Mechanism and structure-reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phase

Article abstract of DOI:10.1002/poc.665

The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323-435 °C and pressure range 28.5-242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH₃OCOOCH₂CH₂Z, Z = substituent) give an approximate linear correlation when using the Taft-Topsom method, log(k _z/k_H)= -(0.57 ± 0.19)σ_α + (1.34 ± 0.49)σ_R (r = 0.9256; SD = 0.16) at 400 °C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the C₃- H bond by the oxygen carbonyl. Because ρ_α is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(k_R/k_H versus E_s^c for CH ₃OCOOCH₂CH₂R at 400°C, R = alkyl, δ= -0.17 (r=0.9993, SD = 0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright

Full text of DOI:10.1002/poc.665

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 839–848
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.665
Mechanism and structure–reactivity correlation in the
homogeneous, unimolecular elimination kinetics of
2-substituted ethyl methylcarbonates in the gas phase
Gabriel Chuchani,1* Edgar Marquez,2 Armando Herize,1 Rosa Marı´a Dom´ınguez,1 Mar´ıa Tosta1
and Doris Brusco2
1
´
´
Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
2
´
Departamento de Quımica, Escuela de Ciencias, Universidad de Oriente, Cumana, Sucre, Venezuela
Received 10 February 2003; revised 3 April 2003; accepted 8 April 2003
ABSTRACT: The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static
reaction system over the temperature range of 323–435 ꢀC and pressure range 28.5–242 Torr. The reactions are
homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are
reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z ¼ substituent) give an approx-
imate linear correlation when using the Taft–Topsom method, log(kZ/kH) ¼ ꢁ(0.57 ꢂ 0.19)ꢀꢁ þ (1.34 ꢂ 0.49)ꢀRꢁ
(r ¼ 0.9256; SD ¼ 0.16) at 400 ꢀC. This result implies the elimination process to be sensitive to steric factors, while
the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable
abstraction of the ꢂ-hydrogen of the Cꢂ—H bond by the oxygen carbonyl. Because ꢃꢁ is significant, a good
correlation of the alkyl substituents of carbonates with Hancock’s steric parameters was obtained: log(kR/kH) versus
Esc for CH3OCOOCH2CH2R at 400 ꢀC, R ¼ alkyl, ꢄ ¼ ꢁ0.17 (r ¼ 0.9993, SD ¼ 0.01). An approximate straight line
was obtained on plotting these data with the reported Hancock’s correlation of 2-alkyl ethylacetates. This result leads
to evidence for the ꢂ-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side
of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic
transition state type of mechanism. Copyright # 2003 John Wiley & Sons, Ltd.
KEYWORDS: gas phase; elimination; pyrolysis; kinetics; 2-substituted ethyl methyl carbonates; Taft–Topsom correlation
INTRODUCTION
were recently reported1 and the mechanism was de-
scribed as in reaction (1). This process of decomposition
was believed to be similar to the concerted six-membered
cyclic transition state ascribed to the gas thermal decom-
position of 2-substituted ethyl acetates2,3 [reaction (2)].
The kinetic and thermodynamic parameters for the gas-
phase elimination of 2-substituted ethyl N,N-dimethyl-
carbamates [(CH3)2NCOOCH2CH2Z, Z ¼ substituent]
Several correlation methods for substituent effects for
these series of 2-substituted ethylcarbamates gave ran-
dom points with no meanning for mechanistic interpreta-
´
*Correspondence to: G. Chuchani, Centro de Quımica, Instituto
Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
E-mail: chuchani@quimica.ivic.ve
´
tion. However, plotting log(kZ/kCH ) against the Taft
3
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

840
G. CHUCHANI ET AL.
original ꢀ* values4 gave rise at the origin, ꢀ*(CH3) ¼
0.00, 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,
especially with the electron-withdrawing and multiple-
bonded substituents at the 2-position of ethyl N,N-di-
methylcarbamates. The Taft correlation was found to be
as follows:
Alkyl groups:
ꢃ* ¼ ꢁ1.94 ꢂ 0.30 at
360 ꢀC (r ¼ 0.9768,
SD ¼ 0.0664)
Figure 1. Taft-Topsom correlation for the pyrolysis of
ZCOOCH2CH3:log(k/k0)¼ ꢁ(0.68 ꢂ 0.12)ꢀꢁ þ (2.57 ꢂ 0.12)ꢀF
F ꢁ(1.18 ꢂ 0.27)ꢀꢁR (r¼ 0.984, SD ¼ 0.119) at 400 ꢀC
Electron-withdrawing groups: ꢃ* ¼ ꢁ0.12 ꢂ 0.02 at
360 ꢀC (r ¼ 0.9364,
SD ¼ 0.0571)
Multiple bonded substituents: ꢃ* ¼ 0.49 ꢂ 0.03 at
360 ꢀC (r ¼ 0.9907,
confirms the interaction between the substituent Z and
the incipient negative reaction center. The negative coef-
ficient ꢃꢁR implies a favorable effect for the abstraction of
the ꢂ-hydrogen of the ethyl ester by the oxygen carbonyl
in the transition state.
SD ¼ 0.1036)
However, the correlation analysis for 2-substituted
ethylacetates [reaction (3)] was reported5 as described
below:
According to the linear correlation for ZCOOCH2CH3
(Fig. 1), and to the various correlations applied to alipha-
tic acetates and carbamates, it seemed of interest to
examine esters of a more polar nature such as chlorofor-
mates or carbonates (Fig. 1). However, chloroformates
are difficult to handle and unstable even at room tem-
perature. For this reason, little research on the gas-phase
elimination of these types of compounds has been re-
ported. Consequently, the present work was aimed at
examining the homogeneous molecular elimination of
unsymmetrical 2-substituted ethyl methylcarbonates,
CH3OCOOCH2CH2Z, Z ¼ substituent.
---
CH3OCOOCH2CH2Z ! CH3COOH þ CH2 ---CH2Z
ð3Þ
Alkyl groups:
Polar groups:
Polar groups:
log(k/k0) ¼ ꢄEsc, ꢄ ¼ ꢁ0.12
(r ¼ 0.916 at 400 ꢀC)
log(k/k0) ¼ ꢃ*ꢀ*, ꢃ* ¼ ꢁ0.19
(r ¼ 0.961 at 400 ꢀC)
log(k/k0) ¼ ꢃIꢀI, ꢃI ¼ ꢁ1.03
(r ¼ 0.960 at 400 ꢀC)
Alkyl and polar: log(k/k0) ¼ ꢁ (0.45 ꢂ 0.01)ꢀꢁ
ꢁ (1.29 ꢂ 0.11)ꢀF (r ¼ 0.959 at
400 ꢀC)
It has been reported that esters of carbonic acids
containing at least one ꢂ-hydrogen atom at the alkyl
side, when heated at high temperature, decompose to an
alkene, alcohol (phenol) and CO2.7 O’Connor and Nace8
suggested that symmetrical carbonates undergo a first-
order rate law, give a negative entropy of activation and
proceed through a unimolecular cyclic cis-elimination. In
these sense, two mechanisms were proposed.
Multiple bonds: log(k/k0) ¼ ꢁ (1.81 ꢂ 0.02)ꢀꢁ
ꢁ (0.38 ꢂ 0.03)ꢀF
þ (7.34 ꢂ 0.17)ꢀꢁR
(r ¼ 0.984 at 400 ꢀC)
In view of the several good correlations obtained by the
application of different structureꢁreactivity relationship
methods for 2-substituted carbamates and acetates, it was
considered interest to examine the elimination kinetics of
another type of organic ethyl ester. In this respect, the
kinetic parameters and comparative rates of different
substituents other than carbon at the acid side of an
organic ethyl ester6 were correlated by using the Taft–
Topsom method. An approximate straight line, as shown
in Fig. 1, was obtained.
Mechanism A:
Mechanism B:
The negative value of ꢃꢁ suggested a modest participa-
tion of the polarizability or steric effect of the substituent
Z. According to the greatest absolute value of ꢃF, the field
or electronic effect has the most important influence in
the process of elimination. The use of the value ꢀRꢁ
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

ELIMINATION KINETICS OF ETHYL METHYLCARBONATES
841
A symmetrical carbonate without a ꢂ-hydrogen atom,
such as dimethyl carbonate, was reported to give di-
methyl ether and CO2 gas when pyrolyzed between 209
and 232 ꢀC,9 and also up to 250 ꢀC10 [reaction (4)].
However, further investigation on dimethyl carbonate11
showed it to be extremely stable even up to 350 ꢀC. The
stability of this compound was later confirmed at tem-
peratures as high as 390 ꢀC.12
reason and in association with the pyrolytic elimination
of 2-substituted ethyl N,N-dimethylcarbamates1 and acet-
ates,2,3 in this work the kinetics of the homogeneous,
unimolecular elimination of 2-substituted ethyl methyl-
carbonates were examined.
RESULTS AND DISCUSSION
The products of elimination of 2-substituted ethyl
methylcarbonates in the gas phase are the corresponding
alkene, metanol and carbon dioxide [reaction (5)].
---
Cð--- O Þ
----- -----
O
----- -----
O
----- -----
CH3 ! H3C CH3 þ CO2 ð4Þ
H3C
O
In spite of the few studies on aliphatic carbonates in the
gas phase, there is no evidennce for Mechanism B.
Taylor13 presented several arguments in favor of Mechan-
ism A when describing the correlation of aryl ethylcar-
bonate pyrolysis,14 ArOCOOCH2CH3, with ꢃ ꢀ ¼ 0.19
(Scheme 1). This result suggested that the electron-with-
drawing substituents in the benzene ring increased the
rate, aiding the Cꢁ—O polarization, exactly the opposite
of what part would be obtained if Mechanism B was
involved, since electron withdrawal would make the
ethereal part less nucleophilic, thereby lowering the
reaction rate. This suggested that Mechanism A applies.
Taylor and Thorne additionally pointed out that tert-
butyl N,N-arylcarbamates15 gave a good Hammett corre-
lation of ꢃ ꢀ ¼ 0.63. This value indicated a similar transi-
tion state of aryl ethylcarbonates, and that the abstraction
of the ꢂ-hydrogen to the carbonyl group is very impor-
tant. Hence both elimination reactions must undergo
pyrolysis via Mechanism A. Moreover, methyl alkylcar-
bonates were believed to be less reactive than phenyl
alkylcarbonates. In this sense, 1-phenylethyl phenylcar-
bonate16 was found to react faster than 1-phenylethyl
methylcarbonate,17 with k values of 22.2 ꢃ 10ꢁ3 and
6.98 ꢃ 10ꢁ3 at 600 K, respectively (Scheme 1). Finally,
the relative rates of Et : i-Pr : t-Bu are 1 : 29.6 : 2934,
which was higher than expected when compared with
other esters for which consideration of steric acceleration
was not ruled out.
---
CH3OCOOCH2CH2Z ! ½CH3OCOOHꢄ þ CH2 ---CH2Z
#
CH3OH þ CO2
ð5Þ
The theoretical stoichiometry for the gas-phase pyro-
lysis of the substrates, as described in reaction (5),
requires Pf/P0 ¼ 3.0, where Pf and P0 are the final and
initial pressures, respectively. The average Pf/P0 values at
four different temperatures and 10 half-lives were, within
experimental error, nearly 3.0 (Table 1). The departure of
Pf/P0 to <3.0 for 2-bromoethyl methylcarbonate, 2-
chloroethyl methylcarbonate and 2-phenylethyl methyl
carbonate was due to polymerization of the correspond-
ing olefinic product. The pure unsaturated products of
these substrates when introduced into the static reaction
vessel show a decrease in pressure and formation of a
solid polymer. The stoichiometry of reaction (5), up to
45–80% reaction, was satisfactorily verified by compar-
ing the percentage decomposition of the substrate from
pressure measurements with that obtained from the gas
chromatographic (GC) analyses of the corresponding
alkene formation (Table 2).
These elimination reactions can be said to be homo-
geneous since no significant effects on the rates were
obtained when using both clean Pyrex and seasoned
Pyrex vessels with a surface-to-volume ratio of 6.0
relative to the normal vessel, which is equal to 1.0. The
effect of the addition of different proportions of the free
radical inhibitor toluene or cyclohexene is described in
Table 3. Nevertheless, the pyrolyses of these carbonates
were carried out in the presence of at least twice the
amount of the inhibitor in order to prevent any possible
free radical chain reactions (Table 3). The k values are
reproducible with a standard deviation not greater than
5% at a given temperature.
Even though these previous studies give to some extent
reasonable arguments for Mechanism A, additional work
may well be needed for a definite mechanistic elucidation
of carbonate decomposition in the gas phase. For this
The rate coefficients for elimination, calculated from
k1 ¼ (2.303/t)log[2P0/(3P0 – Pt)], are invariable to initial
pressures (Table 4), and the first-order plots of log
(3P0 – Pt) against time t gave a good straight lines up to
40–80% decomposition, suggesting that the elimination
process is unimolecular in nature. The temperature depen-
dence of the rate coefficients and the corresponding
Arrhenius equation is shown in Table 5, where 90% confi-
dence limits from a least-squares procedure are given.
Scheme 1
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

842
G. CHUCHANI ET AL.
Table 1. Ratio of final (Pf) to initial (P0) pressures
Substrate
Temperature ( ꢀC)
P0 (Torr)
Pf (Torr)
Pf/P0
Average
Ethyl
methylcarbonate
370.6
380.2
389.8
399.8
360.0
370.0
380.0
390.0
350.0
360.0
370.0
380.0
350.0
360.0
370.0
380.0
350.8
360.4
370.9
379.5
349.8
359.3
370.2
380.4
370.7
380.0
389.3
398.8
360.0
370.0
380.0
390.0
400.0
410.0
420.0
430.2
405.7
415.3
425.8
435.7
344.5
353.0
362.9
372.5
358.3
370.2
380.3
390.4
90
105
107
80
118
135
172
150
96
118
88
92
126
102
94
126
78
28.5
50
50
95
35.5
122
120
96
108
90
105
46
49.5
69
106.5
84
83
66.5
73
252
298
318
238
350
380
490
440
278
344
258
270
300
276
306
368
214.5
78.5
143
144
250
92
351
332
258
288
248
290
141
154
212
320
239.5
246
215
213
372
256
236
311
231
187
249.5
223.5
258
94
2.88
2.83
2.96
2.98
2.97
2.81
2.85
2.93
2.90
2.92
2.93
2.93
2.94
2.94
2.94
2.92
2.75
2.75
2.86
2.88
2.63
2.59
2.87
2.77
2.69
2.67
2.76
2.76
3.07
3.11
3.07
3.00
2.85
2.96
3.07
2.92
2.73
3.04
3.06
2.83
2.68
2.56
2.51
2.48
2.90
3.10
2.98
3.07
2.91 ꢂ 0.03
Propyl
methylcarbonate
2.89 ꢂ 0.04
2.92 ꢂ 0.01
2.94 ꢂ 0.01
2.81 ꢂ 0.03
2.71 ꢂ 0.06
2.72 ꢂ 0.02
3.06 ꢂ 0.07
2.95 ꢂ 0.05
2.91 ꢂ 0.04
2.56 ꢂ 0.03
3.01 ꢂ 0.09
1-Butyl
methylcarbonate
3-Methyl-1-butyl
methylcarbonate
3,3-Dimethyl-1-butyl
methylcarbonate
2-Bromoethyl
methylcarbonate
2-Chloroethyl
methylcarbonate
3-Chloropropyl
methylcarbonate
2-Methoxyethyl
methylcarbonate
2-Phenoxyethyl
methylcarbonate
136
84
77
110
88
73
99.5
90
89
30
41
25.5
2-Phenylethyl
methylcarbonate
3-Phenylpropyl
methylcarbonate
122
78
The kinetic and thermodynamic parameters for the gas-
phase elimination of 2-substituted ethyl methylcarbonate,
CH3OCOOCH2CH2Z, Z ¼ substituent, examined in this
work are listed in Table 6. The negative entropy of
the elimination of these carbonates is not spontaneous,
unstable and endergonic. The application of several
correlations of substituent effects, polar ꢀ*, inductive
ꢀI, steric ES, ESC and ꢅ values,4 gave random points with
no reasonable mechanistic interpretation for the carbo-
nates. However, the data in Table 7 gave a relative good
correlation by means of the Taft–Topsom equation (Fig. 2).
The size of the substituent described by ꢃꢁ is sensitive
to polarizability or steric effects. Since the standard
error for the coefficient ꢃF is extremely high, the field/
electronic effect appears to be unimportant and may be
deleted from the above equation. However, the size and
¼
activation, ÁS , of these eliminations implies a symme-
trical arrangement and possible planarity of the transition
state. However, the substituents (CH3)3C and CH2Cl with
¼
a very small positive ÁS may be attributed to steric
factors, which may hinder some approximation to a
symmterical concerted transition state. The enthalpy of
¼
activation ÁH , suggests endothermic processes. The
values of the free energy of activation ÁG , indicate that
¼
Copyright # 2003 John Wiley & Sons, Ltd.
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ELIMINATION KINETICS OF ETHYL METHYLCARBONATES
Table 2. Stoichiometry of the reaction
843
Substrate
Temperature ( ꢀC) Parameter
Value
13
Ethyl
methylcarbonate
371.0
370.0
375.0
350.0
Time (min)
5
20
19.8
5
24
23
5
26
26.1
10
20.8
21.3
8
15
50
21
62
Reaction (%) (pressure)
Ethylene (%) (GC)
Time (min)
Reaction (%) (pressure)
Propene (%) (GC)
Time (min)
Reaction (%) (pressure)
Butene (%) (GC)
Time (min)
30.6
29.5
7
30
29.8
7
34.4
33.1
15
29.5
30.0
44.8
45.5
10
40.7
39.4
9
42
42.7
20
51.7
13
60.7
18
Propyl
methylcarbonate
49.3
51
61
62
1-Butyl
methylcarbonate
12
52
15
60
51.8
25
60.3
30
3-Methyl-1-butyl
methylcarbonate
Reaction (%) (pressure)
3-Methyl-1-butene
(%) (GC)
37.3
38.2
44.2
44.1
50.4
52
3,3-Dimethyl-1-butyl
methylcarbonate
350.8
Time (min)
10
16
50
33.5
2.1
21
26
Reaction (%) (pressure)
3,3-Dimethyl-1-butene
3,3-Dimethyl-1-butyl
methyl ether (%) (GC)
Time (min)
Reaction (%) (pressure)
Vinyl bromide
(%) (GC)
34.3
22.9
7.6
58.9
37.8
3.0
68.5
50.1
2.0
2-Bromoethyl
methylcarbonate
360.3
388.8
370.0
410.0
405.3
353.4
358.5
2
28.0
21.0
4
49.0
43.0
5
52.0
45.0
2-Chloroethyl
methylcarbonate
Time (min)
3
18
20
6
32
30
10
56
51
15
80
77
Reaction (%) (pressure)
Vinyl chloride
(%) (GC)
3-Chloropropyl
methylcarbonate
Time (min)
6
31.1
34.2
11
49
52.9
16
59.2
58.8
23
70.5
68.6
31
78.1
76.8
Reaction (%) (pressure)
3-Chloropropene
(%) (GC)
2-Methoxyethyl
methylcarbonate
Time (min)
2
15
16.8
3
22.4
21
4
25.6
27.8
5
35.8
33.6
6
43.5
45.1
Reaction (%) (pressure)
Methyl vinyl ether
(%) (GC)
2-Phenoxyethyl
methylcarbonate
Time (min)
1.5
11.6
10.8
3
20.1
20.4
6
33.6
34.8
9
41.8
40.8
12
56.4
56.3
Reaction (%) (pressure)
Phenyl vinyl ether
(%) (GC)
2-Phenylethyl
methylcarbonate
Time (min)
1.5
11.1
13.2
3
20.8
22.7
4.5
31.4
34.7
6
36.6
40.7
7
47.1
51.7
Reaction (%) (pressure)
Styrene
(%) (GC)
3-Phenylpropyl
methylcarbonate
Time (min)
15.2
31.6
28.9
23.1
43.1
29.9
44.0
53.9
49.6
75.6
64.7
61.0
Reaction (%) (pressure)
Allylbenzene
(%) (GC)
the positive sign of ꢃꢁR has the greatest influence in the
elimination reaction, which implies that the Cꢁ—O bond
polarization in the transition state is the limiting factor.
This result also suggests a favorable effect for the
abstraction of the ꢂ-hydrogen of the Cꢂ—H bond by
the oxygen carbonyl in the elimination of these carbo-
nates. This indicates that Mechanism A as the most
probable process of decomposition.
As described before, the 2-substituted alkylethyl acet-
ates was reported to give an approximate straight line
with Hancock steric parameter ESC values (ꢄ ¼ꢁ0.12,
r ¼ 0.916 at 400 ꢀC). Any similarity of the transition state
between carbonates and acetates is demonstrated by a
linear correlation between logkrel. of 2-substituted
alkylethyl methylcarbonates and logkrel. of 2-substituted
alkylethyl acetates (Table 9).
Since ꢃꢁ ¼ ꢁ0.63 is significant in the elimination
reaction of these carbonates, the 2-substituted alkylethyl
methylcarbonates may well give a good correlation with a
steric parameter such as Hancock’s ESC values. This is
supported by the results shown in Table 8 and Fig. 3
(ꢄ ¼ ꢁ0.17, r ¼ 0.9993, SD ¼ 0.0102 at 400 ꢀC).
Figure 4 shows a good straight line with a correlation
coefficient r ¼ 0.9857 and a standard deviation SD ¼
0.0457 at 400 ꢀC.
The results obtained in this work appear to give addi-
tional support to Taylor’s idea13–17 that Mechanism A is
the most reasonable transition-state type of mechanism
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

844
G. CHUCHANI ET AL.
Table 3. Effect of the free radical inhibitor on rates
b
Pi (Torr)
Substrate
Temperature ( ꢀC)
370.3
Ps a (Torr)
Pi=Ps
104 k1 (sꢁ1
)
Ethyl
methylcarbonate
130
76
80
85
—
75
120
165
180
—
138
160
124
260
—
108
120
162
200
—
—
1.0
1.5
1.9
3.0
—
1.0
2.0
2.1
3.0
—
1.1
1.5
2.1
2.2
—
1.6
2.0
2.5
2.8
—
0.7
1.0
1.8
3.0
—
0.3
0.7
1.2
4.3
—
0.2
0.6
1.2
2.1
—
0.9
1.1
3.4
—
0.8
1.8
2.2
2.9
—
0.6
1.2
2.1
3.9
—
0.4
1.3
3.0
6.6
—
1.6
2.4
3.8
5.2
7.88
7.78
7.80
7.88
7.79
9.20
9.19
9.18
9.20
9.17
9.94
9.91
9.96
9.98
9.98
4.03
60
Propyl
methylcarbonate
372.0
370.4
350.0
370.9
359.1
389.1
126
136
81
60
88
134
102
80
80
90
66
74
80
60
48
50
61
72.5
1-Butyl
methylcarbonate
3-Methyl-1-butyl
methylcarbonate
120
160
150
136
—
4.00
4.02
4.05
3.96
3,3-Dimethyl-1-butyl
methylcarbonate
22.8
44
22.53
22.34
22.71
22.69
25.36
7.77
6.88
7.76
7.54
71.5
115
106.5
—
91
89
77
155
—
32
55
96
129
—
47
65
63
36
101
242
136
63
2-Bromoethyl
methylcarbonate
35.5
2-Chloroethyl
methylcarbonate
84
151
91
78
61
11.2
10.28
10.50
10.80
10.15
6.34
5.37
5.10
5.10
4.74
4.41
4.50
4.78
4.52
11.31
11.72
11.29
11.11
11.68
45.25
32.23
33.12
33.32
33.08
13.98
13.89
13.98
13.65
14.24
3-Chloropropyl
methylcarbonate
360.0
388.9
60.5
52
60
52
95.5
90
86
77
64
122
125
96
73
47
101.1
155
84
176
—
2-Methoxyethyl
methylcarbonate
75.5
155.5
167.5
183.5
—
2-Phenoxyethyl
methylcarbonate
405.7
372.5
380.3
77
117
153
183
—
2-Phenylethyl
methylcarbonate
64
110
298.0
277
—
99.5
42.5
46.5
48.9
45
3-Phenylpropyl
methylcarbonate
79
110
175
234
46.5
45
a
Ps ¼ pressure of substrate.
b
Pi ¼ pressure of free radical inhibitor.
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

ELIMINATION KINETICS OF ETHYL METHYLCARBONATES
Table 4. Invariability of the rate coefficients from initial pressure
845
Substrate
Temperature ( ꢀC)
370.3
Parameter
Value
115
Ethyl
methylcarbonate
Propyl
methylcarbonate
1-Butyl
P0 (Torr)
)
80
7.98
60
9.20
80
9.92
60
3.96
28.5
12.09
35
7.54
30
9.99
46
5.42
47.5
8.91
47
11.68
42.5
33.08
27
95
7.80
80
9.20
90
9.92
92
3.94
46
12.08
63
7.76
55
10.38
54
5.45
56
9.00
91
11.11
84
125
7.88
120
9.14
122
9.90
124
3.96
170
7.84
140
9.25
140
9.90
140
4.00
104 k1 (sꢁ1
7.80
100
9.20
100
9.91
104
3.94
78
12.12
74
7.93
90
10.86
68
5.60
84
8.95
118
11.00
95
372.0
370.4
350.0
360.4
359.6
389.3
360.0
400.0
405.7
372.5
358.3
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
P0 (Torr)
104 k1 (sꢁ1
)
methylcarbonate
3-Methyl-1-butyl
methylcarbonate
3,3-Dimethyl-1-butyl
methylcarbonate
2-Bromoethyl
methylcarbonate
2-Chloroethyl
methylcarbonate
3-Chloropropyl
methylcarbonate
2-Methoxyethyl
methylcarbonate
2-Phenoxyethyl
methylcarbonate
2-Phenylethyl
methylcarbonate
3-Phenylpropyl
methylcarbonate
)
)
117.5
12.22
242
)
)
7.77
129
219
)
10.15
77
5.30
92.5
8.92
136
10.37
90.5
5.32
141
8.67
149
)
)
)
11.58
155
11.88
)
33.12
39
4.57
32.67
50
4.67
32.93
78
4.52
89
4.44
)
4.47
Table 5. Temperature dependence of rate coefficients
Substrate
Parameter
Value
370.3
7.83
Ethyl
methylcarbonate
Temperature ( ꢀC) 342.0
351.2
2.65
360.9
4.44
380.1
13.00
389.7 399.8
21.90 36.80
104 k1 (sꢁ1
)
1.55
Rate equation
log k1 (sꢁ1) ¼ (12.21 ꢂ 0.12) ꢁ (188.7 ꢂ 1.5) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9998
Propyl
methylcarbonate
Temperature ( ꢀC) 340.0
104 k1 (sꢁ1
1.77
Rate equation
351.0
3.02
360.7
5.20
372.0
8.70
382.0
14.90
390.0 400.0
25.11 42.74
)
log k1 (sꢁ1) ¼ (11.72 ꢂ 0.44) ꢁ (182.0 ꢂ 2.3) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9987
1-Butyl
methylcarbonate
Temperature ( ꢀC) 340.3
104 k1 (sꢁ1
1.98
Rate equation
350.0
3.42
360.0
5.82
370.0
9.91
380.0
16.86
390.5 400.5
28.60 48.67
)
log k1 (sꢁ1) ¼ (11.77 ꢂ 0.10) ꢁ (181.9 ꢂ 0.5) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9998
3-Methyl-1-butyl
methylcarbonate
Temperature ( ꢀC) 331.0
104 k1 (sꢁ1
1.35
Rate equation
340.0
2.29
350.0
3.91
360.0
6.66
370.0
11.30
380.0 390.0
19.13 32.55
)
log k1 (sꢁ1) ¼ (11.60 ꢂ 0.12) ꢁ (178.3 ꢂ 1.2) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9990
3,3 Dimethyl-1-butyl Temperature ( ꢀC) 331.2
methylcarbonate
341.4
3.42
350.8
6.26
360.4
11.74
370.9
20.93
379.5
34.10
104 k1 (sꢁ1
Rate equation
)
1.80
log k1 (sꢁ1) ¼ (13.60 ꢂ 0.15) ꢁ (200.7 ꢂ 1.8) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9998
2-Bromoethyl
methylcarbonate
Temperature ( ꢀC) 329.0
104 k1 (sꢁ1
1.31
Rate equation
338.6
2.54
348.7
4.36
359.6
7.76
370.5
13.20
380.4
22.30
)
log k1 (sꢁ1) ¼ (11.53 ꢂ 0.28) ꢁ (177.3 ꢂ 3.4) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9993
2-Chloroethyl
methylcarbonate
Temperature ( ꢀC) 360.4
104 k1 (sꢁ1
1.97
Rate equation
370.7
3.69
380.0
6.02
389.3
10.33
398.8
17.20
)
log k1 (sꢁ1) ¼ (12.71 ꢂ 0.44) ꢁ (199.1 ꢂ 5.5) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9999
3-Chloropropyl
methylcarbonate
Temperature ( ꢀC) 340.6
104 k1 (sꢁ1
1.57
Rate equation
350.6
3.35
360.4
5.43
370.1
10.39
380.5
18.31
390.3
3.36
)
log k1 (sꢁ1) ¼ (13.59 ꢂ 0.07) ꢁ (204.3 ꢂ 0.8) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9999
2-Methoxyethyl
methylcarbonate
Temperature ( ꢀC) 380.0
104 k1 (sꢁ1
2.87
Rate equation
388.9
4.60
396.3
7.03
400.0
8.84
410.0
14.30
420.0 430.2
23.74 38.36
)
log k1 (sꢁ1) ¼ (12.30 ꢂ 0.14) ꢁ (198.0 ꢂ 1.8) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9998
2-Phenoxyethyl
methylcarbonate
Temperature ( ꢀC) 385.0
104 k1 (sꢁ1
3.73
Rate equation
395.3
6.25
405.3
10.40
415.3
16.74
425.6
30.40
435.7
45.47
)
log k1 (sꢁ1) ¼ (11.92 ꢂ 0.28) ꢁ (193.5 ꢂ 3.6) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9993
2-Phenylethyl
methylcarbonate
Temperature ( ꢀC) 323.9
104 k1 (sꢁ1
2.37
Rate equation
332.5
4.07
344.5
7.72
353.5
12.29
363.5
19.54
372.5
33.32
)
log k1 (sꢁ1) ¼ (10.82 ꢂ 0.23) ꢁ (164.9 ꢂ 2.7) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9995
3-Phenylpropyl
methylcarbonate
Temperature ( ꢀC) 340.3
104 k1 (sꢁ1
1.45
Rate equation
358.3
4.57
370.2
8.74
380.3
13.94
390.4
21.31
)
log k1 (sꢁ1) ¼ (12.04 ꢂ 0.10) ꢁ (186.5 ꢂ 1.3) kJ molꢁ1 (2.303 RT)ꢁ1, r ¼ 0.9999
J. Phys. Org. Chem. 2003; 16: 839–848
Copyright # 2003 John Wiley & Sons, Ltd.

846
G. CHUCHANI ET AL.
Table 6. Kinetic and thermodynamic parameters for pyrolisis of CH3OCOOCH2CH2Z at 400 ꢀC
¼
¼
¼
ÁG
k1 ꢃ 10ꢁ4
kH ꢃ 10ꢁ4
Ea
(kJ molꢁ1
Loꢁg1A
(s
ÁS
ÁH
Z
(sꢁ1
)
(sꢁ1
)
)
)
(J molꢁ1 Kꢁ1
)
(kJ molꢁ1
)
(kJ molꢁ1
)
H
CH3
CH2CH3
CH(CH3)2
C(CH3)3
Br
36.65
39.27
44.86
57.71
105.35
58.73
18.04
54.10
8.55
12.22
19.63
22.43
28.85
52.68
29.36
9.03
27.05
4.28
3.58
188.7 ꢂ 1.5
182.0 ꢂ 2.3
181.9 ꢂ 1.4
178.3 ꢂ 1.2
200.7 ꢂ 1.8
177.3 ꢂ 3.4
199.1 ꢂ 5.5
204.3 ꢂ 0.8
198.0 ꢂ 1.8
193.5 ꢂ 3.6
164.9 ꢂ 2.7
186.5 ꢂ 1.3
12.21 ꢂ 0.12
11.72 ꢂ 0.44
11.77 ꢂ 0.10
11.60 ꢂ 0.22
13.60 ꢂ 0.15
11.53 ꢂ 0.28
12.71 ꢂ 0.44
13.59 ꢂ 0.07
12.30 ꢂ 0.14
11.92 ꢂ 0.28
10.82 ꢂ 0.23
12.04 ꢂ 0.10
ꢁ26.23
ꢁ35.61
ꢁ34.66
ꢁ37.91
0.37
ꢁ39.25
ꢁ16.66
0.18
ꢁ24.51
ꢁ31.78
ꢁ52.84
ꢁ29.49
183.1
176.4
176.3
172.7
195.1
171.7
193.5
198.7
192.4
187.9
159.3
180.9
200.8
200.4
199.6
198.2
194.9
198.1
204.7
198.6
208.9
209.3
194.9
200.8
Cl
CH2Cl
OCH3
OC6H5
C6H5
7.15
105.02
36.31
52.51
18.15
CH2C6H5
Table 7. Parameters of substituents Z in CH3OCOOCH2-
CH2Z at 400 ꢀC
Z
Log (kZ/kH)
ꢀꢁ
ꢀF
ꢀRꢁ
H
CH3
CH2CH3
CH(CH3)2
C(CH3)3
Br
0.0000
0.20
0.26
0.37
0.63
0.00
ꢁ0.35
ꢁ0.49
ꢁ0.62
ꢁ0.75
ꢁ0.59
ꢁ0.43
ꢁ0.54
ꢁ0.17
ꢁ0.38
ꢁ0.81
ꢁ0.70
0.00
0.00
0.00
0.00
0.00
0.45
0.45
0.23
0.25
0.38
0.10
0.05
0.00
0.03
0.02
0.01
0.00
0.38
ꢁ0.13
ꢁ0.10
ꢁ0.12
0.02
ꢁ0.27
ꢁ0.26
0.22
Cl
CH2Cl
OCH3
OC6H5
C6H5
0.34
ꢁ0.46
ꢁ0.54
0.63
CH2C6H5
0.17
0.02
Figure 3. Log(kZ/kH) versus Esc for CH3OCOOCH2CH2Z at
400 ꢀC. ꢄ ¼ ꢁ0.17, r ¼ 0.9993, SD ¼ 0.0102
Table 9. Logarithm of relative rates of carbonates and
acetates [log(kZ/kH)] at 400 ꢀC
Z
Carbonates
Acetates
H
CH3
CH2CH3
CH(CH3)2
C(CH3)3
0.00
0.20
0.26
0.37
0.63
0.00
0.11
0.20
0.19
0.46
Figure 2. Taft–Tompson correlation for the pyrolysis of
CH3OCOOCH2CH2Z: log(kZ/kH) ¼ ꢁ(0.63 ꢂ 0.18)ꢀꢁ þ (1.29
ꢂ 0.45)ꢀꢁR (r ¼ 0.9447; SD ¼ 0.14) at 400 ꢀC
for the gas-phase elimination of carbonates with a Cꢂ—H
bond at the alkyl side of the ester.
Table 8. Log krel. versus Esc at 400 ꢀC
Z
Carbonates log (kZ/kH)
Esc
EXPERIMENTAL
H
CH3
CH2CH3
CH(CH3)2
C(CH3)3
0.00
0.20
0.26
0.37
0.63
0.00
ꢁ1.24
ꢁ1.62
ꢁ2.32
ꢁ3.70
General procedure. The 2-substituted ethyl methyl-
carbonates listed below were prepared by mixing
0.2 mol of methyl chloroformate (Aldrich) with 0.2 mol
corresponding substituted alcohol in 40 ml of chloroform.
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

ELIMINATION KINETICS OF ETHYL METHYLCARBONATES
847
2-Chloroethyl methylcarbonate. B.p. 95 ꢀC at 82 Torr,
yield 38%. H NMR, ꢄ 3.5 (m, 2H), 4.0 (m, 2H). MS,
1
m/z 138 (Mþ), 103 (CH3OCOOCH2CH2þ), 59 (CH3O
COþ), 44 (CO2).
3-Chloro-1-propyl methylcarbonate. B.p. 152 ꢀC at
1
65 Torr, yield 54%. H NMR, ꢄ 2.1 (q, 2H), 3.6 (t, 2H),
3.8 (s, 3H), 4.3 (t, 2H). MS, m/z 153 (Mþ), 117 (CH3
OCOOCH2CH2CHþ2 ), 103 (CH3OCOOCH2CHþ2 ), 73
(COOCH2CHþ2 ), 59 (CH3OCOþ), 44 (CO2).
2-Methoxyethyl methylcarbonate. B.p. 95 ꢀC at 82 Torr,
yield 37%. 1H NMR, ꢄ 3.0 (s, 3H), 4.2 (t, 2H), 3.6 (s, 3H),
4.5 (t, 2H). MS, m/z 134 (Mþ), 103 (CH3
OCOOCH2CHþ2 ), 59 (CH3OCOþ), 45 (CH3OCHþ2 ).
Figure 4. Log(kZ/kH) (carbonates) versus log(kZ/kH) (acet-
ates) at 400 ꢀC. r ¼ 0.9857, SD ¼ 0.045697
2-Phenoxyethyl methylcarbonate. B.p. 131 ꢀC at 5 Torr,
yield 49%. 1H NMR, ꢄ 3.8 (s, 3H), 4.2 (t, 2H), 4.5 (t, 2H),
6.9–7.3 (m, 5H). MS, m/z 196 (Mþ), 121 (C6H5
OCH2CHþ2 ), 103 (CH3OCOOCH2CHþ2 ), 77 (C6Hþ5 ), 31
(CH3Oþ).
The reaction mixture was refluxed until no more HCl
was evolved. The product of the reaction was distilled
several times to better than 98.8% purity as determined
by GC–MS (Saturn 2000, Varian) with a DB-5MS
capillary column, 30 ꢃ 0.53 mm i.d., 0.53 mm film
thickness.
2-Phenylethyl methylcarbonate. B.p. 145 ꢀC at 23 Torr,
yield 45%. 1H NMR, ꢄ 2.8 (t, 2H), 3.7 (s, 3H), 4.5 (t, 2H),
7.1–7.2 (m, 5H). MS, m/z 180 (Mþ), 104 (C6H5
CH2CHþ2 ), 59 (CH3OCOþ).
Ethyl methylcarbonate. B.p. 108 ꢀC at 630 Torr (1 Torr ¼
1
133.3 Pa), yield 46% (lit.,13 105–107 ꢀC at 760 Torr). H
NMR, ꢄ 1.3 (t, 3H), 3.8 (s, 3H), 4.2 (m, 2H). MS, m/z 105
(Mþ), 59 (CH3OCOþ), 45 (OCH2CHþ3 ), 29 (CH2CHþ3 ).
3-Phenylpropyl methylcarbonate. B.p. 180 ꢀC at 25 Torr,
1
Propyl methylcarbonate. B.p. 125 ꢀC at 630 Torr, yield
90%. 1H NMR, ꢄ 0.9 (t, 3H), 1.6 (m, 2H), 3.9 (s, 3H), 4.0
(t, 2H). MS, m/z 119 (Mþ), 75 (CH3OCOOþ), 59
(CH3OCOþ), 43 (CH3CH2CHþ2 ).
yield 58%. H NMR, ꢄ 2.0 (m, 2H), 2.7 (t, 2H), 3.8 (s,
3H), 4.2 (t, 2H), 7.2 (m, 5H). MS, m/z 195 (Mþ), 117
(CH3OCOOCH2CH2CHþ2 ), 103 (CH3OCOOCH2CHþ2 ),
91 (C6H5CHþ2 ), 77 (C6Hþ5 ).
1-Butyl methylcarbonate. B.p. 85–87 ꢀC at 61 Torr, yield
82%. 1H NMR, ꢄ 0.9 (t, 3H), 1.4 (m, 2H), 3.7 (s, 3H), 4.1
(t, 2H). MS, m/z 133 (Mþ), 117 (OCOOCH2CH2CH2
CHþ3 ) 73 (OCH2CH2CH2CH3þ), 59 (CH3OCOþ), 43
(CH3CH2CHþ2 ).
Analyses. Quantitative analyses of the olefinic hydro-
carbon products were carried out by GC with a Porapak
Q column, 80–100 mesh. Quantitative analyses of
the carbonate substrates, together with the identifications
of products, were performed by GC–MS (Saturn 2000,
Varian) with a DB-5MS capillary column, 30 ꢃ 0.53 mm.
i.d., 0.53 mm film thickness.
3-Methyl-1-butyl methylcarbonate. B.p. 149 ꢀC at
630 Torr, yield 49%. H NMR, ꢄ 0.9 (d-d, 6H), 1.57 (m,
1
2Hþ), 1.6 (m, 1H), 3.8 (s, 3H), 4.2 (t, 2H). MS, m/z 147
(M ), 103 (CH3OCOOCH2CHþ2 ), 87 [OCH2CH2CH
ðCH3Þþ], 71 [CH2CH2CHðCH3Þþ], 59 (CH3OCOþ), 43
Kinetics. The kinetic determinations were carried out in a
static system as reported18,19 with an Omega DP41-TC/
DP41-RTD high-performance digital temperature indica-
tor. The rate coefficients were determined manometri-
cally or by the quantitative analyses of the starting
material and/or by the corresponding olefinic product.
The temperature was controlled by a Shinko DC-PS
resistance thermometer controller and an Omega
SSR280A45 solid-state relay, maintained at ꢂ0.2 ꢀC
and measured with a calibrated platinum–platinum–
13% rhodium thermocouple. No temperature gradient
was observed along the reaction vessel with a syringe
through a silicone-rubber septum. The amount of sub-
strate used for each reaction was ꢅ0.05–0.2 ml.
2
2
[CHðCH3Þþ].
2
3,3-Dimethyl-1-butyl methylcarbonate. B.p. 46 ꢀC at
2 Torr, yield 99%. H NMR, ꢄ 0.9 (s, 9H), 1.6 (t, 2H),
1
3.8 (s, 3H), 4.2 (t, 2H). MS, m/z 160 (Mþ), 103
(CH3OCOOCH2CHþ), 85 [CH2CH2CðCH3Þþ], 57
2
3
[CðCH3Þþ].
3
2-Bromoethyl methylcarbonate. B.p. 55 ꢀC at 2 Torr,
1
yield 81%. þH NMR, ꢄ 3.8 (m, 2H), 4.0 (m, 2H). MS,
m/z 183 (M ), 103 (CH3OCOOCH2CHþ2 ), 44 (CO2).
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 839–848

848
G. CHUCHANI ET AL.
´
6. Herize A, Domınguez RM, Rotinov A, Nun˜ez O, Chuchani G.
J. Phys. Org. Chem. 1999; 12: 201–206.
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