Steric factors in the gas phase elimination kinetics of ethyl N-benzyl-N-cyclopropylcarbamate and ethyl diphenylcarbamate

Article abstract of DOI:10.1002/kin.10010

The elimination kinetics of ethyl N-benzyl-N-cyclopropylcarbamate and ethyl diphenylcarbamate were investigated over the temperature range of 349.9-440.0°C and the pressure range of 31-106 Torr. These reactions have been found to be homogeneous, unimolecular, and obey a first-order rate law. The products are ethylene, carbon monoxide, and the corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius equations: For ethyl N-benzyl-N-cyclopropylcarbamate log k₁ (s^-1) = (1 2.94 ± 0.09) - (198.5 ± 0.9) kJ mol^-1 (2.303RT)^-1. For ethyl diphenylcarbamate log k₁(S^-1 = (12.91 ± 0.18) - (208.2 ± 2.4) kJ mol^-1 (2.303RT)^-1. The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed a decrease in the rate of elimination. Steric factor may be operating during the process of decomposition of these substrates. These reactions appear to undergo a semipolar six-membered cyclic transition type of mechanism.

Full text of DOI:10.1002/kin.10010

Steric Factors in the Gas
Phase Elimination Kinetics
of Ethyl N-Benzyl-N-
Cyclopropylcarbamate and
Ethyl Diphenylcarbamate
MARNIEV LUIGGI,1 ROSA M. DOMINGUEZ,2 ALEXANDRA ROTINOV,2 ARMANDO HERIZE,2
MARY CORDOVA,1 GABRIEL CHUCHANI2
1
´
Departamento de Quımica, Universidad de Oriente, Cumana, Sucre, Venezuela
2
´
´
Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (I.V.I.C.), Apartado 21827,
Caracas 1020-A, Venezuela
Received 26 September 2000; accepted 21 June 2001
ABSTRACT: The elimination kinetics of ethyl N-benzyl-N-cyclopropylcarbamate and ethyl
diphenylcarbamate were investigated over the temperature range of 349.9–440.0 C and the
pressure range of 31–106 Torr. These reactions have been found to be homogeneous, uni-
molecular, and obey a first-order rate law. The products are ethylene, carbon monoxide, and the
corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius
equations:
For ethyl N-benzyl-N-cyclopropylcarbamate
log k1(s 1) = (12.94 0.09) (198.5 0.9) kJ mol 1(2.303RT)
1
For ethyl diphenylcarbamate
log k1(s 1) = (12.91 0.18) (208.2 2.4) kJ mol 1(2.303RT)
1
The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed
a decrease in the rate of elimination. Steric factor may be operating during the process of de-
composition of these substrates. These reactions appear to undergo a semipolar six-membered
C
cyclic transition type of mechanism. 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 67–71,
2002
Correspondence to: Gabriel Chuchani; e-mail: chuchani@
quimica.ivic.ve.
INTRODUCTION
Contract grant sponsor: Consejo Nacional de Investigaciones
Cient´ıficas y Tecnolo´gicas of Venezuela (CONICIT), Proyect No.
The first pyrolysis of a carbamate in the gas phase, the
ethyl ester of N-methyl-N-phenylcarbamate, showed
S1-97000005.
c
2001 John Wiley & Sons, Inc.
DOI 10.1002/kin.10010
to give N-methylaniline, ethylene, and CO2 gas [1].

68
LUIGGI ET AL.
Table I Ratio of Final (Pf) to Initial (P0) Pressure
Substrate
Temperature ( C)
Po (Torr)
Pf (Torr)
Pf/Po
Average
2.98
Ethyl N-benzyl-
382.2
391.2
400.7
412.0
400.0
410.1
420.7
430.6
78
35
66
76
51
50
59
37
231
106
195
225
151
140
158
101
2.96
3.03
2.95
2.96
2.87
2.81
2.69
2.73
N-cyclopropylcarbamate
Ethyl diphenylcarbamate
2.78
Additional works on the thermal decomposition of
alkyl esters of dimethylcarbamates suggested the
mechanism of a six-membered cyclic transition state
[reaction (1)], which is similar to the designated tran-
sition state for the gas phase pyrolyses of acetates, car-
bonates, and xanthates [2–6].
A recent investigation showed that branching of
alkyl groups at the 2-position of ethyl N,N- diethylcar-
bamates increased the rate of elimination in the order
tert-butyl > isopropyl > ethyl [8]. Moreover, the pres-
ence of different substituents other than carbon at the
acid side of the ethyl ester gave a good Taft-Topsom
method of correlation [8]. According to this relation-
ship, the field (inductive effect) of the substituent has
the greatest influence on rate increase, while the polar-
izability (steric) and resonance effects assist very little
to the elimination process.
With this background, some additional informa-
tion on carbamates pyrolyses may be of interest, such
as how the presence of bulky or crowded groups at-
tached to the nitrogen atom may affect the rate of
elimination. Consequently, the present work aimed at
examining the pyrolysis kinetics of ethyl N-benzyl-
N-cyclopropylcarbamate and ethyl N, N-diphenyl-
carbamate in the gas phase.
EXPERIMENTAL
A later work on the gas phase pyrolysis of tert-butyl
arylcarbamates [7] revealed enhancement in rates with
electron-withdrawing substituents at the aromatic nu-
clei, whereas electron-releasing substituents decreased
the k-values.
Ethyl N-benzyl-N-cyclopropylcarbamate (98%) and
ethyl N,N-diphenylcarbamate (99%) were acquired
Table II Homogeneity of the Reaction
S/V
k1
k1
(10 4 s
1 a
1 b
Compound
(cm 1) (10 4 s
)
)
Ethyl N-benzyl-N-
cyclopropylcarbamate
at 369.8 C
1
6
7.61
8.89
7.54
8.42
Ethyl diphenylcarbamate
at 400.0 C
1
6
5.75
8.34
5.82
8.72
Figure 1 Plot of log [(3Po-P )/2Po] vs. time for the pyrol-
ysis kinetics of ethyl N-benzyl-N-cyclopropylcarbamate at
392.2 C.
t
aClean Pyrex vessel.
bVessel seasoned with allyl bromide.

STERIC FACTORS IN THE GAS PHASE ELIMINATION KINETICS
69
Table III Effect of the Inhibitor Toluene on Rates
4
1
Substrate
Po (Torr)
P (Torr)
i
P /Po
i
k1 (10
s
)
Ethyl N-benzyl-N-cyclopropylcarbamate
at 382.2 C
40
51
50
66
59
–
57.5
77
245
–
–
13.54
13.81
13.29
13.57
9.76
1.1
1.5
3.7
–
Ethyl diphenylcarbamate
at 410.1 C
106.5
78
41
82
126
91
0.8
1.6
2.2
9.55
9.89
9.36
P0 = pressure of the substrate; P = pressure of toluene inhibitor.
i
Table IV Variation of Rate Coefficients with Initial
Pressure
4
1
Substrate
Po (Torr)
k1 (10
s
)
Ethyl N-benzyl-
N-cyclopropylcarbamate
at 380.4 C
31
40
60
78
98
35
42
51
68
106
13.48
13.31
13.37
13.17
13.10
9.70
9.35
9.09
9.89
9.55
Ethyl diphenylcarbamate
at 410.0 C
Figure 2 Plot of log [(3Po-P )/2Po] vs. time for the pyrol-
t
ysis kinetics of ethyl N,N-diphenylcarbamate at 410.2 C.
from Aldrich. The quantitative analyses of the sub-
strates were determined by GLC (Pennwalt 223/KOH,
80–100 mesh). The product ethylene was analyzed in
a column of Porapak Q (80–100 mesh). The verifica-
tion of the substrates and identification of the products
were carried out by GLC/MS (Saturn 2000, Varian).
Capillary column DB-5MS, 30 mm 0.250 mm., i.d.
0.25 ␮m.
The pyrolysis experiments were carried out in a
static reaction system as described before [9,10] with
some additions and modifications of modern elec-
tronic and electric devices. The reaction vessel was
seasoned with the product of decomposition of al-
lyl bromide. The rate coefficients were determined by
measurements of pressure increase. The temperature
was controlled by a SHINKO DIC-PS 25TR resistance
thermometer controller maintained within 0.2 C
and measured with a calibrated platinum/platinum–
13%rhodium thermocouple. No temperature gradient
was found along the reaction vessel. The substrates
ethyl N-benzyl-N-cyclopropylcarbamate and ethyl N-
N-diphenylcarbamate were dissolved in dimethyl-
formamide, and injected directly into the reaction
vessel through a silicone–rubber septum.
Table V Variation of Rate Coefficient with Temperature
Temperature
4
1
Substrate
( C)
k1 (10
s
)
Ethyl N-benzyl-
349.5
360.4
372.2
382.2
391.2
400.7
412.0
380.8
390.5
400.8
410.1
420.7
430.6
440.6
1.90
3.80
7.54
13.30
21.60
35.70
63.20
1.94
3.40
5.66
9.69
14.40
29.27
46.42
N-cyclopropylcarbamatea
Ethyl diphenylcarbamateb
aRate equation: log k1 (s 1) = (12.94 0.09) (198.5 0.9)
1
kJ mol (2.303RT ) 1;r = 0.99997.
bRate equation: log k1 (s 1) = (12.91 0.18) (208.2 2.4)
1
kJ mol (2.303RT ) 1;r = 0.99967.

70
LUIGGI ET AL.
Table VI Comparative Rates and Kinetic Parameters for the Pyrolysis of ZCOOCH2CH3 at 380.0 C
k1
4
Ea
(kJ/mol)
1S=
(J/mol K)
1H=
(kJ/mol)
1G=
(kJ/mol)
1
1
Z
(10
s
)
Rel. rate
log A (s
)
Ref.
14
(CH3)2N
CH3PhN
PhCH2(C3H5)N
Ph2N
18.20
17.75
11.52
1.80
1.0
185.5 5.3
189.9 4.5
198.5 0.9
208.2 2.4
12.10 0.22
12.44 0.19
12.94 0.09
12.91 0.18
6.74
5.18
2.89
3.03
180.1
184.5
193.1
202.8
184.5
187.9
195.0
204.8
0.98
0.63
0.10
1
a
a
aThis work.
The clean and seasoned Pyrex vessel had no effect
on rate. However, the packed clean and packed sea-
soned Pyrex vessel resulted in a significant heteroge-
neous effect in the rate coefficients. The pyrolysis of
these carbamate substrates were not affected by the
presence of different proportions of a free radical in-
hibitor such as toluene (Table III). No induction period
was obtained. The k-values are reproducible with a rel-
ative standard deviation not greater than 5% at a given
temperature.
RESULTS AND DISCUSSION
The products formation of these carbamates are de-
scribed in reaction (2)
The first-order rates of the carbamates, calculated
from k1 = (2.303/t) log [2Po/(3Po P )], were found
t
to be invariable of the initial pressure. A plot of
log (3Po P ) vs time t gave a good straight line up
t
to 60–70% decomposition (Table IV, Figs. 1 and 2).
The temperature dependence of the rate coefficients
and the corresponding Arrhenius equations are given
in Table V (90% confidence coefficient from the least-
squares method).
The influence of the phenyl group at the N atom of
the ethyl carbamates shown in Table VI suggests that
steric factors may be affecting the rate of elimination.
The replacement of a methyl group in the (CH3)2N by a
phenyl group results in a small decrease in k-value due
to steric factor. This consideration appears to be evident
when the remaining CH3 at N atom of the carbamate
is replaced by an additional phenyl group i.e. diphenyl
carbamate (Table VI). Augmentation in bulkiness of
CH3PhN by PhCH2(C3H5)N also gives rise to a small
but significant decrease in rate, which may also be due
to steric factor. The effect of the substituent PhNH as
Z in ZCOOCH2CH3, i.e. PhNHCOOCH2CH3, was not
included in Table VI, because this compound was re-
ported to give poor kinetic reproducibility [11]. It was
believed that some parallel reaction should take place;
the formation of isocyanate and alcohol has been re-
ported [1,12–14], when the alkyl side of the phenylcar-
bamate is primary or secondary.
The stoichiometry based on reaction (2) required
that, for long reaction time Pf/Po = 3.0, where Pf
and Po are the final and initial pressure, respectively.
The average experimental Pf/Po values at four differ-
ent temperatures and 10 half-lives were 2.98 for ethyl
N-benzyl-N-cyclopropylcarbamate and 2.78 for ethyl
N,N-diphenylcarbamate (Table I). The small depar-
ture in ethyl diphenylcarbamate was due to a small
polymerization of the product diphenylamine.
The homogeneity of these reactions 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, taken as 1.0 (Table II).
With regard to the results of the present work, the
mechanisms for reactions described in reaction (2) may
be explained in terms of a semipolar six-membered
cyclic transition state as pictured in reaction (1).

STERIC FACTORS IN THE GAS PHASE ELIMINATION KINETICS
71
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