The kinetics and mechanisms of gas phase elimination of the ethyl ester of amino acid hydrochlorides

Article abstract of DOI:10.1002/poc.1073

The kinetics of the gas phase elimination of the ethyl ester of four α-amino acid hydrochlorides have been examined over the temperature range of 339-451 °C and pressure range of 8-108 Torr. The reactions, in a static reaction system, are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficient is expressed by the following Arrhenius equations: Glycine ethyl ester hydrochloride: log k₁(sec^-1) = (12.29 ±0.24) - (203.7 ±3.2) kJ mol^-1 (2.303 RT)^-1 Sarcosine ethyl ester hydrochloride: log k₁ (sec^-1) = (13.64 ±0.60) - (215.0 ±7.8) kJ mol^-1 (2.303 RT) ^-1 DL-Alanine ethyl ester hydrochloride: log k₁) (sec ^-1 = (12.49 ± 0.46) - (200.2 ±5.9) kJ mol^-1 (2.303 RT) L-Phenylalanine ethyl ester hydrochloride log k₁(sec ^-1) = (12.49 ±0.09)-(194.4. ±1.1)kJ mol^-1 (2.303 RT)^-1 The elimination of these amino ester hydrochlorides leads to the formation of the corresponding α-amino acid and ethylene. However, the amino acid intermediates, except sarcosine, under the condition of reaction temperatures, undergo an extremely rapid decarboxylation process. These results apparently support previous reported mechanistic consideration where α-amino acids decompose to the corresponding amines and CO₂ gas. Copyright

Full text of DOI:10.1002/poc.1073

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 326–332
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1073
The kinetics and mechanisms of gas phase elimination
of the ethyl ester of amino acid hydrochlorides
Gabriel Chuchani,* Armando Herize, Rosa Mar ´ı a Dom ı´ nguez, Alexandra Rotinov and Maria Tosta
Centro de Qu ´ı mica, Instituto Venezolano de Investigaciones Cient ı´ ficas (I.V.I.C.), Apartado 21827, Caracas 1020-A, Venezuela
Received 7 February 2006; revised 17 March 2006; accepted 25 March 2006
ABSTRACT: The kinetics of the gas phase elimination of the ethyl ester of four a-amino acid hydrochlorides have
been examined over the temperature range of 339–4518C and pressure range of 8–108 Torr. The reactions, in a static
reaction system, are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficient is expressed by
the following Arrhenius equations:
Glycine ethyl ester hydrochloride:
log k1ðsecꢀ1Þ ¼ ð12:29 ꢁ 0:24Þ ꢀ ð203:7 ꢁ 3:2Þ kJ molꢀ1 ð2:303 RTÞꢀ1
Sarcosine ethyl ester hydrochloride:
log k1ðsecꢀ1Þ ¼ ð13:64 ꢁ 0:60Þ ꢀ ð215:0 ꢁ 7:8Þ kJ molꢀ1 ð2:303 RTÞꢀ1
DL-Alanine ethyl ester hydrochloride:
log k1ðsecꢀ1Þ ¼ ð12:49 ꢁ 0:46Þ ꢀ ð200:2 ꢁ 5:9Þ kJ molꢀ1 ð2:303 RTÞ
L-Phenylalanine ethyl ester hydrochloride
log k1ðsecꢀ1Þ ¼ ð12:49 ꢁ 0:09Þ ꢀ ð194:4: ꢁ 1:1Þ kJ molꢀ1 ð2:303 RTÞꢀ1
The elimination of these amino ester hydrochlorides leads to the formation of the corresponding a-amino acid and
ethylene. However, the amino acid intermediates, except sarcosine, under the condition of reaction temperatures,
undergo an extremely rapid decarboxylation process. These results apparently support previous reported mechanistic
consideration where a-amino acids decompose to the corresponding amines and CO gas. Copyright # 2006 John
2
Wiley & Sons, Ltd.
KEYWORDS: elimination; gas-phase kinetics; mechanism; ethyl glycine hydrochloride; ethyl sarcosine hydrochloride;
ethyl DL-alanine; ethyl L-phenyl alanine hydrochloride
INTRODUCTION
The acid product of these esters with a substituent at the
a-position, under the reaction conditions, may further
decompose into another products. In this respect, the gas
The homogeneous unimolecular gas phase eliminations
of ester of carboxylic acids are generally known to
phase elimination of several types of 2-substituted
proceed through a six-membered cyclic transition state
type of mechanism to yield the corresponding carboxylic
3–9
carboxylic acids
have been described, both exper-
imentally and theoretically, in terms of a decarbonylation
process as depicted in reaction (2).
1
acid and the olefin, respectively [reaction (1)] . For
,2
molecular elimination, the presence of a C —H bond at
b
O
O
the alkyl side of the ester is necessary .
C
L
COOH
C
C
C
C
O + HL
O
C
H
C
β
H
O
L
O
O
C
C
C
H
Z
O
C
ZCOOH
+
C
C
α
Z
C=O + CO
Z=Substituent
(1)
L = Leaving Group: Cl, Br, OH, OR, OPh, OAc.
(2)
*
Correspondence to: Prof. G. Chuchani, Centro de Qu ´ı mica, Instituto
Venezolano de Investigaciones Cient ´ı ficas (I.V.I.C.), Apartado 21827,
Caracas 1020-A, Venezuela.
E-mail: chuchani@ivic.ve
However, 2-substituted amino carboxylic acids pro-
ceed a decarboxylation process as reported in the
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

ELIMINATION OF THE ETHYL ESTER OF AMINO ACID HYDROCHLORIDES
327
homogeneous, unimolecular gas phase pyrolysis of N,N-
forming zwitterions species, limits their examination as
neutral molecules in the gas phase. Consequently, the
objective of this work is to investigate on how the nitrogen
atom at the a-position of the carboxylic acid intermedi-
ates would affect the elimination process. Therefore, the
substrate for this investigation are the ethyl ester
hydrochlorides of glycine, sarcosine, DL-alanine, and L-
phenylalanine.
1
0
11
dimethylglycine, picolinic acid, and N-phenylgly-
12
cine [reaction (3)].
(3)
10–12
The above-mentioned substrates
have been found
to be very reactive species in the gas phase. The fact that
neutral a-amino acids decompose rapidly in the gas phase
is supported from the experimental results on the
elimination kinetics of the said compounds when
compared as intermediate products of their corresponding
ethyl ester pyrolysis. However, the gas-phase pyrolysis of
RESULTS AND DISCUSSION
Glycine ethyl ester hydrochloride
The molecular gas phase pyrolytic elimination of this
substrate proceeds according to reaction (4).
13
2
-(N-phenylamino)propanoic acid
was thought to
decarbonylate through a five-membered cyclic transition
state type of mechanism similar to reaction (2).
Consequently, this result differs from the decarboxylation
process of the a-amino acids described in reaction (3).
Moreover, the pyrolysis products of 2-(N-arylamino)pro-
.
ClH NH
2
CH
2
COOCH
2
CH
3
→ [ NH
2
CH
2
2 3
COOCH CH ] + HCl
↓
[
NH
2
CH
2
COOH ] + CH
2
CH
3
↓
2
13
CH
3
NH
+
CO
2
panoic acids were found to yield ArNH , CH CHO, and
2
3
CO, which appears to justify reaction (2).
Recently, the elimination kinetics of ethyl esters of
(4)
14
The stoichiometry of reaction (4) requires that for long
reaction time the final pressure should be four times the
initial pressure, that is, P /P ¼ 4. The average exper-
pipecolinic and N-methylpipecolinic acids in the gas
phase indicated the formation of the corresponding
carboxylic acids and ethylene. The acid intermediate with
the N atom at the a-position of the carboxylic acid, an
amino acid derivative, proceeds to a very rapid
decarboxylation process as described in reaction (3).
This work apparently supports the elimination of neutral
a-amino acids through a five-membered cyclic transition
state to produce the corresponding substituted amine and
f
0
imental results at 4 different temperatures and 10 half-
lives is 3.9 (Table 1). To check stoichiometry (4), up to
5
0% reaction, was possible by comparing the pressure
measurements with the quantitative chromatographic
analysis of ethylene gas (Table 2).
The effect of the surface on the rate of elimination was
examined by carrying out several runs in a vessel with a
surface-to-volume ratio of 6.0 relative to that of the
normal vessel, which is equal to 1.0 (Table 3). Packed and
unpacked Pyrex vessel seasoned with allyl bromide and
the unpacked clean Pyrex Vessel showed no effect on the
reaction rates. However, the packed clean Pyrex vessel
had a small heterogeneous effect. The addition of
different proportion of the free radical inhibitor propene
exhibited no effect on rates (Table 4). Moreover, no
induction period was observed.
CO gas.
2
Because of different products formation of the 2-amino
1
2,13
acids
substrates
3), the present work aimed at determining the kinetic and
as described in reaction (2) with those
9–11,14
undergoing a process as in reaction
(
thermodynamic parameters of the gas phase elimination
of a-amino acids intermediates through a consecutive
reaction from the corresponding ethyl ester hydrochloride
decomposition. The amino ester hydrochlorides were
chosen by virtue that NH Cl decomposes at high
4
The first order rate coefficient for elimination of this
substrate showed no significant variation change of the
initial pressure (Table 5) and the first order plots of log
temperatures into NH and HCl. Under such condition,
3
the NH Cl is attributed to be a strong acid. In this respect,
4
this investigation thought as a challenge to examine if the
aminoester hydrochloride salts may decompose, at high
temperature, into the corresponding free ester and HCl.
Consequently, the ester may then proceed to pyrolytic
elimination as reported. This mode of experimental
approach was applied because most a-amino acids of low
molecular weight are solids and difficult to determine
their elimination kinetics in the gas phase. These
compounds on heating sinter or decompose into a
vitreous materials, and unfortunately are insoluble in
most organic solvents. Their high solubility in water,
(
4P ꢀ P) against time t are linear up to 50% reaction.
0
Further decomposition leads to a relative standard
deviation greater than ꢁ5%. The rate equation of four-
product formation by pressure increase is described as
follows:
ClHꢂNH2CH2COOCH2CH3
P0ꢀP
!
HCl þ CH ¼¼ CH þ CH NH þ CO
2
2
3
P
2
2
P
P
P
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

3
28
G. CHUCHANI ET AL.
Table 1. Ratio of final (P ) to initial (P )
f
0
Substrate
Temperature (8C)
410.0
P0 (Torr)
Pf (Torr)
Pf/P0
Average
3.9
Glycine ethyl esterꢂHCl
40
67.5
65
46
22
40
23.5
31.5
18.6
18.9
18.5
19.5
26.1
19.5
15.8
36.6
31.8
153
264
258
177
3.8
3.9
4.0
3.9
3.2
3.3
3.2
3.3
3.8
3.8
4.2
4.0
3.8
3.8
3.9
3.8
3.9
4
4
4
20.5
30.7
40.5
Sarcosine ethyl esterꢂHCl
399.6
71
3.3
4.0
3.8
4
4
4
10.8
20.2
30.2
130.5
74.5
105
DL-Alanine ethyl esterꢂHCl
379.3
70.7
71.4
78.0
78.5
100.3
74.3
61.3
138.8
122.7
3
4
4
90.1
00.5
09.8
L-Phenylalanine ethyl esterꢂHCl
359.5
3
3
3
3
70.0
80.0
89.6
99.6
Table 2. Stoichiometry of the reaction
Substrate
Temperature (8C)
Parameters
Value
Glycine ethyl esterꢂHCl
430.7
Time (min)
1.5
11.3
12.0
10
25.3
23.2
6
25.9
23.4
3
24.9
28.5
3
5
8
Reaction (%) (pressure)
Ethylene (%) (GLC)
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GLC)
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GLC)
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GLC)
25.0
28.4
15
32.8
27.1
8
33.5
32.5
6
39.2
42.7
38.0
41.5
21
46.4
41.2
10
37.7
36.7
9
44.8
47.7
48.1
53.1
30
56.8
53.8
12
42.7
43.8
12
59.8
61.5
Sarcosine ethyl esterꢂHCl
DL-Alanine ethyl esterꢂHCl
L-Phenylalanine ethyl esterꢂHCl
389.1
399.5
389.9
15
48.6
54.6
18
62.7
65.0
Table 3. Homogeneity of the elimination reaction
Substrate
ꢀ
1 a
)
4
10 k (sec
ꢀ1 b
4
10 k (sec
ꢀ1 c
S/V (cm
)
)
1
1
Glycine ethyl esterꢂHCl at 420.58C
1
6
1
6
1
6
1
6
8.45
9.13
8.26
8.37
11.13d
21.76
7.95
8.10
13.99
13.96
2
-Sarcosice ethyl esterꢂHCl at 401.48C
10.81
21.12
d
DL-Alanine ethyl esterꢂHCl at 400. 58C
8.07
8.14
14.23
13.81
L-Phenylalanine ethyl esterꢂHCl at 390.08C
a
S ¼ surface area; V ¼ volume.
b
Clean Pyrex vessel.
Vessel seasoned with the products decomposition of allyl bromide.
Average k-values
c
d
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

ELIMINATION OF THE ETHYL ESTER OF AMINO ACID HYDROCHLORIDES
Table 4. Effect of the free radical chain inhibitor on rates
329
a
Ps (Torr)
b
Pi (Torr)
4
10 k (sec
ꢀ1
Substrate
Temperature (8C)
Pi/Ps
)
1
c
Glycine ethyl esterꢂHCl
430.7
85
—
78
91
151
—
42
95.5
140.5
183
—
—
0.8
1.7
4.7
—
0.9
2.4
3.9
6.4
—
2.1
5.8
6.8
—
1.5
2.1
3.2
4.1
14.17
14.80
14.50
14.28
17.81
17.94
18.41
18.46
18.25
7.93
8.02
8.27
8.04
15.65
14.33
14.25
14.67
14.21
9
5
3
6
5
2
d
Sarcosine ethyl esterꢂHCl
410.8
23
4
4
3
2
5
0
6.5
8.5
d
DL-Alanine ethyl esterꢂHCl
400.5
389.6
21.5
1
1
1
8.7
5.5
8.5
40
90.5
125.5
—
58.0
89.5
110.5
146.5
d
L-Phenylalanine ethyl esterꢂHCl
38.1
3
4
3
3
7.9
3.0
4.3
5.9
a
P
P
s
¼ Pressure of substrate.
b
c
d
i
¼ Pressure of free radical inhibitor.
Propene inhibitor.
Toluene inhibitor.
Table 5. Invariability of the rate coefficients from initial pressure
Substrate
Temperature(8C)
430.7
Parameters
Value
Glycine ethyl esterꢂHCl
Sarcosine ethyl esterꢂHCl
DL-Alanine ethyl esterꢂHCl
L-Phenylalanine ethyl esterꢂHCl
P (Torr)
0
0 k (sec
35.5
14.14
28.5
18.25
8.6
8.09
22.0
14.33
55
14.50
31
18.03
15.5
8.27
34.3
14.17
96
14.00
40
18.41
21.5
8.04
43.0
14.25
108
45
17.94
25.2
8.02
50.7
14.03
4
ꢀ1
1
1
1
1
)
)
)
)
14.06
1
P (Torr)
410.8
55
18.21
0
0 k (sec
4
ꢀ1
1
P (Torr)
400.5
0
0 k (sec
4
ꢀ1
1
P (Torr)
389.6
0
0 k (sec
4
ꢀ1
1
where P ¼ the initial pressure of the substrate
then
0
P ¼ amount reacted of substrate or formed product then
pressure at time t,
C ¼ P ꢀ ½ðP ꢀ P Þ=3ꢃ or C ¼ ð3P ꢀ P þ P Þ=3
0
t
0
0
t
0
C ¼ ð4P ꢀ P Þ=3
0
t
P ¼ P ꢀ P þ P þ P þ P þ P ¼ P þ 3P
therefore,
t
0
0
consequently k ¼ ꢀ1/t ln (4 P ꢀ P )/3 P or k ¼ ꢀ2.303/t
0
t
0
log [(4 P ꢀ P )/3 P ]
0
t
0
The temperature dependence of the rate coefficient
and the corresponding Arrhenius equation is given in
Table 6 (90% confidence coefficient from least square
procedure).
P ¼ ðP ꢀ P Þ=3
t
0
for unimolecular process
k ¼ ꢀ1=t ln C=C0
C ¼ initial concentration and C ¼ concentration at time t
0
Sarcosine ethyl ester hydrochloride
if
The products formation from the elimination of sarcosine
ethyl ester hydrochloride described in reaction (5) suggest
a threefold increase in the final pressure P , that is, P /
C ¼ P
and
C ¼ P ꢀ P
0
0
0
f
f
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

3
30
G. CHUCHANI ET AL.
Table 6. The variation of the rate coefficients with temperatures
Glycine
ethyl ester.HCl
Temp. (8C)
390.7
1.78
400.3
3.18
410.0
5.35
420.5
8.45
430.7
14.17
440.4
23.40
451.0
40.93
4
10 k (sec
ꢀ1
)
1
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (sec ) ¼ (12.29 ꢁ 0.24) ꢀ (203.7 ꢁ 3.2)kJ mol (2.303 RT) , r ¼ 0.9994
1
Sarcosine
ethyl ester.HCl
Temp. (8C)
389.1
4.75
399.6
410.8
18.08
420.2
27.96
430.2
44.57
4
10 k (sec
ꢀ1
)
8.42
1
Rate equation log k (sec ) ¼ (13.64 ꢁ 0.60) ꢀ (215.0 ꢁ 7.8)kJ mol (2.303 RT)ꢀ1, r ¼ 0.9974
ꢀ
1
ꢀ1
1
DL-Alanine
ethyl ester.HCl
Temp. (8C)
379.3
2.73
390.8
400.5
8.07
409.4
13.59
420.0
23.17
430.5
39.38
4
10 k (sec
ꢀ1
)
4.87
1
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (sec ) ¼ (12.49 ꢁ 0.46) ꢀ (200.2 ꢁ 5.9)kJ mol (2.303 RT) , r ¼ 0.9996
1
L-Phenylalanine
ethyl ester.HCl
Temp. (8C)
339.6
0.78
350.4
359.5
2.69
370.0
4.90
380.0
389.6
14.13
399.6
23.99
4
10 k (sec
ꢀ1
)
1.55
8.51
1
ꢀ
1
ꢀ1
ꢀ1
Rate equation log k (sec ) ¼ (12.49 ꢁ 0.09) ꢀ (194.4 ꢁ 1.1)kJ mol (2.303 RT) , r ¼ 0.9999
1
P ¼ 3.0. The average experimental result of P /P at 4
ometry in Equation (6) was possible, when obtaining, up
to 50% decomposition, good agreement between pressure
measurements and the quantitative chromatographic
analyses of ethylene (Table 2).
0
f
0
different temperatures and 10 half-lives is 3.3 (Table 1).
.
ClH CH NHCH COOCH CH
3
→
3
2
2
(5)
CH NHCH COOH + HCl + CH CH2
3
2
2
2.
2 3
CH
CH
3
CH(NH HCl)COOCH
The small departure from the theoretical stoichiometry
was due to the formation of a small amount of CO gas.
→
HCl + CH
2
2 3 2 2
CH + CH CH NH + CO
2
2
(
6)
Verification of stoichiometry (5), up to 60% reaction, was
performed by comparing the pressure measurements with
the quantitative chromatographic analysis of ethylene gas
Table 3 appears to indicate that reaction (6) is
homogeneous, since no significant effect was obtained
in the experiments when using both clean Pyrex and
seasoned Pyrex vessels with a surface-to-volume ratio of
(
Table 2). The homogeneity of reaction (5) was examined
by using a vessel with a surface-to-volume ratio six times
greater than that of the unpacked vessel. The clean packed
and unpacked Pyrex vessel has a significant effect on rates
6
.0 relative to that of the normal vessel. The effect of the
free radical suppressor toluene is shown in Table 4. No
induction period was observed. The rate coefficients are
reproducible with a relative standard deviation not greater
than ꢁ5% at a given temperature.
(
Table 3). However, the packed and unpacked Pyrex
vessel seasoned with allyl bromide had no effect on the
rate coefficients (Table 3). The effect of the free radical
inhibitor toluene is described in Table 4. No induction
period was obtained. The rate coefficients are reprodu-
cible with a relative standard deviation not greater than
The rate coefficient also calculated from k ¼ ꢀ(2.303/
1
t) log [(4P ꢀ P )/3P ] was invariable of their initial
0
t
0
pressure. When plotting first log (4P ꢀ P) against time t a
0
ꢁ
5% at a given temperature.
The first order rate coefficients of this substrate,
good straight line, up to 50% reaction, was obtained
(Table 5). The temperature dependence of the rate
calculated from k ¼ (2.303/t) log [2P /3P ꢀ P ] were
0
0
t
coefficient and the corresponding Arrhenius equation is
described in Table 6. (90% confidence coefficient from
least-squares procedure).
found to be independent of the initial pressure (Table 5).
A plot of log (3P ꢀ P ) versus time t gave a good straight
0
t
line up to 60% decomposition. The variations of the rate
coefficient with temperature are described in Table 6. The
results given in Table 6 lead, by using the least squares
procedure and 90% confidence limits, to the shown
Arrhenius equation.
L-Phenylalanine ethyl ester hydrochloride
The gas phase elimination process of L-phenylalanine
ethyl ester hydrochloride is described in reaction (7). The
stoichiometry based on reaction (7) required that, for long
reaction time P /P ¼ 4.0. The average experimental P /
DL-Alanine ethyl ester hydrochloride
f
0
f
The theoretical stoichiometry for the elimination process
of DL-alanine ethyl ester hydrochloride in the gas phase
P value at 4 different temperatures and 10 half-lives was
0
3.8 (Table 1). Verification of the above stoichiometry (7),
up to 60% reaction, was possible when comparing that the
corresponding ethylene gas produced is equivalent to
pressure increase (Table 2). The elimination products,
[
imental P /P values at 4 different temperatures and 10
reaction (6)] demands P /P ¼ 4.0. The average exper-
f
0
f 0
half-lives was 4.1 (Table 1). Confirmation of stoichi-
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

ELIMINATION OF THE ETHYL ESTER OF AMINO ACID HYDROCHLORIDES
Table 7. Comparative kinetic and thermodynamic parameters at 4008C
331
4
ꢀ1 ꢀ1 6¼ 6¼
6¼
Z
k ꢄ 10 (sec
) Ea (kJ/mol) log A (sec ) DS (J/mol K) DH (kJ/mol) DG (kJ/mol)
1
Glycine ethyl ester.HCl
3.02
8.97
8.94
25.22
203.7 ꢁ 3.2 12.29 ꢁ 0.24
215.0 ꢁ 7.8 13.64 ꢁ 0.60
200.2 ꢁ 5.9 12.49 ꢁ 0.46
194.4 ꢁ 1.1 12.49 ꢁ 0.09
ꢀ24.74
1.10
198.1
209.4
194.6
191.3
214.8
208.7
208.7
205.4
Sarcosine ethyl ester.HCl
DL-Alanine ethyl ester.HCl
L-Phenylalanine ethyl ester.HCl
ꢀ20.91
ꢀ20.92
within the range of rate determination were: phenethy-
lamine, ethylene, carbon dioxide, and hydrogen chloride.
The rate coefficients were reproducible with a relative
standard deviation less than ꢁ5% at a given temperature.
2.HCl )COOCH
2
3
2
2 2 3
CH(NH )COOCH CH ] + HCl
PhCH
2
CH(NH
CH
→ [ PhCH
↓
[
PhCH
2
CH(NH
2
)COOH ] + CH
2
CH
2
ð7Þ
↓
2 2 2 2
PhCH CH NH + CO
The pyrolytic elimination of reaction (7) is homo-
geneous since no significant variations in the rates are
observed when using both clean Pyrex and seasoned
Pyrex vessels with a surface-to-volume ratio of 6.0
relative to that of the normal vessel, which is equal to 1.0
The rate coefficient of this elimination was found to be
independent to initial pressures (Table 5), and the first-order
rate was calculated from k ¼ ꢀ(2.303/t) log [(4P ꢀ P )/
1
0
t
3P ]. A plot of log (4P ꢀ P) against time (t) gave a good
0
0
straight line up to 60% decomposition. The variation of the
rate coefficient with temperature and the corresponding
Arrhenius equation are given in Table 6 (90% confidence
coefficient from the least-squares procedure).
Product formations and the kinetic and thermodynamic
parameters given in Table 7 suggest a consecutive
mechanism as described in reaction (8)
(
Table 3). The pyrolysis experiments of this substrate
were carried out in the presence of at least twice the
amounts of the free radical inhibitor toluene in order to
prevent any possible chain reaction. The effect of
different proportions of toluene in the elimination process
is shown in Table 4. No induction period was observed.
O
O
C
1
H
H
R1
C
OCH CH3
R1
OCH CH
3
+
HCl
C
C
2
2
2
.
NHR2
NHR HCl
1
2
a. R = R = H
1
2
2
b. R = H, R =CH
3
1
2
c. R = CH , R = H
3
1
2
d. R = PhCH , R = H
2
O
C
O
H
H
H
C
R1
C
CH2
R1
+
C
OH
H C CH
2 2
δ-
NHR2
O
CH
NHR2
ð8Þ
2
+
δ
3
O
1
1
R CH
R HC
C
CO2
+
δ-
2
2
R HNH
R HN
O
H
δ+
1
2
R CH NHR
2
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332

3
32
G. CHUCHANI ET AL.
Step 1 is a fast process of dehydrochlorination. The ester
calculated from the pressure increase manometrically, or
by the quantitative chromatographic analyses of ethylene
product. The temperature was controlled by a Shinko DC-
PS resistance thermometer controller and an Omega
SSR280A45 solid-state relay, maintained ꢁ0.28C and
measured with a calibrated Iron Constantan thermo-
couple. No temperature gradient was observed along the
reaction vessel. The starting materials, dissolved in
benzyl alcohol or acetic acid, were all injected directly
into the reaction vessel with a syringe through a silicone
rubber septum. The amount of substrate used for each
reaction was ꢅ0.05–0.2 ml.
intermediate proceeds through a slow determining Step 2
for the formation of the corresponding a-amino acid and
ethylene gas. The ‘in situ’ unstable amino acid from Step 2
rapidly decarboxylates to the corresponding amine
(Step 3). Ethyl ester of sarcosine is the exception since
only produces the corresponding amino acid. These results
10–12
apparently support the consideration reported before
where a-amino acids decarboxylate to the corresponding
amines. It is interesting to report that phenylalanine was
15
found to decompose in a stainless steel reactor, between
4
00–6008C from 4 sec to 1 min, with complex results.
The general concept that electron-withdrawing groups
at the acid side of ethyl, isopropyl, and tert-butyl esters
enhance the elimination rate, while electron-releasing
groups reduce it has been described in a review on the
16
structural effect on gas-phase reactivities. However, the
comparative rates of the gas phase reactivity of the ethyl
esters of amino acid hydrochlorides listed in Table 7 seem
to be contrary to the above-mentioned generalization.
Steric accelerations appear to influence the process of
decomposition. That is, the bigger the size or the more
bulky is the substituent at the acyl side of the amino ester,
the faster is the elimination rate of ethylene formation.
REFERENCES
1. Taylor R. The Chemistry of Functional Group. Acid Derivatives,
Supplementary, vol. B, Saul Patai (ed). Wiley: London, 1979;
Chapter 15.
. Holbrook KA. The Chemistry of Acid Derivatives. Vapor and Gas
phase Reaction of Carboxylic Acids and their Derivatives, vol. 2,
Saul Patai (ed). Wiley: London, 1992; Chapter 12.
2
3
4
5
. Al-Awadi NA, Kaul K, El-Dosouqui OME. J. Phys. Org. Chem.
2
000; 13: 499–504.
. Safont VS, Moliner V, Andres J, Domingo LR. J. Phys. Chem. A
997; 101: 1859–1865.
. Domingo LR, Andres J, Moliner V, Safont VS. J. Am. Chem. Soc.
997; 119: 6415–6422.
1
EXPERIMENTAL
1
Glycine ethyl ester hydrochloride (Aldrich), sarcosine
ethyl ester hydrochloride (Aldrich), DL-alanine ethyl ester
hydrochloride (Aldrich), and L-phenylalanine ester
hydrochloride (Aldrich) of 99% purity were employed
6. Domingo LR, Pitcher MT, Andres J, Moliner V, Safont VS,
Chuchani G. Chem. Phys. Lett. 1997; 274: 422–428.
7
. Domingo LR, Pitcher MT, Safont V, Andres J, Chuchani G.
J. Phys. Chem. A. 1999; 103: 3935–3943.
8. Rotinov A, Chuchani G, Andres J, Domingo LR, Safont VS. Chem.
Phys. 1999; 246: 1–12.
(
GC-MS: Saturn 2000, Varian, with a DB-5MS capillary
9
. Chuchani G, Dom ´ı nguez RM, Rotinov A, Mart ´ı n I. J. Phys. Org.
Chem. 1999; 12: 612–618.
column 30 m ꢄ 0.25 mm. i.d., 0.25 mm film thickness).
The products sarcosine, the hydrochloride salts of
methylamine, ethylamine, and phenethylamine collected
out of the reaction vessel were identified in a GC-MS
10. Ensuncho A, Lafont J, Rotinov A, Dom ´ı nguez RM, Herize A,
Quijano J, Chuchani G. Int. J. Chem. Kinet. 2001; 33: 465–471,
and references cited therein.
1. Lafont J, Ensuncho A, Dom ´ı nguez RM, Rotinov A, Herize A,
Quijano J, Chuchani G. J. Phys. Org. Chem. 2003; 16: 84–88.
2. Dominguez RM, Tosta M, Chuchani G. J. Phys. Org. Chem. 2003;
1
1
1
1
1
1
(
Saturn 2000, Varian) with a DB-5MS capillary column
0 m ꢄ 0.25 mm. i.d., 0.25 mm. The identification of
ethylene and carbon dioxide were performed GC-MS
Saturn 2000, Varian) with megabore capillary column
3
1
6: 869–874.
3. Al-Awadi SA, Abdallah MR, Hasan MA, Al-Awadi NA. Tetra-
hedron 2004; 60: 3045–3049.
4. Rosas F, Monsalbe A, Tosta M, Herize A, Dominguez RM, Brusco
D, Chuchani G. Int. J. Chem. Kinet. 2005; 37: 383–389.
5. Shulman GP, Simmonds PG. Chem. Commun. 1968, 1040–
1042.
6. Chuchani G, Mishima M, Notario R, Abboud JL. Structural Effect
on Gas-Phase Reactivities. Advances in Quantitative Structure-
Property Relationship, vol. 2, Charton M, Charton BI (eds).
JAI Press, Inc.: Stamford, Connecticut, USA, 1999.
7. Maccoll A. J. Chem. Soc. 1955: 965–973.
8. Swinbourne ES. Aust. J. Chem. 1958; 11: 314–330.
9. Dominguez RM, Herize A, Rotinov A, Alvarez-Aular A, Visbal G,
Chuchani G. J. Phys. Org Chem. 2004: 17: 399–408.
(
GS-Q 30 m, i.d. 0.53 mm with a TCD. The quantitative
analysis of the product ethylene (Matheson) was carried
out by using a Gas Chromatograph HP 5710A with a
Porapak Q (80-100 mesh) column.
Kinetics
1
1
1
The kinetic experiments were performed in a static
The rate coefficients were
17–19
reaction system reported.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 326–332