Gas-phase elimination kinetics of selected aliphatic α,β-unsaturated aldehydes catalyzed by hydrogen chloride

Article abstract of DOI:10.1002/poc.3404

The gas-phase elimination of 2-methyl-2-propenal catalyzed by HCl yields propene and CO gas, while E-2-pentenal with the same catalyst gives butene and CO gas. The kinetics determinations were carried out in a static system with the reaction vessels deactivated with allyl bromide and the presence of the free radical inhibitor toluene. Temperature and pressure ranges were 350.0-410.0C and 34-76Torr. The elimination reactions are homogeneous and unimolecular, and follow a first-order rate law. The rate coefficients for the reactions are expressible by the following Arrhenius equations: Product formation from 2-methyl-2-propenalpropene:logk1s-1lmol-1=12.58±0.47-165.0±5.86kJmol-12.303RT-1Product formation from E-2-pentenalbutene:logk1s-1lmol-1=12.82±0.44-171.3±5.52kJmol-12.303RT-1 Data from the kinetic and thermodynamic parameters of these catalyzed elimination reactions implies a mechanism of a concerted five-membered cyclic transition state structure for the formation of the corresponding olefin and carbon monoxide.

Full text of DOI:10.1002/poc.3404

Research Article
Received: 28 August 2014,
Revised: 29 October 2014,
Accepted: 12 November 2014,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3404
Gas-phase elimination kinetics of selected
aliphatic α,β-unsaturated aldehydes catalyzed
by hydrogen chloride
Libia L. Julioa, José R. Moraa*, Alexis Maldonadoa and Gabriel Chuchania*
The gas-phase elimination of 2-methyl-2-propenal catalyzed by HCl yields propene and CO gas, while E-2-pentenal with the
same catalyst gives butene and CO gas. The kinetics determinations were carried out in a static system with the reaction
vessels deactivated with allyl bromide and the presence of the free radical inhibitor toluene. Temperature and pressure
ranges were 350.0–410.0 °C and 34–76 Torr. The elimination reactions are homogeneous and unimolecular, and follow a
first-order rate law. The rate coefficients for the reactions are expressible by the following Arrhenius equations:
Product formation from 2-methyl-2-propenal
À
Á
propene : log k1
s
ꢀ1lmolꢀ1 ¼ ð12:58 0:47Þ ꢀ ð165:0 5:86Þ kJmolꢀ1ð2:303RTÞꢀ1
Product formation from E-2-pentenal
À
Á
butene : log k1
s
ꢀ1lmolꢀ1 ¼ ð12:82 0:44Þ ꢀ ð171:3 5:52Þ kJmolꢀ1ð2:303RTÞꢀ1
Data from the kinetic and thermodynamic parameters of these catalyzed elimination reactions implies a mechanism of a
concerted five-membered cyclic transition state structure for the formation of the corresponding olefin and carbon monoxide.
Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: 2-methyl-2-propenal; E-2-pentenal; gas-phase elimination; kinetics; mechanism
INTRODUCTION
Recently, furaldehyde and benzaldehyde were examined in a
heated microtubular flow reactor.[34] The former substrate sug-
Publications on the gas-phase elimination of simple aliphatic and
aromatic aldehydes revealed complex processes of free radical
type of reactions.[1–33] Yet, few unsaturated aldehydes are found
to proceed via molecular in nature.[32–35] Only one β,γ-
unsaturated aldehyde, i.e. 2,2-dimethyl-3-butenal, was examined
in the gas phase producing 2-methyl-2-butene and carbon mon-
oxide.[33] This elimination reaction was found to be unimolecular
and obeying a first-order rate law. Surprisingly, β,γ-unsaturated
aldehydes of low molecular weight have not been reported in
gas-phase pyrolysis, except the above-mentioned work.[33] Ap-
parently, this fact may be due to difficulties or complexities to
synthesize these types of substrates. On the other hand, α,β-
unsaturated aldehydes have to some extent been experimentally
studied. In this respect, the pyrolysis kinetic of 2-butenal in a flow
system proved to be molecular in nature.[32] This work included
the thermolysis of 2-furaldehyde and benzaldehyde. The former
gave a biradical mechanism, while the later showed to proceed
by a radical reaction. Moreover, the copyrolysis of 2-butenal in a
1:3 mixture of normal and 2-butenal-O-D showed a negligible
kinetic isotope effect. This result suggested the mechanism of
2-butenal to proceed through a concerted three-membered
cyclic transition state structure described in reaction (1).
gested a unimolecular pyrolysis to furan and carbon monoxide
formation, while for benzaldehyde to a complex radical process.
The furan intermediate was thought to rearrange to α-carbene
giving HC ≡ CH and CH2 = C = O. Simultaneously, furan also
rearranges to β-carbene producing CO + CH3C ≡ CH or to the
radical pair: HĊ = O, H ≡ C-ĊH2.
The gas-phase elimination of two α-β-unsaturated aldehydes,
E-2-butenal and E-2-methyl-3-phenylpropenal,[35] in a static
reaction system with deactivated reaction vessel was found to
be homogeneous and unimolecular and obey a first-order law.
The 2-butenal products are propene and CO gas, while E-2-
methyl-3-phenylpropenal yielded α-methylstyrene, cis-trans-β-
methylstyrene, indan and CO gas. Kinetics and thermodynamics
results suggested these reactions to proceed through a three-
membered cyclic transition state type of mechanisms, thus,
supporting previous mechanistic consideration for α,β-
unsaturated aldehydes pyrolysis.[32]
* Correspondence to: J. R. Mora and G. Chuchani, Centro de Química, Instituto
Venezolano de Investigaciones Científicas (I.V.I.C.), Apartado 21827, Caracas,
Venezuela.
E-mail: chuchani@ivic.gob.ve; jrmora@ivic.gob.ve
a L. L. Julio, J. R. Mora, A. Maldonado, G. Chuchani
Centro de Química, Instituto Venezolano de Investigaciones Científicas
(I.V.I.C.), Apartado 21827, Caracas, Venezuela
(1)
J. Phys. Org. Chem. 2015
Copyright © 2015 John Wiley & Sons, Ltd.

L. L. JULIO ET AL.
GC/MS
(Saturn
2000.
Varian,
capillary
column
DB-5MS,
A few years ago, a theoretical work on the gas phase
unimolecular and homogeneous elimination of (E)-2-butenal
and E-2-methyl-3-phenylpropenal[36] was studied at MP2, MP4
and B3LYP level of theories. Two different mechanisms were
considered. The first mechanism proceeds through a concerted
three-membered cyclic transition state, while a second process
proceeds through stepwise with ketene formation by way of a
four-membered cyclic transition state. The authors favored the
second mechanism as shown in reaction (2). However, this work
showed no agreement between experimental and theoretical
results.
30 mm × 0.250 mm i.d., 0.25 μm). The quantitative chromatographic anal-
yses of propene and butene were determined by using a Gas Chromato-
graph Varian Model 3700 GC with a Porapak S column, 80/100 mesh, 1 m
and internal diameter of 2 mm. The products were identified by chro-
matographic comparison of the reaction products with true authentic
samples acquired from Aldrich and in a GC-MS (Saturn 2000. Varian
3600X with a DB-5MS capillary column 30 m × 0.25 mm. i.d. and 0.25 μm).
Kinetics
The experiments have been carried out in a static system,[38–41] where
the reaction vessel was seasoned or deactivated by the products of
thermolysis of allyl bromide and in the presence of a free radical inhibitor
toluene. The rate coefficients were estimated manometrically. The tem-
perature was regulated within 0.2 °C with a SHINKO DIC-PS 23TR resis-
tance thermometer controller with
a calibrated Iron Constantan
(2)
thermocouple. The temperature reading was measured within 0.1 °C
with the Iron Constantan thermocouple attached to a Digital Multimeter
Omega 3465B. No temperature gradient was found at different points in
the reaction vessel. The starting materials were injected directly into the
reaction vessel through a silicone rubber septum. The HCl gas was intro-
duced into the reaction vessel to an amount measured manometrically
from a reservoir of the system. The volume of substrate used for each re-
action was ≈0.05–0.1 ml.
Because α-β-unsaturated aldehydes require high temperatures
for pyrolysis in static reaction system, a very recent work[37]
thought interesting to investigate the elimination kinetic of
these types of substrates by using a hydrogen halide catalyst.
This way, the experiments should be carried out at much lower
temperatures. In this respect, the gas-phase elimination of
trans-α-methylcinamaldehyde and E-2-methyl-2-pentenal cata-
lyzed by HCl (370.0–438.7 °C and 44–165 Torr)[37] were per-
formed in a static system deactivated reaction vessel with the
presence of a free radical suppressor. The pyrolysis kinetics of
uncatalyzed trans-α-methylcinamaldehyde was believed to oc-
cur via Step 1 through a bicyclic transition state type of mecha-
nism to produce α-methylstyrene and CO gas [reaction (2)],
while this substrate under catalysis of HCl [reaction (3)] takes
place through a five-membered cyclic transition state to yield
β-methylstyrene and CO gas. With regard to the gas-phase elimi-
nation of the unsaturated aliphatic aldehyde E-2-methyl-2-
pentenal to yield E-2-pentene implied a concerted five-membered
cyclic transition state type of mechanism [reaction (3)].
RESULTS AND DISCUSSIONS
The theoretical stoichiometry for reaction (4) in a static system
deactivated with allyl
(4)
bromide pyrolysis products and inhibited with toluene demands
the final pressure, Pf, be 2.0 times the initial pressure, Po. The
average experimental values for Pf/P0 at five different tempera-
tures and 10 half-lives were 1.99 0.01 for 2-methyl-2-propenal
and 1.98 0.03 for E-2-pentenal (Table 1).
Table 1. Ratio of final (Pf) to initial pressure (P0)a
2-Methyl-2-propenal
(3)
Temperature (°C)
P0 (Torr) Pf (Torr) Pf/P0
Average
350
360
370
380
390
47
32
42
49
50
93
64
84
98
99
1.98 1.99 0.01
2.00
2.00
2.00
1.98
Since little information is known with regard to the mecha-
nism of the gas-phase elimination of α-β-unsaturated aldehydes
catalyzed by HCl, the purpose of the present work is to obtain
additional ideas about the process of elimination of these types
of compounds; consequently, the pyrolysis kinetics of 2-methyl-
2-propenal and E-2-pentenal catalyzed by HCl gas were to be
examined.
E-2-Pentenal
370
381
390
400
62
43
43
79
51
123
87
84
157
100
1.98 1.98 0.03
2.02
1.95
1.98
1.96
EXPERIMENTAL SECTION
410
aDeactivated vessel in the presence of toluene suppressor
and HCl gas fourfold with respect to the substrate.
The substrate 2-methyl-2-propenal of 98.1% purity was purchased from
Aldrich, while E-2-pentenal of 98.0% purity was acquired from Across Or-
ganic. The purity of the substrates and products was examined by using a
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Copyright © 2015 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2015

ELIMINATION α,β-UNSATURATED ALDEHYDES
Further verification of the stoichiometry described in reaction
(4), up to 45–50% thermolysis, gave a reasonable good agree-
ment between the extents of decomposition of the unsaturated
aldehyde substrate calculated from pressure measurements with
that obtained from the chromatographic analyses of products
formation (Table 2).
Surface effect upon the rate of elimination was examined by
carrying out several runs under toluene chain inhibitor in a
vessel with a surface-to-volume ratio of 6.0 cm-1 times greater
than that of a normal vessel which is equal to one (Table 3).
The rates of both unsaturated aldehydes were unaffected when
the packed and unpacked vessels are seasoned or deactivated
vessels, yet significant effect was found in clean packed and
unpacked Pyrex vessel.
The effect of adding an efficient free radical chain inhibitor
toluene is revealed in Table 4. Accordingly, the gas-phase elimi-
nation of these unsaturated aldehydes had to be carried out in
the presence of at least once the amount of the inhibitor. No
induction period was observed.
The pseudo-first-order rate law was depicted in Eqn (1); the
rate coefficient kobs varies in the presence of the HCl catalyst,
suggesting that by change of PHCl different kobs values are
obtained. Consequently, the second-order rate coefficient is pro-
cured by dividing kobs into PHCl. This means, Eqn (1) is replaced
Table 4. Effect of toluene inhibitor on rates
P0 (Torr)
Pi (Torr)
Pi/P0
106 k1(Torrꢀ1 sꢀ1
)
2-Methyl-2-propenal at 370 °C
71
60
60
53
54
38
36
–
59
94
135
179
159
197
–
3.56 0.61
3.90 0.24
4.04 0.11
4.10 0.09
4.04 0.28
4.10 0.27
4.20 0.14
1.0
1.6
2.5
3.3
4.2
5.5
E-2-Pentenal at 380 °C
54
53
76
43
59
38
35
–
31
93
–
3.25 0. 30
3.42 0. 15
3.47 0. 10
3.45 0. 08
3.36 0. 09
3.46 0.30
3.47 0.16
0.6
1.2
2.2
2.6
4.1
6.3
96
151
157
219
Vessel deactivated with allyl bromide. P0 = pressure of the
substrate. Pi = pressure of the inhibitor toluene. Presence of
fourfold of HCl gas with respect to the substrate
with Eqn (2). Table 5 gives the values of the kobs at different PHCl
values from 104 to 212 Torr in accordance with PT = P0[2ꢀexp
(ꢀkobst)] and by using the OriginPro 8.0 program. Constant
values in the second-order rate coefficient agree with those
foretold by the equation. The second-order rate coefficient k1
is obtained from the slope of the linear fit of the graph kobs
vs PHCl [Eqn (1)].
Table 2. Stoichiometry of the reactionsa
Substrate
Temperature Time
%
% Reaction
(°C)
(min) Productb (pressure)
(GC)
2-Methyl-2-
propenal
370
3.0
6.5
9.0
14.3
3.5
5.5
9.2
11.7
19.0
11.8
21.1
33.0
44.0
11.4
18.7
26.5
31.9
51.5
11.1
21.6
32.7
44.5
10.3
17.7
28.2
32.3
50.0
P0
kobs ¼ PHC1k1 ¼ ð1=tÞln
(1)
ð2P0 ꢀ PTÞ
E-2-Pentenal
380
Table 5. Effect of HCl gas catalyst on the rate of
decarbonylation
PHCl (Torr)
104kobs (sꢀ1
)
106 k1(Torrꢀ1 sꢀ1
)
aDeactivated vessel in the presence of toluene inhibitor and
HCl gas fourfold with respect to the substrate.
2-Methyl-2-propenal at 371 °C
bThese values are the total percentage of products.
104
135
158
169
178
212
4.33 0.40
5.70 0.09
6.40 0.18
6.83 0.29
7.30 0.14
8.88 0.90
4.12 0.16
r = 0.9961
Table 3. Homogeneity of the reaction
S/V (cmꢀ1
)
106 k1(Torrꢀ1 sꢀ1 (a)
)
106 k1(Torrꢀ1 sꢀ1 (b)
)
E-2-Pentenal at 380 °C
2-Methyl-2-propenal at 370 °C
104
124
154
172
180
208
3.60 0.16
4.32 0.10
5.32 0.14
5.98 0.17
6.27 0.20
7.11 0.31
1.0
6.0
7.70 1.31
15.30 2.96
4.10 0.21
4.30 0.25
3.40 0. 06
r = 0.9993
E-2-Pentenal at 380 °C
1.0
5.30 1.57
10.60 3.75
3.42 0.15
3.50 0.20
6.0
S = surface. V = volume. (a) Clean Pyrex vessel. (b) Vessel
deactivated with allyl bromide
Deactivated vessel with the presence of toluene inhibitor. P0
substrate ≈ 63 Torr.
Presence of toluene inhibitor and HCl gas fourfold with
respect to the substrate
The k1 is obtained from the slope of the linear fit of the graph
kobs vs PHCl
.
J. Phys. Org. Chem. 2015
Copyright © 2015 John Wiley & Sons, Ltd.
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L. L. JULIO ET AL.
P0
The variation of the rate coefficients of 2-methyl-2-propenal
and E-2-pentenal thermolysis with temperature, in deactivated
vessel and in the presence of the free radical suppressor toluene
is shown in Table 7 (PHCl/P0 = 4.0). The results described in Table 7
lead, by using the least square procedure and 90% confidence
limits, to the shown Arrhenius equations.
According to the results described in Table 7, the mecha-
nism of the gas phase elimination of 2-methyl-2-propenal
and E-2-pentenal appears to suggest a similar transition
state to the previously reported work[37] of a polar concerted
five-membered cyclic structure. In view of the similarity of
the mechanistic elimination of previous and present work,
Table 8 describes the reactivity order as a function of the
kinetic and thermodynamic parameters of the listed α,β-
unsaturated aldehydes under HCl catalysis. The sequence 2-
methyl-2-propenal (i) > E-2-pentenal (iv) > E-2-methyl-2-pentenal
(ii) > trans-α-methyl-cinamaldehyde (iii) may be rationalized in
terms of substituent effect on the substrate as described in
Scheme 1.
In this sense, a reasonable evaluation of the substituent effect
on the mechanism of the elimination of the compounds de-
scribed in Table 8, the bond polarization Cδαꢀ----δ+CO as
determining step of the reaction was taken into consideration.
The reactivity order described for compounds (i) (R1 = H)
(1.00) > (ii) (R1 = Et) (0.28) > (iii) (R1 = Ph) (0.14) is due to elec-
tronic effect of alkyl group relative to the phenyl group. In the
case of (R1 = Et) in (iv) may destabilize the transition state sug-
gested in Scheme 1, hence reducing the reactivity relative to
(R1 = H) in (i). The lowest reactivity observed in (R1 = Ph) in (iii)
may be attributed to resonance (conjugative effect) of the
phenyl group with C = C and C = O. Consequently, the double
K1 ¼ ð1=tÞð1=PHC1Þln
(2)
ð2P0ꢀPTÞ
At a given temperature, the rate coefficients of these elimina-
tion reactions, at constant HCl gas pressure (PHCl ≈ 200 Torr) were
invariable of the initial pressures of the substrates (Table 6). The
rate coefficients are reproducible with a standard deviation not
greater than 5% at a given temperature.
Table 6. Unchanged rate coefficient with initial pressure of
the substrate
P0 (Torr)
106 k1(Torrꢀ1 sꢀ1
)
2-Methyl-2-propenal at 371 °C
34
42
51
60
8.16 1.00
8.00 0.80
8.28 0.28
8.08 0.22
74
8.32 0.42
E-2-Pentenal at 380 °C
35
43
53
62
76
6.94 0.32
6.90 0.16
6.84 0.30
7.04 0.04
6.94 0.20
Deactivated vessel with the presence of fourfold of toluene
inhibitor. PHCl ≈ 200 Torr.
Table 7. The variation of the rate coefficients with temperatures
Substrate
Parameters
Values
Temp. (°C)
350.0
360.0
370.0
380.0
390.0
2-Methyl-2-propenal (i)
k1(ccmolꢀ1 sꢀ1
)
56.02 0.77 90.04 4.00 165.00 5.21 240.10 6.93 378.00 9.93
Rate equation: log k1(sꢀ1Lmolꢀ1) = (12.58 0.47) ꢀ (165.0 5.86)kJmolꢀ1(2.303RT)ꢀ1 r = 0.9974
Temp. (°C)
370.0
381.0
390.0
400.0
410.0
E-2-Pentenal (iv)
k1 (ccmolꢀ1 sꢀ1
) 80.30 0.70 139.07 2.00 228.11 4.00 353.05 6.00 512.00 7.00
Rate equation: log k1(sꢀ1Lmolꢀ1) = (12.82 0.44) ꢀ (171.3 5.52)kJmolꢀ1(2.303RT)ꢀ1 r = 0.9979
Rate equation: log k1(sꢀ1Lmolꢀ1) = (12.79 0.0.06)-(174.5 0.80)kJmolꢀ1(2.303RT)ꢀ1
Rate equation: log k1(sꢀ1Lmolꢀ1) = (13.19 0.0.06) ꢀ (183.0 0.31)kJmolꢀ1(2.303RT)ꢀ1
E-2-Methyl-2-pentenala (ii)
trans-α-Methyl cinamaldehydea (iii)
aReference 37.
Table 8. Kinetic and thermodynamic parameters of α,β-unsaturated aldehydes at 370 °C (643 K)
Substrate
log A
Ea (kJmolꢀ1
)
ΔS‡ (Jmolꢀ1
)
ΔH‡ (kJmolꢀ1
)
ΔG‡ (kJmolꢀ1
)
Ref.
2-Methyl-2-propenal (i)
E-2-Pentenal (iv)
E-2-Methyl-2-pentenal (ii)
trans-α-Methyl-cinamaldehyde (iii)
12.58 0.47
12.82 0.44
12.79 0.06
13.19 0.06
165.0 5.86
171.3 5.52
174.5 0.80
183.0 0.31
ꢀ27.1
ꢀ22.5
–23.1
–15.4
154.3
160.6
163.7
172.3
171.7
175.1
178.6
182.2
(a)
(a)
(37)
(37)
(a) This work.
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J. Phys. Org. Chem. 2015

ELIMINATION α,β-UNSATURATED ALDEHYDES
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Substrate
R1
R2
k1 (sꢀ1
)
Rel. rate
(i)
H
CH3
CH3
CH3
H
0.16
1.0
(ii)
(iii)
(iv)
Et
Ph
Et
0.044
0.023
0.087
0.28
0.14
0.54
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J. Phys. Org. Chem. 2015
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