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