Kinetic and mechanism of the homogeneous, unimolecular elimination of α, β-unsaturated aldehydes in the gas phase

Article abstract of DOI:10.1002/poc.1140

The gas phase thermal decarbonylation of α,β-unsaturated aldehydes E-2-butenal and E-3-phenyl-2-methylpropenal was studied in a static system over the temperature range 380.5-490.0°C and pressure range 55.5-150Torr. The reactions are homogeneous and unimolecular and obey a first-order rate law. The rate coefficient is represented by the following Arrhenius equations: E-2-Butenal: log k₁, (s^-1) = (13.18 ± 0.16) - (212.0 ± 2.0) kJ mor^-1(2.303 RT)^-1 E-3-phenyl-2-methylpropenal: log k₁, (s^-1) = (13.23 ± 0.22) - (234.6 ± 3.2) kJ mor^-1(2.303 RT)^-1 The elimination products of 2-butenal are propene and CO gas, while 3-phenyl-2-methylpropenal produces α-methylstyrene, cis-trans-β- metriylstyrene, indan, and CO gas. Kinetic and thermodynamic parameters suggest these elimination reactions to proceed through a three-membered cyclic transition state type of mechanisms. However, a two steps mechanisms for the formation of a carbene type of intermediate through a four-membered cyclic transition n structure can not be overlooked. Copyright

Full text of DOI:10.1002/poc.1140

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 307–312
Published online 27 March 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1140
Kinetic and mechanism of the homogeneous,
unimolecular elimination of a,b-unsaturated aldehydes
in the gas phase
1
2
2
2
1
´
´
´
Omar Y. Chaban, Rosa M. Domınguez, Armando Herize, Marıa Tosta, Angela Cuenca
2
*
and Gabriel Chuchani
1
´
´
´
Departamento de Quımica, Universidad Simon Bolıvar, Sartenejas, Edo Miranda, Venezuela
2
´
´
Centro de Quımica, Instituto Venezolano de Investigaciones Cientıficas (I.V.I.C.), Apartado 21827, Caracas, Venezuela
Received 5 October 2006; revised 14 November 2006; accepted 17 November 2006
ABSTRACT: The gas phase thermal decarbonylation of a,b-unsaturated aldehydes E-2-butenal and
E-3-phenyl-2-methylpropenal was studied in a static system over the temperature range 380.5–490.0 8C and pressure
range 55.5–150 Torr. The reactions are homogeneous and unimolecular and obey a first-order rate law. The rate
coefficient is represented by the following Arrhenius equations:
Eꢀ2ꢀButenal : log k1 ðsꢀ1Þ ¼ ð13:18 ꢁ 0:16Þ ꢀ ð212:0 ꢁ 2:0Þ kJ molꢀ1ð2:303 RTÞꢀ1
Eꢀ3ꢀphenylꢀ2ꢀmethylpropenal : log k1 ðsꢀ1Þ ¼ ð13:23 ꢁ 0:22Þ ꢀ ð234:6 ꢁ 3:2Þ kJ molꢀ1ð2:303 RTÞꢀ1
The elimination products of 2-butenal are propene and CO gas, while 3-phenyl-2-methylpropenal produces
a-methylstyrene, cis-trans-b-methylstyrene, indan, and CO gas. Kinetic and thermodynamic parameters suggest
these elimination reactions to proceed through a three-membered cyclic transition state type of mechanisms. However,
a two steps mechanisms for the formation of a carbene type of intermediate through a four-membered cyclic transition
structure can not be overlooked. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: gas phase elimination; pyrolysis; kinetics; mechanisms; 2-butenal; 3-phenyl-2-methylpropenal
INTRODUCTION
2,2-dimethyl- 3-butenal, was reported by Crawford
et al.33 This compound, in a static system at
282–302 8C, yielded 2-methyl-2-butene and carbon
monoxide. Additionally, the kinetic isotope effect of this
elimination reaction gave a kH/kD ¼ 2.8 at 296.9 8C (i.e.,
7.2 at 25 8C). This result indicated that the H attached to
the CO group of the aldehyde has been transferred during
the process of decomposition. Since the reaction proved
to be unimolecular and obeys a first-order rate law, the
mechanism was described by a five-membered cyclic
transition state type of mechanism as depicted in
reaction (1)
An important type of organic molecules, the aldehydes,
have little been studied in thermal decomposition kinetics
in the gas phase. This fact may be due to their tendency to
oxidize in contact with air and to generally undergo a
complex free radical chain processes.1 Gas phase
pyrolyses of saturated aldehydes such as propionalde-
hyde,2–8 acetaldehyde,9–24 and formaldehyde25–27 at
temperatures of 450–600 8C showed to proceed through
free radical reactions. An aldehyde with the CHO group
attached directly to a p-system, that is, benzaldehyde28–32
at 550–1400 8C was described to undergo a radical
mechanism to produce benzene and CO as primary
products, while biphenyl and hydrogen gas as secondary
products. The only work on the gas phase thermal de-
CH3
CH3
C
H3C
H
H3C
H
H3C
H
β
β
∆
Cα
C
CH
C
C
(1)
+ CO
α
γ
γ
C
CH2
H3C
CH
C
CH2
O
O
R
R
R
composition
of
a
b-g-unsaturated
aldehyde,
R = H, D
In the paper describing the pyrolysis of benzaldehyde
in a flow system described of Grela and Colussi,32
2-furaldehyde and 2-butenal were also examined. The
´
*Correspondence to: G. Chuchani, Centro de Quımica, I.V.I.C., Apart
ado 21827, Caracas 1020-A, Venezuela.
E-mail: chuchani@ivic.ve
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 307–312

´
O. Y. CHABAN ET AL.
308
2-furaldehyde suggested to proceed via a biradical
mechanism in bond breaking of C2—O or C5—O
[reaction (2)], while 2-butenal was demonstrated to be
comparing the pressure measurements with the quanti-
tative GLC analysis of olefin formation (Table 2).
The homogeneity of the reaction was studied by using a
vessel with a surface-to-volume ratio 6 relative to the
normal vessel, which had an S/V ¼ 1 (Table 3). The
packed and unpacked clean Pyrex vessels and the packed
and unpacked Pyrex vessels seasoned with allyl bromide
had no marked effect on the rate coefficients. The effect of
the free radical inhibitor is described in Table 4. The
kinetic determinations were carried out in the presence of
at least 2 equivalent of toluene in order to suppress any
possible free radical chain process of the substrate and/or
products. No induction period was observed and the rates
were reproducible with a relative standard deviation not
greater than 5% at a given temperature.
a molecular process. 2-Butenal with mixed CHO and
CDO in a ratio of 1:3 yielded the same product ratio. This
means that H or D at the CO of the aldehyde are
transferred to the b-position through a 1,2-migration.
Therefore, the mechanism of gas elimination of 2-butenal
suggested a concerted three-membered cyclic transition
state structure as shown in reaction (3).
The rate coefficients for the elimination reaction
calculated from k1 ¼ ꢀð2:303=tÞlog½ð2P0 ꢀ PtÞ=P0Þꢂ
were shown to be independent of the initial pressure of
the substrate (Table 5), and the first-order plots of
log½ð2P0 ꢀ PtÞꢂ against time t are linear up to 68%
reaction. The temperature dependence of the rate
coefficients and the corresponding Arrhenius equation
is described in Table 6 (90% confidence limits from a
least-squares procedure).
In view of little information on the thermolysis of
a-b-unsaturated aldehydes, the present work aimed at
examining in a static system, under homogeneous and
molecular conditions, the gas phase elimination kinetics
of 2-butenal and 3-phenyl-2-methylbutenal and to
consider a reasonable mechanism of these reactions
RESULTS AND DISCUSSION
E-2-Butenal
E-3-Phenyl-2-methylpropenal
The elimination kinetics of E-3-phenyl-2-methylpropenal
was examined in the temperature range of 440.6–490.0 8C
and pressure range of 62–142 Torr. The stoichiometry of
reaction (5) requires
The gas phase molecular elimination of this aldehyde was
determined between the temperature range of 380.5 and
430.5 8C and pressure range of 55.5–150 Torr. The
theoretical stoichiometry described in reaction (4)
demands Pf/P0 ¼ 2.0, where Pf and P0 are the final and
initial pressures,
H
CHO
C
C
∆
(5)
C9H10
Olefin isomers
+ CO
CH3
H
CHO
H
H
∆
(4)
C
C
+ CO
C
C
H3C
H
H3C
H
Pf ¼ 2P0. The actual average experimental value of Pf/
P0 at several different temperatures and 10 half-lives is
2.0 (Table 1). Additional verification of stoichiometry (5),
up to 56% reaction, gave a good agreement between the
extent of decomposition as predicted from pressure
respectively. The average experimental results Pf/P0 at
four different temperatures and 10 half-lives for 2-butenal
is 1.9 (Table 1). Additional examination of stoichiometry
(4), up to 68% decomposition, was obtained by
Table 1. Ratio of final (Pf) to initial pressure (P0) of the substrate
Compound
Temperature (8C)
P0 (Torr)
Pf (Torr)
Pf/P0
Aver.
E-2-Butenal
––
––
––
E-3-Phenyl-2-methylpropenal
––
––
400.0
410.3
420.3
430.5
470.2
480.2
490
96
101
96
100.5
103.2
92
178
1.9
1.8
2.0
2.0
2.0
2.0
2.1
1.9
––
––
––
2.0
––
––
184.5
188.5
204
203
186.5
204.5
99.9
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 307–312
DOI: 10.1002/poc

a,b-UNSATURATED ALDEHYDES
309
Table 2. Stoichiometry of the reaction
Substrate
Temperatures (8C)
Parameter
Value
E-2-Butenal
––
––
E-3-Phenyl-2-methylpropenal
––
––
410.3
––
––
470.2
––
––
Time (min)
7
10
12
15
20
Reaction (%) (pressure)
Olefin (%) (GLC)
Time (min)
Reaction (%) (pressure)
Olefin (%) (GLC)
32.8
31.6
6
18.4
18.7
43.4
42.6
10
27.1
25.2
51.5
51.7
15
36.9
34.5
56.3
55.7
20
46.0
45.2
68.3
68.7
35
56.2
55.2
Table 3. Homogeneity of the elimination reactions
Substrate
S/V (cmꢀ1 a
)
k1 ꢃ 104 (sꢀ1 b
)
k1 ꢃ 104 (sꢀ1 c
)
E-2-Butenal at 410.3 oC
1
6
1
6
9.75
9.77
5.73
5.75
9.60
9.71
5.66
5.72
E-3-Phenyl-2-methylpropenal at 470.2 oC
a S, surface area; V, volume.
b Clean Pyrex vessel.
c Vessel seasoned with allyl bromide.
Table 4. Effect of free radical suppressor toluene on rates
Substrate
Temperature (8C)
Ps (Torr)
Pi (Torr)
Pi/Ps
k1 ꢃ 104 (sꢀ1
)
E-2-Butenal
––
––
––
––
––
––
––
410.3
––
––
––
––
––
––
––
470.2
––
––
––
––
103.5
104
150
99
97.5
100
107
101.5
92
80.8
79.7
99.1
79.4
79.4
—
56.5
151
159
197.5
245
307
346
—
102
183.5
298
327
411
—
0.5
1.0
1.6
2.0
2.5
2.9
3.4
—
1.3
2.3
3.0
4.1
5.2
11.4
9.72
9.67
9.52
9.71
9.51
9.65
9.74
7.56
5.74
5.76
5.58
5.64
5.53
E-3-Phenyl-2-methylpropenal
––
––
––
––
––
––
Ps, pressure substrate; Pi, pressure inhibitor.
Table 5. Invariability of the rate coefficients with initial pressure
Substrate
Temperature (8C)
Parameters
Value
E-2-Butenal
––
E-3-Phenyl-2-methylpropenal
––
410.3
––
470.2
––
P0 (Torr)
55.5
9.77
62.7
5.65
82
9.72
79.4
5.53
94
9.66
86.4
5.63
108.5
9.58
91
5.64
130
9.74
108.8
5.67
150
k1 ꢃ 104 (sꢀ1
)
)
9.67
142.3
5.72
P0 (Torr)
k1 ꢃ 104 (sꢀ1
Table 6. The variation of the rate coefficients with temperatures
Substrate
Parameters
Value
E-2-Butenal
––
Temperature (8C)
380.5
1.70
390.5
3.22
400.0
5.51
410.3
9.60
420.3
16.40
430.5
27.40
k1 ꢃ 104 (sꢀ1
)
Rate equation log k1ðsꢀ1Þ¼ ð13:18 ꢁ 0:16Þ ꢀ ð212:0 ꢁ 2:0Þ kJ molꢀ1 ð2:303 RTÞꢀ1 , r ¼ 0.9998
E-3-Phenyl-2-methylpropenal
––
Temperature (8C)
440.6
1.19
450.7
1.92
460.2
3.38
470.2
5.66
480.2
9.15
490.0
15.20
k1 ꢃ 104 (sꢀ1
)
Rate equation log k1ðsꢀ1Þ¼ ð13:23 ꢁ 0:22Þ ꢀ ð234:6 ꢁ 3:2Þ kJ molꢀ1 ð2:303 RTÞꢀ1 , r ¼ 0.9996
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 307–312
DOI: 10.1002/poc

´
O. Y. CHABAN ET AL.
310
Table 7. Distribution of olefins at different percentages of the reaction at 470.2 8C
t (min)
% reaction
a-Methylstyrene
Cis-b-methylstyrene
Trans-b-methylstyrene
Indan
6
18.7
25.2
34.5
45.2
30.8
27.9
29.7
29.0
29.4
14.3
13.7
12.9
13.7
13.7
41.4
41.9
44.2
43.2
42.7
13.5
16.6
13.3
14.1
14.4
10
15
20
––
Average
measurements and the sum of quantitative chromato-
graphic analyses of olefin products (Table 2).
portions of the free radical inhibitor toluene is given in
Table 4. This reaction has also been carried out in the
presence of at least 2 equivalent of suppressor so as to
inhibit any possible free radical chain process of the
substrate and/or products. No induction period was
observed and the rates were reproducible with a relative
standard deviation not greater than 5% at a given
temperature.
The first-order rate coefficients of this aldehyde,
calculated from k1¼ ꢀ ð2:303=tÞlog½ð2P0 ꢀ PtÞ=P0Þꢂ
were found to be independent of the initial pressure of
the substrate (Table 5). A plot of log [(2 P0 ꢀ Pt)] versus
time t gave a good straight line up to 55% decomposition.
The variations of the rate coefficient are described in
Table 6. The results given in Table 6 lead, by using the
least square procedure and 90% confidence limits, to the
shown Arrhenius equation.
The products formation of this substrate within the rate
of coefficient determinations at different reaction
percentages were analyzed at 470.2 8C (Table 7). As
the extent of the reaction increases, the olefin distribution
are nearly constant. This result appears to be due to the
very rapid isomerization of the b-methylstyrene product.
Moreover, Table 8 describes how the temperature
influences, at 50% reaction, the process of olefin
isomerization.
To give support to the above consideration, the
equilibrium process of these products were verified by
the approximate calculations using thermodynamic group
additivity table34 at 470.2 8C (743.35 K) (Table 9).
Tests for surface effects on the rate of elimination, the
packed and unpacked clean Pyrex vessels, and the packed
and unpacked Pyrex vessels seasoned with allyl bromide
showed no effect, and the reaction may be said to be
homogeneous in nature. The effect of different pro-
According to the kinetic and thermodynamic
parameters of Table 10, the small negative value of the
entropy of activation DSz, together with the frequency
Table 8. Distribution of olefins at different temperature
Temperature (8C)
a-Methylstyrene
Cis-b-methylstyrene
Trans-b-methylstyrene
Indan
440.6
470.2
490.0
25.8
29.7
32.4
14.2
12.9
12.8
49.6
44.2
41.4
10.4
13.3
13.4
Table 9. Equilibrium mixtures from thermodynamic additivity34 at 470.2 8C (743.35 K)
Temperature (8C)
470.2
a-Methylstyrene
Cis-b-methylstyrene
Trans-b-methylstyrene
Indan
14.2
12.1
38.8
34.9
Table 10. Kinetic and thermodynamic parameters at 440.6 8C
k1 ꢃ 104
Ea
(kJ molꢀ1
logꢀA1
(sec
DSz
DHz
DGz
(kJ molꢀ1
)
Substrate
(secꢀ1
)
)
)
(J molꢀ1 Kꢀ1
)
(kJ molꢀ1
)
E-2-Butenal
E-3-Phenyl-2-methylpropenal
46.5
1.19
212.0 ꢁ 2.0
234.6 ꢁ 3.2
13.18 ꢁ 0.16
13.23 ꢁ 0.22
ꢀ8.2
ꢀ7.1
209.7
228.7
215.5
233.8
Copyright # 2007 John Wiley & Sons, Ltd.
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a,b-UNSATURATED ALDEHYDES
311
factor of log A values (Table 10) of 13.18 and 13.23
implies a transition state of these elimination reaction of
three- or four-membered cyclic structure.34 Of these two
structures, the most reasonable hypothesis is the
concerted semi-polar three-membered cyclic transition
state structure as described in reaction (6).
by comparing with true authentic samples bought from
Aldrich and in a GC-MS (Saturn 2000, Varian 3600ꢃ
with a DB-5MS capillary column 30 m ꢃ 0.25 mm. i.d.,
0.25 mm).
Kinetics
The kinetic determinations were carried out in a static
reaction system as described before.35–37 At each
temperature, six to nine runs have been carried out in
our experiments. The rate coefficients for the decompo-
sition was calculated from the pressure increase
manometrically. The temperature was maintained with-
in ꢁ 0.2 8C through control with a Shinko DIC-PS 23TR
resistance thermometer and was measured with a
calibrated iron–constantan thermocouple. No tempera-
ture gradient was observed along the reaction vessel. The
substrates were all injected directly into the reaction
vessel with a syringe through a silicone rubber septum.
The amount of substrate used for each reaction was ꢄ
0.05–0.1 mL.
This assumed mechanism may find support from the
pyrolytic elimination of E-2-butenal in a flow system,
with Ea ¼ 249.7 kJ/mol and log A ¼ 13.5 at 770–900 8C,
proposed by Grela and Colussi.32 Otherwise, the
formation of a four-membered cyclic transition structure
[reaction (7)] may not be overlooked and implies the
formation of a carbene type of intermediate and CO gas.
Such intermediate may undergo a 1,2-hydrogen migration
through a three-membered cyclic transition state to give
the corresponding olefin.
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