Gas-phase elimination kinetics of ethyl, isopropyl and tert-butyl N,N-diethylcarbamates. Application of Taft-Topsom correlation for substituents other than carbon at the acid side of organic ethyl esters

Article abstract of DOI:10.1002/(SICI)1099-1395(199903)12:3<201::AID-POC112>3.0.CO;2-G

The elimination kinetics of ethyl, isopropyl and tert-butyl N,N-diethylcarbamates were investigated in a static reaction vessel over the temperature range 220-400°C and pressure range 17-160 Torr. These reactions are homogeneous, unimolecular and follow a first-order rate law. The temperature dependance of the rate coefficients is given by the following equations: for ethyl N,N-diethylcarbamate, log k₁ (s^-1) = (11.47 ± 0.25) - (178.4 ± 3.1) kJ mol^-1 (2.303 RT)^-1, for isopropyl N,N-diethylcarbamate, log k₁ (s^-1) = (12.83 ± 0.70) - (179.8 ± 7.9) kJ mol^-1 (2.303 RT)^-1; and for tert-butyl N,N-diethylcarbamate, log k₁ (s^-1) = (12.87 ± 0.62) - (158.6 ± 6.2) kJ mol^-1 (2.303 RT)^-1. The branching of the alkyl groups at the alcohol side of the ester exerts a significant effect on the rates in the order tert-butyl > isopropyl > ethyl. In addition, the presence of different substituents other than carbon at the acid side of organic ethyl esters gives the best correlation when using the Taft-Topsom equation: log k/k_H = -(0.68 ± 0.12)σ_s + (2.57 ± 0.12)σ_F - (1.18 ± 0.27)σ_R (r = 0.984 ± 0.119 at 400°C). According to this relationship, the field (inductive) effect of the substituent has the greatest influence on rate enhancement, while the polarizability (steric) and resonance factors, although small in effect, favour the elimination process. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(199903)12:3<201::AID-POC112>3.0.CO;2-G

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 12, 201–206 (1999)
Gas-phase elimination kinetics of ethyl, isopropyl and tert-
butyl N,N-diethylcarbamates. Application of Taft±Topsom
correlation for substituents other than carbon at the acid
side of organic ethyl esters
Armando Herize,1 Rosa M. Dominguez,2 Alexandra Rotinov,2 Oswaldo NunÄ ez1 and Gabriel Chuchani2*
1Departamento de QuõÂmica, Universidad Simo n BolõÂvar, Sartenejas, Edo. Miranda, Venezuela
2Centro de QuõÂmica, Instituto Venezolano de Investigaciones CientÂõ®cas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 20 March 1998; revised 1 June 1998; accepted 5 June 1998
ABSTRACT: The elimination kinetics of ethyl, isopropyl and tert-butyl N,N-diethylcarbamates were investigated in a
static reaction vessel over the temperature range 220–400°C and pressure range 17–160 Torr. These reactions are
homogeneous, unimolecular and follow a first-order rate law. The temperature dependance of the rate coefficients is
given by the following equations: for ethyl N,N-diethylcarbamate, log k1 (s 1) = (11.47 Æ 0.25) (178.4 Æ 3.1) kJ
1
1
mol (2.303 RT) 1, for isopropyl N,N-diethylcarbamate, log k1 (s 1) = (12.83 Æ 0.70) (179.8 Æ 7.9) kJ mol
(2.303 RT) 1; and for tert-butyl N,N-diethylcarbamate, log k1 (s 1) = (12.87 Æ 0.62) (158.6 Æ 6.2) kJ mol 1 (2.303
RT) 1. The branching of the alkyl groups at the alcohol side of the ester exerts a significant effect on the rates in the
order tert-butyl > isopropyl > ethyl. In addition, the presence of different substituents other than carbon at the acid
side of organic ethyl esters gives the best correlation when using the Taft–Topsom equation: log k/kH =
(0.68 Æ 0.12)ꢀa ‡ (2.57 Æ 0.12)ꢀF (1.18 Æ 0.27)ꢀR (r = 0.984 Æ 0.119 at 400°C). According to this relation-
ship, the field (inductive) effect of the substituent has the greatest influence on rate enhancement, while the
polarizability (steric) and resonance factors, although small in effect, favour the elimination process. Copyright
1999 John Wiley & Sons, Ltd.
KEYWORDS: alkyl N,N-diethylcarbamates; elimination kinetics; Taft–Topsom correlation
INTRODUCTION
The homogeneous, unimolecular thermal decomposition
of a carbamate in the gas phase was first reported by Daly
and Ziolkowski.1 The substrate ethyl N-methyl-N-
phenylcarbamate was found to produce N-methylaniline,
ethylene and carbon dioxide [reaction 1]. The generally
Scheme 1
methyl N-methylcarbamate5 shows that its decomposi-
tion process differs from that of ethyl N-methyl-N-
phenylcarbamate [reaction 2].
accepted mechanism of this elimination process con-
sisted of a six-membered cyclic transition state (Scheme
1) similar to those described for acetates, carbonates, and
xanthates.1–4
CH3NHCOOCH3 ! CH3NCO ‡ CH3OH
ꢀ2†
The presence of a hydrogen at the nitrogen atom was
believed to be responsible for the different mechanistic
pathway, and the elimination reaction was explained in
terms of a four-membered cyclic transition-state mechan-
ism (Scheme 2).
Further examination of carbamate pyrolysis using
*Correspondence to: G. Chuchani, Centro de Qu´ımica, Instituto
Venezolano de Investigaciones Cientı´ficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
An additional study of the gas-phase pyrolysis of ethyl
N,N-dimethylcarbamate4,6 led to the conclusion that
carbamate decomposition proceeds through an inter-
mediate whose structure lies between a semi-concerted
Contract/grant sponsor: Consejo Nacional de Investigaciones Cientı´-
ficas
y Tecnolo´gicas (CONICIT); Contract/grant number: S1-
97000005.
Copyright 1999 John Wiley & Sons, Ltd.
CCC 0894–3230/99/030201–06 $17.50

202
A. HERIZE ET AL.
The stoichiometry as described in reaction 3 implies
that, for long reaction times, Pf = 3P0, where Pf and P0
are final and initial pressure, respectively (Table 1). The
average experimental Pf/P0 at four different temperatures
and ten half-lives were 2.85 for ethyl N,N-diethylcarba-
mate, 2.84 for isopropyl N,N-diethylcarbamate and 2.83
for tert-butyl N,N-diethylcarbamate.
The small departure of Pf/P0 < 3 was due to the
polymerization of the product diethylamine. Further
examination of the stoichiometry of reaction 3, up to
40–70% reaction, was made by comparing the amount of
decomposition of the substrate calculated from pressure
measurements with that obtained from quantitative gas–
ligun chromatographic (GLC) analyses of the unreacted
carbamate (Table 2).
The decomposition reaction 3 is homogeneous, since
no significant variation of the rates was observed in the
experiments 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. The pyrolyses of
these carbamate substrates were not affected by the
presence of different proportions of a free radical
suppressor such as cyclohexene (Table 3). No induction
period was observed. The rate coefficients are reprodu-
cible with a relative standard deviation not greater than
5% at a given temperature.
Scheme 2
six-centered cyclic transition state and an intimate ion-
pair type of mechanism.
The kinetics of pyrolysis of esters of carboxylic acids
of the type ZCOOR, where R is an alkyl group and Z is an
electron-releasing or -withdrawing aliphatic substituent,
were correlated by plotting the log krel. vs Taft s*
values.7–9 Each of these correlations gave a small
positive r* value. These results suggested that a negative
charge has developed at the g-carbon, where the Ca—O
ꢁ ‡
ꢁ
ÁÁÁO
bond polarization, in the direction of Ca
determining step (Scheme 1).
is rate-
In view of the few earlier studies on carbamate
pyrolysis, the present work was aimed at examining the
elimination kinetics of primary, secondary and tertiary
alkyl N,N-diethylcarbamates and the possibility of
establishing a structure–reactivity relationship of sub-
stituents Z of different ethyl esters of organic acids of the
type ZCOOCH2CH3.
The first-order rate coefficients of the carbamates,
calculated from k1 = (2.303/t)log[2P0/(3P0-Pt)], were
found to be independent of the initial pressure. A plot
of log(3P0-Pt) against time t gave a good straight line up
to 40–70% decomposition.
The temperature dependence of the rate coefficient and
the corresponding Arrhenius equations are given in Table
4 (90% confidence coefficient from the least-squares
procedure).
The order of reactivity, tertiary > secondary > prim-
ary alkyl esters of N,N-diethylcarbamates (Table 5), is
consistent with the results of the comparative rates for the
corresponding N,N-dimethylcarbamates10 (Table 5) and
acetates.17 This sequence is to be expected from electron
RESULTS AND DISCUSSION
The products of elimination of ethyl, isopropyl and tert-
butyl esters of N,N-diethylcarbamic acid are the corre-
sponding olefin, diethylamine and carbon dioxide [reac-
tion 3].
(CH3CH2†2NCOOCRR1CH3 ! RR1C CH2
‡ ꢀCH3CH2†2NH ‡ CO2
i R ˆ R1 ˆ H
ꢀ3†
ii R ˆ H, R1 ˆ CH3
iii R ˆ R1 ˆ CH3
Table 1. Ratio of ®nal to initial pressure of the substrate (Pf/P0)
Compound
Temperature (°C)
P0 (Torr)
Pf (Torr)
Pf/P0
Average
2.85
Ethyl N,N-diethylcarbamate
359.7
379.9
389.7
399.6
301.6
311.2
330.3
350.7
241.7
249.9
261.0
271.0
48
42
50.5
66
56
51.5
58
135
122
144
186
160
147
166
169
89
2.81
2.90
2.85
2.82
2.86
2.88
2.76
2.84
2.83
2.82
2.91
2.77
Isopropyl N,N-diethylcarbamate
tert-Butyl N,N-diethylcarbamate
2.84
2.83
59.5
31.5
25.5
47
72
137
115
41.5
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 201–206 (1999)

GAS-PHASE ELIMINATION KINETICS OF ALKYL N,N-DIETHYLCARBAMATES
Table 2. Stoichiometry of the reaction
203
Parameter
Ethyl N,N-diethylcarbamate at 380.2°C
Time (min)
Reaction (%) (pressure)
Substrate (%) (GLC)
1
11.7
12.2
3
29.0
28.4
5
42.2
41.5
8
52.0
53.5
12
71.5
67.0
Isopropyl N,N-diethylcarbamate at 330.0°C
Time (min)
Reaction (%) (pressure)
Substrate (%) (GLC)
1
8.6
8.5
2
19.1
21.3
3
26.6
28.7
4
34.9
34.5
5
39.5
41.0
tert-Butyl N,N-diethylcarbamate at 250.4°C
Time (min)
1.5
9.6
10.0
3
17.8
21.6
6
26.5
29.3
10
44.8
40.4
17
60.2
64.5
Reaction (%) (pressure)
Substrate (%) (GLC)
Table 3. Effect of the radical inhibitor cyclohexene on rate
1
Substrate
Temperature (°C)
Ps (Torr)a
Pi (Torr)a
PiPs
104 k1 (s
)
Ethyl N,N-diethylcarbamate
375.5
17
160.5
82
—
37
70
94
118
—
58
69
79
133
—
46
65
97
161
—
0.2
0.9
1.9
4.5
—
0.9
1.3
2.6
6.3
—
10.57
10.38
10.95
10.91
10.56
17.65
17.76
17.97
17.46
17.04
10.43
10.16
10.90
10.09
10.87
49
26
54
66
52.5
31
21
Isopropyl N,N—diethylcarbamate
tert-Butyl N,N-diethylcarbamate
329.9
250.0
25.5
111
59.5
49
0.4
1.1
2.0
9.5
17
a
Ps = pressure of the substrate and Pi = pressure of the inhibitor cyclohexene.
Table 4. Variation of the rate coef®cient with temperature
Parameter
Ethyl N, N-diethylcarbamate
359.8 369.9 380.4
Temperature (°C)
340.5
1.71
349.5
2.99
389.5
23.14
399.7
37.89
104 k1 (s
)
5.06 8.52 15–34
1
log k1 (s 1) = 11.47 (0.25) 178.4 (3.1) kJmol (2.303RT) 1; r = 0.99980
1
Isopropyl N,N-diethylcarbamate
Temperature (°C)
291.7
1.48
301.2
3.22
310.4
5.51
320.2
10.30
330.5
18.14
340.0
35.57
350.8
55.37
1
104 k1 (s
)
log k1 (s 1) = 12.83 (0.70) 179.8 (7.9) kJmol (2.303RT) 1; r = 0.99872
1
tert-Butyl N,N-diethylcarbamate
Temperature (°C)
220.1
1.07
228.4
2.45
241.7
6.09
250.1
10.41
260.3
20.68
270.1
43.97
279.5
71.57
1
104 k1 (s
)
log k1 (s 1) = 12.87 (0.62) 158.6 (6.2) kJmol (2.303RT) 1; r = 0.99900
1
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 201–206 (1999)

204
A. HERIZE ET AL.
Table 5. Comparative rates and kinetic parameters at 320°C
1
1
1
1
Compound
104 k1 (s
)
Ea (kJmol)
Log A (s
)
DSÆ (J mol
)
DHÆ (kJ mol
)
Reference
(CH3)2NCOOCH2CH3
0.58
0.56
12.02
9.77
912.00
794.30
185.5
178.4 Æ 3.1
181.2
12.10
27.2
39.3
9.2
13.3
12.3
12.5
180.6
173.5
176.3
174.9
153.0
153.7
10
(CH3CH2)2NCOOCH2CH3
(CH3)2NCOOCH(CH3)2
(CH3CH2)2NCOOCH(CH3)2
(CH3)2NCOOC(CH3)3
11.47 Æ 0.25
This work
10
13.04
179.8 Æ 7.9
12.83 Æ 0.70
12.87
12.87 Æ 0.62
This work
10
This work
157.9
(CH3CH2)2NCOOC(CH3)3
158.6 Æ 6.2
Table 6. Kinetic parameters for the pyrolyses of ZCOOCH2CH3 at 400°C
1
1
Z
104k1(s
)
k/kH
Ea (kJmol)
Log A(s
)
Log k/kH
s*
Reference
H
CH3
C6H5
(CH3)2N
(CH3CH2)2N
CH3O
C6H5O
Cl
5.25
10.00
16.60
50.12
41.69
44.67
97.72
245.47
1.0
1.9
201.7
200.4
199.5
185.5
178.4
187.5
189.5
183.6
12.34
12.55
12.70
12.10
11.47
12.20
12.70
2.64
0.280
0.000
0.220
0.700
0.620
0.650
0.990
1.390
0.49
0.00
0.60
0.68
0.62
1.46
2.24
2.94
11
12
13
3.2
9.6
7.9
8.5
18.6
46.8
14
This work
15
13
16
Table 7. Relatives rates and substituent parameters of ZCOOCH2CH3 at 400°C
Z
Log k/kH
sa
sF
sR
H
CH3
C6H5
(CH3)2N
(CH3CH2)2N
CH3O
C6H5O
Cl
0.000
0.280
0.500
0.980
0.900
0.930
1.270
1.670
0.00
0.35
0.85
0.44
0.56
0.17
0.38
0.43
0.00
0.00
0.10
0.10
0.10
0.25
0.38
0.45
0.00
0.03
0.22
0.26
0.27
0.27
—
0.12
a
C. Hansch, A. Leo, and R. W. Taft, Chem. Rev. 91, 165 (1991).
release to the reaction center, where the elongation of the
Ca—O bond and polarization in the sense of Caꢁ‡ÁÁÁÁOꢁ
is the rate-determining step of these reactions. Even
though the rates for the N,N-diethylcarbamates are
similar to therefor the corresponding N,N-dimethylcar-
bamates (Table 5), the (CH3CH2)2N substituent at the
acid side of the ester tends to decrease the rate slightly
compared with the (CH3)2N group. This difference may
be rationalized in terms of the greater electron-releasing
power of the CH3CH2 group to the N atom than CH3. This
reduces the electron-withdrawing effect of the diethyla-
mino group and a slight decrease in the rate of
elimination results.
and possible planarity of the transition state. This
phenomenon seems to be more difficult with increasing
bulkiness of the alkyl group such as isopropyl or tert-
butyl. An alternative argument with regard to the
conformation and entropy of the unactivated precursor
rather than the transition state is that in the isopropyl and
tert-butyl esters, one of the methyl groups is already in
the vicinity of the carbonyl oxygen in a low-energy
conformation, whereas the ethyl group will preferentially
orient anti to the C—O bond, and thus require rotation to
a less favorable orientation to achieve the cyclic
transition state.
In relation to the organic esters investigated by
Taylor,13 the carbamates examined in this work confirm
the proposed order of reactivity, carbonates <
carbamates < benzoates < phenylacetates < acetates.
The kinetic parameters and comparative rates of
different substituents other than carbon at the acid side
The observed decrease in entropy of activation from
ethyl to tert-butyl N,N-diethylcarbamates suggests a
change in the polarity of the transition state on increasing
the alkyl branch of the ester. The more negative DS‡ for
the ethyl ester suggests a more symmetrical arrangement
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 201–206 (1999)

GAS-PHASE ELIMINATION KINETICS OF ALKYL N,N-DIETHYLCARBAMATES
205
Figure 1. Taft±Topsom correlation for the pyrolysis of ZCOOCH2CH3 (r = 0.984 Æ 0.119 at 400°C)
of an organic ethyl ester are shown in Table 6. It is
reasonable to believe that some additional effect other
than just the electronic character of Z in ZCOOCH2CH3
may be operating in the rate of elimination of these
substrates. To rationalize the above argument, on plotting
log krel. against Taft s* values (Table 6), a poor
relationship is obtained (r* = 0.46 Æ 0.16; r = 0.862 Æ
0.295 at 400°C). However, the Taft–Topsom treatment
of log k/kH = ꢀaꢂa ‡ ꢀFꢂF ‡ ꢀRꢂR takes into account
the steric (sa), electronic (sF) and resonance (sR) factors
in the quantitative study of structural and substituent
effects on chemical reactivity. In this respect, a good
correlation is found between the experimental data and
the three substituent parameters sa, sF and sR [Table 7;
Eqn 4; Fig. 1].
of the substituent Z on the rate of ethylene elimination.
The influence of paramount importance in the process of
ZCOOCH2CH3 decomposition is the field or electronic
effect. This is demonstrated by the fact that the
coefficient of greatest absolute value is rF. The fact that
the best correlation may be obtained by including sR
values confirms the interaction between the substituent
with an incipient negative reaction center. Moreover, the
negative value of the coefficient rR suggests a small
favorable effect for the abstraction of the b-hydrogen of
the ethyl ester by the oxygen of the carbonyl (Scheme 1).
EXPERIMENTAL
log k=kH ˆ ꢀ0:68 Æ 0:12†ꢀꢃ ‡ ꢀ2:57 Æ 0:12†ꢀF
General preparations. The alkyl esters of N,N-diethyl-
carbamic acid were prepared by mixing 0.15 mol of the
corresponding alcohol (Merck) with 0.15 mol of N,N-
diethylcarbamyl chloride (Aldrich) in 45 ml of anhydrous
pyridine. The reaction mixtures were heated in a 100 ml
sealed glass tube as described.18
ꢀ1:18 Æ 0:27†ꢀR
ꢀ4†
ꢀat 400 ꢁC; r ˆ 0:984; SD ˆ 0:119†
The C6H5O substituent is not included in Fig. 1
because the corresponding sR value has not yet been
defined. It is reasonable to assume that the oxygen of
C6H5O, through delocalization to the aromatic nuclei,
must exert a more powerful attraction of electrons than
CH3O. Consequently, the sR be much less negative than
sR of CH3O. If this is so, the position of the data point
for the C6H5O group may well fall close to the line
depicted in Fig. 1.
Ethyl N,N-diethylcarbamate. This product, obtained as
described above, was distilled several times (b.p. 78–
79°C at 28 Torr; yield 55.1%) and the fraction with
97.8% purity as determined by GLC (Pennwalt 223/
KOH, 80–100 mesh) was used. 1H NMR, ꢁ 0.99 (t, 6H),
‡
‡
1.13 (t, 3H), 3.1 (q, 4H), 4.0 (q, 2H). MS, m/z 145 (M ),
‡
‡
The small negative value of the coefficient ra implies a
modest participation of the polarizability, or steric effect,
130 (C6H12NO2 ), 116 (M
C2H5), 100 (M
‡
‡
OCH2CH3), 86 (C3H4NO2 ), 72 (C2H2NO2 ), 58
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 201–206 (1999)

206
A. HERIZE ET AL.
‡
(CON2 ). The primary product ethylene (Matheson) was
determined using a Porapak Q 80–100 mesh column.
Acknowledgements
We are grateful to Professor Jose´ L. Abbud for
correlation calculations and the financial support of
CONICIT (Consejo Nacional de Investigaciones Cient´ı-
ficas y Tecnolo´gicas) of Venezuela, Project No. S1-
97000005.
Isopropyl N,N-diethylcarbamate. This carbamate sub-
strate prepared according to the above general method18
(b.p. 58°C at 15 Torr; yield 63%) was distilled to 99.8%
purity as determined by GLC (Pennwalt 223/KOH, 80–
100 mesh). 1H NMR, ꢁ 0.98 (t, 6H), 1.11 (d, 6H), 3.1 (q,
‡
‡
4H), 4.80 (m, H). MS, m/z 159 (M ), 144 (M
CH3),
‡
‡
116 [M
CH (CH3)2], 102 (C4H8NO2 ), 72
‡
‡
‡
(C2H2NO2 ), 58 (CON2 ), 43 (C3H7 ). Propene (Air
Products) was determined using a Porapak R 80–100
mesh column.
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‡
‡
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Kinetics. The kinetic determinations were carried out in a
static system and the rate coefficients were followed by
measuring the pressure increase. The temperature was
controlled by a SHINKO DIC-PS 25 TR resistance
thermometer controller maintaned within Æ0.2°C and
measured with a calibrated platinum–platinum–13%
rhodium thermocouple. No temperature gradient was
found in the reaction vessel and the carbamate substrates
were injected with a syringe through a silicone-rubber
septum directly into the reaction vessel.
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 201–206 (1999)