A comparative study on the gas-phase and liquid-phase thermal isomerization reaction of α-pinene

Article abstract of DOI:10.1002/poc.3011

In this paper, a method of preparation of ocimene is investigated, which is obtained from isomerization reaction of α-pinene. Two kinds of experimental apparatus are established for the investigation of the thermal isomerization reaction of α-pinene. The behavior of thermal isomerization reaction of α-pinene is respectively discussed in the gas phase and in the liquid phase. Under gas phase conditions, the conversion of α-pinene is 80% and the selectivity of ocimene is 30%-33%. Under liquid phase conditions, the conversion of α-pinene is 60% and the selectivity of ocimene is 50%-54%. According to the kinetic-molecular theory of ideal gases, two kinds of reaction models are proposed to visualize the reaction process. In addition, the mechanism and kinetics of thermal isomerization reaction of α-pinene are respectively discussed. The conclusion is that the gas phase reaction temperature is calculated to be 390-450 °C and the liquid phase reaction temperature is calculated to be 450-550°C. From a bond dissociation energy point of view, results support the hypothesis that the reaction involves biradical intermediates. Copyright

Full text of DOI:10.1002/poc.3011

Research Article
Received: 5 May 2012,
Revised: 7 June 2012,
Accepted: 4 July 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.3011
A comparative study on the gas-phase and liquid-
phase thermal isomerization reaction of a-pinene
Jindong Hea, Yan Gonga, Wentao Zhaoa, Xiangyang Tanga and Xin Qia*
In this paper, a method of preparation of ocimene is investigated, which is obtained from isomerization reaction of
a-pinene. Two kinds of experimental apparatus are established for the investigation of the thermal isomerization
reaction of a-pinene. The behavior of thermal isomerization reaction of a-pinene is respectively discussed in the gas phase
and in the liquid phase. Under gas phase conditions, the conversion of a-pinene is 80% and the selectivity of ocimene is
30%–33%. Under liquid phase conditions, the conversion of a-pinene is 60% and the selectivity of ocimene is 50%–54%.
According to the kinetic-molecular theory of ideal gases, two kinds of reaction models are proposed to visualize the reaction
process. In addition, the mechanism and kinetics of thermal isomerization reaction of a-pinene are respectively discussed.
The conclusion is that the gas phase reaction temperature is calculated to be 390–450 ꢀC and the liquid phase reaction
temperature is calculated to be 450–550 ꢀC. From a bond dissociation energy point of view, results support the hypothesis
that the reaction involves biradical intermediates. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: a-pinene; kinetics; mechanism; ocimene; thermal isomerization
retention times and mass spectra with those compounds.[14] GC112A-
INTRODUCTION
FID: SE-54 15 m ꢁ 0.25 mm ꢁ 0.33 mm capillary column, program: 65 ꢀC
(hold 25–30 min); injector temperature: 250 ꢀC; detector temperature:
Turpentine, an important renewable resource, is mainly composed
250 ꢀC; split flow: 40 mL/min; split ratio: 40; carrier gas: N2. Results are
of a-pinene and b-pinene. Turpentine can be obtained by the
treated with an N2000 workstation by the method of area normalization
extraction of pine resins or by distillation from crude sulfate
(N2000 workstation: Zhejiang University, China). TRACEDSQ (Thermo
turpentine. Currently, shortages of resources, environmental pollu-
Finnigan, US) gas chromatography–mass spectrometry (GC-MS): program:
65ꢀC(hold 2min), 1 ꢀC/min up to 75ꢀC (hold 2min), 25ꢀC/min up to
150ꢀC (hold 2 min); injector temperature: 230 ꢀC; split flow: 42mL/min;
split ratio: 30; carrier gas: He; EI (70 eV).
tion, and the pursuit of sustainable development have accelerated
the shift towards chemical products, which are derived from
renewable, biological feedstocks.[1]
The thermal isomerization reaction of a-pinene is a long-known
investigated reaction.[2–4] However, it is difficult to obtain a high
selectivity of the ocimene (3).[5,6] Generally, in the isomerization of
a-pinene (1) at a relatively high temperature, it has been reported
that the total amount of concomitantly formed dipentene (2) and
allo-ocimene (4) always exceeds that of ocimene (3)[7–10] (Scheme 1).
According to Table 1, ocimene (3) is known as a thermally unstable
compound. Stolle et al.[11] has reported that (3Z)-ocimene (3)
is mainly obtained from the thermal isomerization reaction of
a-pinene (1). Moreover, a fast isomerization of ocimene (3) to
allo-ocimene (4) at a relatively high temperature has been
proposed by various authors.[11–13] In this paper, the thermal
isomerization reaction of a-pinene is discussed once more from a
mechanistic and kinetic point of view.
Gas-phase pyrolysis experiments
Experiments were carried out in the apparatus (Figure 1) of the gas-phase
pyrolysis of a-pinene. In a typical experiment, 250mL a-pinene was charged
in the flask, the inhibitor added such as butylated hydroxytoluene 1.00g,
then the system was protected by nitrogen. Because the vapor of a-pinene
passing through Nichrome wire may explode at a high temperature, the
system must be oxygen-free. The mixture was boiled at 13–20 mmHg
pressure. The vapor of a-pinene was pyrolyzed at a reflux ratio of
1 : 1–4 :1 and voltage of 56–60 volts. The process was stopped after
10–40 h. The mixtures of pyrolysis were analyzed by gas chromatography-
flame ionization detector (GC-FID) and GC-MS.
Liquid-phase pyrolysis experiments
Experiments were carried out in the apparatus (Figure 2) of the liquid-phase
pyrolysis of a-pinene. In a typical experiment, 250mL a-pinene was charged
in the flask, the inhibitor added such as butylated hydroxytoluene 1.00g,
then the system protected by nitrogen. The mixture was boiled at
25–30 mmHg pressure. The liquid of a-pinene was pyrolyzed at a reflux
ratio of 1: 1–3: 1 and voltage of 17–20volts. The process was stopped after
EXPERIMENTAL
Materials
Standard sample (69% ocimene, mainly impurity is limonene) was purchased
from IFF (International Flavors & Fragrances Inc. US). The a-pinene (98%) in
this work was provided by the manufacturer (Shuinan Natural Spices
Co. Ltd. China). Other chemical reagents were purchased from Tianjin
Guangfu Fine Chemical Research Institute (Tianjin, China).
* Correspondence to: Xin Qi, Department of Chemistry, Tianjin University, Tianjin
300072, P. R. China.
E-mail: xinqi@tju.edu.cn
Analyses
Analyses of the contents of products were carried out in a GC112A series
a J. He, Y. Gong, W. Zhao, X. Tang, X. Qi
(GC112A: Shang Fen, China). Products are identified by comparing
Department of Chemistry, Tianjin University, Tianjin 300072, P. R. China
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

J. HE ET AL.
a-pinene is changed at each pass, a recycling system is designed.
A packing fractionating column (25 mm in diameter by 500 mm)
is employed to separate effectively a-pinene.
The products of pyrolysis are identified by GC-MS (Figure 3).
Yields (Yi) of pyrolysis products of a-pinene and content of
remaining a-pinene are expressed by the corrected peak area
from GC. Conversions of starting materials (Xj) are calculated
according to Eqn (2). Selectivities (Si) for the isomerization
reaction products are calculated according to Eqn (3).
2
racemic limonene
(dipentene)
1
α-pinene
Yj
100
Xj ¼ 1 ꢂ
(2)
(3)
4
3
4E,6Z-allo-ocimene
3Z-β-ocimene
Scheme 1. Reaction products resulted from the thermal isomerization
of a-pinene
Yi
Si ¼
Xj
20–50h. The mixtures of pyrolysis were analyzed by gas chromatography-
flame ionization detector (GC-FID) and GC-MS.
Experiments were carried out in the apparatus (Fig. 1) of the
gas-phase pyrolysis of a-pinene. The experimental results are
shown in Table 2. Because the surface temperature of the heated
filament cannot be easily measured, the influence of the pyrolysis
voltages on selectivities (Si) for the reaction products can be
studied easily. When the pyrolysis voltage increases, the pyrolysis
temperature also increases. When the pyrolysis voltage increases,
the selectivity of ocimene (S3) declines, the selectivity of allo-
ocimene (S4) increases, and the conversion of a-pinene (X1)
increases. Thus, the optimum voltage used to heat the wire is
about 57volts. In addition, the influence of the vapor flow rates
on selectivities (Si) for the reaction products is researched. When
the vapor flow rate decreases, the selectivity of ocimene (S3)
increases, the selectivity of allo-ocimene (S4) decreases, and the
conversion of a-pinene (X1) decreases. Thus, the optimum vapor
flow rate passing the heating cage is 0.6–0.8mL per minute.
Fractional distillation
The product mixtures were fractionated carefully at 15–20 mmHg
pressure in a packing distillation column (25 ꢁ 1500 mm). The first
fraction was mainly residual starting material. The second fraction was
the mixture of dipentene and ocimene, because the difference of its
boiling was only 2–3ꢀC. We suggested that the time of total reflux be
prolonged up to 6 h and the reflux ratio increased up to 20: 1–15 : 1. The
temperature of the distillation flask was kept below 100ꢀC. The residue
consisted of allo-ocimene and its polymers in the bottom of the flask.
RESULTS AND DISCUSSIONS
Gas-phase pyrolysis of a-pinene
The apparatus of the gas phase pyrolysis of a-pinene is shown in
Fig. 1. Gas phase pyrolysis experiments were carried out in an
electrically heated glass cage (Fig. 1(c)). The cage is made of
Nichrome wire by winding it in a glass frame. The cage is
suspended in the centre of the glass condenser (Fig. 1(b)). The
current in the wire is usually AC of low voltage, for example
56–60 volts, which with a wire of length 5.00 m requires a current
of up to 1.20 A. The surface area of pyrolysis reaction is
calculated by the Eqn (1) where l is length of the wire and d is
diameter of the wire (d = 0.40 mm.).
Liquid-phase pyrolysis of a-pinene
The apparatus of the liquid-phase pyrolysis of a-pinene is shown
in Fig. 2. Liquid-phase pyrolysis experiments were carried out
with an electrically heated reaction chamber (Fig. 2(b)). The
reaction chamber is made of Nichrome wire by spiral winding
in a liquid environment. The spiral is loosened and suspended
vertically in the center of the glass chamber. The current in the
wire is usually AC of low voltage, for example 17–20 volts, which
with a wire of length 0.50 m requires a current of up to 3.12 A.
When length of the wire in the pyrolysis zone is 300 mm, the
surface area of pyrolysis reaction is calculated to be 376.8 mm2.
The volume of the liquid in the chamber is designed about
50 mL, while the total volume of the chamber is about 100 mL.
The liquid of a-pinene is passed through the electrically heated
Nichrome wire, while the temperature of the liquid environment
is 55–60 ꢀC under 25–30 mmHg pressure. Because a small part of
the a-pinene is changed at each pass, a recycling system is
S ¼ pdl
(1)
When the length of the wire in pyrolysis zone is 4500 mm, the
surface area of pyrolysis reaction is calculated to be 5652 mm2.
a-Pinene vapor (bp. 45–50 ꢀC) at 13–20 mmHg pressure is passed
rapidly through the electrically heated cage, then is cooled
rapidly by the glass condenser. Because a small part of the
Table 1. Thermodynamic data1 for the isomerization reaction of a-pinene
Compound
a-Pinene
Dipentene
Ocimene
Allo-ocimene
ΔG(kJ/mol)
ΔE(kJ/mol)
0
0
ꢂ63.18
ꢂ55.65
ꢂ15.06
ꢂ55.23
ꢂ28.03
11.72
1Thermodynamic data is calculated by density functional theory. The calculation method is Becke, three-parameter, Lee–Yang–Parr/
6-311++G**. G is Gibbs energy and E is electronic energy. These values are the relative values. a-Pinene is regarded as ground-level,
delta G and delta E values are calculated by Becke, three-parameter, Lee–Yang–Parr/6-311++G**.
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THERMAL ISOMERIZATION REACTION OF a-PINENE
(a)
(b)
(c)
Figure 1. (a) Apparatus for production of ocimene by gas-phase pyrolysis of a-pinene. (1) Charging flask; (2) and (6) thermometer; (3) packing
distillation column (25 ꢁ 500 mm); (4) joint; (5), (8) and (9) condenser; (7) thermocouple; (10) heating cage; (11) return line; (12) Nichrome wire; (13) glass
frame; (14) magneton. (b) and (c) show a more detailed view of the condenser and heating cage (30 ꢁ 75 mm), respectively
designed. The packing fractionating column (25 mm in diameter
by 500 mm) is employed to separate effectively a-pinene from its
isomerization products. Experiments were carried out in the
apparatus (Fig. 2) of the liquid-phase pyrolysis of a-pinene. The
experimental results are shown in Table 3.
The relationship between the pyrolysis voltages on selectivities
(Si) for the isomerization reaction products is researched. When
the pyrolysis voltage increases, the pyrolysis temperature also
increases. When the pyrolysis voltage increases, the conversion of
a-pinene (X1) increases, but the selectivity of ocimene (S3) declines.
Thus, the optimum voltage used to heat the wire is 18–19 volts.
In addition, the relationship between the liquid flow rates on
selectivities (Si) for the reaction products is investigated. When
the liquid flow rate increases, the selectivity of ocimene (S3)
increases, and the selectivity of allo-ocimene (S4) decreases.
However, the conversion of a-pinene (X1) decreases sharply.
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J. HE ET AL.
(a)
Figure 3. Gas chromatograms (GC-MS) of the pyrolysis products of
a-pinene. (a) Standard sample: ocimene (69%); (b) the gas phase pyrolysis
products; (c) the liquid phase pyrolysis products. (1) a-Pinene; (2) limonene
(dipentene); (3) 3Z-ocimene; (4) 4E,6Z-allo-ocimene and 4E,6E-allo-ocimene
ocimene is up to 54%, based on the conversion of a-pinene of
60%. Results show that the selectivity (S3) of ocimene obtained
from pyrolysis in liquid phase is higher than that in gas phase.
Model of pyrolysis reaction of a-pinene
(b)
To study deeply the reaction process concerning pyrolysis of
a-pinene, the kinetic-molecular theory of ideal gas will be
employed. In this work, the vapor will be simply seen as an
ideal gas. The gas-phase and liquid-phase pyrolysis behavior of
a-pinene are respectively shown in Figs 4 and 5.
We will first focus our attention on a single molecule. The
molecule moves to the wire surface and absorbs energy, leading
to chemical bonds cracking. The heated molecule diffuses to the
gas environment (Fig. 4). However, the reaction molecule is cooled
until it arrives at the cooling wall. According to the kinetic theory of
ideal gases, we can obtain the collision frequency of molecules and
the mean free path. Under our experimental conditions, the mean
free path is so short that the molecules cannot be cooled rapidly.
Moreover, it is impossible that the heating wall is too close to the
cooling wall. Thus, ocimene obviously isomerizes to allo-ocimene.
Figure 2. (a) Apparatus for production of ocimene by liquid phase pyrolysis
of a-pinene. (1) Charging flask; (2) and (4) thermometer; (3) packing distilla-
tion column (25 ꢁ 500 mm); (5) and (8) condenser; (6) electromagnet;
(7) valve; (9) reaction chamber; (10) Nichrome wire; (11) vacuum pump;
(12) the liquid of a-pinene. (b) A more detailed view of the reaction chamber
Thus, the optimum liquid flow rate passing the heating cage is
2.0–3.0 mL per minute to obtain more ocimene.
Under optimum conditions, the experimental results are summa-
rized in Table 4. When the gas-phase pyrolysis reaction of a-pinene
is carried out under optimum reaction conditions, the selectivity of
ocimene is 30%–33%, based on the conversion of a-pinene of
80%. When the liquid-phase pyrolysis reaction of a-pinene is
carried out under optimum reaction conditions, the selectivity of
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THERMAL ISOMERIZATION REACTION OF a-PINENE
Table 2. The experimental results for thermal isomerization of a-pinene in the gas phase1
Voltages
Temperature2
(ꢀC)
Flow rate
(mL/min)
Y1
Y2
Y3
Y4
X1
S2
S3
S4
(Y3+ Y4)/ Y2
(ꢃ 0.5 V)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
56
57
57
57
58
60
432
447
400
390
434
474
0.8–1.2
0.8–1.2
0.6–0.8
0.4–0.6
0.8–1.2
0.8–1.2
55.05
59.95
73.04
82.09
47.92
41.61
17.03
14.51
11.02
7.31
19.43
19.65
13.07 13.32 43.81 38.87 29.83
11.85 12.11 38.91 37.29 30.45
30.40
31.12
23.00
25.10
32.11
37.29
1.55
1.65
1.32
1.30
1.56
1.83
8.56
5.31
5.74 25.82 42.68 33.15
4.21 16.77 43.59 31.66
13.96 16.36 50.94 38.14 27.40
14.69 21.35 57.25 34.32 25.66
1Reaction time 6 h; Y1 is content of remaining of a-pinene; Y2 is yield of dipentene; Y3 is yield of ocimene; Y4 is yield of allo-ocimene;
X1 is conversion of a-pinene; S2 is selectivity of dipentene; S3 is selectivity of ocimene; S4 is selectivity of allo-ocimene.
2Theoretical calculation.
Table 3. The experimental results for thermal isomerization of a-pinene in the liquid phase1
Voltages
Temperature2
(ꢀC)
Flow rate
(mL/min)
Y1
Y2
Y3
Y4
X1
S2
S3
S4
(Y3+ Y4)/ Y2
(ꢃ 0.5 V)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
17
18
19
20
20
20
400
452
456
488
552
495
0.5–1.0
0.5–1.0
0.5–1.0
0.5–1.0
1.0–2.0
2.0–3.0
93.95
83.36
84.46
55.68
66.46
84.63
1.04
4.58
4.61
13.03
9.16
4.21
1.32
6.46
6.30
12.34
13.03
6.80
0.08
1.26
1.59
12.86
8.96
1.50
2.44
12.30
12.50
38.23
31.15
12.51
42.62
37.24
36.88
34.08
29.41
33.65
54.10
52.52
50.40
32.27
41.83
54.36
3.28
1.35
1.68
1.71
1.93
2.40
1.97
10.24
12.72
33.64
28.76
11.99
1Reaction time 20 h; Y1 is content of remaining of a-pinene; Y2 is yield of dipentene; Y3 is yield of ocimene; Y4 is yield of
allo-ocimene; X1 is conversion of a-pinene; S2 is selectivity of dipentene; S3 is selectivity of ocimene; S4 is selectivity of allo-ocimene.
2Theoretical calculation.
Table 4. The comparative results for thermal isomerization of a-pinene in the gas phase and in the liquid phase1
Method
Temperature2 Flow rate Time
Y1
Y2
Y3
Y4
X1
S2
S3
S4
(Y3+ Y4)/ Y2
(ꢀC)
(mL/min)
(h)
(%)
(%)
(%)
9.26
(%)
(%)
(%)
(%)
(%)
Gas
Liquid
Gas
395
528
445
532
0.6–0.8
2.0–3.0
0.6–0.8
2.0–3.0
10
30
42
56
73.20 11.03
69.06
5.18 25.66 42.99 36.09 20.19
4.84 28.43 31.02 51.95 17.02
1.31
2.22
1.64
2.25
8.82 14.77
14.99 30.92 24.82 26.06 81.80 37.80 30.34 31.86
38.05 18.33 30.27 11.08 59.68 30.71 50.72 18.57
Liquid
1Y1 is content of remaining of a-pinene; Y2 is yield of dipentene; Y3 is yield of ocimene; Y4 is yield of allo-ocimene; X1 is conversion
of a-pinene; S2 is selectivity of dipentene; S3 is selectivity of ocimene; S4 is selectivity of allo-ocimene;
2Theoretical calculation.
However, when pyrolysis reaction is carried out in the liquid
phase, the molecules heated absorb energy, leading to chemical
bonds cracking. The reaction molecules rapidly diffuse to the
liquid environment (Fig. 5). Because the temperature of the
liquid environment is 55–60 ꢀC under pressure of 25–30 mmHg,
these molecules can be cooled rapidly. Because surface
tension exists, it is difficult to form bubbles on the wire surface.
Moreover, it also is difficult for these bubbles to diffuse to the
surrounding liquid. Thus, the wire surface must have higher
temperature to keep boiling. We believe that the molecules have
more opportunity to absorb energy, leading to chemical bonds
cracking. However, we must control the flow rate of the liquid
to eliminate the opportunity that the reaction molecules are
heated again. Thus, the conclusion is that the reaction molecules
are cooled more rapidly in the liquid environment than that in
the gas environment.
Mechanism of pyrolysis reaction of a-pinene
Mechanism of pyrolysis reaction of a-pinene has been reported
by various authors in the literatures,[11,15–18] but it seems to be
appropriate to investigate the reaction once more from a bond
dissociation energy point of view. The initial formation of a
biradical has been assumed as an intermediate of pyrolysis
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J. HE ET AL.
Figure 4. (a) The model concerning the gas phase pyrolysis of a-pinene. (b) The relationship of temperature and distance in the gas phase
Figure 5. (a) The model concerning the liquid phase pyrolysis of a-pinene. (b) The relationship of temperature and distance in the liquid phase
reaction of a-pinene.[11,15] Generally, cracking carbon–carbon
bond is difficult because carbon–carbon s bond is very stable.
However, because the strain energy of cyclobutane ring exists,
it is most possible to crack the carbon–carbon bond (positions
C1 and C6) leading to forming the biradical intermediate
(Scheme 2). It is a stable intermediate because this model has a
resonance-stabilized allyl-type radical and tert-butyl radical.
The intermediate probably undergoes two different paths
to generate different products. Racemic limonene is formed
(path: A) when the model has intramolecular [1,5] sigmatropic
hydrogen shift (positions C1, C8, or C9). At the same time, when
the model breaks the carbon–carbon bond (positions C5, C7, or
C4), the acyclic ocimene is formed (path: B). Probably, the acyclic
ocimene undergoes very fast [1,5] sigmatropic hydrogen shift
leading to fully conjugated allo-ocimene. The energy diagram
of the reaction process is shown in Fig. 6.
reaction of a-pinene is assumed as first-order kinetics. It is
reasonable that the model is competitive parallel first-order
reaction, combined with consecutive first-order reaction. On
the basis of the kinetic model, the reaction process is shown in
Scheme 3. According to the competitive parallel first-order
reaction, the concentration of products 2 and 3 can be
expressed by Eqn (4).
½3ꢄ þ ½4ꢄ k3
¼
(4)
½2ꢄ
k2
Arrhenius activation parameters for the reaction steps can
be calculated according to Eqn (5), where Ea2 = 160.8 kJ/mol,
Ea3 = 178.5 kJ/mol, lgA2 = 12.61, lgA3 = 14.11.[11]
ꢂE
a
RT
k ¼ Ae
(5)
(6)
k3
lg ¼ lgk3 ꢂ lgk2 ¼
k2
Ea2 ꢂ Ea3
2:303RT
Kinetic analysis of the reaction
þ lgA3 ꢂ lgA2
The thermal isomerization reaction of a-pinene has been inten-
sively subjected to kinetic studies in gas phase,[5,11,16,17] liquid
phase,[19] or supercritical alcohols.[10,20] It is the assumption that
the rate-determining reaction step is to form the biradical
intermediate [TS]. The kinetic model of the isomerisation
According to Eqn (6), we can calculate the reaction tempera-
ture, although it is difficult to measure directly the reaction
temperature. From Table 2, the temperature of the gas-phase
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J. Phys. Org. Chem. (2012)

THERMAL ISOMERIZATION REACTION OF a-PINENE
8
1
2
6
6
3
1
=
9
5
5
6
4
2
4
3
4
7
3
7
1
7
2
5
10
biradical intermediate
α
-pinene
8
6
H
5
path: A
path:B
H
5
4
3
4
3
7
2
1
1
2
7
H
=
=
=
=
H
racemic limonene
(dipentene)
3Z-ocimene
3E-ocimene
3
2
4
H
5
1
6
8
7
4E,6Z-allo-ocimene
3Z-β-ocimene
Scheme 2. Possible mechanism of the thermal rearrangement of a-pinene
gas-phase pyrolysis reaction of a-pinene is carried out under
optimum reaction conditions, the selectivity of ocimene is
30%–33%, based on the conversion of a-pinene of 80%. When
the liquid-phase pyrolysis reaction of a-pinene is carried out
under optimum reaction conditions, the selectivity of ocimene
is 50%–54%, based on the conversion of a-pinene of 60%.
Results are shown that the selectivity of ocimene obtained
from pyrolysis in liquid phase is higher than that in gas phase.
According to the model of pyrolysis reaction of a-pinene, the
conclusion is that the reaction molecules are cooled more
rapidly in the liquid environment than that in the gas environ-
ment. From the bond dissociation energy point of view, results
support the hypothesis that the pyrolysis reaction involves
biradical intermediates. According to the kinetic model, the
temperature of the gas-phase pyrolysis reaction of a-pinene is
calculated to be 390–450 ꢀC. However, the temperature of the
liquid-phase pyrolysis reaction of a-pinene is calculated to be
450–550 ꢀC.
Figure 6. The energy diagram of reaction process is schematically painted
k2
2
k1
[TS]
Acknowledgements
1
k4
k3
3
4
This work was supported by Professor Xiangyang Tang and
Professor Wentao Zhao. In addition, we thank Professor Haitao
Zhao for his quantum calculation expertise.
Scheme 3. The kinetic model of the isomerisation reaction of a-pinene
pyrolysis reaction of a-pinene is calculated to be 390–450 ꢀC.
From Table 3, the temperature of the liquid-phase pyrolysis
reaction of a-pinene is calculated to be 450–550 ꢀC.
REFERENCES
[1] A. Behr, L. Johnen, ChemSusChem 2009, 2, 1072.
[2] L. A. Goldblatt, S. Palkin, J. Am. Chem. Soc. 1941, 63, 3517.
[3] T. R. Savich, L. A. Goldblatt, J. Am. Chem. Soc. 1945, 67, 2027.
[4] R. E. Fuguitt, J. E. Hawkins, J. Am. Chem. Soc. 1945, 67, 242.
[5] J. E. Hawkins, H. G. Hunt, J. Am. Chem. Soc. 1951, 73, 5379.
[6] J. E. Hawkins, W. A. Burris, J. Org. Chem. 1959, 24, 1507.
[7] R. L. Blackmore, US3281485, 1966.
CONCLUSIONS
Two kinds of experimental apparatus are established for the
investigation of the pyrolysis reaction of a-pinene. When the
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J. HE ET AL.
[8] L. Lemee, M. Ratier, J. G. Duboudin, B. Delmond, Synth. Commun.
1995, 25, 1313.
[9] A. Stolle, B. Ondruschka, W. Bonrath, Eur. J. Org. Chem. 2007, 2310.
[10] A. Yermakova, A. M. Chibiryaev, I. V. Kozhevnikov, P. E. Mikenin, V. I.
Anikeev, Chem. Eng. Sci. 2007, 62, 2414.
[15] R. L. Burwell, J. Am. Chem. Soc. 1951, 73, 4461.
[16] J. J. Gajewski, C. M. Hawkins, J. Am. Chem. Soc. 1986, 108, 838.
[17] J. J. Gajewski, I. Kuchuk, C. Hawkins, R. Stine, Tetrahedron 2002, 58,
6943.
[18] A. Stolle, B. Ondruschka, H. Hopf, Helv. Chim. Acta 2009, 92,
[11] A. Stolle, B. Ondruschka, M. Findeisen, J. Org. Chem. 2008, 73, 8228.
[12] A. Stolle, B. Ondruschka, J. Anal. Appl. Pyrolysis 2009, 85, 252.
[13] T. Sasaki, S. Eguchi, H. Yamada, Tetrahedron Lett. 1971, 12, 99.
[14] R. Ryhage, E. von Sydow, Acta Chem. Scand. 1963, 17, 2025.
1673.
[19] R. E. Fuguitt, J. E. Hawkins, J. Am. Chem. Soc.1947, 69, 319.
[20] A. Yermakova, A. M. Chibiryaev, I. V. Kozhevnikov, V. I. Anikeev,
J. Supercrit. Fluids 2008, 45, 74.
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Research Article
A comparative study on the gas-phase and liquid-phase thermal isomerization reaction of a-pinene
Jindong He, Yan Gong, Wentao Zhao, Xiangyang Tang and Xin Qi
path:A
2
racemic limonene
(dipentene)
path: B
1
α-pinene
biradical intermediate
4
3
3Z-β-ocimene
4E,6Z-allo-ocimene
path: B
path:A
Energy
biradical intermediate
α-pinene
3Z-β-ocimene
dipentene
4E,6Z-allo-ocimene
This work reports that the selectivity of ocimene obtained from pyrolysis of a-pinene in liquid phase is higher than that
in gas phase. According to the model of pyrolysis reaction of a-, the conclusion is that the reaction molecules are cooled
more rapidly in the liquid environment than that in the gas environment. From a bond dissociation energy point of view,
results support the hypothesis that the reaction involves biradical intermediates