Kinetics and mechanisms of the gas-phase elimination of N-phenylglycine and its ethyl ester

Article abstract of DOI:10.1002/poc.676

The elimination kinetics of the title compounds were determined over the temperature range 290.4420.3 °C and pressure range 14.5-44.0 Torr. The reactions, carried out in seasoned vessels and in the presence of the free radical inhibitor toluene, are homogeneous, unimolecular and follow a first-order rate law. The rate coefficient is expressed by the following Arrhenius equation: for N-phenylglycine ethyl ester, log [k₁ (s ^-1)] = (12.13±0.38)(193.6±4.9) kJ mol^-1 (2.303 RT)^-1; and for N-phenylglycine, log [k₁ (s ^-1)] = (12.95±0.52)-(177.4±5.8) kJ mol^-1 (2.303 RT)^-1. Both substrates yield mainly N-methylaniline and a small amount of aniline. N-Phenylglycine as a neutral amino acid decomposes 130 times faster than its ethyl ester at 380 °C. The mechanisms of these elimination reactions are presented and discussed.Copyright

Full text of DOI:10.1002/poc.676

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 869–874
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.676
Kinetics and mechanisms of the gas-phase elimination
of N-phenylglycine and its ethyl ester
Rosa M. Domı´nguez, Maria Tosta and Gabriel Chuchani*
´
´
Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 31 March 2003; revised 30 May 2003; accepted 2 June 2003
ABSTRACT: The elimination kinetics of the title compounds were determined over the temperature range 290.4–
420.3 ꢀC and pressure range 14.5–44.0 Torr. The reactions, carried out in seasoned vessels and in the presence of the
free radical inhibitor toluene, are homogeneous, unimolecular and follow a first-order rate law. The rate coefficient is
expressed by the following Arrhenius equation: for N-phenylglycine ethyl ester, log [k1 (sꢁ1)] ¼ (12.13 ꢂ 0.38)ꢁ
(193.6 ꢂ 4.9) kJ molꢁ1 (2.303 RT )ꢁ1; and for N-phenylglycine, log [k1 (sꢁ1)] ¼ (12.95 ꢂ 0.52)ꢁ(177.4 ꢂ 5.8) kJ
molꢁ1 (2.303 RT )ꢁ1. Both substrates yield mainly N-methylaniline and a small amount of aniline. N-Phenylglycine as
a neutral amino acid decomposes 130 times faster than its ethyl ester at 380 ꢀC. The mechanisms of these elimination
reactions are presented and discussed. Copyright # 2003 John Wiley & Sons, Ltd.
KEYWORDS: pyrolysis; elimination; kinetics; gas phase; phenylglycines
INTRODUCTION
the present work we studied the homogeneous, molecular
pyrolysis kinetics of N-phenylglycine and its ethyl ester.
Low molecular weight amino acids are solids and on
heating sinter or decompose into amorphous materials. In
addition, their insolubility in most organic solvents and
high solubility as zwitterions in water are difficult to
examine in gas-phase elimination reactions. However,
two recent papers reported the homogeneous, unimole-
cular elimination of N,N-dimethylglycine1 and picolinic
acid.2 These substrates as 2-substituted amino carboxylic
acids undergo decarboxylation as shown in reaction (1).
RESULTS AND DISCUSSION
N-Phenylglycine ethyl ester
The elimination products of this substrate described by
reaction (3) requires that for long reaction times Pf/
P0 ¼ 3, where Pf and P0 are the final and initial pressures,
respectively.
ð1Þ
ð3Þ
This process of decomposition differs from the gas-
phase elimination of several types of 2-substituted car-
boxylic acids which are found to decarbonylate3–9 as
described in reaction (2).
The average experimental Pf/P0 values at four tem-
peratures and 10 half-lives is 2.9 (Table 1). Additional
verification of the above stoichiometry (3), up to 92%
reaction, was possible by comparing the ethylene forma-
tion with the pressure increase of the substrate decom-
position (Table 2).
ð2Þ
The homogeneity of reaction (3) 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). The elimination may be
said to be homogeneous since no significant effects on the
rates were obtained in clean and seasoned Pyrex vessels.
Toluene, a free radical inhibitor, used in different
proportions had no effect on the rates (Table 4). No
induction period was observed on plotting pressure
To further our understanding of the mechanistic path-
ways of neutral amino acid elimination in the gas phase, in
´
*Correspondence to: G. Chuchani, Centro de Quımica, Instituto
Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827,
´
Caracas 1020-A, Venezuela.
E-mail: chuchani@quimica.ivic.ve
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874

´
R. M. DOMINGUEZ, M. TOSTA AND G. CHUCHANI
870
Table 1. Ratio of final pressure (Pf) to initial pressure (P0)
Substrate
Temperature ( ꢀC)
P0 (Torr)a
Pf (Torr)a
Pf/P0
Average
2.9
N-Phenylglycine ethyl ester
390.6
402.3
411.6
420.3
26.6
24.8
21.3
28.1
71.6
73.8
67.3
80.1
2.7
3.0
3.1
2.9
N-Phenylglycine
319.6
330.2
340.5
32
21.5
21
62
41.5
40
1.9
1.9
1.9
1.9
a
Torr ¼ 133.3 Pa.
Table 2. Stoichiometry of the reactiona
Substrate
Temperature ( ꢀC)
Parameterb
Value
N-Phenylglycine ethyl ester
411.5
Time (min)
Reaction (%) (pressure)
Ethylene (%) (GC)
2.5
28.2
31.2
6.5
67.6
64.2
10.5
77.3
74.4
14.5
92.0
92.6
N-Phenylglycine
309.0
Time (min)
3
5
6
8
Reaction (%) (pressure)
N-Methylaniline(%) (GC)
Aniline (%) (GC)
Sum (%) (GC)
21.4
17.2
2.2
32.1
26.9
4.0
37.5
30.3
4.7
46.4
39.2
5.7
19.4
30.9
35.0
44.9
a
Maximum error ꢂ 0.5.
b
GC, gaschromatography.
Table 3. Homogeneity of pyrolysis reactions
Substrate
S/V (cmꢁ1 a 104 k1 (sꢁ1)b 104 k1 (sꢁ1 c
) )
N-Phenylglycine
ethyl ester at 402.3 ꢀC
1
6
15.26
14.89
14.27
15.07
N-Phenylglycine
at 320.9 ꢀC
1
6
23.95
23.80
22.27
22.76
a
S ¼ surface area; V ¼ volume.
b
Clean Pyrex vessel.
Vessel seasoned with allyl bromide.
c
Figure 1. Pressure against time for N-phenylglycine ethyl
ester at 420.4 ꢀC
Table 4. Effect of free radical inhibitor toluene on rates
Substrate
Temperature ( ꢀC)
402.3
Psa (Torr)
Pib (Torr)
Pi/Ps
104 k1 (sꢁ1
)
N-Phenylglycine ethyl ester
34.6
25.2
24.5
24.1
—
58
112
146
—
2.3
4.6
6.1
15.26
14.68
15.04
14.87
c
N-Phenylglycine
319.6
38
44
22.5
32
—
64
83
130
102
—
1.4
3.7
4.1
4.9
—
21.49
20.48
20.32
20.58
21
a
Ps, pressure of the substrate.
Pi, pressure of the inhibitor.
The k-value decreases rapidly.
b
c
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874

GAS-PHASE ELIMINATION OF N-PHENYLGLYCINE
Table 5. Variation of rate coefficients with initial pressure
871
Substrate
Temperature ( ꢀC)
411.6
Parameter
Value
N-Phenylglycine ethyl ester
P0 (Torr)
18
24.72
25
24.80
31
24.39
34
24.80
104 k1 (sꢁ1
)
N-Phenylglycine
309.0
P0 (Torr)
14.5
11.04
21
11.52
32.5
11.30
38
11.28
104 k1 (sꢁ1
)
Figure 3. Arrhenius plot for N-phenylglycine ethyl ester
Figure 2. Plot of log(3P0 ꢁ Pt) vs time for N-phenylglycine
ethyl ester at two temperatures
P0 ¼ 1.93 (Table 1). Determination of the stoichiometry
of reaction (4), up to 45% decomposition, was possible
by comparing the percentage decomposition of the sub-
strate from pressure measurements with the sum of the
quantitative gas chromatographic (GC) analyses of the
products N-methylaniline and aniline (Table 2).
against time t (Fig. 1). The rate coefficients are reprodu-
cible with a relative standard deviation of <10% at a
given temperature, and the precision of manometric
measurements is ꢂ 0.5 mm.
The first-order rate coefficients of the substrate, calcu-
lated from k1 ¼ (2.303/t)log [2P0/(3P0 ꢁ Pt)] were found
to be independent of the initial pressure (Table 5). A plot
of log(3P0–Pt) against time t gave a good straight line up
to 92% decomposition (Fig. 2). The temperature depen-
dence of the rate coefficients is shown in Table 6. The
results in Table 6 lead, by using the least-squares proce-
dure and 90% confidence coefficients, to the Arrhenius
graphs (Fig. 3) and equations shown.
ð4Þ
To examine the influence of the surface area on the rate
of elimination, several runs were carried out in a vessel
with a surface-to-volume ratio six times greater than that of
the normal vessel (Table 3). The packed and unpacked
clean Pyrex vessels gave slightly different kinetic results,
indicating a small amount of heterogeneous reaction, while
the seasoned vessels gave identical results, indicating no
heterogeneous reaction within the experimental precision.
N-Phenylglycine
The elimination process of N-phenylglycine described in
reaction (4) gives an average experimental value of Pf/
Table 6. Variation of the overall rate coefficients with temperature
Substrate
Parameter
Value
N-Phenylglycine ethyl ester
Temperature (ꢀC)
380.4
4.44
390.6
7.52
402.3
14.82
411.6
24.47
420.3
35.73
104 k1 (sꢁ1
)
Rate equation: log[k1 (sꢁ1)] ¼ (12.13 ꢂ 0.38) ꢁ (193.6 ꢂ 4.9)kJ molꢁ1 (2.303 RT)ꢁ1; r ¼ 0.9990
N-Phenylglycine
Temperature (ꢀC)
104 k1 (sꢁ1
290.4
3.23
300.9
6.09
309.0
11.43
319.6
20.50
329.5
36.72
340.5
70.89
)
Rate equation: log [k1 (sꢁ1)] ¼ (12.95 ꢂ 0.52) ꢁ (177.4 ꢂ 5.8)kJ molꢁ1 (2.303 RT)ꢁ1; r ¼ 0.9995
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874

´
R. M. DOMINGUEZ, M. TOSTA AND G. CHUCHANI
872
Details of the chromatographic analyses of the elim-
ination products under these conditions are given in
Tables 7 and 8. Within the experimental errors, the
distribution of products does not vary at a given tempera-
ture as a function of extent of reaction, and varies very
insignificantly as a function of temperature. In this
respect, it is not unreasonable to assume that the forma-
tion of the products proceeds under kinetic control.
The partial rates of product formation described in
reaction (4) were determined by quantitative GC analyses
of N-methylaniline and aniline. The temperature depen-
dence of the rate coefficients for the formation of these
products (Table 9) gives by the least-squares procedure
and with 90% confidence limits the following Arrhenius
equations: for N-methylaniline formation, log[k1
(sꢁ1)] ¼ (12.46 ꢂ 0.57) ꢁ (172.6 ꢂ 6.4)kJ molꢁ1 (2.303
RT)ꢁ1, r ¼ 0.9993, and for aniline formation, log[k1
(sꢁ1)] ¼ (14.85 ꢂ 0.38) ꢁ (208.6 ꢂ 4.3)kJ molꢁ1 (2.303
RT)ꢁ1, r ¼ 0.9998. The Arrhenius plots are described in
Fig. 5. The large numbers for the kinetic parameters of
aniline formation (Table 9) may be due to mechanical
errors during experimental measurements. Even though
log A ¼ 14.85 is not a preferred value for a five-
membered cyclic transition state, it is compensated by a
larger value of Ea ¼ 208.6 kJ molꢁ1. This occurs in
many examples of pyrolyses described in the literature.
Figure 4. Pressure against time for N-phenylglycine at
319.7 ꢀC
Table 7. Product distribution (%) at different extents of
reaction at 309 ꢀC
Extent of reaction (%)
Aniline
N-Methylaniline
19.4
30.9
35.0
45.2
13.6
13.5
13.4
12.7
86.4
86.5
86.6
87.3
Table 8. Product distribution (%) at different reaction
temperatures
Temperature ( ꢀC)
Aniline
N-Methylaniline
309.0
319.6
330.2
340.5
13.5
13.8
12.5
14.7
86.5
86.2
87.5
85.3
Different proportions of the free radical inhibitor
toluene in the elimination process are shown in Table 4.
The pyrolysis experiments with N-phenylglycine were
carried out under maximum inhibition of toluene in order
to prevent any possible free radical chain reaction. No
induction period was observed according to Fig. 4. The
rate coefficients were reproducible with a relative stan-
dard deviation of <10% at a given temperature, and the
precision of manometric measurements is ꢂ0.5 mm.
Figure 5. Arrhenius plots for N-methylaniline and aniline
formation from N-phenylglycine
Table 9. Temperature dependence of partial rate coefficients of products formation from pyrolysis of N-phenylglycine
Product
Aniline
Parameter
Value
309.0
Temperature ( ꢀC)
290.4
0.31
300.9
0.74
319.6
2.88
329.5
5.69
340.5
12.10
104 k1 (sꢁ1
)
1.39
Rate equation: log[k1 (sꢁ1)] ¼ (14.85 ꢂ 0.38) ꢁ (208.6 ꢂ 4.3)kJ molꢁ1 (2.303 RT)ꢁ1; r ¼ 0.9998
N-Methylaniline
Temperature ( ꢀC)
104 k1 (sꢁ1
290.4
2.92
300.9
5.35
309.0
10.04
319.6
17.62
329.5
31.03
340.5
58.79
)
Rate equation: log[k1 (sꢁ1)] ¼ (12.46 ꢂ 0.57) ꢁ (172.6 ꢂ 6.4)kJ molꢁ1 (2.303 RT)ꢁ1; r ¼ 0.9993
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874

GAS-PHASE ELIMINATION OF N-PHENYLGLYCINE
873
The negative entropy of activation ꢀSz of these elimina-
tions suggests a symmetrical arrangement and a possible
approximation to planarity of the transition state. Log
A ¼ 12.13 for N-phenylglycine ethyl ester is a reasonable
value for a six-membered cyclic transition-state mechan-
ism, while log A ¼ 12.95 for N-phenylglycine is accep-
table for a five-membered cyclic transition state type of
mechanism. The enthalpy of activation ꢀHz implies
endothermic eliminations, whereas the free energy of
activation ꢀGz indicates that these reactions are not
spontaneous, unstable and endergonic.
The N-phenylglycine produced as an intermediate in
reaction (3) gives an extremely fast decomposition, 130
times faster than that for N-phenylglycine ethyl ester at
380 ꢀC (Table 10). This result appears to support the
previous idea that neutral amino acid type molecules
undergo a rapid decomposition at low temperatures.1,2
According to the present results, together with the data in
Tables 6 and 9, the overall mechanism for the elimination
of N-phenylglycine ethyl ester together with N-phenyl-
glycine may be rationalized as in reaction (5).
Figure 6. Plots of log (2P0 ꢁ Pt) vs time for N-phenylglycine
at two temperatures
Therefore, comparative elimination reactions must be
made through k values rather than Ea values.
The overall rate coefficients for N-phenylglycine elim-
ination, in seasoned vessels and in the presence of toluene
inhibitor, were found to be independent of the initial pre-
ssure of the substrate {k1 ¼ (2.303/t) log [P0/(2P0 ꢁ Pt)]}
and the first-order plots, log (2P0 ꢁ Pt) vs t, are satisfac-
torily linear up to 45% reaction (Table 5, Fig. 6). The
variation of the overall rate coefficients with temperature,
at 90% confidence limits with the least-squares method,
is given in Table 6. The Arrhenius plot is shown in Fig. 7.
ð5Þ
N-Phenylglycine ethyl ester, as an organic ester with a
Cꢀ—H bond, pyrolyses through a six-membered cyclic
transition state10,11 to yield N-phenylglycine and ethylene
[reaction (5)]. At the reaction temperature, the intermedi-
ate N-phenylglycine is unstable and decomposes rapidly
through decarboxylation to N-methylaniline (k1) as
described for neutral type amino acid elimination in
reaction (1). That is, the nucleophilicity of the N atom
will abstract the acidic H of the COOH group to form a
five- centered transition state, which through CO2 elim-
ination yields N-methylaniline. Surprisingly, and con-
trary to the idea that an amino nitrogen substituent is a
difficult leaving group in the gas-phase elimination of
Figure 7. Arrhenius plot for the overall elimination of N-
phenylglycine
Table 10. Kinetic and thermodynamic parameters at 380 ꢀC
Substrate
k1 ꢃ 104
Ea
(kJ molꢁ1
Loꢁg1A
(s
ꢀSz
ꢀHz
ꢀGz
(sꢁ1
)
)
)
(J molꢁ1 Kꢁ1
)
(kJ molꢁ1
)
(kJ molꢁ1
)
N-Phenylglycine ethyl ester
N-Phenylglycine
4.40
193.6 ꢂ 4.9
177.4 ꢂ 5.8
12.13 ꢂ 0.38
12.95 ꢂ 0.52
ꢁ27.6
ꢁ11.9
188.3
172.1
206.3
179.9
574.3
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874

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R. M. DOMINGUEZ, M. TOSTA AND G. CHUCHANI
874
Table 11. Comparative kinetic parameters at 300 ꢀC
Substrate
k1 ꢃ 104 (sꢁ1
7.97
)
Ea (kJ molꢁ1
)
Log A (sꢁ1
)
Ref.
N,N-Dimethylglycine
N-Phenylglycine
176.6
13.0
1
5.32
172.6 ꢂ 6.4
12.46 ꢂ 0.57
This work
2-substituted carboxylic acids, a small amount of aniline
was also obtained (k2). This suggests that the oxygen
carbonyl may assist to stabilize the Cꢁꢂþꢄ ꢄ ꢄꢂ ꢁ NHPh
bond polarization in the transition state, which then
decomposes to aniline and the unstable lactone as shown
in reaction (2). The latter product rapidly decarbonylates
to produce formaldehyde.
Kinetics. The elimination kinetics were examined in a
static system described before12,13 with an Omega DP41-
TC/DP41-RTD high-performance digital temperature in-
dicator. The rate coefficients were determined manome-
trically and chromatographic analyses were performed
with a precision of 0.5 mm. The temperature was con-
trolled by a Shinko DC-PS resistance thermometer con-
troller and an Omega Model SSR280A45 solid-state
relay, maintained within ꢂ 0.2 ꢀC and measured with a
calibrated platinum–platinum–13% rhodium thermocou-
ple. No temperature gradient was detected along the
reaction vessel. N-Phenylglycine ethyl ester was dis-
solved in dioxane and injected directly into the reaction
vessel with a syringe through a silicone-rubber septum.
The amount of substrates used for each run was ꢅ0.05–
0.1 ml.
The recently reported gas-phase elimination of N,N-
dimethylglycine1 is found to be slightly faster in the rate
of decarboxylation than N-phenylglycine (Table 11). This
small difference may be explained by the OH bond
polarization of the carboxylic group, in the sense of
O
ꢂ ꢁ ꢄ ꢄ ꢄ Hꢂþ, while the nucleophilicity of the N atom of
the amino acid is determinant. Since the N atom of the
(CH3)2N substituent is more nucleophilic than the N of
the PhNH group for the abstraction of the acidic hydro-
gen, a higher rate of decarboxylation of N,N-dimethyl-
glycine is expected [reaction (5), k1]. The fact that N-
phenylglycine yields a small amount of aniline, the
nitrogen atom in the PhNH substituent, as an activating
group of the aromatic nuclei, may delocalize its available
electrons towards the benzene ring. Therefore, a bond
polarization as reported above of Cꢂþꢄ ꢄ ꢄ ꢂ ꢁ NHPh favors
the assistance of the oxygen carbonyl of the COOH group
to the partial positive carbon and the product formation
depicted in reaction (5) (k2).
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Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 869–874