Kinetics and mechanisms of the gas-phase elimination of arylethyl N,N-dimethylcarbamates and ω-phenylalkyl N,N-dimethylcarbamates

Article abstract of DOI:10.1002/poc.567

The gas-phase elimination kinetics of several arylethyl N,N-dimethylcarbamates and ω-phenylalkyl N,N-dimethylcarbamates were determined in the temperature range 299.6-399.9 °C and pressure range 18-95 Torr. The reactions in a static system, seasoned with allyl bromide, and in the presence of a free radical suppressor are homogeneous and unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius equations: for 4-phenethyl N,N-dimethylcarbamate, log [k₁ (s^-1)] = (11.32 ± 0.22) - (166.9 ± 2.5) kJ mol^-1 (2.303RT)^-1; for methylphenethyl N,N-dimethylcarbamate, log[k₁ (s^-1)] = (12.07 ± 0.36) - (178.6 ± 4.3) kJ mol^-1 (2.303RT)^-1; for 4-methoxyphenethyl N,N-dimethylcarbamate, log[k₁ (s^-1)] = (11.03 ± 0.60) - (167.3 ± 7.1) kJ mol^-1 (2.303RT)^-1; for 4-nitrophenethyl N,N-dimethylcarbamate, log[k₁ (s^-1)] = (11.31 ± 0.54) - (163.7 ± 6.1) kJ mol^-1 (2.303 RT)^-1; for 3-(4-methoxyphenyl) propyl N,N-dimethylcarbamate, log [k₁ (s^-1)] = (13.52 ± 0.54) - (208.4 ± 6.8) kJ mol^-1 (2.303 RT)^-1; for 4-phenyl-1-butyl N,N-dimethylcarbamate, log [k₁ (s^-1)] = (12.00 ± 0.34) - (185.2 ± 4.2) kJ mol^-1 (2.303 RT)^-1; and for 5-phenyl-1-pentyl N,N-dimethyl carbamate, log [k₁ (s^-1)] = (11.79 ± 0.31) - (182.2 ± 3.9) kJ mol^-1 (2.303RT)^-1. The results imply the absence of anchimeric assistance of the phenyl group, while the acidity of the benzylic β-hydrogen appears to be responsible for a small but significant rate augmentation in these eliminations. Copyright

Full text of DOI:10.1002/poc.567

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 40–46
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.567
Kinetics and mechanisms of the gas-phase elimination of
arylethyl N,N-dimethylcarbamates and v-phenylalkyl N,N-
dimethylcarbamates
Gabriel Chuchani,* Rosa M. Dominguez, Alexandra Rotinov and Armando Herize
Centro de Qu õÂ mica, Instituto Venezolano de Investigaciones Cient Âõ ®cas (IVIC), Apartado 21827, Caracas 1012-A, Venezuela
Received 8 May 2002; revised 31 July 2002; accepted 2 August 2002
ABSTRACT: The gas-phase elimination kinetics of several arylethyl N,N-dimethylcarbamates and o-phenylalkyl
N,N-dimethylcarbamates were determined in the temperature range 299.6–399.9°C and pressure range 18–95 Torr.
The reactions in a static system, seasoned with allyl bromide, and in the presence of a free radical suppressor are
homogeneous and unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius
À1
À1
À1
À1
equations: for 4-phenethyl N,N-dimethylcarbamate, log[k1 (s )] = (11.32 Æ 0.22) À(166.9 Æ 2.5) kJ mol
À1
(
2.303RT) ; for methylphenethyl N,N-dimethylcarbamate, log[k (s )] = (12.07 Æ 0.36) À(178.6 Æ 4.3) kJ mol
À1 À1
1
(
2.303RT) ; for 4-methoxyphenethyl N,N-dimethylcarbamate, log[k (s )] = (11.03 Æ 0.60) À(167.3 Æ 7.1)
À1 À1 À1
1
kJ mol
(2.303RT) ; for 4-nitrophenethyl N,N-dimethylcarbamate, log[k (s )] = (11.31 Æ 0.54) À(163.7 Æ
À1 À1 À1
1
6
.1) kJ mol
(2.303RT)
;
for 3-(4-methoxyphenyl)propyl N,N-dimethylcarbamate, log[k
(s )] =
1
À1
À1
(13.52 Æ 0.54) À(208.4 Æ 6.8) kJ mol
(2.303 RT) ; for 4-phenyl-1-butyl N,N-dimethylcarbamate, log[k1
À1
À1
À1
(s
)] = (12.00 Æ 0.34) À(185.2 Æ 4.2) kJ mol (2.303 RT) ; and for 5-phenyl-1-pentyl N,N-dimethyl carbamate,
À1
À1
À1
log[k (s )] = (11.79 Æ 0.31) À(182.2 Æ 3.9) kJ mol (2.303RT) . The results imply the absence of anchimeric
1
assistance of the phenyl group, while the acidity of the benzylic b-hydrogen appears to be responsible for a small but
significant rate augmentation in these eliminations. Copyright  2002 John Wiley & Sons, Ltd.
KEYWORDS: arylethyl N,N-dimethylcarbamates; o-phenylalkyl N,N-dimethylcarbamates; gas-phase elimination;
kinetics; mechanism
2
INTRODUCTION
2-phenethyl chloride, the presence of the methoxy
(
OCH ) group at the 4-position of pheneylethyl chloride
3
The use of several structure–reactivity relationships for
the gas-phase elimination kinetics of a considerable1
number of 2-substituted-ethyl N,N-dimethylcarbamates
gave meaningless results, except for Taft original s*
values. Three good slopes were originated at s*
suggested the participation of the aromatic nuclei by a
significant increase in the rate of HCl elimination. In
addition to this work, the phenyl susbtituent was also
found to assist anchimerically the gas-phase elimination
3
of o-phenylalkyl methanesulfonates. The C H sub-
6
5
(
CH ) = 0.00. Mechanisms were suggested on the basis
stituent at the 2- and 4-positions with respect to the C—O
bond of the methanesulfonate confirmed participation in
the rate of pyrolysis. Moreover, the five-membered
structure which is favorable for anchimeric assistance
yielded to some extent a cyclic product, tetralin.
Neighboring phenyl group participation at the 3-position
was found to be absent.
3
of this relationship. However, the point positions of
phenyl (C H ) and isopropenyl [CH2=C(CH )] as
6
5
3
substituents Z in (CH ) NCOOCH CH Z were found to
3
2
2
2
fall far above the three slopes of the lines. These
substituents were believed to enhance the rate of
elimination due to neighboring group participation or to
the acidity of the benzylic and allylic C —H bond.
In association with the above considerations, and
previous work on the gas-phase pyrolytic elimination of
Because of the dichotomy as to whether the anchimeric
assistance of the phenyl substituent or the acidity of the
b
benzylic C —H bond enhanced the rate of elimination of
b
2-substituted-ethyl N,N-dimethylcarbamates, the present
work was aimed at examining the gas-phase pyrolysis
kinetics of several arylethyl N,N-dimethylcarbamates and
o-phenylalkyl N,N-dimethylcarbamates, and it was
possible to reveal which of these two factors affects the
rate of elimination of these carbamates.
*
Correspondence to: G. Chuchani, Centro de Qu ´ı mica, Instituto
Venezolano de Investigaciones Cient ı´ ficas (IVIC), Apartado 21827,
Caracas 1012-A, Venezuela.
Contract/grant sponsor: Fondo Nacional de Ciencia, Tecnolog ´ı a e
Innovaci o´ n (FONACIT); Contract/grant number: S1-97000005.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

GAS-PHASE ELIMINATION OF ARYLALKYL N,N-DIMETHYLCARBAMATES
41
Table 1. Stoichiometry of the reactions
Substrate
Temperature (°C)
Parameter
Time (min)
Value
20
4
-Methylphenethyl N,N-dimethylcarbamate
339.6
5
10
31
Reaction (%) (pressure) 20.0 37.1 58.9 73.6
Olefin (%) (GLC)
Time (min)
18.5 35.8 59.2 73.3
10 15 20
4
-Methoxyphenethyl N,N-dimethylcarbamate
349.9
6
8
Reaction (%) (pressure) 29.4 37.7 42.7 58.8 62.8
Olefin (%) (GLC)
—
Time (min)
28.2 36.8 43.0 56.5 62.4
a
4
3
-Nitrophenethyl N,N-dimethylcarbamate
—
379.6
—
5
—
6
—
7
—
8
—
-(4-Methoxyphenyl)propyl N,N-dimethylcarbamate
Reaction (%) (pressure) 17.4 22.2 25.0 30.8
Olefin (%) (GLC)
Time (min)
16.9 22.5 25.7 31.9
10
4
5
-Phenyl-1-butyl N,N-dimethylcarbamate
369.6
379.1
4
6
8
15
Reaction (%) (pressure) 22.7 31.6 39.6 46.4 59.0
Olefin (%) (GLC)
Time (min)
Reaction (%) (pressure) 28.9 33.9 39.2 44.6 53.9
Olefin (%) (GLC)
21.8 31.6 38.3 46.0 58.5
10
-Phenyl-1-pentyl N,N-dimethylcarbamate
4
5
6
8
29.0 33.5 40.0 44.2 54.1
a
The stoichiometry was difficult to determine because the olefin product could not be obtained quantitatively.
RESULTS AND DISCUSSION
ogeneous, since no significant variations in rates were
obtained when using packed and unpacked seasoned
vessels with allyl bromide with a surface-to-volume ratio
of 6.0 relative to that of the normal vessel. (Table 2).
However, the packed and unpacked clean Pyrex vessels
showed a marked effect on 4-nitrophenethyl N,N-
dimethylcarbamate and 5-phenyl-1-pentyl N,N-dimethyl-
carbamate.
The effect of addition of different proportions of the
free radical inhibitor cyclohexene or toluene is described
in Table 3. Nevertheless, the pyrolyses of these
carbamates were carried out in the presence of at least
twice the amount of the inhibitor in order to avoid any
possible free radical chain reactions. No induction period
was observed. The rate coefficients were reproducible
with a relative standard deviation not greater than 5% at a
given temperature.
The unimolecular elimination of 2-substituted-ethyl N,N-
dimethylcarbamates in the gas phase is described by the
reaction
ꢀ
CH † NCOOCH CH Z!ꢀCH † NCOOH‡CH =CHZ
3
2
2
2
3 2
2
#
ꢀ1†
ꢀCH † NH ‡ CO
3
2
2
where Z = 4-CH C H , 4- CH OC H , 4- NO C H ,
3
6
4
3
6
4
2 6 4
4
- CH OC H CH , 4- C H CH CH or 4- C H CH -
3 6 4 2 6 5 2 2 6 5 2
CH CH . The stoichiometry of reaction (1) implies for a
2
2
long reaction time P /P = 3, where P and P are the final
f
0
f
0
and initial pressure, respectively. The average P /P0
values at four temperatures and ten half-lives were 2.8 for
f
4
-methylphenethyl N,N-dimethylcarbamate, 2.9 for 4-
The first-order rate coefficients of the carbamates
calculated from k = (2.303/t)log[2P /(3P À P )] were
methoxyphenethyl N,N-dimethylcarbamate, 3.0 for 4-
nitrophenethyl N,N-dimethylcarbamate, 2.1 for 3-(4-
methoxyphenyl)propyl N,N-dimethylcarbamate, 3.0 for
1
0
0
t
found to be independent of the initial pressure (Table 4).
A plot of log(3P À P ) vs time gave a good straight line
0
t
4
-phenyl-1-butyl N,N-dimethylcarbamate and 2.9 for 5-
up to 30–70% decomposition. The temperature depen-
dence of the rate coefficient and the corresponding
Arrhenius equations are given in Table 5 (95%
confidence coefficient from least-squares procedure).
According to the data in Table 6, if neighboring group
participation is the paramount factor affecting the rate of
elimination of these phenethyl N,N-dimethylcarbamates,
the 4-CH and 4-OCH substituents in the benzene ring
phenyl-1-pentyl N,N-dimethylcarbamate. The departure
from P /P0 <3 for 3-(4-methoxyphenyl)propyl N,N-
f
dimethylcarbamate may be due to some significant
condensation or polymerization of the corresponding
product 3-(4-methoxyphenyl)-1-propene. An authentic
sample of this unsaturated product, when introduced into
the reaction vessel, gave rise to a decrease in pressure to
yield an unidentified solid. The stoichiometry of reaction
3
3
should have given a significant augmentation in rates
(
1), up to 30–74% reaction, was checked by comparing
compared with 4-H and 4-NO groups. On the other hand,
2
the extent of decomposition of the substrates from
pressure measurements with that obtained from quanti-
tative gas—liquid chromatographic (GLC) analyses of
the corresponding olefin formation (Table 1).
the relative rates depicted in Scheme 1 suggest that the
assistance of the benzylic b-hydrogen may be responsible
for a small but significant rate increase in the rate of
elimination.
The elimination reaction (1) was found to be hom-
A further approach to elucidating whether neighboring
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

4
2
G. CHUCHANI ET AL.
Table 2. Homogeneity of the reactions
À1 a
4
À1 b
4
À1 c
Compound
S/V (cm
)
10 k (s
)
10 k (s )
1
1
4-Methylphenethyl N,N-dimethylcarbamate at 339.6°C
4-Methoxyphenethyl N,N-dimethylcarbamate at 349.9°C
4-Nitrophenethyl N,N-dimethylcarbamate at 330.2°C
3-(4-Methoxyphenyl)propyl N,N-dimethylcarbamate at 379.6°C
4-Phenyl-1-butyl N,N-dimethylcarbamate at 369.6°C
5-Phenyl-1-pentyl N,N-dimethylcarbamate at 379.1°C
1
6
1
6
1
6
1
6
1
6
1
6
7.29
7.18
9.01
7.30
7.07
9.55
9.50
13.0
13.6
7.21
7.31
9.55
9.39
15.5
15.8
9.20
d
12.1
d
12.5
7.96
8.16
9.40
9.22
13.2
10.7
a
b
c
d
S = Surface area; V = volume.
Clean Pyrex vessel.
Vessel seasoned with allyl bromide.
k Value up to 20% decomposition.
group participation may influence the rate of elimination
is to examine the effect of the C H ring along the carbon
structures. This argument is derived from the lack
of rate increase and possible formation of ring-closed
products. The experimental findings imply that the
6
5
chain of o-phenylalkyl N,N-dimethylcarbamates. The
comparative rates in Scheme 2 suggest the absence of
anchimeric assistance of the C H group at the 2-position
acidity of the benzylic C —H bond assists the
b
elimination process, while the rate decreased on inter-
position of a CH2 group with respect to the N,N-
dimethylcarbamate.
6
5
for a three-membered structure and at the 4- and 5-
positions for more favorable five- and six-membered
a
Table 3. Effect of the free radical inhibitor on rates
b
b
4
À1
Substrate
Temperature (°C)
Ps (Torr)
Pi (Torr)
Pi/Ps
10 k (s
)
1
4
-Methylphenethyl N,N-dimethylcarbamate
349.7
77.5
—
52
83.5
187.5
136
—
73.5
108
129
—
90.5
40
51.5
107.5
163.5
—
45.5
86.5
146.5
168
—
—
0.8
1.6
2.5
3.7
—
1.5
1.6
3.2
—
1.1
1.5
2.9
4.4
5.1
—
0.9
1.4
3.4
5.3
—
11.6
11.7
11.6
11.8
11.7
9.42
9.54
9.35
9.03
13.0
13.2
13.1
13.2
13.0
13.2
22.1
22.2
22.0
21.9
22.4
9.61
9.63
9.39
9.83
27.7
15.1
15.6
15.5
15.7
6
5
7
3
5
1
5.5
7
31
0.4
8
0
4
-Methoxyphenethyl N,N-dimethylcarbamate
-Nitrophenethyl N,N-dimethylcarbamate
349.9
330.2
5
6
4
4
41
8
2
1
2
3
4
7.5
8
4.5
2
3
-(4-Methoxyphenyl)propyl N,N-dimethylcarbamate
399.2
54.5
5
6
4
3
1
4
3
2
4
-Phenyl-1-butyl N,N-dimethylcarbamate
-Phenyl-1-pentyl N,N-dimethylcarbamate
369.6
379.1
54
6
6
5
2.5
4.5
1
47
97
173
—
54
74
134.5
125.5
0.8
1.5
3.4
—
0.9
1.4
2.5
3.4
5
32
6
5
5
3
3
2
3.5
7
a
Temperature <360°C with cyclohexene inhibitor, >360°C, with toluene inhibitor.
, Pressure of the substrate; P , pressure of the inhibitor.
P
s
i
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

GAS-PHASE ELIMINATION OF ARYLALKYL N,N-DIMETHYLCARBAMATES
43
Table 4. Invariability of the rate coef®cients from initial pressure
Substrate Temperature (°C) Parameter
Value
4-Methylphenethyl N,N-dimethylcarbamate
4-Methoxyphenethyl N,N-dimethylcarbamate
4-Nitrophenethyl N,N-dimethylcarbamate
3-(4-Methoxyphenyl)propyl N,N-dimethylcarbamate
4-Phenyl-1-butyl N,N-dimethylcarbamate
5-Phenyl-1-pentyl N,N-dimethylcarbamate
359.1
349.9
330.2
390.6
369.6
379.1
P (Torr)
27
19.8
21.4
9.35
18
13.2
40.5
12.7
48
45
20.2
31.4
79
20.0
40
9.03
32
13.2
63
12.9
64
9.39
52
15.6
95
20.2
50.4
9.54
41
13.1
82.5
12.9
79.5
0
4
À1
1
0 k (s
)
)
)
)
)
)
1
P (Torr)
0
4
À1
1
0 k (s
9.42
1
P (Torr)
24.5
13.0
51
12.8
51
84
13.2
0
4
À1
1
0 k (s
1
P (Torr)
0
4
À1
1
0 k (s
1
P (Torr)
0
4
À1
1
0 k (s
9.49
30
15.5
9.83
48
15.3
9.50
63
15.3
1
P (Torr)
0
4
À1
1
0 k (s
1
EXPERIMENTAL
determined by GLC (DB-5MS capillary column, 30 m
1
 0.250 mm i.d., 0.25 mm). H NMR, ꢀ 1.8–1.9 (q, 2H,
General procedure. The arylethanol or the o-phenylalk-
anol (0.15 mol) was added to N,N-dimethylcarbamyl
chloride (0.15 mol) in 50 ml of carbon tetrachloride and
the reaction mixture was heated until no more HCl gas
was evolved.
C—CH —C), 2.6–2.8 (m, 2H, CH —C), 2.8–2.9 [s, 6H,
2
2
(CH ) N], 3.7 (s, 3H, OCH ), 3.8–4.1(t, 2H, —CH O—),
3
2
3
2
‡
6.8–7.1 (m, 4H, C H ). MS, m/z 237 (M ), 147
6
4
‡
[CH OC H CH CH CH ] , 133 [CH OC H CH -
3
6
4
2
2
2
3
6
4
2
‡
‡
‡
CH ] , 119 [CH OC H CH ] , 72 [(CH ) NCO] , 44
2
3
6
4
2
3 2
(CO2).
2
-Phenethyl N,N-dimethylcarbamate. This compound
1
was prepared as reported previously.
4-Phenylbutyl N,N-dimethylcarbamate. The above
method was used for the preparation of this substrate.
B.p. 150°C at 22 Torr, yield 67%. Several distillations
gave a 98.9% purity by GLC (DB-5MS capillary column,
4
-Methylphenethyl N,N-dimethylcarbamate. This sub-
strate was distilled several times to 99.5% purity as
determined by GLC (DB-5MS capillary column, 30
1
30 m  0.250 mm i.d., 0.25 mm). H NMR, ꢀ 1.7 (m, 4H,
m  0.250 mm i.d., 0.25 mm). B.p. 124°C at 2 Torr
CH CH ), 2.6 (t, 2H, CH Ph), 2.9 [s, 6H, (CH ) N], 4.1
2
2
2
3 2
1
‡
(
1 Torr = 133.3 Pa), yield 61%. H NMR, ꢀ 2.3 (s,
(t, 2H, OCH ), 7.2–7.3 (m, 5H, C H ). MS, m/z 221 (M ),
133 [C H CH CH CH CH ] , 119 [C H CH CH -
2 6 5
‡
3
2
H, CH ), 2.8 (m, 8H, 2CH , —CH O—), 4.2 (t,
H, CH ), 7.1 (m, 4H, C H ). MS, m/z 207 (M ), 119
3
3
2
6
5
2
2
2
2
6
5
2
2
‡
‡
‡
‡
CH ] , 105 [C H CH CH ] , 91 [C H CH ] , 72
2
6
5
2
6
5
2
2
6
5
2
‡
‡
[
CH C H CH CH ] .
[(CH ) NCO] , 44 (CO ).
3 2 2
3
6
4
2
2
4
-Methoxyphenethyl N,N-dimethylcarbamate. Several
5-Phenylpentyl N,N-dimethylcarbamate. This com-
pound was prepared by the above procedure. B.p.
158°C at 2 Torr, yield 80.0%. Several distillations gave
a 99.0% purity by GLC (DB-5MS capillary column, 30 m
distillations of this carbamate gave a purity of 99.6% as
determined by GLC (FFAP–Chromosorb W AW, 80–100
1
mesh). B.p. 160°C at 2 Torr, yield 70%. H NMR, ꢀ 2.9
1
(
m, 8H, 2CH , —CH O—), 4.2 (t, 2H, CH ), 6.8–7.1 (m,
 0.250 mm i.d., 0.25 mm). H NMR, ꢀ 1.4 (m, 2H, C—
3
2
2
‡
4
H, C H ), 7.8 (s, 3H,CH O). MS, m/z 237 (M ), 133
CH —C), 1.7 (m, 4H, CH —C—CH ), 2.6 (t, 2H,
6
4
3
2
2
2
‡
‡
[
CH OC H CH CH ] 121 [CH OC H CH ] .
CH Ph), 2.9 [s, 6H, (CH ) N], 4.0 (t, 2H, OCH ), 7.1–7.3
3
6
4
2
2
3
6
4
2
2 3 2 2
‡
(
m, 5H, C H ). MS, m/z 235 (M ), 147 [C H CH -
6
5
6
5
2
‡
‡
4
-Nitrophenethyl N,N-dimethylcarbamate. This solid
CH CH CH CH ] , 133 [C H CH CH CH CH ] , 119
2
2
2
2
6
5
2
2
2
2
‡
‡
compound was recrystallized several times with water to
[C H CH CH CH ] , 105 [C H CH CH ] , 91
6 5 2 2 2 6 5 2 2
‡
1
‡
9
9.0% purity. M.p. 81°C, yield 66%. H NMR, ꢀ 2.8–2.9
[C H CH ] , 72 [(CH ) NCO] , 44 (CO ).
6 5 2 3 2 2
(
s, 3H, 3CH ), 3.1 (t, 2H, —CH O—), 4.3 (t, 2H, CH )
Quantitative chromatographic analyses of the unsatu-
rated olefinic products were performed by GLC with a
DB-5MS capillary column, 30 m  0.250 mm i.d.,
0.25 mm film thickness. However, 4-Methoxystyrene
was analyzed by GLC with an FFAP–Chromosorb W
AW, 80–100 mesh column. The solid product 4-
nitrostyrene was difficult to analyze quantitatively using
different types of GLC columns. The identifications of
substrates and products were performed by GLC–MS
3
2
2
‡
7
.4–8.2 (m, 4H, C H ). MS, m/z 236 (M ), 150
6 4
‡
‡
‡
[
NO C H CH CH ] , 120 [NO C H ] , 88 [(CH ) -
2 6 4 2 2 2 6 4 3 2
‡
‡
NCOO] , 72 [(CH ) NCO] , 46 [NO ] , 44 (CO ).
3
2
2
2
3-(4-Methoxyphenyl)-propyl N,N-dimethylcarbamate.
This starting material was prepared by the above-
mentioned method. B.p. 165°C at 1 Torr, yield 65%.
After several distillations the purity was 99.4% as
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

4
4
G. CHUCHANI ET AL.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

GAS-PHASE ELIMINATION OF ARYLALKYL N,N-DIMETHYLCARBAMATES
45
Table 6. Kinetic and thermodynamic parameters for pyrolysis of ZCH CH OCON(CH ) at 360°C
2
2
3 2
k1Â10À4 (sÀ1
)
Ea (kJ mol
À1
)
Log A (s
À1
)
DS (J mol
≠
À1 À1
K
)
DH (kJ mol
≠
À1
)
DG (kJ mol
≠
À1
)
Z
C H
35.3
5.72
2.10
5.22
5.69
21.5
16.8
63.4
166.9 Æ 2.5
181.2 Æ 3.2
208.4 Æ 6.8
185.2 Æ 4.2
182.2 Æ 3.9
178.6 Æ 4.3
167.3 Æ 7.1
163.7 Æ 6.1
11.32 Æ 0.22
11.71 Æ 0.26
13.52 Æ 0.54
12.00 Æ 0.34
11.79 Æ 0.31
12.07 Æ 0.36
11.03 Æ 0.60
11.31 Æ 0.54
À42.74
161.6
175.9
203.1
179.9
176.9
173.3
162.0
158.4
188.7
198.2
203.5
198.7
198.3
191.3
192.6
185.6
6
5
a
C H CH
À35.27
À0.62
6
5
2
4
-CH OC H CH
3 6 4 2
C H CH CH
À29.72
À33.73
À28.38
À48.28
À42.92
6
5
2
2
C H CH CH CH
6
5
2
2
2
4
4
4
-CH C H
3 6 4
-CH OC H
-NO C H
6 4
3
6 4
2
a
From Ref. 1.
Scheme 1
Scheme 2
measured with a calibrated platinum–platinum–13%
rhodium thermocouple. No temperature gradient was
found along the reaction vessel. The substrates 4-
methoxyphenethyl N,N-dimethylcarbamate dissolved in
dioxane, 4-nitrophenethyl N,N-dimethylcarbamate dis-
solved in chlorobenzene and the other carbamates in pure
liquid form were injected with a syringe through a
silicone rubber septum directly into the reaction vessel.
(
3
Saturn 2000, Varian) with a DB-5MS capillary column,
0 m  0.250 mm i.d., 0.25 mm film thickness.
Kinetics. The pyrolyses kinetics were studied in a static
system as described
4,5
with some modifications and
additions of modern electronic and electrical devices.
The reaction vessel was seasoned with allyl bromide, and
the experiments were performed in the presence of at
least twice the amount of the free radical inhibitor
cyclohexene or toluene. The rate coefficients were
determined manometrically or by quantitative chromato-
graphic analyses of the olefinic products. The tempera-
ture was controlled by a resistance thermometer
controller, Shinko DIC-PS 25RT, and an Omega Solid
State SSR240AC45, maintained within Æ0.2°C and
Acknowledgement
We are grateful for the financial support of the Fondo
Nacional de Ciencia, Tecnolog ı´ a e Innovaci o´ n (FONA-
CIT) of Venezuela, Project No. S1-97000005.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46

4
6
G. CHUCHANI ET AL.
2
3
. Hernandez JA, Chuchani G. Int. J. Chem. Kinet. 1978; 10: 923–929.
. Chuchani G, Dom ´ı nguez RM, Rotinov A, Alvarez J. J. Phys. Chem.
990; 94: 3341–3343.
. Maccoll A. J. Chem. Soc. 1955; 965–973.
REFERENCES
1
1
. Chuchani G, Nu n˜ ez O, Marcano N, Napolitano S, Rodriguez H,
Dom ´ı nguez M, Ascanio J, Rotinov A, Dominguez RM, Herize A. J.
Phys. Org. Chem. 2001; 14: 146–158.
4
5. Swinbourne ES. Aust. J. Chem. 1958; 11: 314–330.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 40–46