108
MONSALVE ET AL.
EXPERIMENTAL
the static reaction system. However, these compounds
as intermediates from pyrolysis of the corresponding
ethyl ester may through consecutive reaction give ki-
netic and mechanistic information during the process
of elimination.
The substrates ethyl 3-(piperidin-1-yl) propionate
(
(
Aldrich), ethyl 1-methylpiperidine-3-carboxylate
Aldrich), andethylpiperidine-3-carboxylate(Aldrich)
were distilled until 98.6% purity (GC-MS: Saturn
2
3
000, Varian, with a DB-5MS capillary column
0 m × 0.53 mm. i.d., 0.53 µm film thickness).
Ethyl 3-(Piperidin-1-yl) Propanoate
The pure samples of β-(1-piperidyl)propanoic acid
The elimination products of reaction (4)
(Aldrich), 3-piperidine carboxylic acid (Aldrich), and
1-methyl-3-piperidine carboxylic acid (by treating
ethyl 1-methyl-3-piperidine carboxylate with NaOH
and then HCl) could not be examined in the gas
phase because they are solid, decompose on heating,
and are insoluble in most of the inert organic sol-
vents. The quantitative chromatographic analysis of
ethylene was determined by using a gas chromato-
graph HP 5710A with a Porapak Q (80–100 mesh).
The identifications of the products piperidine and
N-methylpiperidine and N-ethylpiperidine were made
by comparing with true authentic samples bought from
Aldrich and in a GC-MS (Saturn 2000, Varian with
a DB-5MS capillary column 30 m × 0.25 mm. i.d.,
(
4)
demand a theoretical stoichiometry P /P = 3.0,
f
0
where P and P are the final and initial pres-
f
0
sure, respectively. The average experimental P /P0
at four different temperatures and 10 half-lives is
3.0 (Table I). Further confirmation of stoichiome-
try (4), up to 60% decomposition, was obtained
by comparing the pressure measurements with the
quantitative GLC analysis of ethylene formation
f
0
.25 µm).
(
Table II).
Kinetics
The reaction was found to be homogeneous since
no significant effects on the rates were obtained on
using both clean Pyrex and seasoned Pyrex ves-
sels with a surface-to-volume ration of 6.0 relative
to the normal clean and seasoned vessels in these
experiments (Table III). The effect of different pro-
portions of toluene inhibitor had no effect on the
rates (Table IV). No induction period was observed,
and the rates were reproducible with a relative stan-
dard deviation of not greater that 5% at a given
temperature.
The kinetic determinations were performed in a static
reaction system as described before [14–16]. At each
temperature, 6–9 runs are carried out in our experi-
ments. The rate coefficients for the ethyl ester decom-
position were calculated from the pressure increase
manometrically and/or by formation of ethylene prod-
◦
uct. The temperature was maintained within ± 0.2 C
through control with a Shinko DIC-PS 23TR resis-
tance thermometer and was measured with a calibrated
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 sili-
cone rubber septum. The amount of substrate used for
each reaction was ∼0.05–0.1 mL.
The rate coefficients for ethyl 3-(piperidin-1-
yl)propionate, calculated from k1 = (2.303/t) log
[
2P0/(3P0 − Pt)], were found to be independent of the
initial pressures (Table V). A plot of log (3P0 – Pt)
against time t gave a good straight line up to 55–60%
reaction. The temperature dependence of the rate co-
efficients and the corresponding Arrhenius equation
shown in Table VI, where 90% confidence limits from
a least-squares procedure, are given.
RESULTS AND DISCUSSION
Authentic samples of β-(1-piperidyl)propanoic acid
(
1
Aldrich), 3-piperidine carboxylic acid (Aldrich), and
-methyl-3-piperidine carboxylic acid (from heating
Ethyl 1-Methylpiperidine-3-carboxylate
ethyl 1-methyl-3-piperidine carboxylate with NaOH
and then HCl) were difficult to examine in the gas
phase. These solid materials were found to be insoluble
in most of the organic solvents limiting their studies on
The product formation in the molecular elimi-
nation of ethyl-1-methylpiperidine-3-carboxylate is
N-methylpiperidine, ethylene, and carbon dioxide
(reaction (5)).