Kinetic and Mechanisms of the Homogeneous, Unimolecular Elimination of Phenyl Chloroformate and p-Tolyl Chloroformate in the Gas Phase

Article abstract of DOI:10.1002/kin.20939

The gas-phase elimination of phenyl chloroformate gives chlorobenzene, 2-chlorophenol, CO₂, and CO, whereasp-tolyl chloroformate produces p-chlorotoluene and 2-chloro-4-methylphenol CO₂ and CO. The kinetic determination of phenyl chloroformate (440-480^oC, 60-110 Torr) and p-tolyl chloroformate (430-480°C, 60-137 Torr) carried out in a deactivated static vessel, with the free radical inhibitor toluene always present, is homogeneous, unimolecular and follows a first-order rate law. The rate coefficient is expressed by the following Arrhenius equations: Phenyl chloroformate: Formation of chlorobenzene, log k_I = (14.85 ± 0.38) - (260.4 ± 5.4) kJ mol^-1 (2.303RT)^-1; r = 0.9993 Formation of 2-chlorophenol, log k_II = (12.76 ± 0.40) - (237.4 ± 5.6) kJ mol^-1(2.303RT)^-1; r = 0.9993 p-Tolyl chloroformate: Formation of p-chlorotoluene: log k_I = (14.35 ± 0.28) - (252.0 ± 1.5) kJ mol^-1 (2.303RT)^-1; r = 0.9993 Formation of 2-chloro-4-methylphenol, log k_II = (12.81 ± 0.16) - (222.2 ± 0.9) kJ mol^-1(2.303RT)^-1; r = 0.9995 The estimation of the k_I values, which is the decarboxylation process in both substrates, suggests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom through a semipolar, concerted four-membered cyclic transition state structure; whereas the k_II values, the decarbonylation in both substrates, imply an unusual migration of the chlorine atom to the aromatic ring through a semipolar, concerted five-membered cyclic transition state type of mechanism. The bond polarization of the C-Cl, in the sense C^δ+Cl^δ-, appears to be the rate-determining step of these elimination reactions.

Full text of DOI:10.1002/kin.20939

Kinetic and Mechanisms
of the Homogeneous,
Unimolecular Elimination
of Phenyl Chloroformate
and p-Tolyl Chloroformate
in the Gas Phase
JESUS LEZAMA, GABRIEL CHUCHANI
Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (I.V.I.C.), Apartado, 21827, Caracas, Venezuela
Received 19 February 2015; revised 3 June 2015; accepted 14 July 2015
DOI 10.1002/kin.20939
Published online 24 August 2015 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The gas-phase elimination of phenyl chloroformate gives chlorobenzene, 2-
chlorophenol, CO2, and CO, whereas p-tolyl chloroformate produces p-chlorotoluene and
2-chloro-4-methylphenol CO2 and CO. The kinetic determination of phenyl chloroformate
(440–480oC, 60–110 Torr) and p-tolyl chloroformate (430–480°C, 60–137 Torr) carried out in
a deactivated static vessel, with the free radical inhibitor toluene always present, is homoge-
neous, unimolecular and follows a first-order rate law. The rate coefficient is expressed by the
following Arrhenius equations:
Phenyl chloroformate:
Formation of chlorobenzene, log kI = (14.85
0.38) – (260.4
5.4) kJ mol−1 (2.303RT)−1
5.6) kJ mol−1(2.303RT)−1
;
;
r = 0.9993
Formation of 2-chlorophenol, log kII = (12.76
r = 0.9993
0.40) – (237.4
p-Tolyl chloroformate:
Formation of p-chlorotoluene: log kI = (14.35
0.28) – (252.0
1.5) kJ mol–1 (2.303RT)−1
;
r = 0.9993
Formation of 2-chloro-4-methylphenol, log kII = (12.81
mol−1(2.303RT)–1; r = 0.9995
0.16) – (222.2
0.9) kJ
The estimation of the kI values, which is the decarboxylation process in both substrates, sug-
gests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom
Correspondence to: Gabriel Chuchani; e-mail: chuchani@ivic.
gob.ꢀCve.
2015 Wiley Periodicals, Inc.

HOMOGENEOUS, UNIMOLECULAR ELIMINATION OF CHLOROFORMATES IN THE GAS PHASE
665
through a semipolar, concerted four-membered cyclic transition state structure; whereas the
kII values, the decarbonylation in both substrates, imply an unusual migration of the chlorine
atom to the aromatic ring through a semipolar, concerted five-membered cyclic transition state
type of mechanism. The bond polarization of the C–Cl, in the sense Cδ+ . . . Clδ−, appears to be
ꢀC
the rate-determining step of these elimination reactions.
Chem Kinet 47: 664–670, 2015
2015 Wiley Periodicals, Inc. Int J
INTRODUCTION
The kinetics of the gas-phase elimination of alkyl chlo-
roformates has been for years the subject of different
mechanistic interpretations according to experimental
conditions, structure of the substrates, and products for-
mation [1–12]. In this sense, it is noteworthy to classify
the several types of esters of chloroformic acid studied
at high temperatures and at subatmospheric pressures
according to structures: (a) presence of the Cβ–H bond
at the alkyl side of the chloroester; (b) absence of the
Cβ–H bond but the presence of a Cα–H bond at the
alkyl side of the chloroester; (c) absence of both active
Cβ–H bond and Cα–H bond at the alkyl side of the
chloroester.
Mechanism via (b)
Absence of the Cβ–H Bond, but the
Presence of the Cα–H Bond
The elimination kinetics of methyl chloroformate [10],
lacking a Cβ–H bond, under homogeneous and molec-
ular conditions (425–480ºC) yielded CH3Cl and CO2
gas (log k1 = (14.26
0.20) – (251.0
2.0)
kJ mol−1(2.303RT)−1. The result of log A = 14.26
suggested the reaction to proceed through a four-
membered cyclic transition state type of mechanism.
A recent investigation of a chloroester without
Cβ–H, that is, the gas-phase thermal decomposition of
benzyl chloroformate (475–523 K) [15] in deactivated
reaction vessels and in the presence of a free-radical
inhibitor, was found to be unimolecular homogeneous
and obeys a first-order rate law: log k1 = (13.30 0.38)
Presence of the Cβ–H Bond at the
Alkyl Side
Many works of alkyl chloroformates with the presence
of a Cβ–H bond resulted in several discrepancies in
product formation ascribed to surface effects. Some
types of these substrates were found to undergo parallel
elimination reactions [9,11] as depicted in reaction (1).
– (152.9
3.6) kJ mol−1 (2.303RT)−1; r = 0.9989.
According to the results of the kinetic and thermody-
namic parameters, the reaction was thought to proceeds
through a concerted four-membered cyclic transition
state to give benzyl chloride and CO2 gas. Addition-
ally, the homogeneous, unimolecular gas-phase elim-
ination of neopentyl chloroformate (563–622 K, 37–
70 Torr) [15] proceeded to a parallel reaction, where the
formation of neopentyl chloride appears to be obtained
through a polar four-membered cyclic transition state,
whereas the mixture of olefins, 2-methyl-2-butene, and
2-methyl-1-butene is produced from a carbene inter-
mediate. This intermediate supposed to be originated
from a concerted five-membered cyclic transition state
type of mechanism.
This parallel elimination reaction (1) was thought
that through via (a) to be homogeneous, whereas via
(b) heterogeneous, yet both molecular in nature. Two
different mechanisms were suggested to account for
the formation of products: mechanism via (a), which is
similar to the gas-phase elimination reaction of alkyl
acetates [reaction (2)]; whereas mechanism via (b) is
a concerted process with direct participation of the Cl
atom [reaction (3)] [13,14],
Absence of Both Active Cβ–H Bond and
Cα–H Bond
The
gas-phase
decomposition
kinetics
of
trichloromethyl chloroformate [16] at 533–583 K
was shown to be first order and homogeneous, except
with some wall catalysis. The final products were two
molecules of phosgene. Deactivation of the reaction
vessel and the presence of a free radical inhibitor were
Mechanism via (a)
International Journal of Chemical Kinetics DOI 10.1002/kin.20939

666
LEZAMA AND CHUCHANI
not reported. The yield of phosgene, instead of the
formation of CCl4 and CO2, is surprising.
from a pressure increase measured manometrically and
by quantitative gas chromatograph analysis. The tem-
perature was controlled by a SHINKO DIC-PS 23TR
resistance thermometer controller, maintained to better
than 0.2°C and measured with a calibrated iron con-
stantan thermocouple. The temperature reading was
measured within 0.1°C with a thermopar of iron con-
stantan attached to a digital multimeter Omega 3465B.
No temperature gradient was found at different points
of the reaction vessel. The substrates were directly in-
jected into the reaction vessel of approximately 250 mL
with a syringe through a silicone rubber septum. The
amount of reactant employed for each kinetic run was
ࣈ0.05–0.1 mL.
With this background of information, the present
work is addressed to examine the elimination kinetics
of chloroesters of a special structure to bring about
the possibility of a greater degree of polarizations in
the transition state. Consequently, rearrangement, iso-
merization, intramolecular migrations, and probably
other interesting mechanistic pathways may well be
obtained. From the consideration that only a single in-
formation was reported in the chemical literature with
regard to decomposition of substrates of the type (c),
which is the absence of both active Cβ–H bond and
Cα–H bond at the alkyl side of the chloroester [16],
this investigation aimed at studying the mechanisms
of the gas-phase elimination kinetics of phenyl chloro-
formate and p-tolyl chloroformate under homogeneous
and molecular conditions of the experiments
RESULTS AND DISCUSSION
The elimination reaction of both phenyl and p-tolyl
chloroesters, in a static system, deactivated reaction
vessel with allyl bromide, and in the presence of the
free radical inhibitor toluene was determined over
the temperature range of 430–480°C and the pressure
range of 60–137 Torr. The products formed are de-
scribed in reactions (4) and (5).
EXPERIMENTAL
The starting materials phenyl chloroformate and
p-tolyl chloroformates were bought from Aldrich,
Milwaukee, Wisconsin, USA and used after distilla-
tion to better than 99.0% purity. Analyses of these sub-
strates and products were carried out using a GC/MS
(Saturn 2000, Varian 3600X), utilizing a DB–5MS
capillary column, 30 mm ×0.250 mm. id. 0.25 μm.
For quantitative analysis of phenyl chloroformate,
chlorobenzene, and 2-chlorophenol was made by the
use of n-propyl benzene as an internal standard. A
Varian 3700 gas chromatograph with 15% OV-101,
Chrom. G 100/120 mesh 3 m column was employed.
The same column of 15% OV-101 was utilized for
quantitative determination of p-tolyl chloroformate,
p-chlorotoluene, and 2-chloro-4-methylphenol when
using chlorobenzene as an internal standard. The quan-
titative analysis of CO2 gas, with propene as an in-
ternal standard, was performed on a Varian 3600X
chromatograph provided with a thermal conductivity
detector, and the capillary column used was GS-Q,
30m, id.53 mm.
The theoretical stoichiometry based on reactions
(4) and (5) required that, for long reaction times, Pf =
2P0, where Pf and P0 are the final and initial pressure,
respectively. The average experimental Pf/P0 values
at five different temperatures and 10 half-lives was in
both cases approximately 2.0 (Table I).
The homogeneity of these elimination reactions was
determined 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 II).
The packed and unpacked Pyrex vessels, deactivated
with allyl bromide products of decomposition, had no
effect on rates. However, the packed and unpacked
clean Pyrex vessels caused a marked heterogeneous
effect on the rate coefficients.
Kinetics
The kinetic measurements were performed in a static
reaction system described in detail and with the figure
of the apparatus as previously reported [17–20]. The
wall of the Pyrex reaction vessel was deactivated with
the products of decomposition of allyl bromide [21].
The reaction products were collected in a storage reser-
voir cooled in liquid nitrogen and retired at different
reaction times. The rate coefficients were calculated
The effect of different proportions of the free radical
suppressor toluene in the elimination process is shown
in Table III. Apparently, these reactions proceed in a
molecular nature; yet the experiments were carried out
in the presence of at least equal amounts of the inhibitor
toluene to prevent any possible radical reactions. No
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HOMOGENEOUS, UNIMOLECULAR ELIMINATION OF CHLOROFORMATES IN THE GAS PHASE
667
Table I Rate of Final (Pf) to Initial (P0) (Pressure)a
Substrate
Temperature (°C)
P0 (Torr)
Pf (Torr)
Pf / P0
Average
Phenyl chloroformate
440.1
450.6
460.4
470.6
480.1
90
83
55
121
135
176
105
105
243
271
1.96
2.05
1.91
2.01
2.04
1.99 0.06
p-Tolyl chloroformate
430.2
440.3
449.8
460.2
469.8
80
59
70
76
53
154
122
144
150
109
1.93
2.07
2.06
1.97
2.06
2.02 0.06
aReaction vessels deactivated with allyl bromide.
Table II Homogeneity of the Elimination Reactions
decomposition. The variations of the rate coefficients
with temperature are given in Table V. The results
given in Table V, led by using least-squares procedure
and 90% confidence limits, to the shown Arrhenius
equations (Figs. 1 and 2).
×104 k1
×104 k1
Substrate
S/V (cm−1 a
)
(s−1 b
)
(s−1 c
)
Phenyl chlo-
roformate
1
6
5.21 1.17 1.15 0.02
12.49 5.10 1.17 0.03
The results of the kinetic and thermodynamic pa-
rameters shown in Table VI demand a careful consid-
eration for a reasonable interpretation in the elimina-
tion process of these substrates. The value of log A of
14.85 appears to be consistent with the idea that phenyl
chloroformate decomposition may proceed through via
I in concerted polar four-membered cyclic transition
state. The positive value of ꢀS‡ of 23.5 J mol−1 K−1
at 450.0°C
p-Tolyl chlo-
roformate
1
6
11.49 3.75 4.37 0.09
34.16 7.16 4.32 0.12
at 440.3°C
aS = surface area; V = Volume. bClean Pyrex vessel. cVessel
deactivated with allyl bromide.
(5.6 cal mol−1
K
−1) obtained for the formation of
induction period was observed. The rate coefficients
were reproducible with a relative standard deviation
less 5% at a given temperature.
The first-order rate coefficients of these
chloroesters, calculated from k1 = (2.303/t) log
[2P0/(3P0 – Pt), were found to be independent of the
initial pressures (Table IV). A plot of log (3P0 – Pt)
against time t gave a good straight line up to 60%
chlorobenzene indicates that the entropy of the tran-
sition state, S‡, is a little greater than the entropy of
the reactant, Sr, suggesting a cyclic transition struc-
ture to be less ordered in comparison with the starting
material; this means a more loose and polar in nature.
The formation of the four-membered cyclic transition
state requires the bonds to be very elongated, with-
out breaking the Cδ+—Clδ−, diminishing the torsional
stress and angles from electronic interactions. Con-
sequently, the significant increase in the polarization
generates gains in degree of translational freedom in
Table III Effect of Free Radical Inhibitor on Ratesa
Temperature
×104 kI × 104 kII
Substrate
(°C)
Pi/Ps
(s−1
)
(s−1
)
Phenyl chlo-
roformate
470.6
—
1.4
1.6
2.0
2.5
3.52
3.54
3.45
3.53
3.04
1.16
1.17
1.18
1.17
1.10
Table IV Invariability of the Rate Coefficients with
Initial Pressures
Phenyl chloroformate (450.6ºC)
P0 (mmHg)
60
85
96
107
110
p-Tolyl chlo-
roformate
460.2
—
0.8
1.6
2.8
3.5
2.64
2.59
2.68
2.68
2.60
9.54
9.51
9.58
9.43
9.61
kI (×104 s−1
)
1.18 1.17 1.15 1.14 1.10
0.41 0.39 0.41 0.39 0.41
kII (×10 ×104 s−1
)
p-Tolyl chloroformate (460.2ºC)
P0 (mmHg)
63 84 98
2.56 2.64 2.63 2.59 2.62
9.62 9.60 9.76 9.51 9.53
120
137
kI (×104 s−1
)
Vessel deactivated with allyl bromide. Ps = pressure substrate.
Pi = pressure inhibitor.
kII (×10 ×104 s−1
)
aToluene inhibitor.
International Journal of Chemical Kinetics DOI 10.1002/kin.20939

668
LEZAMA AND CHUCHANI
–2.5
−3
Table V Variation of the Rate Coefficients with
Temperature
T (ºC)
kI (×104 s−1
)
kII (×104 s−1
)
y = –13602x + 14.848
r = 0.9993
Phenyl chloroformate
–3.5
–4
440.1
450.6
460.4
470.6
480.1
0.61 0.02
0.24 0.01
0.41 0.02
0.69 0.01
1.17 0.03
2.03 0.02
1.15 0.02
1.94 0.03
3.54 0.03
6.42 0.11
y = –12401x + 12.755
r = 0.9993
–4.5
log kI = (14.85 0.38)
– (260.4 5.4) kJ mol−1 (2.303RT)−1; r = 0.9993
–5
0.00132
0.00134
0.00136
1/T (K–1
0.00138
0.0014
0.00142
log kII = (12.76 0.40)
)
– (237.4 5.6) kJ mol−1(2.303RT)−1; r = 0.9993
Figure
1
Arrhenius plot for the formation of (•)
p-Tolyl chloroformate
chlorobenzene and (ꢀ) 2-chlorophenol in the gas-phase elim-
ination of phenyl chloroformate.
430.1
440.7
449.8
460.2
470.6
480.6
0.43 0.02
0.77 0.05
1.43 0.02
2.62 0.09
4.28 0.08
7.68 0.19
1.99 0.06
3.60 0.12
5.79 0.09
9.54 0.26
15.21 0.13
26.53 0.15
–2
–2.5
y = –11607x + 12.81
r = 0.9995
log kI = (14.35 0.28)
–3
– (252.0 1.5) kJ mol–1 (2.303RT)–1; r = 0.9993
log kII = (12.81 0.16)
–3.5
– (222.2 0.9) kJ mol–1 (2.303RT)–1; r = 0.9995
–4
y = –13165x + 14.351
r = 0.9993
–4.5
0.00132
0.00134
0.00136
0.00138
1/T (K–1
0.0014
0.00142
0.00144
)
some centers involved in the transition state when com-
pared to the reactant. Likewise, we may well consider
that this transition state has a late formation with re-
spect to the reaction coordinate. With regard to via
II, the formation of 2-chlorophenol ꢀS‡ of –15.4 J
Figure 2 Arrhenius plot for the formation of (•) p-
chlorotoluene and (ꢀ) 2-chloro-4-methylphenol in the gas-
phase elimination of p-tolyl chloroformate.
mol−1 K−1 (–3.7 cal mol−1
K
−1) indicates that S‡ <
Sr, suggesting a more rigid transition state close to pla-
narity and with less bond polarization in comparison
to via I. Also, there is a loss of degree of freedom aris-
ing from the restrictions of movements arising from
the formation of the transition state. The value of log
A of 12.82 implies the reaction to occur through the
formation of a five-membered cyclic transition state in
agreement with the ꢀS‡ value.
In the case of p-tolyl chloroformate, the results of
log A and ꢀS‡ obtained for both reaction pathway
suggest the transition state to be with similar geometry
described for phenyl chloroformate.
According to products formation and the results of
the kinetic and thermodynamic parameters of these aryl
chloroformates elimination (Fig. 3), the polarization of
the C–Cl bond of both substrates, in the direction of
Table VI Kinetic and Thermodynamic Parameters
Ea
ꢀS‡
(J mol−1 K−1
ꢀH‡
(kJ mol−1
ꢀG‡
(kJ mol−1
)
Product
×104 k (s−1
)
(kJ mol−1
)
log A
)
)
Phenyl chloroformate (460.4ºC)
Chlorobenzene
2-Chlorophenol
1.99
0.81
260.4 5.4
237.4 5.6
14.85 0.38
12.82 0.40
23.5
−15.4
254.3
231.3
237.1
242.6
p-Tolyl chloroformate (460.2ºC)
p-Chlorotoluene
2-Chloro-4-methylphenol
2.50
9.56
252.0 1.5
222.2 0.9
14.35 0.28
12.81 0.16
13.9
−15.6
245.9
216.1
235.7
227.5
International Journal of Chemical Kinetics DOI 10.1002/kin.20939

HOMOGENEOUS, UNIMOLECULAR ELIMINATION OF CHLOROFORMATES IN THE GAS PHASE
669
O
O
O
Cl
O
C
Cl
1
Cl
2
kI
Vía I
CO2
+
X = CH3, H
X
X
X
Vía II
O
C
O
O
H
Cl
Cl
kII
CO
+
H
X
X
fast
OH
Cl
X
Figure 3 Mechanistic consideration for the gas-phase elimination of aryl chloroformates.
I; additionally, a smaller value of ꢀG‡ means a faster
reaction rate. Moreover, formation of a five-membered
cyclic transition state is more favored when compared
to that of a four-membered cyclic transition state, since
an augmentation in the size of the cyclic structure there
is a relative decrease in the stress. In addition to these
facts, chlorine migration at the position 2 of the aro-
matic ring appears to be favored by electron release
(+I) of the methyl group present at the para position
of the benzene ring. The above argument may explain
the difference in reactivity with respect to phenyl chlo-
roformate. The products formations are different, that
is, 71.1% of chlorobenzene v´ıa I, and 28.9% of 2-
chlorophenol v´ıa II. Apparently, the most important
process appears to be through four-membered cyclic
structure. In this case, there is less stabilization on the
carbonyl carbon adjacent to the electron-withdrawing
chlorine atom, which may be the absence of the
C
δ+.. . . . Clδ−, appears to be the rate-determining step.
Moreover, the consideration of a reasonable mecha-
nism of via I suggests that the chlorine atom undergoes
in both cases an intramolecular substitution at positions
1 and 2 of the benzene ring, where decarboxylation oc-
curs through a four-membered cyclic transition state,
whereas decarbonylation by way of a five-membered
cyclic transition state takes place through via II.
Because of a more complex interpretation is re-
quired for p-tolyl chloroformate, it was thought to be
first discussed. Consequently, this substrate undergoes
through via I to produce about 20.7% p-chlorotoluene
and 79.1% by via II with the formation of 2-chloro-
4-methylphenol. This result suggests an intramolec-
ular competition of aromatic migration by the chlo-
rine atom. A difference in the energy of activation of
ࣈ30 kJ mol−1 lower in the 2-chloro-4-methylphenol
formation, implies a more favorably process than via
International Journal of Chemical Kinetics DOI 10.1002/kin.20939

670
LEZAMA AND CHUCHANI
electron release of the CH3 substituent at the para po-
sition of the ClCOO substituent.
7. Lewis, E.; Herndon, W.; Duffey, D. J Am Chem Soc
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CONCLUSIONS
The gas-phase elimination of aryl chloroformates un-
dergoes a homogeneous, unimolecular reaction and
follows a first-order rate law. The aryl chloroformates
appears to undergo through a mechanisms involving
two competitive intramolecular reactions, where decar-
boxylation occurs on the one side and decarbonylation
on the other side. The proposed mechanisms were con-
sidered to proceed through a concerted, polar four- and
five-membered cyclic transition states. The bond polar-
ization of the C–Cl, in the sense Cδ+ . . . Clδ−, appears
to be the rate-determining step of these elimination
reactions.
10. Johnson, R.; Stimson, R. Aust J Chem 1977, 30, 1917–
20.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20939