Kinetic and mechanistic study on the thermal isomerization of ocimene in the liquid phase

Article abstract of DOI:10.1002/poc.1925

The rate of thermal isomerization of ocimene in the liquid phase has been investigated in the range 90-150°C. The rate constant for the disappearance of ocimene may be expressed by k=1.3×10¹⁰e ^-11994.2/T(min^-1), from which we can infer that the activation energy is 99.7kJmol^-1 and the pre-exponential factor is 1.3×10¹⁰min^-1. The half-life for the disappearance of ocimene may be expressed by t_1/2=5.2×10^-11e ^11994.2/T(min). The conclusion has been supported by the study results that the ocimene is safe when temperature is below 100°C. A discussion of the mechanism concerning the conversion is included. Copyright

Full text of DOI:10.1002/poc.1925

Research Article
Received: 9 June 2011,
Accepted: 18 July 2011,
Published online in Wiley Online Library: 7 September 2011
(wileyonlinelibrary.com) DOI: 10.1002/poc.1925
Kinetic and mechanistic study on the thermal
isomerization of ocimene in the liquid phase
Jindong Hea, Meng Xiea, Xiangyang Tanga and Xin Qia*
The rate of thermal isomerization of ocimene in the liquid phase has been investigated in the range 90–150 ꢀC. The
rate constant for the disappearance of ocimene may be expressed by k = 1. 3 ꢁ 1010e– 11994. 2/T(min– 1), from which we
can infer that the activation energy is 99.7 kJ mol–1 and the pre-exponential factor is 1.3 ꢁ 1010 min–1. The half-life
for the disappearance of ocimene may be expressed by t1/2 = 5. 2 ꢁ 10– 11e11994. 2/T(min). The conclusion has been
supported by the study results that the ocimene is safe when temperature is below 100 ꢀC. A discussion of the mech-
anism concerning the conversion is included. Copyright © 2011 John Wiley & Sons, Ltd.
Keywords: ocimene; allo-ocimene; kinetics; mechanism; isomerization
Analyses
INTRODUCTION
Analyses of the contents of products are carried out in a GC112A series.
Ocimene is an acyclic monoterpene found within a variety of
Products are identified by comparing the retention times and mass
plants and fruits. It has been found naturally as mixtures of the
spectra with those compounds.[13] GC112A-FID: SE-54 15 m ꢁ 0.25 mm
various forms. Ocimene exists as four isomers, and allo-ocimene
0.33 mm capillary; program, 65 ꢀC (hold 25–30 min); injector temperature,
also exists as four isomers. The mixture of ocimene is an oil with
250 ꢀC; detector temperature, 250 ꢀC. Results are treated with an N2000
a pleasant odor. Thus, ocimene and its derivatives (ocimenol,
workstation (Zhejiang University, Hangzhou, China) by the method of
esters and 6,7-expoxy ocimene, etc.) are mainly used as perfume
area normalization. TRACEDSQ (Thermo Finnigan, US) GC-MS: program,
components. In addition, they are widely used as building
65 ꢀC (hold 2 min), 1 K/min up to 75 ꢀC (hold 2 min), 30 K/min up to
blocks for pharmaceuticals, flavors, fragrances, food supple-
150 ꢀC (hold 2 min); injector temperature, 230 ꢀC; carrier gas He,
ments and pheromones.[1–4] It is known that ocimene is
EI(70 eV). Now we use the statistical software R version 2.11.1 (University
obtained by thermal isomerization of a-pinene; however, under
of Auckland, New Zealand) to calculate linear regression equations by the
method of the least squares.
high temperature, ocimene isomerizes readily to allo-ocimene
(Scheme 1).[5,6] a-Pinene has been intensively subjected to
kinetic studies in gaseous, liquid or supercritical phase;[7–9]
however, the isomerization reaction of ocimene is rarely
investigated.[10–12]
Kinetic experiments
The first work of this reaction was reported by Hawkins[5] that
Kinetic experiments with the mixture of dipentene and ocimene were
conducted. The mixture 20.00 g (25 ml) was added 0.2–0.5% inhibitor
butylated hydroxytoluene (BHT), then protected by nitrogen. The mixture
the activation energy was 108.8 kJ mol–1. Then, kinetic results
were reported by Sasaki et al.[10] that activation energy was
118.4 kJ mol–1. Stolle et al.[11] again investigated the pyrolysis
was heated at constant temperature in an oil bath, with a maximum varia-
of ocimene, and the activation energy was calculated to be
tion of ꢂ1ꢀC at each temperature. At regular intervals, about 0.05 ml was
125.7 kJ mol–1. Although many kinetic results have been
quickly drawn from the sample with a syringe, then diluted with precooled
published in the literature concerning the isomerization of
acetone to 0.50ml to quench the reaction. Finally, the rate of reaction was
ocimene, it seems to be appropriate to study the reaction once
more from a kinetic and mechanistic point of view. Performing
experiments with dipentene–ocimene mixtures under identical
conditions allow for a detailed study. The calculation of activa-
calculated according to the results from gas chromatography.
tion parameters according to Arrhenius and Eyring equation
RESULTS AND DISCUSSION
are given.
Calculation of the rate of reaction
Kinetic experiments with the mixture of dipentene and ocimene
were conducted at 141.5 ꢀC. The plot is depicted in Fig. 1. The
EXPERIMENTAL
Materials
* Correspondence to: Xin Qi, Department of Chemistry, Tianjin University, Tianjin
300072, China.
E-mail: xinqi@tju.edu.cn
Standard samples are provided by the manufacturers. The mixtures of
dipentene and ocimene are produced by fractional distillation the pyrol-
ysis products from a-pinene. Other chemical reagents are purchased
a J. He, M. Xie, X. Tang, X. Qi
Department of Chemistry, Tianjin University, Tianjin 300072, China
from Tianjin Guangfu Reagents Company.
J. Phys. Org. Chem. 2012, 25 373–378
Copyright © 2011 John Wiley & Sons, Ltd.

J. HE ET AL.
Scheme 1. Reaction products resulted from the thermal isomerization
of a-pinene
rate of the consumption of ocimene was fast at 141.5 ꢀC, the
half-life of the reaction was about 190 min according to Fig. 1.
Figure 2. The relationship lnw of ocimene and t
Calculation of the rate constant
Hawkins[5] first reported that the thermal isomerization of oci-
mene was a first-order reaction. From the first-order kinetic equa-
tion, the rate constant for the reaction is calculated by Eqn (1).
If the Arrhenius law applies, a plot of ln k against 1/T will be a
straight line, and the slope will be –Ea/R. The plot is depicted in
Fig. 3. Results suggest that the activation energy for the isomer-
ization of ocimene is independent of the reaction temperature
within the range of 90–150 ꢀC. The activation energy for the
isomerization of ocimene is calculated to be 99.7 kJ mol–1; the
pre-exponential factor is calculated to be 1.3 ꢁ 1010 min–1.
The dependence of the rate constant of the reaction may be
expressed by Eqn (4)
ln w ¼ ꢃkt þ ln w0
(1)
where k is the rate constant for the consumption of ocimene,
w is the weight fractions of ocimene at a certain time t, w0 is
the weight fractions of ocimene at the beginning of the reaction.
Graphical methods can be employed to test Eqn (1) and to ob-
tain the rate constant. The plot is depicted in Fig. 2.
Results suggest that the isomerization of ocimene is approxi-
mately a first-order reaction at 141.5ꢀC. The rate constant is calcu-
lated to be 3.619 ꢁ 10–3 min–1, and the value of the half-life is
calculated to be 191.5 min, approaching the experimental resuzlts.
À
Á
k ¼ 1:3 ꢁ 1010eꢃ11994:2=T minꢃ1
(4)
The dependence of the half-life of the reaction may be
expressed by Eqn (5)
Effect of temperature on isomerization of ocimene
t1=2 ¼ 0:693=k ¼ 5:2 ꢁ 10ꢃ11e11994:2=T ðminÞ
(5)
According to Eqn (1), kinetic experiments were investigated at
different temperatures. The rate constant and the half-life were
calculated, and then summarized in Table 1.
Graphical methods can be employed to test the (5). When the
half-life for the isomerization of ocimene is plotted against reac-
tion temperature, we can obtain a simulative curve. The plot is
depicted in Fig. 4.
From Table 1, we can see that the rate constant is increased
with increasing temperature. This relationship can be expressed
by the Arrhenius equation
k ¼ AeꢃE =RT
(2)
a
Generally, the preparation of ocimene is conducted by the py-
rolysis of a-pinene.[5,6] According to Eqn (5), the half-life for the
reaction is calculated to be only 0.7 s when the pyrolysis temper-
ature is conducted generally about 350 ꢀC. If a desirable
product is ocimene, the pyrolysis products have to be cooled
sharply. However, through general pyrolysis apparatus, for
example a heated tube, pebble reactor or fluidized bed, the
product of allo-ocimene rather than ocimene is obtained, be-
cause of the thermal isomerization of the ocimene initially
produced.[5,6,11,14,15] There has been no satisfactory solution to
the problem of cooling the pyrolysate molecules sufficiently
quickly to the temperature at which there is no appreciable
reaction. The temperature at which there is no appreciable
reaction is known as the safe temperature. We suggest that oci-
mene is safe when temperature is below 100 ꢀC. Thus, we
designed the pyrolysis apparatus to produce the thermally un-
stable ocimene. The pyrolysis apparatus is shown in Fig. 5.
In addition, the reaction mixture of a-pinene is then subjected
to fractional distillation under vacuum in a Φ 25 mm ꢁ 1500 mm
column and then the qualified product can be obtained. We sug-
gest that the boiling temperature of the reaction mixture of
a-pinene is kept at below 100 ꢀC under vacuum.
To test the Arrhenius Eqn (2), we can take logarithms of both
sides:
ln k ¼ ln A ꢃ Ea=RT
(3)
Figure 1. The content of each compound depending on the time at
141.5 ꢀC
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J. Phys. Org. Chem. 2012, 25 373–378

THERMAL ISOMERIZATION OF OCIMENE IN THE LIQUID PHASE
Table 1. The rate constants at different temperaturesa
T (ꢀC)
k (min–1)
t1/2 (min)
Simulative
curveb
R2 c
p-valued
90.5
102
112
125.5
141.5
6.025 ꢁ 10–5
2.030 ꢁ 10–4
1.219 ꢁ 10–3
1.219 ꢁ 10–3
3.619 ꢁ 10–3
11,503.7
3414.3
2026.6
568.6
ln w = ꢃ6.025 ꢁ 10–5 t + 3.290
ln w = ꢃ2.030 ꢁ 10–4 t + 3.227
ln w = ꢃ3.420 ꢁ 10–4 t + 3.213
ln w = ꢃ1.219 ꢁ 10–3 t + 3.174
ln w = ꢃ3.619 ꢁ 10–3 t + 3.067
0.9827
0.9742
0.9647
0.9839
0.9947
1.347 ꢁ 10–5
3.658 ꢁ 10–5
7.702 ꢁ 10–8
3.974 ꢁ 10–12
7.383 ꢁ 10–14
191.5
aThe content of ocimene in the mixture is 24–25%.
b,c,dCalculated by the statistical software R version 2.11.1.
Table 2. Results indicate that the rate constant of the reaction
is independent of the diluent of dipentene; a strong base cata-
lyst (for example potassium tert-butyl alcohol); or an inhibitor
(for example BHT). Thus, the isomerization of ocimene falls into
the category of concerted pericyclic reaction in which there are
no intermediates and proceeds through cyclic transition struc-
tures. In addition, the rate constant is also calculated by use of
transition-state theory. The entropy of activation, enthalpy of ac-
tivation and Gibbs energy of activation are calculated according
to the Eyring equation (6).
À
Á
À
Á
{
{
k ¼ ðkBT=hÞexp 5 S=R exp ꢃ5 H=RT
À
Á
{
¼ ðkBT=hÞexp ꢃ5 G=RT
(6)
where kB is the Boltzmann constant and h is the Planck constant.
According to the Eyring equation (6), we can obtain the equa-
tions
Figure 3. The plot of ln k against 1/T
{
ln k ¼ lnðkBT=hÞ ꢃ 5 G=RT
(7)
(8)
{
ln A ¼ lnðkBT=hÞ ꢃ 5 S=R
{
{
{
5 H ¼ 5 G þ T5 S
(9)
{
Ea ¼ 5 H þ RT
(10)
At 125.5 ꢀC, Gibbs energy of activation is calculated to be
107.9 kJ mol–1; entropy of activation is calculated to be ꢃ19.7 J
k–1 mol–1; enthalpy of activation is calculated to be 100 kJ mol–1;
activation energy is calculated to be 103.3 kJ mol–1, while the
experimental result is 99.7 kJ mol–1. Activation entropy indicates
a tight transition state, being typical for reactions passing
through a six-membered ring transition state.[11]
Figure 4. The plot of the half-life depending on T
Mechanism of the isomerization of ocimene
The conservation principle of the molecular orbital symmetry
proposed by Woodward and Hoffmann and the frontier orbital
theory proposed by Kenichi Fukui interpret perfectly the isom-
erization of ocimene. The six-membered ring transition state
and molecular orbital are given in Scheme 3. According to the
Woodward and Hoffmann rules, 1,3-hydrogen shift is forbidden,
while 1,5-hydrogen shift is allowed. The thermal 1,5-hydrogen
migration of ocimene is shown in Scheme 4.
The thermal isomerization of ocimene has been recognized for a
long time; the mechanism for the rearrangement was proposed
by Sasaki et al.[10] Deuterium-labeling experiments performed
by Gajewski et al.[16,17] supported the hypothesis that the ther-
mal isomerization of ocimene proceeded as 1,5-hydrogen shift
reaction. Stolle et al.[11] reported that activation entropy and
frequency factor indicated a tight transition state, being typical
for reactions passing through a six-membered ring transition
state (Scheme 2). Because the proton in ocimene is bisallylic,
the reaction probably is carried out through the carbon anion
intermediate or the allylic radical intermediate (Scheme 2).
Kinetic experiments are investigated at different conditions to
test those possible mechanisms. Results are summarized in
CONCLUSION
The rate of thermal isomerization of ocimene in the liquid phase
is investigated in the range 90–150 ꢀC. The rate constants for the
J. Phys. Org. Chem. 2012, 25 373–378
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J. HE ET AL.
Figure 5. (a) Apparatus for the preparation of ocimene by pyrolysis of a-pinene: (1) charging flask; (2) and (6) thermometer; (3) distillation column Φ25
mm ꢁ 500 mm; (4) joint; (5), (8), (9) condenser; (7) thermocouple; (10) heating cage; (11) return line; (12) nichrome wire; (13) glass frame; and (14) mag-
neton. (b) and (c) show a more detailed view of the condenser and heating cage (Φ30 mm ꢁ 75 mm), respectively
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J. Phys. Org. Chem. 2012, 25 373–378

THERMAL ISOMERIZATION OF OCIMENE IN THE LIQUID PHASE
Scheme 2. Three kinds of possible mechanisms of the thermal rearrangement of ocimene to allo-ocimene
Table 2. The rate constants of the reaction under different conditiona
Content of ocimene (%)
Cat.(w%)
k (min–1)
Simulative equationb
R2 c
p-valued
24–25
35–36
35–36
35–36
0.2% BHT
0.2% BHT
0% BHT
1.219 ꢁ 10–3 ln w = ꢃ1.219 ꢁ 10–3 t + 3.174 0.9839 3.974 ꢁ 10–12
9.563 ꢁ 10–4 ln w = ꢃ9.563 ꢁ 10–4 t + 3.504 0.9833 3.247 ꢁ 10–10
9.165 ꢁ 10–4 ln w = ꢃ9.165 ꢁ 10–4 t + 3.471 0.9874 8.272 ꢁ 10–12
0.5% (CH3)3COK and (CH3)3COH 1.025 ꢁ 10–3 ln w = ꢃ1.025 ꢁ 10–3 t + 3.518 0.9950 5.06 ꢁ 10–14
aReaction temperature 125.5 ꢀC.
b,c,dCalculated by the statistical software R version 2.11.1.
The average value of k = 1.0 ꢁ 10–3 (min–1).
Scheme 3. The six-membered ring transition state and molecular orbital
Scheme 4. The thermal 1,5-hydrogen migration of ocimene
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J. HE ET AL.
first-order reaction may be expressed by k = 1.3ꢁ 1010e– 11994. 2/T
(min– 1), and the activation energy is calculated to be 99.7 kJ
mol–1. We suggest that the ocimene is safe when temperature
is below 100 ꢀC. Thus, we design the apparatus for the prepara-
tion of ocimene by the pyrolysis of a-pinene. We suggest that
the boiling temperature of the reaction mixture of a-pinene is
kept at below 100 ꢀC under vacuum. The experiment results indi-
cate that the rate constant of the reaction is independent of the
diluent of dipentene, a strong base catalyst (for example potas-
sium tert-butyl alcohol) or an inhibitor (for example BHT). The
mechanism of the isomerization of ocimene probably undergoes
the 1,5-hydrogen shift pathway.
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Acknowledgments
We thank Professor Xiangyang Tang for support of this work and
Professor Tong Lin for his GC-MS expertise.
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J. Phys. Org. Chem. 2012, 25 373–378