Mechanisms of the homogeneous, unimolecular gas-phase elimination kinetics of triethyl orthoacetate and triethyl orthopropionate

Article abstract of DOI:10.1002/poc.1353

Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of 291-351°C and pressure range of 80-170 Torr. The elimination reactions of these orthoesters in seasoned vessels are homogeneous, unimolecular, and follow a first-order rate law. The reaction products are ethanol, ethylene and the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for triethyl orthoacetate, log k₁ (s^-1) = (13.76 ± 0.09) - (187.6 ± 1.1) KJ mol^-1 (2.303 RT)^-1 (r = 0.9993), and for triethyl orthopropionate, log k₁ (s^-1) = (13.63 ± 0.07) - (193.3 ± 1.8) kJ mol^-1 (2.303 RT) ^-1 (r = 0.9992). A reasonable mechanism of these elimination is to consider that the C-OCH₂CH₃ bond, as C^δ+. ..^δ+ OCH₂CH₃ in the TS, is the rate-determining step. The nucleophilicity of the oxygen atom of OCH ₂CH₃ may abstract the hydrogen of the adjacent C-H bond for a four-membered cyclic structure to give the corresponding unsaturated ketal. The unstable ketal intermediate decomposes, in a six-membered cyclic transition state, into ethylene and the corresponding ethyl ester. Copyright

Full text of DOI:10.1002/poc.1353

Special Issue
Received: 12 November 2007,
Revised: 31 January 2008,
Accepted: 1 February 2008,
Published online in Wiley InterScience: 21 May 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1353
Mechanisms of the homogeneous,
unimolecular gas-phase elimination kinetics
of triethyl orthoacetate and triethyl
orthopropionate
Edgar Marqueza, Maria Tostaa, Rosa M. Domı´ngueza, Armando Herizea
a
*
and Gabriel Chuchani
Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of
291–351-C and pressure range of 80–170 Torr. The elimination reactions of these orthoesters in seasoned vessels are
homogeneous, unimolecular, and follow a first-order rate law. The reaction products are ethanol, ethylene and
the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for
triethyl orthoacetate, log k1 (sS1) ¼ (13.76 W 0.09) S (187.6 W 1.1) kJ molS1 (2.303 RT)S1 (r ¼ 0.9993), and for triethyl
orthopropionate, log k1 (sS1) ¼ (13.63 W 0.07) S (193.3 W 1.8) kJ molS1 (2.303 RT)S1 (r ¼ 0.9992). A reasonable mech-
anism of these elimination is to consider that the C—OCH2CH3 bond, as CdR. . .dS OCH2CH3 in the TS, is the
rate-determining step. The nucleophilicity of the oxygen atom of OCH2CH3 may abstract the hydrogen of
the adjacent C—H bond for a four-membered cyclic structure to give the corresponding unsaturated ketal. The
unstable ketal intermediate decomposes, in a six-membered cyclic transition state, into ethylene and the correspond-
ing ethyl ester. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: triethyl orthoacetate; triethyl orthopropionate; gas-phase elimination; kinetics; mechanisms
In view of the interesting reactions undergone by different
INTRODUCTION
methods of pyrolysis of orthoesters as described above,[1–3] the
present work aimed at studying the kinetics of this type of
compounds in the gas phase, especially under homogeneous and
molecular conditions. In this respect, this investigation examines
the gas-phase elimination kinetics of triethyl orthoacetate and
triethyl orthopropionate.
The chemical literature does not describe the pyrolysis kinetics of
orthoesters in the gas phase. However, it seemed interesting to
find some related works of thermal decomposition carried out
under different conditions. In 1922, Staudinger and Rathsam[1]
carried out the pyrolysis of some triethyl orthoesters in the
presence of nickel at 250–2608C to produce ethyl ether and the
corresponding ethyl ester. They also pyrolyzed triethyl ortho-
phenylacetate with the simultaneous formation of ethyl
phenyl acetate and corresponding phenylketene acetal after
distillation. No ethyl ether was detected as a final product. These
authors proved that pure phenylketene diethyl acetal when
RESULTS
The gas-phase elimination of triethyl orthoacetate and triethyl
orthopropionate at the temperature range of 291–3518C and
pressure range of 80–170 Torr were found to give ethanol, ethylene,
and the corresponding ethyl ester, as described in reaction (1).
heated in a bomb at 260–2708C yielded mainly ethyl
phenylacetate, presumably ethylene gas, and an unidentified
solid.
Diethylbenzyl orthoacetate[2] in the glass liner of a steel bomb
were electrically heated at 2008C for several hours. This reaction
was found to give o-tolylacetate from the rearrangement of the
ketene ethylbenzylacetal intermediate. In the case of triethyl
orthobenzoate, which has no a-hydrogen, gave the normal ester
and diethyl ether on pyrolysis.
ð1Þ
The substrate triethyl orthophenyl acetate through slow
distillation gave phenylketene diethyl acetal and ethyl pheny-
lacetate.[3] Otherwise, when trimethyl orthophenyl acetate was
heated at 250–2608C yielded phenylketene dimethyl acetal and
methyl phenylacetate in a better yield than ethyl phenylacetate.
Moreover, the vigorous evolution of methanol formation in the
pyrolysis of the latter orthoester led to believe the interaction of
two molecules of trimethyl orthophenyl acetate.
*
Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (IVIC),
Apartado 21827, Caracas 1020-A, Venezuela.
E-mail: chuchani@ivic.ve
a E. Marquez, M. Tosta, R. M. Dom´ınguez, A. Herize, G. Chuchani
Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (IVIC),
Apartado 21827, Caracas 1020-A, Venezuela
J. Phys. Org. Chem. 2008, 21 666–669
Copyright ß 2008 John Wiley & Sons, Ltd.

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION
Table 1. Ratio of final (Pf) to initial pressure P0 of the sub-
strate
Table 2. Stoichiometry of the reactions
´
Time
Reaccion
Ethylene
Susbtrate
T (8C) Po (Torr) Pf (Torr) Pf/Po Average
Susbtrate
T (8C) (min) (pressure) (%) (GC) (%)
Triethyl
orthoacetate
301.8
90
262
2.9
2.9
2.9
Triethyl
orthoacetate
301.8
3
5.6
5.7
310.6
320.2
330.5
340.7
301.8
102
107
80
60
75
298
318
230
176
218
2.9
3.0
2.9
2.9
2.9
10
15
18
25
35
48
2
17.5
25.1
29.3
38.2
49.0
60.3
3.3
17.8
25.3
29.5
38.5
49.2
60.8
3.4
Triethyl
orthopropionate
312.3
323.3
331.3
341.4
90
98
80
70
262
286
236
205
2.9
2.9
3.0
2.9
Triethyl
orthopropionate
312.3
5
10
15
25
35
45
55
8.0
15.4
22.2
34.1
44.2
52.8
60.0
8.4
15.5
22.7
34.3
44.5
53.0
61.0
Stoichiometry (1) demands that, for long reaction times,
Pf ¼ 3P0, where Pf and P0 are the final and initial pressure,
respectively. The average experimental results for Pf/P0 values at
five different temperatures and 10 half-lives were 2.9 for both
triethyl orthoacetate and triethyl orthopropionate (Table 1).
Further confirmation of the above stoichiometry (1) was made by
comparing the per cent decomposition of the orthoester
substrate from pressure measurements against the quantitative
chromatographic analyses of the product ethylene gas (Table 2).
The homogeneity of this elimination was examined by using a
packed reaction vessel with a surface-to-volume ratio six times
greater than that of the unpacked vessel (Table 3). The rates were
unaffected by the packed and unpacked seasoned vessels,
whereas a marked heterogeneous effect was obtained with the
packed and unpacked clean Pyrex vessels. The absence of a free
radical chain reaction was verified by carrying out several runs in
the presence of different proportions of toluene as inhibitor
(Table 4). No induction period was observed. The rates are
reproducible with a standard deviation not greater than 5% at a
given temperature.
these reactions carried out in seasoned vessels are homo-
geneous, unimolecular, and follow a first-order rate law. The rate
coefficient is expressed by the following Arrhenius equations:
Triethyl orthoacetate; log k1ðsꢀ1Þ ¼ ð13:76 ꢁ 0:09Þ
ꢀ ð187:6 ꢁ 1:1Þ kJ molꢀ1ð2:303 RTÞꢀ1
ꢂ ðr ¼ 0:9993Þ
Triethyl orthopropionate; log k1ðsꢀ1Þ ¼ ð13:63 ꢁ 0:07Þ
ꢀ ð193:3 ꢁ 1:8Þ kJ molꢀ1
ꢂ ð2:303 RTÞꢀ1ðr ¼ 0:9992Þ
DISCUSSION
The rate coefficients for the orthoesters calculated from
k1 ¼ (2.303/t) log [2P0/(3P0–Pt)] were found to be independent of
the initial pressure (Table 5). A plot of log (3P0–Pt) against time t
gave a good straight line up to 60% reaction. The temperature
dependence of the rate coefficients and the corresponding
Arrhenius equation are given in Table 6 and Fig. 1 (95%
confidence coefficients from a least-squares method). Therefore,
The data described in Table 7 shows, for both substrates, that the
log of A value is greater than 13.2. Such value of log A > than 13.2
implies a four-membered cyclic structure in the transition state as
postulated by Benson et al.[4,5] Moreover, the positive values of
the entropy of activations suggest a very polar concerted cyclic
transition state. Consequently, a reasonable explanation of the
Table 3. Homogeneity of the elimination reaction
Substrate
Temperature (8C)
S/V (cmꢀ1
)
104 k (sꢀ1 a
)
104 k (sꢀ1 b
)
Triethyl orthoacetate
301.8
1
6
1
6
51.60 ꢁ 3.10
79.20 ꢁ 10.10
31.12 ꢁ 6.40
61.12 ꢁ 12.10
5.20 ꢁ 0.20
5.25 ꢁ 0.10
3.31 ꢁ 0.30
3.35 ꢁ 0.12
Triethyl ortopropionate
312.3
aClean pyrex vessel.
bVessel seasoned with allyl bromide.
J. Phys. Org. Chem. 2008, 21 666–669
Copyright ß 2008 John Wiley & Sons, Ltd.
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E. MARQUEZ ET AL.
Table 4. Effect of the free radical chain suppresor on rates
Substrate
Temperature (8C)
P0 (Torr)
Pi (Torr)
Pi /P0
104 k (sꢀ1
)
Triethyl orthoacetate
301.8
80
86
82
60
0
88
120
150
0
1
1.5
2.5
5.28 ꢁ 0.20
5.22 ꢁ 0.15
5.21 ꢁ 0.17
5.20 ꢁ 0.30
Triethyl orthopropionate
312.3
80
70
86
80
0
106
170
180
0
1.5
2
3.31 ꢁ 0.33
3.37 ꢁ 0.32
3.39 ꢁ 0.35
3.35 ꢁ 0.28
3
Table 5. The invariability of the rate constant with initial pressure
Substrate
Temperature (8C)
P0 (mmHg)
104 k1 (sꢀ1
)
80
95
115
125
170
5.25 ꢁ 0.11
5.30 ꢁ 0.16
5.32 ꢁ 0.14
5.27 ꢁ 0.50
5.23 ꢁ 0.38
Triethyl orthoacetate
301.8
Triethyl ortopropionate
312.3
80
92
102
120
3.32 ꢁ 0.33
3.37 ꢁ 0.51
3.31 ꢁ 0.25
3.38 ꢁ 0.23
Table 6. Variation of rate coefficient with temperature
Substrate
Temperature (8C)
104 k1 (sꢀ1
)
Triethyl orthoacetate
291.5
301.8
310.6
320.2
330.5
340.7
301.3
312.8
323.1
331.3
340.8
351.2
2.40 ꢁ 0.14
5.23 ꢁ 0.20
1.05 ꢁ 0.49
18.9 ꢁ 0.92
34.00 ꢁ 1.97
61.20 ꢁ 2.86
1.27 ꢁ 0.22
3.30 ꢁ 0.30
5.75 ꢁ 0.50
9.35 ꢁ 0.25
18.37 ꢁ 0.35
33.60 ꢁ 0.55
Triethyl ortopropionate
Figure 1. Graphic representation of the Arrhenius plot for the gas-phase
elimination of triethyl orthoacetate (––) and triethyl orthopropionate
(-----)
unsaturated ketal. This intermediate, under the reaction
condition of temperature, rapidly decomposes into ethylene
and the corresponding ethyl ester. To explain this latter
mechanism of elimination, if one of the two OCH2CH3 of the
unsaturated ketal is considered as any other substituent, then
the corresponding ethyl vinyl ether decomposes through a
six-membered cyclic transition state to give ethylene and the
corresponding ethyl ester [reaction (2), step 2] as reported in the
literature.[6] Further data of the kinetic and thermodynamic
mechanism in the elimination of these orthoesters is to consider
that breakage of the C—OCH2CH3 bond, such as Cdþ. . .dꢀ
OCH2CH3 in the TS, is the rate-determining step [reaction (2), step
1]. The nucleophilicity of the oxygen atom of the OCH2CH3 will
then abstract the hydrogen of the adjacent C—H bond for a
four-membered cyclic structure to give the corresponding
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 666–669

HOMOGENEOUS, UNIMOLECULAR GAS-PHASE ELIMINATION
Table 7. Kinetic and thermodynamic parameters
¼
¼
Substrate
104 k1 (sꢀ1 a
)
Ea (kJ/mol)
log A (sꢀ1
)
DH (kJ/mol)
DS (J/mol K)
Triethyl orthoacetate
Triethyl orthopropionate
10.05
2.06
187.6 ꢁ 1.1
193.3 ꢁ 1.8
13.76 ꢁ 0.09
13.63 ꢁ 0.07
182.8 ꢁ 1.07
188.5 ꢁ 1.76
4.62 ꢁ 0.64
2.14 ꢁ 0.25
aAt 3108C.
parameters show that triethyl orthoacetate is about five times
greater in elimination than triethyl orthopropionate. This
difference may be rationalized from the fact that the abstraction
of the adjacent hydrogen of the C—H bond by the nucleophilic
OCH2CH3 is less acidic in the propionate than in the acetate.
SHINKO DIC-PS 23TR resistance thermometer controller with a
calibrated Iron-Constantan thermocouple. The temperature
reading was measured within ꢁ0.18C with a thermopar of
Iron-Constantan attached to a Digital Multimeter Omega 3465B.
The reaction vessel showed no temperature gradient at different
ð2Þ
points, and the substrate was injected directly into the reaction
vessel through a silicone rubber septum. The volume of substrate
used for each reaction was ꢃ0.05–0.1 ml.
EXPERIMENTAL
Triethyl orthoformate (97%) and triethyl orthopropionate (97%)
were bought from Aldrich. The purity of the substrates and
products and their identifications were determined by GC/MS/MS
(Saturn 2000, Varian). Capillary column DB – 5MS, 30 mm ꢂ
0.250 mm, i.d. 0.25 mm. The olefin product ethylene was analyzed
using a chromatograph Varian Star 3600 CX with a thermal
conductivity detector (capillary column: GS-Q, 30 m long and 0.53
i.d., Helium gas carrier).
REFERENCES
[1] H. Staudinger, G. Rathsam, Helv. Chim. Acta. 1922, 5, 645–655 and
references cited in.
[2] S. M. McElvain, H. I. Anthes, S. H. Shapiro, J. Am. Chem. Soc. 1942, 64,
2525–2531.
[3] S. M. McElvain, C. L. Stevens, J. Am. Chem. Soc. 1946, 68, 1917–1961.
[4] S. Benson, W. O’Neal, J. Phys. Chem. 1967, 71, 2903–2921.
[5] S. B. Benson, Thermochemical Kinetics, John Wiley & Sons, New York,
1968.
[6] G. G. Smith, F. W. Kelly, Prog. React. Kinet. 1971, 8, 75–234.
[7] A. Maccoll, J. Chem. Soc. 1955, 965–973.
[8] E. S. Swinbourne, Aust. J. Chem. 1958, 11, 314–330.
[9] R. M. Dominguez, A. Herize, A. Rotinov, A. Alvarez-Aular, G. Visbal, G.
Chuchani, J. Phys. Org. Chem. 2004, 17, 399–408.
Kinetics
The kinetics determinations were carried out in a static reaction
system as described before.[7–9] The reaction vessel was
deactivated with the product of decomposition of allyl bromide.
The rate coefficients were determined manometrically. The
temperature was found to be within ꢁ0.28C when controlled by a
J. Phys. Org. Chem. 2008, 21 666–669
Copyright ß 2008 John Wiley & Sons, Ltd.
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