., Chem. Commun. 1972;
The pyrolysis products of ethyl N,N-dimethyloxamate
were examined and identified as dimethylformamide,
ethylene and CO by GC–MS (Varian Saturn 2000).
1
1
1
911–912.
1. Luiggi M, Dominguez RM, Rotinov A, Herize A, Cordova M,
Chuchani G. Int. J. Chem. Kinet. 2002; 34: 1–5.
2. Chuchani G, Nu n˜ ez O, Marcano N, Napolitano S, Rodr ´ı guez H,
Dom ´ı nguez M, Ascanio J, Rotinov A, Dom ´ı nguez RM, Herize A.
J. Phys. Org. Chem. 2001: 14: 146–158.
2
Kinetics
1
3. Hansch C, Leo AJ. Substituent Constants for Correlation Analysis
in Chemistry and Biology. Wiley: New York, 1979; Hansch C,
Leo A, Taft R. Chem. Rev. 1991; 91: 165–195.
The kinetic determinations were carried out in a static
At each tem-
22–24
reaction system as reported previously.
1
4. Chuchani G, Dom ´ı nguez RM, Rotinov A, Herize A. J. Phys. Org.
Chem. 1999; 16: 40–46.
15. Taylor R. Int. J. Chem. Kinet. 1987; 19: 709–713.
perature, 6–12 runs were carried out. The rate coefficients
were calculated from the pressure increase measured
manometrically and/or by the sum of ethylene and
ethanol products. The temperature was maintained within
16. Taylor R. Int. J. Chem. Kinet. 1991; 23: 247–250.
17. Cordova T, Rotinov A, Chuchani G. J. Phys. Org. Chem. 2004;
1
18. Clark L. J. Phys. Chem. 1961; 65: 659–661.
7: 148–151.
ꢀ
ꢂ 0.2 C through control with a Shinko DIC-PS 23TR
19. Clark L. J. Phys. Chem. 1961; 65: 180–182.
20. Clark L. J. Phys. Chem. 1961; 65: 1651–1652.
21. Lapidus G, Barton D, Yankwich P. J. Phys. Chem. 1964; 68:
1863–1865.
resistance thermometer and was measured with a cali-
brated platinum–platinum–13% rhodium 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.
2
2
2
2. Maccoll A. J. Chem. Soc. 1955; 965–973.
3. Swinbourne ES. Aust. J. Chem. 1958; 11: 314–330.
4. Dominguez RM, Herize A, Rotinov A, Alvarez-Aular A,
Visbal G, Chuchani G. J. Phys. Org. Chem. 2004; 17: 399–
4
08.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 539–545