Kinetics of the gas-phase elimination reaction of benzyl chloroformate and neopentyl chloroformate

Article abstract of DOI:10.1002/kin.20896

The gas-phase eliminations of benzyl chloroformate (475-523 K, 31-103 Torr) and neopentyl chloroformate (563-622 K, 37-70 Torr), in a deactivated static reaction vessel, and in the presence of a free radical suppressor, are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following Arrhenius equations: Benzyl chloroformate log κ_I = (13.30 ± 0.38) - (152.9 ± 3.6) kJ mol-1(2.303RT)-1; r = 0.9989 Neopentyl chloroformate Formation of neopentyl chloride: log κ_I = (14.29 ± 0.48) - (196.3 ± 5.5) kJ mol^-1(2.303RT)^-1; r = 0.9986 Formation of 2-methylbutenes: log κ_II = (12.12 ± 0.73) - (178.2 ± 8.3) kJ mol^-1(2.303RT)^-1; r = 0.9960 The derived kinetic and thermodynamic parameters for benzyl chloroformate decomposition indicate the reaction proceeds through a concerted four-membered cyclic transition state to give benzyl chloride and CO₂ gas. Neopentyl chloroformate undergoes a parallel reaction, where neopentyl chloride formation may arise from a polar-concerted four-membered cyclic transition state, whereas the mixture of olefins, 2-methyl-2-butene, and 2-methyl-1-butene appears to be produced from a carbene intermediate. This intermediate seems to be originated from a concerted five-membered cyclic transition state of the neopentyl substrate.

Full text of DOI:10.1002/kin.20896

Kinetics of the Gas-Phase
Elimination Reaction of
Benzyl Chloroformate and
Neopentyl Chloroformate
´
JESUS LEZAMA, ROSA M. DOMINGUEZ, GABRIEL CHUCHANI
Centro de Qu ´ı mica, Instituto Venezolano de Investigaciones Cient ´ı ficas (I.V.I.C.), Apartado 21827, Caracas, Venezuela
Received 18 March 2014; revised 17 November 2014; accepted 18 November 2014
DOI 10.1002/kin.20896
Published online 16 December 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The gas-phase eliminations of benzyl chloroformate (475–523 K, 31–103 Torr) and
neopentyl chloroformate (563–622 K, 37–70 Torr), in a deactivated static reaction vessel, and
in the presence of a free radical suppressor, are homogeneous, unimolecular, and follow a
first-order rate law. The rate coefficients are expressed by the following Arrhenius equations:
Benzyl chloroformate
log k = (13.30 ± 0.38) − (152.9 ± 3.6) kJ mol 1(2.303RT )−1; r = 0.9989
−
1
Neopentyl chloroformate
Formation of neopentyl chloride:
log kI = (14.29 ± 0.48) − (196.3 ± 5.5) kJ mol−1(2.303RT )−1; r = 0.9986
Formation of 2-methylbutenes:
log kII = (12.12 ± 0.73) − (178.2 ± 8.3) kJ mol−1(2.303RT )−1; r = 0.9960
The derived kinetic and thermodynamic parameters for benzyl chloroformate decomposition
indicate the reaction proceeds through a concerted four-membered cyclic transition state to
give benzyl chloride and CO2 gas. Neopentyl chloroformate undergoes a parallel reaction,
where neopentyl chloride formation may arise from a polar-concerted four-membered cyclic
transition state, whereas the mixture of olefins, 2-methyl-2-butene, and 2-methyl-1-butene
appears to be produced from a carbene intermediate. This intermediate seems to be originated
from a concerted five-membered cyclic transition state of the neopentyl substrate. ꢀC 2014
Wiley Periodicals, Inc. Int J Chem Kinet 47: 104–112, 2015
INTRODUCTION
gas phase, i.e., isobutyl chloroformate, in a static sys-
tem, and at only of two temperatures of 267 and 302°C
Lessig [1] appears to be the first to report the kinet-
ics of thermal decomposition of a chloroformate in the
(540 and 575 K). It is important to notice that one of
the gas-phase reactions reached above the atmospheric
pressure, which may lead to a different process. The
formation of three products was mentioned but with-
out identification. Consequently, it is difficult to have
Correspondence to: Gabriel Chuchani; e-mail: chuchani@
ivic.gob.ve.
ꢀ
C
2014 Wiley Periodicals, Inc.

ELIMINATION REACTION OF BENZYL CHLOROFORMATE AND NEOPENTYL CHLOROFORMATE
105
Scheme 2
Scheme 1 Parallel reactions.
elimination of isopropyl chloroformate via process (b)
in Scheme 1 [5].
some reasonable idea about the process of decomposi-
tion. The reaction was thought to be unimolecular and
to follow a first-order equation in their decomposition.
However, the kinetic data were regarded as semiquan-
titative, suggesting further investigations. Later, the ki-
netics of gas-phase decomposition of trichloromethyl
chloroformate [2] between 260–310°C (533–583 K)
and pressures of 4–17 mmHg was found to be first
order and homogeneous except for traces from wall
catalysis. Two molecules of phosgene were found as a
final product. Deactivation of the glass reaction vessel
and the use of a free radical inhibitor was not men-
tioned. The yield of phosgene, instead the formation of
CCl4 and CO2, is surprising
The kinetics of ethyl chloroformate decomposi-
tion showed, in a static system, a homogeneous uni-
molecular reaction (150–200°C (423–473 K)) to give
ethyl chloride and CO2 gas [3]. Moreover, some sec-
ondary reactions and significant surface effects were
not observed. As above [2], deactivation of the ves-
sel and the use of a radical suppressor were not men-
tioned. However in later work on ethyl chloroformate
elimination [4], the use of different free radical in-
hibitors did not affect the rate coefficient of this re-
action and a mechanism of an internal rearrangement
with the formation of CO2 was considered. The ki-
netics of isopropyl chloroformate decomposition [5]
in both static and flow systems at 180–232°C (453–
Lewis and Herndon [7] described the effect of α-
methylation by an intramolecular nucleophilic substi-
tution (SNi) on the gas-phase decomposition rates of
alkyl chloroformates (ROCOCl → RCl + CO2). The
kinetics of these reactions was carried out in a flow
system. This system has the advantage that the time of
residence at the reaction site is in a fraction of seconds.
The kinetic parameters are many times reasonable. In
this respect, at 240ºC (513 K), with the respective R
group equals to methyl, ethyl, isopropyl, and sec-butyl,
the relative rate was 1:2.2:225.1:643.5. This result sug-
gests a significant augmentation in rates byα-alkyl sub-
stitution. That is, electron-releasing groups decrease
the energy of activation due to a greater stability of the
incipient positive carbon atom in the transition state.
Consequently, the authors thought that alkyl chloride
formation occurs through a four-membered cyclic tran-
sition state as depicted in Scheme 2. The polar nature of
the transition state appears to be indicated by the rela-
tive reactivities of R = 3° > 2° > 1°, thus suggesting the
positive nature of the alkyl carbon group. Based on the
assumed polar nature of the transition state, this work
considered that the low reactivity of trichloromethyl
chloroformate (Cl3C : Me = 1 : 3.3) [2] may be at-
tributed to the low stability expected in the transition
state when R = Cl3C. An additional work, the gas-
phase elimination of neopentyl chloroformate, a sub-
strate without a Cβ–H bond at the alkyl side of the ester,
was decomposed in a flow system, but only at one tem-
perature (240°C) (513 K) into neopentyl chloride and a
mixture of 2-methyl butenes [7]. The authors believed a
carbene intermediate to be responsible for the produc-
tion of 2-butenes. However, they considered an alter-
nate mechanism whereby 1,1-dimethylcyclopropane
was found as an intermediate and then breaks
down to the 2-methylbutenes. The idea of a posi-
tive carbon or an ion-pair intermediate was thought
improbable.
5
05 K) was found to be homogeneous and first or-
der. However, compared with the simple ethyl chloro-
formate reaction, two parallel channels were reported
as shown in Scheme 1. One channel, (a), appears to
occur via migration of the Cl atom though a four-
membered cyclic transition state, whereas the second,
(b), proceeds through a six-membered cyclic transi-
tion state, as in ester pyrolysis, to give the correspond-
ing olefin and the unstable intermediate ClCOOH. The
latter intermediate rapidly decomposes into HCl and
CO2.
Another investigation on ethyl chloroformate de-
composition [6], but at much higher temperature of
The stereospecific cis nature of these elimination re-
actions was demonstrated from the deuterium content
of the olefin products of the gas-phase thermal decom-
positions of erythro- and threo-2-butyl-3-d1 chlorofor-
mates [8]. In addition, the observed isotope effects did
not seem to require the formation of an intermediate or
2
80–330ºC (553–603 K), reported a more complex
product spectrum, including ethyl chloride, CO2, HCl,
and ethylene. This work concluded that a simultaneous
reaction may be occurring, analogous to the gas-phase
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

1
06
LEZAMA, DOM ´I NGUEZ, AND CHUCHANI
The reported data on the kinetics of the gas-phase
thermal decomposition of alkyl chloroformates have
resulted in two proposed mechanisms: (a) a six-
membered cyclic transition state requiring a Cβ–H
bond at the alkyl side of the ester: This mechanism
is similar to that proposed in gas-phase eliminations
organic carboxylic esters [12,13]; and (b) a four-
membered cyclic transition state in the absence of a
Cβ–H bond.
C
H
O
C
C
O
Cl
Scheme 3
An interesting problem in gas-phase elimination
or thermal decomposition of organic molecules, un-
der true homogeneous and molecular conditions, is
to understand the influence of marked polarity in the
transition states on unusual product channels. Because
of the scarce information reported on alkyl chloro-
formate decomposition in the absence of a Cβ–H
bond, it seemed interesting to study substrates con-
taining only Cα–H bonds. The aim is to induce some
significant heterolytic nature in the transition states
and possibly promote rearrangement, intramolecular
migrations, and/or other types of mechanistic path-
ways. Consequently, the present work was designed to
study the kinetics of the gas-phase molecular elimina-
tions of benzyl chloroformate and neopentyl chloro-
formate under homogeneous experimental conditions.
The kinetic study of the latter substrate was under-
taken because of the reported experimental limitations
at one temperature (240°C; 513 K) and the proba-
bility of surface effects [8]. Moreover, it is also im-
portant to obtain the Arrhenius parameters for alkene
formation.
a transition state with any appreciable positive charge
on the involved formation carbon atom.
As indicated above, considerable disagreement ex-
ists in the reported kinetic and analytical results for gas-
phase pyrolyses of alkyl chloroformates [1–8], such as
in the case of ethyl chloroformate [3,6]. The mech-
anism of the gas-phase elimination of this substrate,
in a static system, was subsequently investigated with
the good procedure of taking several precautions to
avoid free radical contributions, including deactivation
of the glass reaction vessel, and addition of a free rad-
ical inhibitor [9]. The results implied a six-membered
cyclic transition state, yielding ethylene, HCl and CO2,
without formation of ethyl chloride as an intermedi-
ate. The Arrhenius equation was reported as log k1 =
−
1
−1
.
(
12.64 ± 0.20) – (183.6 ± 2.0) kJ mol (2.303RT)
The kinetic study of methyl chloroformate [10], under
conditions of a deactivated reaction vessel and in the
presence of an inhibitor, decomposed at very high tem-
peratures of 425–480ºC (698–753 K) to give CH3Cl
and CO2 as final products, with parameters of (log k1 =
−1
−1
.
(
14.26 ± 0.20) – (251.0 ± 2.0) kJ mol (2.303RT)
The rate of decomposition was found to be 4000 times
slower than ethyl chloroformate. The log A value of
EXPERIMENTAL
1
4.26, in the absence of a Cβ–H bond, suggested a
four-membered cyclic transition state. Furthermore,
the decomposition of isopropyl chloroformate [11] was
found to be 160 times faster than ethyl chloroformate,
where the effect of α-substitution is an indication of
the polar nature of the transition state. The suggested
transition state is as depicted in Scheme 3.
The investigations described in the Introduction
about the gas-phase elimination of alkyl chlorofor-
mates reveal different results not only on the kinetic
parameters but also in product formation. This dis-
crepancy may be due to the working conditions and
the type of equipments available at that time. Also, the
use of different methods for kinetic determination may
reflect in different experimental data. In addition to this
fact, the precautions of deactivation of the glass reac-
tion vessel to avoid the surface effect, and the presence
of a free radical suppressor, were not used for an ade-
quate homogeneity and molecularity in the gas-phase
decomposition process.
Benzyl chloroformate (95%) and neopentyl chlorofor-
mate (97%) were acquired from Aldrich. These sub-
strates were distilled several times, and the fractions
of purity better than 99% were used. The identities
of the chloroester substrates and products were deter-
mined by GC/MS (Saturn 2000, Varian 3600X), utiliz-
ing a DB–5MS capillary column, 30 mm × 0.250 mm.
id. 0.25 μm. The starting material neopentyl chlo-
roformate and the products neopentyl chloride and
benzyl chloride were quantitatively analyzed using a
Varian 3600X chromatograph. The column used was
10% SP-1200, 1% H3PO4 CHROM W AW 80/120
mesh, 2 m. The quantitative analysis of 2-methyl-
2-butene was carried out on the same 10% SP col-
umn. However, the mixture of the olefin products 2-
methyl-2-butene and 2-methyl-1-butene was separated
and analyzed on a 20% BMEA, Chromosorb P col-
umn AW (80/100 mesh). The CO2 gas was quanti-
tatively analyzed on a Varian 3600X chromatograph
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

ELIMINATION REACTION OF BENZYL CHLOROFORMATE AND NEOPENTYL CHLOROFORMATE
107
Table I Ratio of the Final (P ) to Initial Pressure (P0)
Benzyl Chloroformate
RESULTS AND DISCUSSION
Benzyl Chloroformate
f
a
Temperature (K) P0, (Torr) Pf, (Torr) Pf/P0 Average
4
4
4
5
5
5
74.1
82.7
92.7
03.0
12.1
22.6
53
64
68
49
81
56
101
126
130
97
158
114
1.91
1.97
1.91
1.97
1.95
2.04
The decomposition kinetics of benzyl chloroformate
was determined in the temperature range of 475–522 K
and pressure range 31–103 Torr. The theoretical stoi-
chiometry of reaction (1) demands P = 2P . The P
1
.96 ± 0.05
f
0
f
and P0 are the final and initial pressure, respectively.
The average experimental result of Pf/P0 at six dif-
ferent temperatures and ten-half lives is 1.96 ± 0.05
a
Reaction vessels deactivated with allyl bromide.
(Table I). The stoichiometry (1) was also checked by
comparing the percent reaction as derived by pressure
with the quantitative chromatographic analysis of the
provided with a thermal conductivity detector, and
the capillary column used was GS-Q, 30 m, id.
product CO gas (Table II).
2
5
3 mm.
Kinetics. The experiments were carried out in a
static system described before in [14–16], and the re-
action vessel was deactivated with the product of allyl
bromide decomposition [17]. Product samples were
collected in a storage reservoir cooled in liquid ni-
trogen and withdrawn at different times of reaction.
The rate coefficients were determined in the following
manner: (a) for benzyl chloroformate, by chromato-
graphic analyses of the products benzyl chloride and
CO2 gas; and (b) for neopentyl chloroformate, from
chromatographic analyses of the methylbutenes. The
temperature was maintained to better than ± 0.2°C with
a Shinko DIC-PS 23TR resistance thermometer con-
troller with a calibrated iron-constantan thermocouple.
The temperature reading was measured within ± 0.1°C
with a thermopar of iron-constantan attached to a dig-
ital multimeter Omega 3465B. At different points of
the reaction vessel, there was no temperature gradi-
ent. The starting materials were directly injected into
the reaction vessel of approximately 250 mL with a
syringe through a silicone rubber septum. The vol-
ume of substrate used for each reaction was ࣈ0.05–
The homogeneity of this decomposition was ex-
amined by carrying out several runs in a vessel with
a surface-to-volume ratio of about 6.0 relative to
that of the normal vessel (Table III). The rates were
Table III Homogeneity of the Reaction
S/V (cm 1)
−
10 k1(s
4
−1 a
)
10 k1(s )
4
−1 b
°
Benzyl chloroformate at 209.5 C
1.0
6.0
26.86 ± 6.10
6.17 ± 0.13
6.66 ± 0.33
60.24 ± 10.21
Neopentyl chloroformate at 320.7°C
1
6
.0
.0
28.78 ± 7.12
11.85 ± 0.11
11.76 ± 0.23
78.65 ± 13.14
Abbreviations: S, surface; V, volume.
a
Clean Pyrex vessel.
Vessel deactivated with allyl bromide.
b
0
.1 mL.
Table II Stoichiometry of the Reactiona
Substrate
Temperature (K)
512.1
Parameter
Value
Benzyl chloroformate
Time (min)
Reaction (%) (pressure)
CO2 (%) (GLC)
Benzyl chloride (%) (GLC)
Time (min)
2
3.25
65.3
65.8
66.2
5
4
5
7
49.0
47.8
45.8
3
73.5
74.7
75.3
7
81.6
82.7
83.4
12
89.8
89.1
89.9
15
Neopentyl chloroformate
593.9
Reaction (%) (GLC)
Neopentyl chloride (%) (GLC)
18.2
15.3
2.0
29.8
21.1
3.6
39.2
29.1
4.8
56.5
39.6
6.5
65.9
46.4
9.1
2
-Methyl-2-butene (%) (GLC)
-Methyl-1-butene
2
1.6
2.9
3.7
5.1
7.1
a
Reaction vessels deactivated with allyl bromide.
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

1
08
LEZAMA, DOM ´I NGUEZ, AND CHUCHANI
Table IV Effect of Free Radical Inhibitor on Rates
4
−1
4
−1
)
Substrate
Temperature (°C)
Pi / Ps
10 kI (s
)
10 kII (s
a
Benzyl chloroformate
249.4
–
106.1
107.8
106.2
105.1
108.1
9.30
9.39
9.58
9.41
9.49
–
1.2
1.7
3.1
3.5
–
–
–
–
b
Neopentyl chloroformate
320.7
–
2.28
2.44
2.38
2.35
2.37
1
.9
.6
.0
.3
2
3
3
Vessel was deactivated with allyl bromide.
Abbreviations: P , pressure substrate; P , pressure inhibitor.
s
i
a
Cyclohexene inhibitor.
Toluene inhibitor
b
.
unaffected in the packed and unpacked deactivated
vessels, yet a marked heterogeneous effect was found
in the clean-packed and clean-unpacked Pyrex vessel.
The significant catalytic effect in the clean vessels is
probably related to the known acidic nature of Pyrex
glass. The presence of different proportions of cyclo-
hexene, a free radical inhibitor, had no effect on rates
in deactivated vessels, and no induction period was
observed (Table IV).
The rate coefficients were shown to be indepen-
dent of the initial pressure of the benzyl chlorofor-
mate; the first-order plots are linear up to 80% of
the reaction (Table V). The rate coefficients are re-
producible with a standard deviation not greater than
± 5% at any given temperature. The variation of the
rate coefficients with temperature is listed in Table VI.
The kinetic and thermodymanic parameters are given
in Table VII. The results described in Table V lead,
by using the least-squares procedure and 90% confi-
dence coefficient, to the following Arrhenius equation
(Fig. 1):
Benzyl chloroformate
−1
log k = (13.30 ± 0.38) − (152.9 ± 3.6) kJ mol
1
−
1
× (2.303RT ) ; r = 0.9989
Table V Variation of the Rate Coefficients from Initial Pressure
Substrate
Temperature (K)
522.6
Parameter
Value
Benzyl chloroformate
P0 (Torr)
31
108.1
37
9.41
2.35
50
106.2
44
9.49
2.37
66
106.1
51
9.58
2.38
75
109.6
60
103
107.8
70
4
−1
1
0 k1 (s
)
Neopentyl chloroformate
593.9
P0 (Torr)
4
−1
1
0 kI (s
)
)
9.39
2.44
9.30
2.48
4
−1
1
0 kII (s
Table VI Temperature Dependence of the Rate Coefficients
Benzyl chloroformate
492.7 503.0
Temperature (K)
475.1
482.7
512.1
522.6
4
−1
)
1
0 k1 (s
3.07 ± 0.10 6.17 ± 0.13 11.33 ± 0.16 25.22 ± 0.08 53.71 ± 0.82 107.66 ± 1.65
Neopentyl chloroformate
Temperature (K)
563.2
574.7
582.7
593.9
9.48 ± 0.06 19.82 ± 0.07 42.87 ± 0.09 68.67 ± 0.13
2.37 ± 0.06 4.73 ± 0.07 9.72 ± 0.07 16.15 ± 0.25
603.9
614.6
622.6
4
−1
1
0 kI (s
)
1.39 ± 0.04 2.59 ± 0.02 5.01 ± 0.05
4
−1
)
1
0 kII (s
0.48 ± 0.02 0.75 ± 0.03 1.30 ± 0.05
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ELIMINATION REACTION OF BENZYL CHLOROFORMATE AND NEOPENTYL CHLOROFORMATE
109
Table VII Kinetic and Thermodynamic Parameters at 548.2 K
4
−1
−1
)
ꢀSࣔ (J/mol K) ꢀHࣔ (kJ/mol) ꢀGࣔ (kJ/mol)
Substrate
k1 × 10 (s
)
Ea (kJ/mol) log A (s
Benzyl chloroformate
Formation neopentyl chloride
Formation of the olefins
528.2
0.376
0.135
152.9 ± 3.6 13.30 ± 0.38
196.3 ± 5.5 14.29 ± 0.48
178.2 ± 8.3 12.12 ± 0.73
−3.7
15.5
−26.3
148.3
191.7
173.6
150.3
183.2
188.0
for the neopentyl chloride and 2-methylbutene prod-
ucts (Table II). Results agreed within an experimental
error. It is important to point out that the product
neopentyl chloride from reaction (2) does not decom-
pose to 2-methylbutenes under the conditions of the
present experiments. On our timescales, this com-
pound requires higher temperatures of 410–496ºC
(683–769 K), to yield 2-methylbutenes [18] through
an intramolecular Wagner–Merwein rearrangement.
As a check, experiments with of neopentyl chloride
(1-chloro-2,2-dimethylpropane; 99% pure; Aldrich)
showed no decomposition in our apparatus at temper-
atures of 350–380 ºC (623–653 K). Thus, formation of
2-methylbutenes from neopentyl chloroformate must
take place via another mechanism. In association with
the work reported in [18], a DFT study of the reaction
mechanism of the gas-phase thermal decomposition
kinetics of neopentyl halides at 451ºC (721 K) was
reported [19].
Figure 1 Arrhenius plot for gas-phase decomposition of
benzyl chloroformate.
Neopentyl chloroformate
Formation of neopentyl chloride:
−
1
log kI = (14.29 ± 0.48) − (196.3 ± 5.5) kJ mol
−1
×
(2.303RT ) ; r = 0.9986
Formation of 2-methylbutenes:
log kII = (12.12 ± 0.73) − (178.2 ± 8.3) kJ mol
−
1
−1
×
(2.303RT ) ; r = 0.9960
In the present work, we find a distribution of 57%
of 2-methyl-2-butene and 43% of 2-methyl-1-butene.
This distribution is believed to reflect an equilibrium
under reaction conditions. The isomerization process
was confirmed when a pure sample of 2-methyl-1-
butene (99.5% pure; Aldrich) with and without the
presence of equimolecular amount of HCl gas was
found to give about 60% to 2-methyl-2-butene under
the reaction conditions of 593 K.
The test for surface effects on the rate of decom-
position showed a dramatic heterogeneous effect in
packed and unpacked clean Pyrex vessels, whereas sea-
soned vessels were unaffected (Table III). Toluene, an
Neopentyl Chloroformate
The gas-phase elimination of this substrate was carried
out in the temperature range of 563–622 K and pres-
sures of 37–110 Torr. Values for Pf/P0 were not deter-
mined because two parallel reactions are present, and
the substrate does not decompose in equal amounts to
products. Moreover, the distribution of products varies
with temperature. Verification of the stoichiometry of
reaction (2) was possible by comparing, at up to 66%
conversion, the percentage decomposition of the sub-
strate derived from pressure measurements with the ex-
pectations derived from the chromatographic analyses
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

1
10
LEZAMA, DOM ´I NGUEZ, AND CHUCHANI
Product formation in neopentyl chloroformate de-
composition [reaction (2)] occurs via two parallel
processes. First, an intramolecular migration of the
incipient chlorine atom (Step 1) for the production
of neopentyl chloride through a very polar four-
membered cyclic transition state type of mechanism
ࣔ
[
reaction (4)], log A = 14.29; ꢀS = 15.5 J/mol K].
In the case of the two methylbutene products (Step
), they may directly be formed from a carbene
2
intermediate originated by the decomposition of a
five-membered cyclic transition state of the sub-
strate [reaction (4)]. The possible formation of 1,1-
dimethylcyclopropane from the carbene intermedi-
ate and subsequent decomposition to the 2-butenes
seems to be improbable. This consideration is derived
from a work on the thermal isomerization of 1,1-
dimethylcyclopropane, at 447–511°C (720–784 K),
producing as main products with an equal amount of
3-methyl-1-butene, 2-methyl-1-butene, traces of low
molecular hydrocarbons, and about 1% of 2-methyl-
1-butene [22]. This decomposition took place at about
100°C (273 K) higher than the elimination process
of the present work. Thus, one would expect the cy-
clopropane product to be isolated in the present ex-
periment if it is an intermediate. Given that olefinic
double bonds substrates may isomerize in the presence
of hydrogen halides [23], it is possible to conceive
that a cyclopropane ring will isomerize in the presence
of HCl gas. As mentioned in the Introduction, Lewis
and co-workers [8] considered the possible occurrence
of either carbene or cyclopropane intermediates in the
decomposition mechanism of neopentyl chloroformate
and that formation of a carbocation or ion-pair interme-
diates of this elimination process could be excluded. In
this respect, they suggested the reaction to be heteroge-
neous. Since our results suggest that we have isolated
the homogeneous and molecular process, it is possible
to suppose the mechanism of this substrate may pro-
ceed through a polar five-membered cyclic transition
state to give a carbene intermediate. This intermedi-
ate proceeds through a bicyclic four-membered struc-
ture to give directly 2-methyl-1-butene. Apparently,
2-methyl-1-butene is first formed and then isomerizes
further to 2-methyl-2-butene until reaching an equi-
librium mixture. The fact that the yield of 2-methyl-1-
butene is a little less in amount than 2-methyl-2-butene;
it may be attributed to a greater thermodynamic stabil-
ity of the latter.
Figure 2 Arrhenius plot for the formation of neopentyl
chloride (♦) and 2-butenes (•) in the gas-phase decomposition
of neopentyl chloroformate.
effective free radical suppressor, had no effect on the
rate coefficient in seasoned vessels (Table IV). No in-
duction period was observed, and the rate coefficients
were reproducible with a standard deviation not greater
than ± 5% at a working temperature.
At a given temperature, the rate coefficients were
independent of the initial pressures, and the first-
order plots are satisfactorily linear at up to 65% of
the reaction (Table V). The temperature dependence
of the rate coefficient and the corresponding Arrhe-
nius expressions are described in Table VI (90%
confidence limit from the least-squares procedure)
(
Fig. 2).
The value for benzyl chloroformate of log A =
3.30 is consistent with a four-membered cyclic struc-
1
ture in the transition state as postulated by Benson
et al. [20,21]. Consequently, the estimated low value
ࣔ
of ꢀS = –3.7 J/mol K suggests a concerted semipo-
lar transition state. This information leads us to be-
lieve that bond polarization of the Cl–C(=O) bond,
δ−
δ+
in the direction of Cl —C (=O) is the rate-
determining process in this reaction. Therefore, dur-
ing the decarboxylation process, the formation of a
relatively stable benzylic positive carbon atom, due
to a resonance interaction from the benzene ring,
results in a structure that is susceptible to nucle-
ophile attack from the incipient negative chlorine atom.
Therefore, we suggest that a reasonable mechanism
for the formation of benzyl chloride is shown in
reaction (3).
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

ELIMINATION REACTION OF BENZYL CHLOROFORMATE AND NEOPENTYL CHLOROFORMATE
111
The only two compounds lacking a Cβ–H bond on
the alkyl side of the chloroesters for which the decom-
position kinetics has been reported in the literature are
methyl chloroformate [10] and neopentyl chlorofor-
mate [7]. In spite of this limited information, the rates
obtained in the present work may be compared with
The Arrhenius parameters of ethyl chloroformate,
where a Cβ–H is present, and neopentyl chloroformate
are surprisingly similar. Since the postulated transi-
tion states of the two substrates differ substantially,
it is unclear how to interpret this result. Ethyl chlo-
roformate, where the polarization between the car-
the compound of reference methyl chloroformate, R
δ−
δ−
=
H [reaction (5)]. Since bond polarization of Cl —
bonyl carbon and the alkyl oxygen [ClC(=O)O –
δ+
δ+
C (=O) appears to be rate determining, the oxygen
CH2CH3] is the rate-determining step, decomposes
δ−
δ+
at the alkyl side [Cl —C (=O)O–CH2R] delocal-
through a six-membered cyclic transition state mech-
anism similar to the gas-phase eliminations organic
carboxylic esters [12,13]. The products are CH2=CH2
and ClCOOH. The latter intermediate is unstable, pro-
izes its electrons for CO2 formation. This fact causes
δ+
RCH2 to be partially positive in nature. The stabi-
lization of the partially positive RCH2 depends of
δ+
δ−
δ+
electron donation of the substituent present at the Cα.
The sequence described in reaction (5) suggests that the
electron release of the R = tert-butyl group stabilizes
ducing HCl and CO2. Since Cl — C (=O) is the
rate determining in neopentyl chloroformate decom-
position to 2-butenes appears to proceed by way of
a five-membered cyclic transition state. Apparently,
the similarity in the Arrhenius parameters may be
fortuitous.
δ+
the positive CαH2 and gives a marked increase in
the rate, whereas the resonance interaction of R = aro-
δ+
matic ring toward partial positive CαH2 as a benzyl
group causes a dramatic increase in the rate coefficient.
350ºC
RCH OCOCl
RCH Cl + CO
2
(5)
2
2
Relative rate Rel. rate
R = H
CH ) C
1
(
41,161
3
3
C6H5 18,277,000
4
1
0 k
1
E
183.6
178.2
a
kJ/mol
log A
12.64
12.12
Temp. 573 K
CH
3
CH
2
OCOCl
OCOCl
0.79
0.74
(
CH
3
)
3
CCH
2
International Journal of Chemical Kinetics DOI 10.1002/kin.20896

1
12
LEZAMA, DOM ´I NGUEZ, AND CHUCHANI
CONCLUSIONS
8. Lewis, E. S.; Herndon, W. C.; Duffey, D. C. J Am Chem
Soc 1961, 83, 1959–1961.
9
. Johnson, R. L.; Stimson, V. R. Aust J Chem 1976, 29,
389–1392.
0. Johnson, R. L.; Stimson, V. R. Aust J Chem 1977, 30,
917–1920.
1. Johnson, R. L.; Stimson, V. R. Aust J Chem 1982, 35,
49–852.
The gas-phase thermal decomposition of benzyl chlo-
roformate and neopentyl chloroformate, in seasoned or
deactivated vessels and in the presence of a free-radical
inhibitor, are homogeneous, unimolecular and obey
a first-order rate law. The mechanism of decomposi-
tion of benzyl chloroformate seems to occur through a
concerted four-membered cyclic transition state struc-
ture. Neopentyl chloroformate undergoes a parallel
reaction, where the formation of neopentyl chloride
takes place by way of a polar four-membered tran-
sition state, whereas the mixture of 2-methylbutenes
products appears to be obtained from a carbene
intermediate.
1
1
1
1
1
8
2. Taylor, R. The Chemistry of Functional Group, Sup-
plementary Volume B: Acid Derivatives; Patai, S, Ed.;
Wiley: London, 1979; Ch. 15, pp. 859–914.
13. Holbrook, K. A. The Chemistry of Acid Derivatives,
Volume 2: Vapor and Gas Phase Reaction of Carboxylic
Acids and Their Derivatives; Patai, S, Ed.; Wiley: Lon-
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1
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4. Maccoll, A. J Chem Soc 1955, 965–973.
5. Swinbourne, E. S. Aust J Chem 1958, 11, 314–330.
6. (a) Espitia, L.; Meneses, R.; Dominguez, R. M.; Tosta,
M.; Herize A.; Lezama, J.; Lafont, J.; Chuchani, G. Int J
Chem Kinet 2009, 41, 145–152; (b) Dominguez, R. M.;
Herize A.; Rotinov, A.; Alvarez-Aular, A.; Visbal, G.;
Chuchani, G. J Phys Org Chem 2004, 17, 399–408.
We are grateful to Jos e´ R. Mora for helpful discussion and to
Alexis Maldonado for technical assistance.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20896