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ESPITIA ET AL.
the substrate with substituent Z and k0 is the rate of
the substrate of reference with substituent Z = CH3)
versus Taft original σ∗ (polar substituent constant) val-
ues yielded a slope of ρ∗ (reaction constant) = 0.315; r
(correlation coefficient) = 0.976, at 673.15 K (400◦C)
[4]. This study supported the general concept that
electron-withdrawing groups at the acyl or acid side of
the ester enhance the elimination rate, while electron-
releasing groups tend to decrease it. The Taft–Topsom
method [5] of log k/kH = σαρα + σFρF + σRρR takes
into consideration the steric (σα), electronic (σF),
and resonance (σR) contributions in the quantitative
study of structural and substituent effects on chem-
ical reactivity. Using this treatment, a correlation
of log k/k0 = (2.09 0.11)σF at 673.15 K (400◦C),
r = 0.979, SD = 0.078, was obtained. The polarizabil-
ity effect (σα) and the resonance interactions (σR) were
found to be minimal and may be neglected, while the
field inductive effect (σF) appears to be the important
factor influencing the elimination rates of these esters.
Although the elimination kinetics of a large number
of ZCOOCH2CH3 species have been examined [3], the
effect of a heteroaromatic substituent at the acid side of
the ethyl ester, such as 2-furyl and 2-thienyl groups, has
not been studied. Therefore, the present work aimed at
examining the extent to which these heteroaromatic
groups may affect the elimination kinetics of ethyl 2-
furoate and ethyl 2-thiophenecarboxylate [reaction (2),
step 1]. An additional question is whether the initial
carboxylic acid intermediate will decarboxylate under
the conditions of the reaction, due to assistance of the
nucleophilicity or basicity of the heteroatom at C-2
for the abstraction of the H of the COOH group. De-
carboxylation could also be promoted by double bond
character or resonance interaction with the heteroaro-
matic nuclei [reaction (2), step 2].
ness) were used. The quantitative chromatographic
analysis of ethylene product was determined by us-
ing a Gas Chromatograph Varian 3600× with a cap-
illary column GS-Q 30 mm × 0.53 mm. The ther-
mal conductivity detector was used to identify CO2
gas. Identification of the products 2-furoic acid and 2-
thiophene carboxylic acid were made by comparison
with authentic samples (Aldrich) and by GC-MS anal-
ysis (Saturn 2000, Varian with a DB-5MS capillary
column 30 m × 0.25 mm. i.d., 0.25 μm). In some ex-
periments propene was used as an inhibitor to suppress
free radical reactions. The reaction vessel was seasoned
by the products of decomposition of allyl bromide to
prevent surface effects on the decomposition of the
substrate.
Kinetics
The kinetic experiments were carried out in a static
reaction system as reported before [6–8] and depicted
in Fig. 1. The reaction vessel (14) of approximately
250 mL is enclosed in a thermostatic furnace (15),
which is a cylindrical aluminum block 20.5 cm in di-
ameter and 36 cm high with a central circular well 10
cm in diameter. A Nichrome heating coil of resistance
90 ohms was wound on it after insulation with asbestos
(19). This furnace (15) is united to a glass diaphragm
(12) and to a mercury manometer (9). The substrate for
analysis is injected to the reaction vessel (14) with a sy-
ringe Perfektum of 1.0 mL through a capillary silicone
rubber septum (11). The increase of pressure in the sys-
tem due to the thermal decomposition of the substrate
causes a small deformation of the glass diaphragm (12),
which is then compensated with the introduction of air
by using a valve (8). The diaphragm (12) lighted by a
lamp (10) produces an indicating line that moves from
the reference point when pressure increases. The varia-
tion of pressure increase seen in the mercury manome-
ter (9) with time is measured with a chronometer. The
initial pressure of the reaction at time zero is estimated
by extrapolation in a graphic of pressure versus time.
The temperature was maintained within 273.35 K
(0.2◦C) through control with a Shinko DIC-PS 23TR
resistance thermometer (16). The temperatures were
determined by using a calibrated iron–constantan ther-
mocouple (13) and measured in a Digital Multimeter
Omega 3465B (17). The vacuum of this static system is
accomplished by a rotatory vacuum pump (1) Hitachi
LTD 3VP-C2 and can reach approximately 5.0 × 10−4
Torr. The vacuum may be improved when using the
Mercury diffusion pump (2) Edwards EMG 150 W.
The pyrolysis products were trapped in the reactant
storage reservoir (4). The amount of substrate used for
each reaction was ∼0.05–0.1 mL.
1
COOCH2CH3
+
H2C CH2
X
COOH
X
>380
slowly
C
X = O,
S
2
+
CO2
X
(2)
EXPERIMENTAL
The substrates ethyl 2-furoate (Aldrich) and ethyl
2-thiophenecarboxylate (Aldrich) of 99.0% purity
(GC-MS: Saturn 2000 Varian, with a DB-5MS capil-
lary column 30 m × 0.53 mm. i.d., 0.53 μm film thick-
International Journal of Chemical Kinetics DOI 10.1002/kin