Kinetic Determination of the Gas-Phase Decarbonylation of Butyraldehyde in the Presence of HCl Catalyst

Article abstract of DOI:10.1002/kin.21078

The gas-phase kinetics and mechanism of the homogeneous elimination of CO from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is homogeneous and follows the second-order kinetics with the following rate expression: log k₁ (s^?1 L mol^?1) = (13.27 ± 0.36) – (173.2 ± 4.4) kJ mol^?1(2.303RT)^?1. Experimental data suggested a concerted four-membered cyclic transition state type of mechanism. The first and rate-determining step occurs through a four-membered cyclic transition state to produce propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and HCl gases.

Full text of DOI:10.1002/kin.21078

Kinetic Determination of
the Gas-Phase
Decarbonylation of
Butyraldehyde in the
Presence of HCl Catalyst
LIBIA L. JULIO,1 LORIETT CARTAYA,1 ALEXIS MALDONADO,1 YELJAIR MONASCAL,1
2
3
1
´
JOSE R. MORA, TANIA CORDOVA, GABRIEL CHUCHANI
1Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (I.V.I.C), Caracas, 1020A, Venezuela
2Departamento de Ingenier´ıa Qu´ımica, Universidad de San Francisco de Quito, Quito, Ecuador
3Department of Chemistry, Florida A & M University, Tallahassee, FL 32307
Received 25 July 2016; revised 13 September 2016; accepted 13 January 2017
DOI 10.1002/kin.21078
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The gas-phase kinetics and mechanism of the homogeneous elimination of CO
from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is
homogeneous and follows the second-order kinetics with the following rate expression: log
k1 (s−1 L mol−1) = (13.27
0.36) – (173.2
4.4) kJ mol−1(2.303RT)−1. Experimental data
suggested a concerted four-membered cyclic transition state type of mechanism. The first
and rate-determining step occurs through a four-membered cyclic transition state to produce
propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and
ꢀC
HCl gases.
2017 Wiley Periodicals, Inc. Int J Chem Kinet 1–6, 2017
aldehydes, 2,2-dimethyl-3-butenal [3], was found to
INTRODUCTION
produce 2-methyl-2-butene and carbon monoxide
through a three-membered cyclic transition state type
of mechanism.
The homogeneous, unimolecular gas-phase elimina-
tion kinetics of α,β-unsaturated aldehydes have been
described to undergo decarbonylation through a three-
membered cyclic transition state as depicted in reaction
(1) [1,2], whereas a single reported β,γ -unsaturated
H
CHO
R2
H
H
α
H
H
-CO
O
C
C
C
C
C
C
β
R1
R2
R1
R2
R1
Correspondence to: G. Chuchani; e-mail: chuchani@ivic.gob.ve.
2017 Wiley Periodicals, Inc.
(1)
ꢀC

2
JULIO ET AL.
With regard to low molecular saturated aliphatic
From the considerations of the proposed mechanism
of the uncatalyzed unsaturated aldehydes depicted in
reaction (1), the labile hydrogen at the carbonyl carbon
appears to migrate toward the unsaturated Cα in the
transition state. This observation led to believe that this
type of substrate may well be catalyzed by a hydrogen
halide in the gas phase. In this sense, this idea was
confirmed from the reasonable good results given in
Table I and the mechanism described in reaction (3).
aldehydes: formaldehyde [4–6], acetaldehyde [7–22],
propionaldehyde [23–29], showed to proceed via
complex-free radical processes. Additionally, an aro-
matic aldehyde, such as benzaldehyde [30–34], re-
quired temperatures of 550–1400°C to produce ben-
zene, CO gas, small amounts of biphenyl and hydro-
gen, through a radical reaction.
In 2014, an interesting work [35] used a shock tube
system for the thermal decomposition kinetics of 2-
methyl butyraldehyde in the gas phase. This decom-
position was performed at 1075–1250 K (802–977°C)
with the pressure range 1.5–6.0 atm. This substrate was
found to decompose into five reaction channels, where
four of these processes were via radical, whereas the
other one appears to undergo molecular decomposition
via a retro-ene reaction as described in reaction (2).
H
O
C
O
HC OH
C
H
+
(2)
H3C CH
CH2
CH2
CH2
H
CH
H3C
H3C CH
CH2
CH2
CH3
O
C
H
H3C CH2
According to the previous results, a recent compu-
tational study of the above-mentioned molecular de-
composition of the saturated aldehyde [36] at the “ab
initio” MP2/6-31G(d) level of theory showed a good
agreement with the experimental kinetic parameters,
thus providing a good support on the mechanism of
reaction (2).
It is noteworthy to mention that in these elimina-
tion reactions enol tautomerism of the α,β-unsaturated
aldehydes does not occur. Similarly, the experimental
conditions of these works do not appear to favor enol
tautomerism with aliphatic saturated aldehydes prior
to decomposition, whereas higher temperature leads to
radical reactions.
Since the kinetics of molecular elimination of the
selected α,β-unsaturated aldehydes [1,2], in a static
reaction system, required very high temperatures, fur-
ther investigations on the decomposition of these sub-
strates were carried out by using hydrogen chloride as
a catalyst. [37,38]. Under these conditions, a molecular
decarbonylation reaction was possible at much lower
temperature. The kinetic and thermodynamic parame-
ters of the α,β-unsaturated aldehydes under HCl catal-
ysis are given in Table I.
In association with the catalytic decarbonylations
described above, aliphatic-saturated aldehydes may
possibly undergo molecular elimination considering
two polarization possibilities of the CHO group
(Scheme 1).
Consequently, the present work aimed at proposing
a reasonable mechanism for the homogeneous molec-
ular decarbonylation of a saturated aliphatic alde-
hyde in the presence of the hydrogen chloride cata-
lyst. This process is expected to occur at much lower
Table I Kinetic and Thermodynamic Parameters of α,β-Unsaturated Aldehydes with HCl Catalyst at 360ºC (633 K)
Substrate
Ea (kJ mol−1
)
log A
࢞S‡ (J mol−1) ࢞H‡ (kJ mol−1) ࢞G‡ (kJ mol−1) Reference
2-Methyl-2-propenal
E-2-Pentenal
E-2-Methyl-2-pentenal
165.0 5.86 12.58 0.47
171.3 5.52 12.82 0.44
174.5 0.80 12.79 0.06
–18.7
–14.1
–14.7
–7.1
159.7
166.0
169.2
177.7
171.5
174.9
178.5
182.2
[38]
[38]
[37]
[37]
α-Methyl-trans-cinamaldehyde 183.0 0.31 13.19 0.06
International Journal of Chemical Kinetics DOI 10.1002/kin.21078

KINETIC DETERMINATION OF THE GAS-PHASE DECARBONYLATION OF BUTYRALDEHYDE
3
Table II Ratio of Final (Pf) to Initial Pressure (P0)a
Temperature
P0
Pf
(°C)
(Torr)
(Torr)
Pf /P0
Average
1.98 0.03
Scheme 1
339.0
350.0
360.0
371.0
57
69
71
68
112
134
142
136
1.96
1.94
2.00
2.00
temperatures. In this sense, the experimental gas-phase
elimination kinetics of butyraldehyde (butanal) in the
presence of hydrogen chloride was carried out.
aDeactivated vessel in the presence of toluene suppressor. Four-
fold of HCl catalyst.
pressure range 40–230 Torr.
EXPERIMENTAL
CH3CH2CH2CHO H→Cl CH3CH2CH3 + CO (4)
The purity of better than 99.0% of the substrate bu-
tyraldehyde (butanal) (Aldrich California, USA) was
examined by using a GC/MS (Saturn 2000; Varian);
capillary column DB–5MS 30 m × 0.250 mm, id 0.25
μm. The substrate was quantitatively analyzed with
the above-described GC/MS. The product propane was
identified in a Varian model 3700 GC, with Porapak S
80/100 mesh, 1 m, id 2 mm column.
ꢀ
The stoichiometry based on reaction (4) required
that, for long reaction times, Pf = 2P0, where Pf, and
P0 are the final and initial pressure, respectively. The
average experimental value for Pf/P0 at four different
temperatures and 10 half-lives for butyraldehyde in
HCl was 1.98 0.03 (Table II).
Verification of the above stoichiometry (4), up to
70% decomposition, was possible by comparing the
propane formation to be equivalent to the pressure in-
crease (Table III).
The homogeneity of this reaction was established
by carrying out several runs in a vessel with a surface-
to-volume ratio of 6.0 relative to the normal vessel,
which is equal to 1.0 (Table IV). The packed and
unpacked clean Pyrex vessel resulted in a signifi-
cant surface effect, whereas the packed and unpacked
Kinetics
Butyraldehyde was decomposed in a static system
reaction vessel as described before [39–42], deacti-
vated by the products of decomposition of allyl bro-
mide and in the presence at least equal amount of a
free radical suppressor toluene. The rate coefficients
were determined manometrically. The temperature was
controlled within 0.2oC with a SHINKO DIC-PS
23TR resistance thermometer controller with a cali-
brated iron–Constantan thermocouple. The tempera-
ture reading was measured within 0.1°C with the
iron–Constantan thermocouple attached to a digital
multimeter Omega 3465B. No temperature gradient
was found in the reaction vessel when measured with a
thermometer introduced around the heated aluminum
block. The substrate was injected directly into the re-
action vessel through a silicone rubber septum. The
HCl gas was introduced into the reaction vessel to an
amount measured manometrically from a reservoir of
the system. The volume of butyraldehyde used for each
reaction was ࣈ 0.05–0.1 mL.
Table III Stoichiometry of the Reactiona
Temperature Time % Propane % Reaction
Substrate
(°C)
(min)
(GC)
(Pressure)
Butyraldehydeb
350
3.5
7.5
15.0
32.0
50.0
6.8
6.6
14.8
22.5
34.2
72.5
14.4
20.4
33.0
73.4
aDeactivated vessel in the presence of toluene inhibitor.
bFourfold of HCl catalyst.
Table IV Homogeneity of the Reaction at 350°C
S/V (cm−1
)
106k1(Torr−1 s−1 a
)
106k1(Torr−1 s−1 b
)
RESULTS AND DISCUSSION
1.0
6.0
9.45 5.17
15.30 7.41
1.47 0.13
1.50 0.18
The elimination process of butyraldehyde, in a static
system, with the reaction vessel deactivated with al-
lyl bromide, and with equal amount of the free radical
toluene, was determined in the presence of HCl cata-
lyst over the temperature range of 339.0–382.0°C and
Abbreviations: S, surface; V, volume.
aClean Pyrex vessel.
bVessel deactivated with allyl bromide.
The presence of toluene inhibitor; fourfold of HCl catalyst.
International Journal of Chemical Kinetics DOI 10.1002/kin.21078

4
JULIO ET AL.
Table V Effect of Toluene Inhibitor on Rates at 360.0°C
Table VII Invariability of Rate Coefficient with Initial
Pressure of the Substrate at 360°C
P0 (Torr)
Pi (Torr)
Pi/P0
106k1(Torr−1 s−1
)
P0 (Torr)
106k1 (Torr−1 s−1
)
73
78
76
71
41
–
93
111
148
120
–
2.46 0.11
2.53 0.07
2.54 0.05
2.64 0.06
2.58 0.14
1.2
1.5
2.1
2.9
41
68
76
95
5.16 0.28
5.20 0.10
5.08 0.10
5.08 0.12
Vessel deactivated with allyl bromide.
Deactivated vessel with toluene inhibitor. PHCl ࣈ 200 Torr.
P0 = pressure of the substrate.
Pi = pressure of the inhibitor toluene; Presence fourfold of HCl.
ꢀ
ꢁ
P0
k1 = (1/t) 1/PHCl ln
(6)
Table VI Effect of HCl Gas Catalyst on the Rate of
Decarbonylation at 362.2°C
(2P0 − PT)
PHX (Torr)
104kobs (s−1
)
106k1 (Torr−1 s−1
)
The rate coefficients of this elimination reaction,
at constant catalysts gas pressure (PHCl ࣈ 200 Torr),
were found to be independent of the initial pressure of
the starting material (Table VII). The rate coefficients
were reproducible with a relative standard deviation
not greater than 5% at a given temperature.
99
2.59 0.09
3.34 0.14
4.42 0.02
5.81 0.11
6.41 0.12
2.57 0.05 r = 0.9993
127
169
225
246
The temperature dependence of the rate coefficients
in the deactivated vessel and in the presence of the
free radical suppressor toluene, together with the cor-
responding Arrhenius equation, is given in Table VIII
(Fig. 1). The errors were estimated to 90% confidence
coefficient from the least-squares method.
Deactivated vessel with the presence of toluene inhibitor. P0
substrate ࣈ 70 Torr.
The 106k1 is obtained from the slope of the linear fit of the graph
kobs versus PHCl
.
From the experimental results, a four-membered
cyclic transition state type of mechanism as depicted
in reaction (7) is proposed.
vessel deactivated with allyl bromide had no effect on
rates.
The kinetic experiments were carried out in the pres-
ence of at least an equal amount of the free radical sup-
pressor toluene to prevent any possible radical chain
reactions. The effect of different proportions of toluene
in the elimination process is given in Table V. No
induction period was observed. The rate coefficients
were reproducible with a relative standard deviation
not greater than 5%.
In the pseudo–first-order rate law shown in Eq. (5),
the variation of the rate coefficient kobs in HCl cata-
lyst implies that by change of PHCl gives different kobs
values. Therefore, the second-order rate coefficient is
obtained by dividing kobs into PHCl, causing Eq. (5) to
be substituted by Eq. (6). Table VI describes the values
of the kobs at different PHCl values from 99 to 246 Torr,
in accord with PT = P0[2 – exp(–kobst)] and by using
OriginPro 8.0 program. The second-order rate coef-
ficient gives constant values which agrees with those
predicted by the equation. The second-order rate coef-
ficient k1 is obtained from the slope of the linear fit of
the graph kobs versus PHCl (Eq. (5)).
CONCLUSIONS
The present work reports an experimental study on
the kinetics and mechanism of the gas-phase elimi-
nation reaction of butyraldehyde in the presence of
HCl. The reaction is molecular, homogeneous and fol-
lows a second-order kinetics with rate equation: log
P0
kobs = PHClk1 = (1/t) ln
(5)
k1 (s−1 L mol−1) = (13.27
0.36) – (173.2
4.4)
(2P0 − PT)
International Journal of Chemical Kinetics DOI 10.1002/kin.21078

KINETIC DETERMINATION OF THE GAS-PHASE DECARBONYLATION OF BUTYRALDEHYDE
5
Table VIII The Variation of the Rate Coefficients with Temperatures
Substrate
Parameter
Value
360.0
Butyraldehyde
(HCl catalyst)
Temperature. (°C)
339.0
350.0
371.0
382.0
281.0 10.6
k1(cc mol−1 s−1
)
30.8 2.3
57.0 2.7
102.0 4.3
177.0 4.8
Rate equation: log k1(s−1 L mol−1) = (13.27 0.36) – (173.2 4.4) kJ mol−1 (2.303RT)−1 r = 0.9987.
Figure 1 Arrhenius plots of gas-phase elimination of butyraldehyde in HCl catalyst.
kJ mol−1(2.303RT)−1. The result suggests a stepwise
mechanism; a first slow step reaction of butyralde-
hyde and the HCl through a four-membered cyclic TS
to yield propane and the corresponding formyl chlo-
ride. The intermediate formyl chloride decomposes in
a faster step to carbon monoxide and HCl.
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International Journal of Chemical Kinetics DOI 10.1002/kin.21078