F. ROSAS ET AL.
tained within ꢁ0.2 8C through control with a Shinko DIC-PS 23TR
resistance thermometer. The vacuum of this static system is
accomplished by a rotatory vacuum pump Hitachi LTD 3VP – C2
and can reach approximately 5.0 ꢃ 10ꢀ4 Torr. The vacuum may
be improved when using the Mercury diffusion pump Edwards
EMG 150 W. The pyrolysis products were trapped in the reactant
storage reservoir. The amount of substrate used for each reaction
was ꢄ0.05–0.1 ml.
CONCLUSIONS
The products of the thermal elimination of 2-methoxytetra-
hydropyran are methanol and dihydropyran. The reaction is
unimolecular, homogeneous, and follows first-order reaction
kinetics. The observed rate coefficient is represented by the
equation: log k (sꢀ1) ¼ (13.95 ꢁ 0.15) ꢀ (223.1 ꢁ 2.1) (kJ molꢀ1
)
(2.303RT)ꢀ1. The reactant 2-methoxytetra-hydropyran exists in
two low energy chair-like conformations, with the 2-methoxy
group in axial or equatorial position. Theoretical calculations of
this reaction were carried out for two putative mechanisms from
the two reactant conformations, using DFT methods. The small
energy difference between the conformers, axial–equatorial
indicates that they are in thermal equilibrium. Calculated
averages of the activation parameters considering the equi-
librium populations of reactant and TS are in good agreement
with the experimental values. Better parameters were obtained
using B3LYP/6-31þþG(d,p) method. The use of diffuse function is
important in the treatment of this reaction. These results suggest
that the two reactant conformers are in equilibrium. NBO charges
and bond orders imply that the rate-determining step is the
breaking of the Ca—O bond while other reaction coordinates
show less progress in the TS, implying the reaction is polar and
moderately asynchronous.
Acknowledgements
T.C. thanks the Consejo de Desarrollo Cient´ıfico y Human´ıstico
(C.D.C.H.) for Grant No. PG-03-00-6499-2006.
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EXPERIMENTAL
The substrate 2-methoxytetrydropyran of 98% purity (GC-MS
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Kinetics
The kinetic determinations were performed in a static reaction
system as reported[24–26] at each temperature; several runs are
carried out in our experiments. According to the static system
figure described before,[26] the reaction vessel of approximately
250 ml is enclosed in a thermostatic furnace which is a cylindrical
aluminum block 20.5 cm in diameter and 36 cm high with a
central circular well 10 cm diameter. A nichrome heating coil of
resistance 90 V was wound on it after insulation with asbestos.
This furnace is united to a glass diaphragm and to a mercury
manometer. The substrate for analysis is injected to the reaction
vessel with a syringe Perfektum of 1.0 ml through a capillary
silicone rubber septum. The increase of pressure in the system
due to the thermal decomposition of the substrate causes a small
deformation of the glass diaphragm, which is then compensated
with the introduction of air by using a valve. The diaphragm
lighted by a lamp produces an indicating line which moves from
the reference point when pressure increases. The variation of
pressure increase seen in the mercury manometer 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 temperatures were determined by using a
calibrated iron-constantan thermocouple and measured in a
Digital Multimeter Omega 3465B. The temperature was main-
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[26] R. M. Dominguez, A. Herize, A. Rotinov, A. Alvarez-Aular, G. Visbal, G.
Chuchani, J. Phys. Org. Chem. 2004, 17, 399–408.
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J. Phys. Org. Chem. 2010, 23 1127–1136