Experimental and theoretical study of the homogeneous, unimolecular gas-phase elimination kinetics of methyl oxalyl chloride

Article abstract of DOI:10.1002/poc.705

The products of the gas-phase elimination of methyl oxalyl chloride are methyl chloroformate and carbon monoxide. The kinetics were determined in a static system over the temperature range 280.3-320.7°C and pressure range 56-140.4 Torr. The reaction, in vessels seasoned with allyl bromide and in the presence of the free-radical inhibitor cyclohexene, is homogeneous, unimolecular and follows a first-order rate law. The rate coefficients are given by the Arrhenius equation: log[k₁(s^-1)] = (12.73 ± 0.58)-(174.3±6.3) kJ mol^-1 (2.303 RT)^-1. Theoretical studies of the mechanism at the semi-empirical PM3 and MP2/6-31G* levels imply a concerted non-synchronous process. Analysis of the three-membered ring transition state suggests that polarization of the Cl-C(=O) bond is rate limiting in the elimination process. The kinetics and thermodynamic parameters are in good agreement with the experimental values.

Full text of DOI:10.1002/poc.705

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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2004; 17: 148–151
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.705
Experimental and theoretical study of the homogeneous,
unimolecular gas-phase elimination kinetics of methyl
oxalyl chloride
Tania Cordova,1 Alexandra Rotinov2 and Gabriel Chuchani2*
1
´
´
´
Laboratorio de Fisico-Quımica Organica, Escuela de Quımica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 1020-A,
Caracas, Venezuela
2
´
´
Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 5 June 2003; revised 13 August 2003; accepted 18 August 2003
ABSTRACT: The products of the gas-phase elimination of methyl oxalyl chloride are methyl chloroformate and
ꢀ
carbon monoxide. The kinetics were determined in a static system over the temperature range 280.3–320.7 C and
pressure range 56–140.4 Torr. The reaction, in vessels seasoned with allyl bromide and in the presence of the free-
radical inhibitor cyclohexene, is homogeneous, unimolecular and follows a first-order rate law. The rate coefficients
are given by the Arrhenius equation: log[k1 (sꢁ1)] ¼ (12.73 ꢂ 0.58)ꢁ(174.3 ꢂ 6.3) kJ molꢁ1 (2.303 RT)ꢁ1. Theoretical
studies of the mechanism at the semi-empirical PM3 and MP2/6–31G* levels imply a concerted non-synchronous
—
process. Analysis of the three-membered ring transition state suggests that polarization of the Cl—C( O) bond is
—
rate limiting in the elimination process. The kinetics and thermodynamic parameters are in good agreement with the
experimental values. Copyright # 2004 John Wiley & Sons, Ltd.
KEYWORDS: kinetics; unimolecular elimination; pyrolysis; methyl oxalyl chloride; semi-empirical and ab initio
calculations; reaction mechanism; transition-state structure
INTRODUCTION
decarboxylation was described in the decomposition of
methyl bromoformate in the gas phase.4 An additional
investigation on methyl chloroformate, under homoge-
neous conditions and in the presence of the free-radical
inhibitor propene, was reported to eliminate methyl
The presence of a Cꢀ —H bond on the alkyl side of an
organic ester, such as acetates, leads to a gas-phase
pyrolytic elimination through a six-membered cyclic
transition state type of mechanism as described in reac-
tion (1):1,2
5
ꢀ
chloride and CO2 at 425–480 C. [reaction (2),
step 2].
ð1Þ
ð2Þ
However, methyl esters without a Cꢀ —H bond generally
decompose by way of complex concurrent molecular–
radical mechanisms and at very high temperatures. Con-
trary to this rule, the gas-phase pyrolysis of methyl
chloroformate was reported to give methyl chloride and
A curious organic molecule with which to investigate
the elimination kinetics in the gas phase is methyl oxalyl
chloride (methyl chlorooxoacetate). The thermal decom-
3
ꢀ
carbon dioxide at 299–491 C. A similar process of
`
position of this substrate may proceed via a consecutive
reaction, where methyl oxalyl chloride, as an acyl
chloride, may decarbonylate [reaction (2), step 1], while
the intermediate methyl chloroformate decarboxylates
[reaction (2), step 2] to yield methyl chloride [reaction
´
*Correspondence to: G. Chuchani, Centro de Quımica, Instituto
Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827,
Caracas 1020-A, Venezuela.
E-mail: chuchani@quimica.ivic.ve
´
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 148–151

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GAS-PHASE ELIMINATION KINETICS OF METHYL OXALYL CHLORIDE
149
ꢀ
(2)]. An alternative mechanistic process for the decom-
position of the above-mentioned substrate may be
thought to proceed through a five-membered cyclic
structure to methyl chloride and an assumed unstable
intermediate C2O3 [reaction (3)]. A extremely rapid
simultaneous fragmentation to methyl chloride, CO and
CO2 may be another process of elimination [reaction (4)].
by standard methods12 for a temperature of 300 C and
1 atm pressure.
The first-order rate coefficient k(T) was calculated using
the TST13 and assuming that the transmission coefficient
is equal to unity, as expressed in the following relation:
kðTÞ ¼ ðKT=hÞexpðꢁÁGz=RTÞ
ð5Þ
where ÁGz is the Gibbs free energy change between the
reactant and the transition state and K and h are the
Boltzmann and Plank constants, respectively. ÁGz was
calculated using the following relations:
ð3Þ
ÁGz ¼ ÁHz ꢁ TÁSz
ð6Þ
and
ÁHz ¼ Vz þ ÁZPVE þ ÁEðTÞ þ PV
ð7Þ
ð4Þ
where Vz is the potential energy barrier and ÁZPVE and
ÁE(T) are the differences in ZPVE and temperature
corrections between the transition state and the reactant,
respectively. ÁSz values were obtained from the differ-
ence between the reactant and the corresponding transi-
tion-state entropies directly from the Gaussian output.
ÁSz values were used to calculate Arrhenius pre-expo-
nential factor using the relation
In order to achieve a reliable interpretation concerning
the mechanistic elimination of the assumed reactions (2),
(3) and (4), this investigation was aimed at examining the
elimination kinetics of methyl oxalyl chloride and to
perform a combination of kinetic experiments and theo-
retical studies of the said substrate. The intention was to
obtain the kinetic parameters and the characterization of
the potential energy surface (PES) in order to elucidate
the nature of the molecular mechanism for the elimina-
tion process.
A ¼ ðemkT=hÞexpðÁSz=RÞ
ð8Þ
where m is the molecularity of the reaction.
RESULTS AND DISCUSSION
COMPUTATIONAL METHOD AND MODEL
The elimination process of methyl oxalyl chloride, in a
static system, seasoned with allyl bromide and in the
presence of the free-radical suppressor cyclohexene, was
The kinetics for the gas-phase elimination reaction of
ClCOCOOCH3 into CO and ClCOOCH3 suggest a con-
certed mechanism. Theoretical calculations using the
semi-empirical PM3 method and the MP2 level of theory
with the 6–31G* basis set6,7 were performed with the
Gaussian 94 W program. The Berny analytical gradient
ꢀ
determined over the temperature range 280–320 C and
the pressure range 56–140 Torr (1 Torr ¼ 133.3 Pa). The
stoichiometry based on reaction (9) demands that, for
long reaction times, Pf ¼ 2P0, where Pf and P0 are the
final and initial pressure, respectively. The experimental
Pf/P0 value at four different temperatures and 10 half-
lives was 1.94.
optimization routines were used for optimization.8,9
A
transition-state search was performed using the Quadratic
Synchronous Transit method. The nature of stationary
points was established by calculating and diagonalizing
the force constant matrix to determine the number of
imaginary frequencies.10 Intrinsic reaction coordinate
(IRC) calculations were performed to verify transition-
state structures.11 Frequency calculations provided
thermodynamic quantities such as zero-point vibrational
energy (ZPVE), temperature corrections and absolute
entropies, and consequently the rate coefficient can be
estimated assuming that the transmission coefficient is
equal to unity. Temperature corrections and absolute
entropies were obtained assuming ideal gas behavior,
from the harmonic frequencies and moments of inertia
ClCOCOOCH3 ! ClCOOCH3 þ CO
ð9Þ
The yields of pyrolysis products within the range of
rate determination, that is, up to 65% reaction, were
methyl chloroformate and CO gas [reaction (9)]. Methyl
chloroformate did not decompose further, since the
experimental working temperature is less than the
5
ꢀ
425 C of the investigation reported previously.
The homogeneity of the elimination was examined by
using a reaction vessel with a different surface-to-volume
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150
T. CORDOVA, A. ROTINOV AND G. CHUCHANI
Table 1. Homogeneity of the reaction
Temperature
( ꢀC)
S/ꢁV1 a 104 k1 104 k1
Substrate
(cm
)
(sꢁ1 b
)
(sꢁ1 c
)
Methyl oxalyl chloride 311.5
1
6
17.62d 14.65
29.62d 14.91
a S ¼ surface area (cm2); V ¼ volume (cm3).
b Clean Pyrex vessel.
c Vessel seasoned with allyl bromide.
d Average k values.
Figure 1. The transition state for this reaction is a three-
membered ring structure showing a very large C2—C4
distance as well as a large Cl1—C2 distance. Also, chlorine
is closer to C4 in the transition state. Angles ꢁ and ꢀ are also
shown. Below is the transition state as a three-dimensional
molecule. The transition state is late in the reaction coordi-
nate; the new Cl—C4 bond is almost completely formed in
addition to a C—O triple bond in CO
Table 2. Effect of the free radical inhibitor cyclohexene on
rates
Temperature
( ꢀC)
Ps
Pi
104 k1
(sꢁ1
Substrate
(Torr)a (Torr)a Pi/Ps
)
Methyl oxalyl 320.7
chlorideb
95.5
—
—
27.33
120.5 122
102 169
102.5 262.5
0.6
1.7
2.6
26.05
25.66
25.72
a good straight line up to 65% reaction. The variation of
the rate coefficients with temperature and the correspond-
ing Arrhenius equation is given in Table 4 (90% con-
fidence coefficient from least-squares procedure).
With these results and to establish the most adequate
mechanism for the gas-phase elimination of methyl
oxalyl chloride, a theoretical study was carried out.
a
Ps ¼ pressure of the substrate; Pi ¼ pressure of the inhibitor. Vessel
seasoned with allyl bromide.
b Inhibitor cyclohexene.
ratio. That is, the packed vessel has a 6.0 times greater
surface-to-volume ratio than the unpacked vessel
(Table 1). The packed and unpacked clean Pyrex vessels
had a significant effect on the rates. However, the packed
and unpacked Pyrex vessels seasoned with allyl bromide
had no effect on rates. The methyl oxalyl chloride
reaction, in seasoned vessels, had to be carried out in
the presence of at least twice the amount of the inhibitor
cyclohexene in order to prevent any radical reaction
(Table 2). No induction period was observed and the
rates were reproducible with a standard deviation not
greater than 10% at a given temperature.
THEORETICAL RESULTS
Theoretical studies suggest that the gas-phase elimination
reaction of methyl oxalyl chloride to give methyl chlor-
oformate and carbon monoxide occurs by a concerted
non-synchronous mechanism. The transitions state found
is late in the reaction coordinate in the sense of the
˚
Table 5. Transition-state (TS) atom distances (A)
The rate coefficient of this elimination was found to be
independent of the initial pressure (Table 3), and the first-
order rate was calculated from k1 ¼ (2.303/t) log P0/
(2P0 ꢁ Pt). A plot of log (2P0 ꢁ Pt) against time t gave
TS atoms
PM3 distance
MP2/6–31G* distance
Cl1—C2
C2—O3
C2—C4
C4—O5
C4—Cl1
C4—O6
3.77
1.13
5.20
1.21
1.73
1.36
4.10
1.13
5.64
1.18
1.84
1.36
Table 3. Invariability of the rate coefficients with initial
pressurea,b
Temperature
( ꢀC)
Substrate
Parameter
Value
Table 6. TS bond angles (ꢀ)
Methyl
oxalyl
chloride
320.7
P0 (Torr) 53 102 120.5 140.5
104 k1 (sꢁ1) 25.79 25.66 25.85 25.73
PM3
12.7
MP2/6–31G*
ꢁ ffC4Cl1C2
ꢀ ffC2Cl1C4
11.9
27.58
28.53
a Vessel seasoned with allyl bromide.
b In the presence of the inhibitor cyclohexene.
Table 4. Variation of rate coefficients with temperaturea
Substrate
Parameters
Value
300.2
Methyl oxalyl chloride
Temperature ( ꢀC)
280.3
1.96
290.2
3.74
311.5
14.62
320.7
25.97
104 k1 (sꢁ1
)
7.05
a Rate equation: log k1 (sꢁ1) ¼ (12.73 ꢂ 0.58) ꢁ (174.3 ꢂ 6.3) kJ molꢁ1 (2.303 RT)ꢁ1; r ¼ 0.9980.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 148–151

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GAS-PHASE ELIMINATION KINETICS OF METHYL OXALYL CHLORIDE
Table 7. Comparative theoretical and experimental activation parameters at 300 ꢀC
151
Method
Ea (kJ molꢁ1
)
ÁHz (kJ molꢁ1
)
ÁSz (J molꢁ1 Kꢁ1
)
ÁGz (kJ molꢁ1
)
Log [A (sꢁ1)] 104 k1 (sꢁ1
)
PM3
MP2/6–3G*
Experimental
174.6
173.2
174.3
169.8
168.5
169.5
ꢁ16.7
ꢁ14.0
ꢁ14.9
179.4
176.5
178.1
12.64
12.78
12.73
5.29
9.81
6.94
breaking of Cl1—C2 and C2—C4 bonds and the forma-
tion of a Cl1—C4 bond. The transition-state geometry is
shown in Fig. 1. The geometric parameters shown in
Tables 5 and 6 indicate a product-like transition state.
Calculated activation parameters are in reasonable
agreement with the experimental values (Table 7).
First-order rate coefficients are of the same order of
magnitude as the experimental value.
Kinetic studies. The kinetic experiments were performed
in a static reaction system as previously reported14,15 with
an Omega DP41-TC/DP41-RTD high-temperature per-
formance digital temperature controller. The rate coeffi-
cients were determined manometrically. The temperature
was controlled by a Shinko DC-PS resistance thermo-
meter controller and an Omega Model ꢀSSR280A55 solid-
state relay maintained within ꢂ 0.2 C and measured
with a calibrated platinum–platinum–13% rhodium ther-
mocouple. No temperature gradient was found along the
reaction vessel. The substrate was injected (0.05–0.1 ml)
directly into the reaction vessel with a syringe through a
silicone-rubber septum.
CONCLUSIONS
Results from theoretical calculations combined with
experimental shows that the reaction proceeds in a con-
certed, rather polar mechanism [reaction (10)], the transi-
tion state being product-like or late in the sense of the
reaction progress. The activation parameters are
satisfactory and first-order reaction rate coefficients are
of the same order of magnitude. Semi-empirical PM3
calculations are in slightly better agreement with experi-
ment than MP2/6–31G* calculations. Analysis and com-
parison of the results suggest the validity of the methods
used.
REFERENCES
1. Taylor R. In The Chemistry of Functional Groups. Supplementary
Volume B, Acid Derivatives, Patai S (ed). Wiley: Chichester,
1979; chapt. 15, 859–914.
2. Holbrook KA. In The Chemistry of Acid Derivatives.Volume 2.
Vapor and Gas Phase Reactions of Carboxylic Acids and their
Derivatives, Patai S (ed). Wiley: Chichester, 1992; chapt. 12,
703–746.
3. Szwac M, Murawki J. Trans. Faraday Soc. 1951; 47: 269–274.
4. Stimson VR, Tilley JW. Aust. J. Chem. 1977; 30: 81–86.
5. Johnson RL, Stimson VR. Aust. J. Chem. 1977; 30: 1917–1920.
ð10Þ
6. Frisch MJ, Pople JA, Binkley JS J. Chem Phys. 1984; 80: 3265–
3269
7. Frisch MJ, Truck GW, Schlegel HB, Gill PMW, Johnson BG, Robb
MA, Cheeseman JR, Keith T, Peterson GA, Montgomery JA,
Raghavachari K, Al-Laham MA, Zakzewski VG, Ortiz JV, Fores-
man JB, Cioslowski J, Stefanov BB, Nanayakkara A, Challacombe
M, Peng CY, Ayala PY, Chen W, Wong MW, Andres JL, Replogle
ES, Gomperts R, Martin RL, Fox DJ, Binkley JS, Defrees DJ,
Baker J, Stewart JP, Head-Gordon M, Gonzalez C, Pople JA.
Gaussian 94, Revision B.1. Gaussian: Pittsburgh, PA, 1995.
8. Schlegel HB. J. Comput. Chem. 1982; 3: 214–218.
9. Schlegel HB. J. Chem. Phys. 1982; 77: 3676–3681.
10. McIver JW Jr. Acc. Chem. Res. 1974; 7: 72–77.
11. Fukui K. J. Phys. Chem. 1970; 74: 4161–4163.
12. McQuarrie D. Statistical Mechanics. Harper & Row: New York,
1986.
EXPERIMENTAL
Methyl oxalyl chloride (methyl chlorooxoacetate). This
compound (Aldrich) was distilled several times (b.p.
ꢀ
114–115 C at 633 Torr) to better than 98.8% purity
(GC: Porapak Q, 80–100 mesh, 2 min). The decomposi-
tion products methyl chloroformate was determing using
a 10% SP 1200–1% H3PO4 Chromosorb W AW DMCS
(80–100 mesh) column. The verification of the substrate
and identification of the product were carried out by gas
chromatography–mass spectrometry (Saturn 2000,
Varian) using a DB–5MS capillary column (30 Â 0.25
mm. i.d., 0.25 mm film thickness).
13. Benson SW. The Foundations of Chemical Kinetics. Mc-Graw-
Hill: New York, 1960.
14. Maccoll A. J. Chem. Soc. 1955; 965–973.
15. Swinbourne E. Aust. J. Chem. 1958; 11: 314–330.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 148–151