Homogeneous, unimolecular gas-phase elimination kinetics of ethyl esters of glyoxylic, 2-oxo-propanoic, and 3-methyl-2-oxo-butanoic acids

Article abstract of DOI:10.1002/kin.20240

The rates of elimination of several ethyl esters of 2-oxo-carboxylic acid were determined in a seasoned static reaction vessel over the temperature range 350-430°C and pressure range 33-240 Torr. The reactions, in the presence of a free-radical inhibitor, are homogeneous, unimolecular, and follow a first-order rate law. The overall and partial rate coefficients are expressed by the Arrhenius equation. Ethyl glyoxalate log k_1(CO2)(s^-1) = (13.43±0.25)-(213.1±3.3) kJ mol^-1 (2.303 RT) ^-1, r = 0.9996 log k_2(CO) (s^-1) = (14.06±0.54) - (232.9±7.0) kJ mol^-1 (2.303 RT) ^-1, r = 0.9986 log k_t(overall) (s^-1) = (13.72±0.25) - (216.1±3.3) kJ mol^-1 (2.303 RT) ^-1, r = 0.9997 Ethyl 2-oxo-propionate log k₁ (s ^-1) = (13.03±0.15) - (205.1±2.0) kJ mol^-1 (2.303 RT)^-1, r = 0.9998 Ethyl 3-methyl-2-oxo-butyrate log k ₁ (s^-1) = (12.58±0.31)-(198.4±4.1) kJ mol^-1 (2.303 RT)^-1, r = 0.9994 The mechanisms of these elimination reactions are described in terms of concerted cyclic transition state structures.

Full text of DOI:10.1002/kin.20240

Homogeneous,
Unimolecular Gas-Phase
Elimination Kinetics of Ethyl
Esters of Glyoxylic,
2
3
-Oxo-Propanoic, and
-Methyl-2-Oxo-Butanoic
Acids
ANDREINA REYES, ROSA M. DOMINGUEZ, MARIA TOSTA, ARMANDO HERIZE,
GABRIEL CHUCHANI
Centro de Qu ´ı mica, Instituto Venezolano de Investigaciones Cient ´ı ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 28 July 2006; revised 9 October 2006, 30 November 2006; accepted 13 December 2006
DOI 10.1002/kin.20240
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The rates of elimination of several ethyl esters of 2-oxo-carboxylic acid were deter-
◦
mined in a seasoned static reaction vessel over the temperature range 350–430 C and pressure
range 33–240 Torr. The reactions, in the presence of a free-radical inhibitor, are homogeneous,
unimolecular, and follow a first-order rate law. The overall and partial rate coefficients are
expressed by the Arrhenius equation.
Ethyl glyoxalate
log k1(CO ) (s ) = (13.43 ± 0.25) − (213.1 ± 3.3) kJ mol (2.303 RT )−1, r = 0.9996
−
1
−1
2
log k2(CO) (s 1) = (14.06 ± 0.54) − (232.9 ± 7.0) kJ mol (2.303 RT ) , r = 0.9986
−
−1
−1
−
1
−1
−1
log kt(overall) (s ) = (13.72 ± 0.25) − (216.1 ± 3.3) kJ mol (2.303 RT ) , r = 0.9997
Ethyl 2-oxo-propionate
log k1 (s 1) = (13.03 ± 0.15) − (205.1 ± 2.0) kJ mol−1(2.303 RT )−1, r = 0.9998
−
Correspondence to: Gabriel Chuchani; e-mail: chuchani@
ivic.ve.
c 2007 Wiley Periodicals, Inc.
ꢀ

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF ETHYL ESTERS
269
Ethyl 3-methyl-2-oxo-butyrate
log k1 (s 1) = (12.58 ± 0.31) − (198.4 ± 4.1) kJ mol−1(2.303 RT )−1, r = 0.9994
−
The mechanisms of these elimination reactions are described in terms of concerted cyclic
transition state structures. ꢀC 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 268–275, 2007
INTRODUCTION
electron-releasing groups decrease it. Moreover, using
the Taft–Topsom treatment [4], a resonance interaction
and polarizability effects were found to be minimal and
therefore neglected. However, the field inductive effect
σF appears to be the main factor affecting the elimina-
The homogeneous unimolecular gas phase elimination
of ester of carboxylicacidsisknowntoproceedthrough
a six-membered cyclic transition state type of mech-
anism to give the corresponding carboxylic acid and
the olefin, respectively (reaction (1)) [1,2]. The pres-
ence of a Cβ H bond at the alkyl side of the ester is
necessary for molecular elimination.
tion rates of this ester: log k/k0 = (2.09 ± 0.11)σF at
◦
4
00 C, r = 0.979, SD = 0.078.
The reported kinetic parameters and the compara-
tive rates of different substituents other than carbon at
the acid side of organic ethyl esters [5] give a similar
mechanism as described in reaction (1). These organic
esters showed a good Taft–Topsom correlation as de-
picted in Fig. 1.
O
C
H
C
β
H
O
O
C
C
C
Z
O
C
ZCOOH
+
C
C
α
Z
Z=Substituent
The negative value of ρ = −0.68 suggests a mod-
α
(1)
est participation of the steric effect. The greatest ab-
solute value of ρF− = + 2.57 of the field or electronic
Among the organic esters that contribute to the
mechanistic consideration described in reaction (1) are
those found from the gas-phase elimination of a con-
siderable number of ethyl esters with alkyls attached
to the acid side of α-substituted carboxylic acid [3].
A correlation of these esters log k/k0 versus Taft
effect seems to have the most important influence on
−
the elimination process. The value of ρR = −1.18 is
believed to confirm the interaction of the substituent Z
with an incipient negative reaction center and implies
a favorable effect for the abstraction of the β-hydrogen
of the ethyl ester by the oxygen carbonyl in the transi-
tion state.
∗
∗
original σ values yielded ρ = 0.315 (r = 0.976, at
◦
4
00 C) [4]. This result implied the general argument
that electron-withdrawing groups at the acyl or acid
side of the ester enhance the elimination rate, whereas
Substrates with the interposition of a polar group
such as the carbonyl between the substituents shown
Figure 1 Taft–Topsom correlation for the pyrolysis of ZCOOCH2CH3 log k/k0 = −(0.68 ± 0.12)σα + (2.57 ± 0.12)σF–
−
◦
(
1.18 ± 0.27)σ (r = 0.984, sd = 0.119 at 400 C).
R
International Journal of Chemical Kinetics DOI 10.1002/kin

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70
REYES ET AL.
◦
Table I Comparative Theoretical and Experimental Activation Parameters at 300 C
‡
‡
‡
4
Ea
(kJ/mol)
ꢀH
ꢀS
ꢀG
log A
10 k1
−
1
−1
Method
(kJ/mol)
(J/mol K)
(kJ/mol)
(s
)
(s
)
PM3
MP2/6-3G*
Experimental
174.6
173.2
174.3
169.8
168.5
169.5
–16.7
–14.0
–14.9
179.4
176.5
178.1
12.64
12.78
12.73
5.29
9.81
6.94
in Fig. 1 and the acid side of ethyl ester, that is
ethyl ester of 2-oxo-carboxylic acids, are interesting
to examine for their elimination kinetics in the gas
phase.
Several works related to 2-oxo substrates are found
in the papers of Clark [6–8] on the kinetics of de-
carboxylation of oxalic and oxamic acids in several
solvents (reaction (2)).
(
5)
Recently, a few ethyl esters of 2-oxo-carboxylic
acids with a nitrogen atom attached to the acid side,
that is, ethyl oxamate, ethyl N,N-dimethyloxamate,
and ethyl oxanilinate [13], were reported.
O
C
O
C
Solvent
O
H N
OH
H
C
NH2
+ CO2
2
o
1
27–160 C
(
6)
7)
Solvent: Aniline
o-Toluidine
Quinoline
(2)
8
-Methylquinoline
Dimethylsulfonate
Triethylphosphate
O
Later, Lapidus et al. [9] determined the kinetics and
stoichiometry of oxalic acid decomposition in the gas
phase and the mechanism was described as in reaction
PhNH
1
PhNH OCH2CH3 + CO
OCH CH3
2
O
O
2
3
CH2 CH2 + CO2
(
(
3).
PhNCO + HOCH CH3
+
PhNH2
2
O
C
O
C
27–157o
C
O
1
HO
OH
H
C
OH
+ CO2
O
O
(
CH ) N
3 2
1
(
CH ) N
3 2
+
CH2 CH2
OCH CH
3
HO
O
(3)
2
OH
O
C
C
O
O
O
2
H
Taylor carried out the thermal decomposition of
pyruvic and benzoylformic acids [10,11]. The mecha-
nism of these reactions was thought to proceed through
a four-membered cyclic transition state to give carbon
dioxide and the corresponding aldehyde (reaction (4)).
(CH ) N
H
+ CO
3
2
2
(8)
O
According to the elimination products of ethyl
oxamate (reaction (6)) and ethyl oxanilinate (reac-
tion (7)), decarbonylation was thought to be the
first step of decomposition and the corresponding
ethyl ester intermediate underwent subsequent par-
allel elimination (reactions (6) and (7)). The ethyl
N,N-dimethyloxamate pyrolysis gives different elim-
ination pathways (reaction (8)). This result was sur-
prising, since decarbonylation was expected to be the
first step of the reaction. This fact suggested the need
for further work with regard to the influence of the
(CH3)2N group on the ethyl ester of 2-oxo-carboxylic
acid in order to explain such difference in the
mechanism.
O
C
O
C
RCOCOOH
R
RCOH + CO
2
R = –Me, –C H
H
O
6
5
(4)
A joint theoretical and experimental study on the
homogeneous, unimolecular gas-phase elimination ki-
netics of methyl oxalyl chloride was reported [12].
The reaction was found to proceed in a concerted
semipolar mechanism as described in reaction (5).
The semiempirical PM3 and MP2/6-31G calculations
have been found to be in good agreement with experi-
ments (Table I).
∗
International Journal of Chemical Kinetics DOI 10.1002/kin

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF ETHYL ESTERS
271
With the background described above, the sub-
stituents given in Fig. 1 are to be examined in the
present work, that is, H, CH3, and (CH3)2CH groups.
materials were all injected directly into the reaction
vessel with a syringe through a silicone rubber sep-
tum. The amount of substrate used for each reaction
was ∼0.05–0.2 mL.
EXPERIMENTAL
RESULTS AND DISCUSSION
Ethyl Glyoxalate
Ethyl glyoxalate in 50% toluene solution (Fluka,
St. Louis, MO, USA), ethyl 2-oxo-propanoate (98%
purity, Aldrich, St. Louis, MO, USA), and ethyl 3-
methyl-2-oxo-butyrate (97% purity, Aldrich, St. Louis,
MO, USA) were employed. The identification and
purity of these substrates were verified by GC–MS:
Saturn 2000, Varian, with a DB-5MS capillary column
The stoichiometry based on reaction (9) cannot be
determined from the pressure ratio of Pf/P0, where
Pf is the final and P0 is the initial pressure, respec-
tively. The average experimental Pf/P0 at 4 different
temperatures and 10 half-lives was 2.7. However, an
estimation of stoichiometry (9), up to 70% decom-
position, was possible from quantitative formation
of CO and CO2 gases (Table II). Consequently, total
ethylene formation (reaction (9)) is needed in order to
estimate CO and CO2 gases. The quantitative amount
of CO2 gas, i.e., k1, is equivalent to the pressure of
ethylene formation from ethyl glyoxylate decom-
position, i.e., Ptotal ethylene = Pethylene from ethyl formate +
3
0 m × 0.25 mm. i.d., 0.25-µm film thickness. The
products ethyl formate, formic acid, ethylene, acetalde-
hyde, isobutyraldehyde, ethylene, and carbon dioxide
were identified in a GC–MS (Saturn 2000, Varian) with
a DB-5MS capillary column 30 m × 0.25 mm. i.d.,
0.25-µm film thickness. The quantitative analysis of
the product ethylene (Matheson) and CO2 was carried
out by using a gas chromatograph Varian 3600X with
a thermal conductivity detector (capillary column: GS-
Q, 30 m long and 0.53 i.d., helium gas carrier).
Ptotal CO . In order to estimate CO gas, we have
2
−k3t
Ptotal ethyl formate = PCO = (Ptotal ethylene − PCO )/1 − e
,
2
where k3 is the rate coefficient of ethyl formate pyrol-
ysis as reported in the literature [17]. In this manner,
the k2 value of decarbonylation was obtained from
the decomposition of ethyl glyoxylate. The rate co-
Kinetics
The kinetic experiments were performed in a static
reaction system as reported in [14–16]. The rate
coefficients were calculated from the pressure in-
crease manometrically, or by the quantitative chro-
matographic analyses of ethyl formate, ethylene, and
CO2 products. The temperature was controlled by
a Shinko DC-PS resistance thermometer controller
−4
efficient of ethyl formate pyrolysis, k = 1.64 × 10
3
◦
at 390 C, is not negligible. Therefore, the formation
of ethyl formate or CO gas from ethyl glyoxylate
must be calculated as explained above. The total rate
coefficient for the decomposition of ethyl glyoxylate
was calculated from the sum k + k . The elimination
1
2
◦
maintained at ± 0.2 C and measured with a calibrated
products within the range of rate determination were
ethyl formate, formic acid, formaldehyde, ethylene,
CO, and CO2.
iron constantan thermocouple. No temperature gradi-
ent was observed along the reaction vessel. The starting
Table II Stoichiometry of the Reaction
◦
Substrate
Temperature ( C)
390.0
Parameters
Time (min)
Values
Ethyl glyoxalate
5
15
20
30
45
CO2(%) (GC)
CO formation (%)
Total (%) (GC)
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GLC)
Time (min)
11.9
1.3
13.2
3
20.0
22.6
2
34.7
5.5
40.2
5
30.0
28.3
3
43.6
6.7
50.3
7
40.0
41.5
5
54.0
8.7
62.7
10
50.0
51.2
7
72.2
11.9
84.1
13
60.8
59.6
10
a
Ethyl 2-oxo-propionate
398.0
399.5
Ethyl 3-methyl-2-oxo-butyrate
Reaction (%) (pressure)
Ethylene (%) (GLC)
16.6
17.3
23.6
23.5
34.2
36.7
44.9
43.8
60.6
54.6
a
CO formation was estimated from PCO = (Ptotal ethylene– PCO2 )/1 – e−k3t
.
International Journal of Chemical Kinetics DOI 10.1002/kin

2
72
REYES ET AL.
Table III Homogeneity of the Elimination Reaction
1a
S/V = 6.2 cm−1a
Substrate
Parameter
S/V = 1 cm−
◦
4
−1
Ethyl glyoxalate at 399.6 C
10 k1 CO2 (s )
7.41
0.90
21.72
14.87
7.92
1.12
21.93
14.95
4
−1
)
1
0 k2 CO (s
◦
4
−1
Ethyl 2-oxo-propionate at 410.2 C
Ethyl 3-methyl-2-oxo-butyrate at 399.6 C
10 k1 (s
)
)
◦
4
−1
10 k1 (s
S is the Surface area; V is the volume.
a
Vessel seasoned with the products decomposition of allyl bromide.
O
O
C
was observed. The rate coefficients were reproducible
with a relative standard deviation of less than ± 5% at
a given temperature.
The rate coefficient of this elimination reaction was
found to be invariable to initial pressures (Table V),
and the first-order rate for decarboxylation was calcu-
O
C
O
C
1
H
C
OCH CH3
+
H C CH2
2
H
OH
2
2
O
CH O
+
CO2
2
HC OCH CH
+
CO
2
3
lated from k1 = −(2.303/t) log[(P0 − PCO )/P0] and
2
3
for decarbonylation [17] from k2 = −(2.303/t) log
(9)
[(P − P )/P ], where P = (P
− P )/
0
CO
−k t
0
CO
total ethylene
CO2
HCOOH + CH2
CH2
9
−40.010 cal/RT
3
[
1 − e ] and k3 = 2.52 × 10 e
. The
The effect of the surface on the rate of elimination
variation in the rate coefficients with temperature
and the corresponding Arrhenius equation are given
in Table VI (90% confidence coefficient from the
least-squares procedure).
was carried out 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 III). Packed and unpacked clean
Pyrex vessel had a marked heterogeneous effect, and
unreliable and irreproducible k values were obtained.
However, packed and unpacked Pyrex vessel seasoned
with allyl bromide showed no effect on the reaction
rates. The pyrolysis experiments of this substrate
were performed in the presence of an equal amount
of the free-radical suppressor toluene in order to
prevent any possible free-radical reaction. The effect
of different proportions of toluene on the elimination
reaction is shown in Table IV. No induction period
Ethyl 2-Oxo-Propionate and Ethyl
3-Methyl-2-Oxo-Butyrate
Theoretical stoichiometry (10) for the gas-phase elim-
ination of the above-mentioned compounds requires
P /P = 3.0. The average experimental P /P value
f
0
f
0
at 4 different temperatures and 10 half-lives for both
substrates was 2.9. Confirmation of the stoichiome-
try of reaction (10), up to 60% decomposition, was
Table IV Effect of the Free-Radical Chain Inhibitor Toluene on Rates
◦
Psa (Torr)
Pib (Torr)
4
−1
4
−1
)
Substrate
Temperature ( C)
Pi/Ps
10 k1 (s
)
10 k2 (s
c
c
d
e
Ethyl glyoxalate
389.7
398.6
399.4
43
4.22
4.77
4.74
0.42
d
e
4
5
5
9.5
0
2
28
84
118
–
142
170
179.5
–
0.6
1.7
2.3
–
1.7
2.2
3.0
–
0.63
d
e
0.58
d
e
4.74
0.57
Ethyl 2-oxo-propionate
79.5
13.60
12.99
12.66
12.47
15.56
14.95
14.86
14.97
80.5
76.5
59
Ethyl 3-methyl-2-oxo-butyrate
63
59
56
50
71
131
149
1.2
2.3
3.0
a
P
s
is the pressure of the substrate.
is the pressure of the free-radical inhibitor.
Substrate dissolved in 50% toluene.
value of CO formation.
value of CO formation.
b
c
d
e
P
i
k
1
2
k
2
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HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF ETHYL ESTERS
273
Table V Invariability of the Rate Coefficients from Initial Pressure
◦
Substrate
Temperature ( C)
Parameters
Values
Ethyl glyoxalate
410.5
P0 (Torr)
32.5
14.40
1.76
29
21.98
46.5
26.04
44
51
68
14.32
1.68
105
21.58
98.5
78.5
14.14
1.88
4
−1
)
1
1
0 k1(CO ) (s
14.08
1.86
55.5
21.87
59
14.47
1.78
71
21.56
78
2
4
−1
)
0 k2(CO) (s
Ethyl 2-oxo-propionate
410.8
409.3
P0 (Torr)
4
−1
1
0 k1 (s
P0 (Torr)
)
)
Ethyl 3-methyl-2-oxo-butyrate
4
−1
1
0 k1 (s
26.15
25.12
24.87
Table VI Variation of the Rate Coefficients with Temperature
◦
Ethyl glyoxalate
Temperature ( C) 389.9 399.4 410.3 419.5 430.3
4
−1
)
)
1
1
0 k1(CO ) (s
4.56
0.52
7.41 14.38 23.30 41.09
2
4
−1
0 k2(CO) (s
0.90
1.84 3.39
5.55
−
1
−1
−1
Rate equation log k1(CO ) (s ) = (13.43 ± 0.25) − (213.1 ± 3.3) kJ mol (2.303 RT) , r = 0.9996
2
−
1
−1
−1
−1
−1
Rate equation log k2(CO) (s ) = (14.06 ± 0.54) − (232.9 ± 7.0) kJ mol (2.303 RT) , r = 0.9986
−
1
Rate equation log kt(overall) (s ) = (13.72 ± 0.25) − (216.1 ± 3.3) kJ mol (2.303 RT) , r = 0.9997
◦
Ethyl 2-oxo-propionate
Temperature ( C) 369.1 378.6 389.6 398.4 410.0 418.9
429.6
62.42
4
−1
1
0 k1 (s
)
2.31
3.88
7.23 12.71 21.78 35.88
Rate equation log k1 (s 1) = (13.03 ± 0.15) − (205.1 ± 2.0) kJ mol (2.303 RT) , r = 0.9998
−
−1
−1
◦
Ethyl 3-methyl-2-oxo-butyrate Temperature ( C) 380.2 389.7 399.6 409.4 420.0
4
−1
1
0 k1 (s
)
5.38
8.48 14.95 25.62 42.38
Rate equation log k1 (s 1) = (12.58 ± 0.31) − (198.4 ± 4.1) kJ mol (2.303 RT) , r = 0.9994
−
−1
−1
possible when comparing between pressure measure-
ments and the quantitative analyses of ethylene gas
Table IV. According to this result, the elimination ex-
periments were carried out in the presence of twice the
amount of toluene with respect to these 2-oxo-esters
in order to prevent any possible radical chain reac-
tions. No induction period was observed, and the rates
were reproducible with a relative standard deviation
not greater than ± 5% at a given temperature.
(Table II).
O
C
O
C
O
C
O
C
1
R
OCH CH3
R
OH + H2C CH2
2
R = CH3, (CH3)2CH
(10)
The first-order rate coefficient of these substrates
calculated from k1 = −(2.303/t) log 2P0/(3P0 − Pt)
was independent of the initial pressure of the substrate
(Table V). A plot of log (3P0 − Pt) versus time (t)
gave a good straight line for up to 60% of the reaction.
The temperature dependence of the rate coefficients
and the corresponding Arrhenius equation are given
in Table VI (90% confidence coefficient from the
least-squares procedure).
R
CHO + CO2
The homogeneity of these reactions (10) was exam-
ined 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 III). The packed and unpacked clean Pyrex
vessel had a marked heterogeneous effect, and unre-
liable k values were obtained. Yet, the packed and
unpacked Pyrex vessel seasoned with allyl bromide
gave no difference in rates. The effect of adding dif-
ferent proportions of the inhibitor toluene is shown in
The comparative kinetic and thermodynamic pa-
◦
rameters at 400 C are shown in Table VII.
◦
Table VII Comparative Kinetic and Thermodynamic Parameters at 400 C
4
−1
−1
ꢁ= ꢁ=
ꢁ=
Z
k1 × 10 (s ) Ea (kJ/mol) log A (s
)
ꢀS (J/mol K) ꢀH (kJ/mol) ꢀG (kJ/mol)
a
Ethyl glyoxalate
8.91
.77
.96
12.92
15.18
216.1 ± 3.3 13.72 ± 0.25
213.1 ± 3.3 13.43 ± 0.25
232.9 ± 7.0 14.06 ± 0.54
205.1 ± 2.0 13.03 ± 0.15
198.4 ± 4.1 12.58 ± 0.31
2.7
−2.9
9.2
210.5
207.8
233.1
199.5
192.8
208.7
209.7
226.9
206.6
205.7
b
c
7
0
Ethyl 2-oxo-propionate
Ethyl 3-methyl-2-oxo-butyrate
–10.6
–19.2
a
Overall rate.
Rate of decarboxylation.
Rate of decarbonylation.
b
c
International Journal of Chemical Kinetics DOI 10.1002/kin

2
74
REYES ET AL.
CONCLUSION
propionate (Step 1, reaction (11)), the intermediate 2-
oxo-propionic acid or pyruvic acid, was reported to
decompose in the gas phase [18] into acetaldehyde
Electron-withdrawing substituents, such as chlorine in
methyl oxalyl chloride and amino in ethyl oxamate and
ethyl oxanilinate, proceed to a decarbonylation [12,13]
process (Step 1, reactions (5)–(7)). An exception is
ethyl N,N-dimethyl oxamate (reaction (8)). However,
by changing the electronic transmission of the sub-
stituent to electron-releasing groups such as methyl
◦
and CO2 in the temperature range 284–334 C with
−
1
an Arrhenius expression log k1 (s ) = 13.53 − 172.4
−1
−1
kJ mol (2.303 RT) . These data suggest that the
pyruvic acid intermediate, under the condition of the
present work, changes to decarboxylate rapidly. The
effect of the increase in rate on the electron release
of the alkyl groups CH3 and (CH3)2CH of the oxy-
esters may be due to the nucleophilic assistance of the
oxygen of the carbonyl to the incipient positive Cγ
in the transition state (reaction (11)). Consequently,
the greater the electron release of the alkyl group R,
the greater the nucleophilicity of oxygen carbonyl,
the more the stabilization of the positive Cγ . There-
fore, an augmentation on the rate of elimination is
obtained.
(ethyl 2-oxo-propionate) and isopropyl (3-methyl-2-
oxo-butyrate), the elimination process proceeds as or-
ganic esters to give the corresponding carboxylic acid
δ+
δ−
and ethylene. Step 1, Cα · · · O , is a rate-determining
step and through a six-membered cyclic transition state
yields the corresponding 2-oxo-acids (reaction (11)).
These acids, under the reaction conditions, rapidly de-
carboxylate, as suggested before [10,11], to give the
corresponding aldehyde. The product of ethyl 2-oxo-
O
C
O
O
H
β
O
R
1
γ
C
CH2
δ
O
δ
+ H C CH2
2
R
C
O
R
OH
O
α H
O
H
R = CH3, (CH3)2CH,
2
O
O
−CO2
O
R
C
H
C
O
R
H
(11)
The presence of H as a substituent in ethyl glyoxalate gives a parallel reaction (12). Step 1 of the predominant
pathway A is the rate-determining step, which proceeds through a six-membered cyclic transition state to produce
ethylene and glyoxylic acid.
O
H
O
O
B
A
H
O
1
−
δ
O
O
O
CH2
δ−
δ+
O
H
CH
H
2
+
δ
CO
O
H
1
O
O
H
O
OH
+
H2C CH2
O
(very fast)
H
O
−
δ
O
CH2
O
O
δ+
CH2
δ−
+
H
δ
H
H
O
2
(very slow)
2
CO2
H
O
O
+
H2C CH2
H
OH
H
(12)
International Journal of Chemical Kinetics DOI 10.1002/kin

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION KINETICS OF ETHYL ESTERS
275
Under the reaction conditions, the acid decomposes
through a four-membered cyclic transition state to
give formaldehyde and CO2. Since the H substituent
4. (a) Hansch, C.; Leo, A. J. Substituent Constants for
Correlation Analysis in Chemistry and Biology; Wiley:
New York, 1979; (b) Hansch, C.; Leo, A.; Taft, R. Chem
Rev 1991, 91, 165–195.
∗
has been defined in the Taft original σ with a positive
5
. Herize, A.; Dom ´ı nguez, R. M.; Rotinov, A.; Nu n˜ ez, O.;
Chuchani, G. J Phys Org Chem 1999, 12, 201–206.
. Clark, L. J Phys Chem 1961, 65, 659–661.
. Clark, L. J Phys Chem 1961, 65, 180–182.
. Clark, L. J Phys Chem 1961, 65, 1651–1652.
. Lapidus, G.; Barton, D.; Yankwich, P. J Phys Chem
value of +0.49 [19], this means that some electron-
withdrawing property, and therefore a small amount
of decarbonylation process (pathway B), is obtained.
The rate-determining Step 2 in the same pathway
occurs in a similar way as in methyl oxalyl chloride
6
7
8
9
(reaction (5)), which is through a three-membered
1964, 68, 1863.
cyclic transition state to give ethyl formate and CO
gas. Ethyl formate slowly decomposes to produce
formic acid and ethylene.
10. Taylor, R. Int J Chem Kinet 1987, 19, 709–713.
11. Taylor, R. Int J Chem Kinet 1991, 23, 247–250.
12. Cordova, T.; Rotinov, A.; Chuchani, G. J Phys Org Chem
2004, 17, 148–151.
13. Chacin, E. V.; Tosta, M.; Herize, A.; Dominguez, R. M.;
Alvarado, Y.; Chuchani, G. J Phys Org Chem 2005, 18,
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International Journal of Chemical Kinetics DOI 10.1002/kin