The unimolecular elimination kinetics of benzaldoxime in the gas phase

Article abstract of DOI:10.1002/kin.20222

The kinetics of the gas-phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C-400°C and 56-140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was fou

Full text of DOI:10.1002/kin.20222

The Unimolecular
Elimination Kinetics
of Benzaldoxime
in the Gas Phase
LIBIA L. JULIO,1 ARMANDO HERIZE,2 MARIA TOSTA,2 ROSA M. DOMINGUEZ,2
JENNIFER LAFONT,1 GABRIEL CHUCHANI2
1Departamento de Qu´ımica, Universidad de Co´rdoba, Monter´ıa, Colombia
2Centro de Qu´ımica, Instituto Venezolano de Investigaciones Cient´ıficas (I.V.I.C.),
Apartado 21827, Caracas 1020-A, Venezuela
Received 1 August 2006; revised 9 October 2006; accepted 10 October 2006
DOI 10.1002/kin.20222
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of the gas-phase elimination of benzaldoxime was determined in a
static reaction system over the temperature and pressure range 350◦C–400◦C and 56–140 Torr,
respectively. The products obtained were benzonitrile and water. The reaction was found to
be homogeneous, unimolecular, and tend to obey a first-order rate law. The observed rate
coefficient is represented by the following Arrhenius equation:
log k1(s − 1) = (12.00 0.19) − (188.0 2.3)kJ mol−1(2.303RT )−1
According to kinetic and thermodynamic parameters, the reaction proceeds thꢀrough a con-
C
certed, semi-polar, four-membered cyclic transition state type of mechanism.
Periodicals, Inc. Int J Chem Kinet 39: 145–147, 2007
2007 Wiley
Years later, Pratt and Purnell [2] carried out the py-
rolysis of acetaldoxime. The reaction was somewhat
heterogeneous, showing some induction period, and
the mechanism was described as a free radical pro-
cess [3]. In 1967, camphor oxime was pyrolyzed at
two different temperatures [4]. When pyrolyzed at
240◦C for 6–7 min and distilled under reduced pres-
sure, it yielded three products as described in re-
action (2). When pyrolysis was carried out in va-
por phase at 500◦C for 1 s, the main product was
found to be 1,2-dimethyl-3-cyanomethylcyclopentene
[reaction (2)].
INTRODUCTION
There are few reported research works on the thermal
decomposition of oximes. The pyrolysis kinetics of
formaldoxime [1], at 350◦C to 415◦C, was found to
produce hydrogen cyanide and water [reaction (1)] and
tend to obey a first-order rate law.
∆
(1)
+
HCN
H2O
CH2NOH
Correspondence to: Gabriel Chuchani; e-mail: chuchani@ivic.
ve.
c
ꢀ
2007 Wiley Periodicals, Inc.

146
JULIO ET AL.
Table III Homogeneity of the Elimination Reactions
for Substrate Benzaldoxime at 380.5◦C
∆
+
+
240oC
S/V (cm−1 a
)
104 × k (s−1 b
)
104 × k (s−1 c
)
HO
1
1
N
O
CH2CN
CH2CN
1
6
9.60 0.22
9.54 0.24
9.50 0.30
9.51 0.14
500oC
a S = surface area; V = volume.
b Clean Pyrex vessel.
c Vessel seasoned with allyl bromide.
CH2CN
(2)
In view of the little information regarding pyroly-
sis of oximes in the gas phase, this work aimed at
examining the homogeneous elimination kinetics of
benzaldoxime in the gas phase, and to consider a rea-
sonable mechanism, if possible.
Table IV Effect of Free Radical Inhibitor Toluene on
Rates for Substrate Benzaldoxime at 380.0◦C
Psa (Torr)
Pib (Torr)
P /P
i
104 × k (s−1
)
s
1
42.9
61.8
56.7
57.3
54.9
42.1
41.1
–
69
69
85
86
85
100
–
9.52
9.50
9.32
9.40
9.40
9.32
9.51
1.1
1.2
1.5
1.6
2.0
2.4
EXPERIMENTAL
The starting material benzaldoxime (Aldrich) of 98.0%
purity was used (GC-MS: Saturn 2000, Varian 3600X,
with a DB-5MS capillary column 30 m × 0.25 mm i.d.,
0.25-µm film thickness). The quantitative chromato-
graphic analysis of benzonitrile product was deter-
mined by using a Gas Chromatograph Hewlett Packard
5710-A with a 10% SP-1200, 1% H3PO4 Chromosorb
WAW 80–100 mesh, 2 m. The identification of the
benzonitrile product was made by comparing with
true authentic samples bought from Aldrich (GC-MS:
Saturn 2000, Varian 3600X with a DB-5MS capil-
lary column 30 m × 0.25 mm i.d., 0.25-µm film
thickness).
aPs = pressure substrate.
bPi = pressure inhibitor.
Table V Invariability of the Rate Coefficients With
Initial Pressure for Substrate Benzaldoxime at 370◦C
Parameter
Value
P (Torr)
61
5.40
70
5.30
72.5
5.35
85
5.25
0
104 × k (s−1
)
1
Kinetics
The reaction kinetics was studied in a static reaction
system as described before [5–7]. At each tempera-
ture, 6 to 9 runs were carried out. The rate coefficients
for the benzaldoxime decomposition was calculated
manometrically from the pressure increases. The tem-
perature was maintained within 0.2◦C through con-
trol with a Shinko DIC-PS 23TR resistance thermome-
ter and was measured with a calibrated iron–constantan
thermocouple. No temperature gradient was observed
along the reaction vessel. The substrates were all in-
jected directly into the reaction vessel with the help of a
Table I Ratio of Final (Pf) to Initial (P0) Pressure for
the Substrate Benzaldoxime
Temperature (◦C) P (Torr) P (Torr) P /P Average
0
f
f
0
360
370
380
390
400
48
70
57.3
57.4
37.2
94
135
114.3
113.4
74.2
1.9
1.9
2.0
2.0
2.0
2.0
Table II Stoichiometry of the Reaction for Substrate Benzaldoxime at 390.0◦C
Parameter
Value
Time (min)
5
7
9
11
16
20
30
40
Reaction (%) (pressure)
Benzonitrile (%) (GLC)
11.2
11.4
20.0
19.2
25.3
25.9
30.0
30.5
38.6
37.7
42.6
41.1
57.3
55.2
68.5
67.2
International Journal of Chemical Kinetics DOI 10.1002/kin

UNIMOLECULAR ELIMINATION KINETICS OF BENZALDOXIME IN THE GAS PHASE
147
Table VI The Variation of the Rate Coefficients With Temperatures for Substrate Benzaldoximea
Parameter
Value
Temp. (◦C)
350.4
1.75 0.07
360.8
3.38 0.11
370.2
5.48 0.03
380.1
9.38 0.10
390.4
15.52 0.02
400.1
26.44 0.01
104 × k (s−1
)
1
a Rate equation: log k1 (s−1) = (12.00 0.19) – (188.0 2.3) kJ mol−1 (2.303RT)−1, r = 0.9997.
Table VII Kinetic and Thermodynamic Parameters at 370◦C for Substrate Benzaldoxime
k × 104 (s−1
5.34 0.03
)
E (kJ mol−1
)
log A (s−1
)
ꢀS‡ (J mol−1 K)
−29.9
ꢀH‡ (kJ mol−1
)
ꢀG‡ (kJ mol−1
)
1
a
188.0 2.3
12.00 0.19
182.7
201.9
syringe through a silicone rubber septum. The amount
of substrate used for each reaction was approximately
0.05–0.1 mL.
be independent of the initial pressure (Table V), and
the first-order plots of log(2P0 – Pt )] against time t are
linear up to 68% of the reaction. The variation of the
rate coefficients with temperature and the correspond-
ing Arrhenius equation are shown in Table VI, where
90% confidence limits from a least-squares procedure
are given.
Data from Tables I to VI and the kinetic and ther-
modynamic parameters given in Table VII imply re-
action (3) to be molecular in nature. Apparently, the
RESULTS AND DISCUSSION
The unimolecular elimination of benzaldoxime in the
gas phase is described by reaction (3). The stoichiom-
etry of this reaction demands for a long reaction time,
Pf/P0 = 2.0,
N OH bond polarization, in the sense Nδ+ · · · OHδ−
,
is rate determining. Then, the OHδ− may abstract
the benzylic hydrogen to yield benzonitrile and wa-
ter through a concerted four-membered cyclic transi-
tion state type of mechanism, as described in reaction
(4)
CH=NOH
C
N
(3)
H2O
+
δ−
where Pf and P0 are the final and initial pres-
sures, respectively. The average experimental Pf/P0
at five different temperatures and 10 half-lives was 2.0
(Table I).
H
C
OH
CH=NOH
C
N
Nδ+
H2O
+
The stoichiometry of reaction (3), up to 68%
decomposition, was checked by comparing the ex-
tent of decomposition of the substrate from pres-
sure measurements with that obtained from quan-
titative GLC analyses of benzonitrile formation
(Table II).
(4)
BIBLIOGRAPHY
The pyrolytic elimination of reaction (3) was found
to be homogeneous, since no significant variations
in rates were obtained when using both clean Pyrex
and allyl bromide seasoned vessels with a surface-to-
volume ratio of 6.0 times greater than normal, which is
equal to 1.0 (Table III). The rates were not affected by
the effect of different proportions of toluene inhibitor
(Table IV). No induction period was observed and the
rates were reproducible with a relative standard devia-
tion of no more than 5% at a given temperature.
The rate coefficient for elimination reaction from
k1 = −(2.303/t) log[(2P0 − Pt ) − P0)] was found to
1. Taylor, H. A.; Bender, H. J Chem Phys 1941, 9,
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International Journal of Chemical Kinetics DOI 10.1002/kin