Polyethylene Glycols as Efficient Catalysts for the Oxidation of Bicyclic Monoterpenes by Ceric Ammonium Nitrate in Acetonitrile under Acid-Free Conditions: Kinetic and Mechanistic Approach

Article abstract of DOI:10.1002/kin.21168

Polyethylene glycols (PEG) acts as efficient catalysts for the oxidation of bicyclic monoterpenes such as borneol, isoborneol, and camphor by ceric ammonium nitrate (CAN), a laboratory desktop reagent, in acetonitrile medium under mineral acid-free conditions. The kinetics of the reactions revealed first-order dependence on in both [CAN] and [bicyclic terpene]. The rate of oxidation is accelerated with an increase in [PEG] linearly, which could be explained by considering PEG-bound oxidant (PEG-CAN) as more reactive species than (CAN) itself. The mechanism of oxidation in PEG media has been explained through the participation of PEG-bound oxidant (PEG-CAN) and bicyclic monoterpene in the slow step.

Full text of DOI:10.1002/kin.21168

Polyethylene Glycols as
Efficient Catalysts for the
Oxidation of Bicyclic
Monoterpenes by Ceric
Ammonium Nitrate in
Acetonitrile under
Acid-Free Conditions:
Kinetic and Mechanistic
Approach
N. RAJITHA, S. SHYLAJA, K. C. RAJANNA, B. YADAGIRI
Department of Chemistry, Osmania University, Hyderabad, 500 007, India
Received 15 October 2017; revised 26 January 2018; accepted 25 February 2018
DOI 10.1002/kin.21168
Published online 26 March 2018 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Polyethylene glycols (PEG) acts as efficient catalysts for the oxidation of bicyclic
monoterpenes such as borneol, isoborneol, and camphor by ceric ammonium nitrate (CAN),
a laboratory desktop reagent, in acetonitrile medium under mineral acid-free conditions. The
kinetics of the reactions revealed first-order dependence on in both [CAN] and [bicyclic ter-
pene]. The rate of oxidation is accelerated with an increase in [PEG] linearly, which could be
explained by considering PEG-bound oxidant (PEG-CAN) as more reactive species than (CAN)
itself. The mechanism of oxidation in PEG media has been explained through theꢀCparticipation
of PEG-bound oxidant (PEG-CAN) and bicyclic monoterpene in the slow step.
Periodicals, Inc. Int J Chem Kinet 50: 383–396, 2018
2018 Wiley
Correspondence to: K. C. Rajanna; e-mail: kcrajannaou@
yahoo.com.
ꢀC
2018 Wiley Periodicals, Inc.

384
RAJITHA ET AL.
INTRODUCTION
EXPERIMENTAL
The camphor is a bicycle monoterpenoid ketone, which
is well known for its uses in medicine and as a plasti-
cizer for nitrocellulose. For several years, camphor was
extracted mainly from the Formosan camphor tree, but
it is now synthesized on a large scale from α-pinene
through isobornyl ethanoate and isoborneol interme-
diates. Isobornyl ethanoate upon hydrolysis affords
isoborneol, which upon oxidation gives camphor. On
the other hand, oxidation of borneol also gives cam-
phor [1,2]. Camphor upon oxidation is known to form
camphoronic acid [3]. During the past few decades,
single-electron transfer (SET) oxidation has received
considerable attention as a means to promote bond-
forming reactions in organic synthesis [2–4]. Oxida-
tion of an organic substrate by a SET reagent gen-
erally proceeds through a radical or radical–ion fol-
lowed by the reduction of oxidizing reagent [5–9].
More so, cerium(IV) ammonium nitrate (CAN) has
recently emerged as a versatile reagent for oxidative
electron transfer; the overwhelming number of reports
serve as a testimony to the unparalleled utility of CAN
in a variety of transformations of synthetic impor-
tance [10–19]. Polyethylene glycol (PEG) is a poly-
oxyethylene, which exhibits a broad spectrum of appli-
cations from industrial manufacturing to medicine. It
is a condensation polymer of ethylene oxide and water
(polyethylene oxide (PEO)) with the general formula
[H(OCH2CH2)nOH], where n is the average number of
repeating oxyethylene groups typically from 4 to about
180. It is a neutral, less expensive, and easily available
hydrophilic polyether. The structure of PEG is shown
below:
All chemicals used were of analytical grade. Doubly
distilled water (distilled over alkaline KMnO4 and acid
dichromate in a glass apparatus) was used whenever
required. Acetonitrile and other solvents were HPLC
grade and used as such throughout the work. CAMP
was procured from Aldrich (India). CAN, polyethy-
lene glycols (PEGs: PEG–200, PEG–300, PEG–400,
and PEG–600) were purchased from Avra or SD-fine
chemicals (Mumbai, India).
Kinetic Method
For a typical first-order reaction, a flask containing
centimolar (0.01 mol/dm3) CAN prepared in acetoni-
trile (MeCN) solvent was clamped in a thermostat at a
desired temperature. In another stoppered cylindrical
tube, BCT (2.0 mL of 0.10 mol/dm3), suitable amount
of PEG (0.50–5.0 mL depending on the reaction), and
the remaining amount was adjusted to 10.0 mL with
acetonitrile (MeCN). This tube was also clamped in
the thermostatic bath for about few minutes until it
attained desired temperature. Reaction was initiated
by transferring 2.0 mL of CAN to the other contents
present in the cylindrical tube. The entire reaction mix-
ture was mixed thoroughly. Aliquots of the reaction
mixture were withdrawn into a cuvette and placed in
the cell compartment of the laboratory visible spec-
trophotometer, which was provided with an inlet and
an outlet for circulation of thermostatic liquid at a de-
sired temperature. Absorbance (A) values of the CAN
content were recorded at 400 nm at different time inter-
vals. Absorbance values were in agreement with each
other with an accuracy of 3% error. However, for a
second-order reaction, equimolar (0.01 mol/dm3) CAN
and BCT were used.
In recent past, PEG has been used as catalysts, cata-
lyst supports, and also have been found as inexpensive,
nonionic, nontoxic, environmentally friendly reaction
medium, which avoids the use of acid or base cata-
lysts [20–23].
A perusal of literature shows that the kinetics of ox-
idation of bicyclic terpenoids (BCT), such as borneol
(BORN), isoborneol (IBORN), and camphor (CAMP)
by CAN have not been attempted so far. Inspired by
this aspect coupled with the striking features of PEG
as a green acid-free catalyst/reaction medium, we have
used the PEG to assist the present study in acetonitrile
medium. It is of interest to note that native CAN oxida-
tion of BCT compounds in a pure acetonitrile medium
did not proceed smoothly and gave reproducible re-
sults. This study does not require Bronsted acid to
onset the reaction.
RESULTS AND DISCUSSION
The kinetic study was taken-up in acetonitrile medium
under acid-free conditions. BCT–CAN reaction kinet-
ics could not be followed smoothly in the absence of
PEG. An irregular trend was obtained. However, the re-
action was smooth, when we conducted the reactions
in PEG–MeCN media.
Determination of the Order of Reaction
and Salient Kinetic Features
1. Reactions were conducted under two different
conditions. Under the conditions
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385
Figure 1 First-order plot of [CAN] in the PEG-mediated
CAN-camphor reaction at 310 K [CAN] = 1.67 ×
10−3 mol/dm3; [PEG-200] = 0.50 mol/dm3; [Camphor] =
1.67 × 10−2 mol/dm3. [Color figure can be viewed at wiley-
onlinelibrary.com]
Figure 3 First-order plot of [CAN] in the PEG-mediated
CAN–BORN reaction at 305
K
[CAN]
=
1.67
×
10−3 mol/dm3; [PEG-400] = 0.25 mol/dm3; [BORN] =
1.67 × 10−2 mol/dm3. [Color figure can be viewed at wiley-
onlinelibrary.com]
Figure
4
Second-order plot of the PEG-mediated
CAN–Camphor reaction at 320 K [CAN] = 1.67 ×
10−3 mol/dm3; [PEG-200] = 0.5 mol/dm3; [Camphor] =
1.67 × 10−3 mol/dm3. [Color figure can be viewed at wiley-
onlinelibrary.com]
Figure 2 First-order plot of [CAN] in the PEG-mediated
CAN–IBORN reaction at 315 K [CAN] = 1.67 ×
10−3 mol/dm3; [PEG-300] = 0.30 mol/dm3; [IBORN] =
1.67 × 10−2 mol/dm3. [Color figure can be viewed at wiley-
onlinelibrary.com]
reactions. The addition of small amounts of PEG
to CAN in acetonitrile medium increases the in-
tensity of CAN color, indicating a hyperchromic
shift. An increase in the [PEG] increased the re-
action rates depending on the nature of PEG. By
and large, reaction rates were found high in PEG-
300 media over other PEGs in increasing order:
PEG-300 > PEG-400 ࣙ PEG-600 > PEG-200.
4. The addition of DPPH (2,2-diphenyl-1-
picrylhydrazyl) to the reaction mixture de-
creased the reaction rate substantially, because it
is an efficient radical trap [24]. The DPPH has a
deep violet color in solution and showed a strong
absorption band at about 520 nm, became col-
orless by trapping substrate-free radical formed
during the course of the reaction [25].
[BCT] ꢀ [CAN], plots of ln (At) versus time
were straight lines with a negative slope, indi-
cating first order with respect to [CAN] to be
unity (Figs. 1–3). First rate constants (kꢁ) are
obtained from slopes of these plots.
2. This reaction is also conducted under second-
order conditions with equal concentrations of
[BCT]0 = [CAN]0. Kinetic plots of [1/(At)]
versus. time have been found to be linear with
a positive gradient and definite intercept on or-
dinate (vertical axis), indicating overall second-
order kinetics (Figs. 4–6). Since the order with
respect [CAN] is already verified as one under
pseudoconditions, this observation suggests that
order in [BCT] is also one.
3. CAN oxidation of BCT compounds in pure ace-
tonitrile medium was generally fast, but kinetics
could not be followed smoothly and obtained re-
sults were not reproducible. Therefore, we have
taken up kinetic studies only in PEG-mediated
Computation of Activation Parameters
Kinetic studies have been taken up at various temper-
atures in the temperature range 300–325 K. The free
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RAJITHA ET AL.
Figure 5 Second-order plot of the PEG-mediated CAN–
IBORN reaction at 320 K [CAN] = 1.67 × 10−3 mol/dm3;
[PEG-200] = 0.5 mol/dm3; [IBORN] = 1.67 × 10−3
mol/dm3. [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 7 ꢀG# versus temperature graph Isoborneol-PEG-
300 (4.0 mL). [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 8 ꢀG# versus temperature graph isoborneol-PEG-
200 (2.0 mL). [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 6 Second-order plot of the PEG-mediated CAN–
IBORN reaction at 310 K [CAN] = 1.67 × 10−3 mol/dm3;
[PEG-600] = 0.17 mol/dm3; [IBORN] = 1.67 × 10−3
mol/dm3. [Color figure can be viewed at wileyonlineli-
brary.com]
energy of activation (ꢀG#) at various temperatures is
calculated using Eyring’s equation according to the
well-known theory of reaction rates [25,26]
ꢀG# = RT ln (RT/Nhk)
Figure 9 ꢀG# versus temperature graph borneol-PEG-600
(3.0 mL). [Color figure can be viewed at wileyonlineli-
brary.com]
Substituting the values for rate constant (k), R
(8.314 J mol−1 K−1), N (6.022 × 1023 mol−1), h (6.626
× 10−34 J s), and T (Kelvins), respectively, free energy
of activation (ꢀG#) can be obtained. Enthalpy and en-
tropies of activation (ꢀH# and ꢀS#) were obtained
from (ꢀG#) versus (T) plots (Figs. 7–9), according to
the Gibbs–Helmholtz equation [25,26]:
Reactive Species and Mechanism of
Oxidation of Bicyclic Terpenes by CAN
In aqueous nitric acid CAN is known to form dif-
2−
ferent types of reactive species such as Ce(NO3)6
,
Ce(NO3)5−, Ce(OH)(NO3)4−, Ce(NO3)4, [Ce(NO3)4
(H2O)2], and Ce(OH)3+ as reported by earlier work-
ers [10–20]. However, our aim is to study the reac-
tions under acid-free conditions. Therefore, we stud-
ied the kinetics of the reactions in moderately a green
acetonitrile (MeCN) solvent. Therefore, the reactive
ꢀG# = ꢀH # − TꢀS#
The data related to activation parameters are com-
piled in Tables I–IV.
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RAJITHA ET AL.
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OXIDATION OF BICYCLIC MONOTERPENES BY CERIC AMMONIUM NITRATE IN ACETONITRILE
389
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RAJITHA ET AL.
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OXIDATION OF BICYCLIC MONOTERPENES BY CERIC AMMONIUM NITRATE IN ACETONITRILE
391
could be entirely different in the MeCN medium. Since
MeCN is very large excess over [CAN], MeCN may
penetrate into the coordination spheres of Ce(IV) and
form solvated CAN species with replacement of water
molecules according to the following equilibrium:
Ce(NO3)4 (H2O2) + 2(CH3CN)2
(CAN)
ꢀ
ꢁ
ꢀ Ce(NO3)4 (CH3CN2) +2H2O
[Solvated CAN]
Figure 10 Benesi–Hildebrand plot for PEG-200. [Color
figure can be viewed at wileyonlinelibrary.com]
When bicyclic terpene is added to the reaction
mixture, kinetics could not be followed smoothly by
(solvated CAN), and an irregular trend was obtained.
However, in polyethylene glycol–acetonitrile media,
the kinetic studies were smooth, and the reaction is
enhanced remarkably in all PEGs A set of polyoxy
ethylene glycols (PEGs) with varied molecular weights
ranging from 200 to 600. Reaction times were reduced
from 24 h to few hours. The catalytic activity was
found to increase in the order: PEG-300 > PEG-400
ࣙ PEG-600 > PEG-200. Furthermore, it is also in-
teresting to note that the absorbance (A) of solvated
Ce(IV) species is increased when PEG is added to
the CAN solution. This observation may indicate that
solvated –CAN species could bind with PEG to form
PEG-supported CAN species according to the follow-
ing equilibrium:
Figure 11 Benesi–Hildebrand plot for PEG-300. [Color
figure can be viewed at wileyonlinelibrary.com]
For the PEG-bound CAN equilibrium, A = CAN,
therefore, [A]0 = [CAN]0 and [D]0 = PEG and in this
case the above equation reduces to
ꢀ
ꢁ
H − (OCH2 − CH2)n −OH + Ce(NO3)4(CH3CN)2
(PEG)
(CAN)
ꢀ H − (OCH2 − CH2)n − O (H) − Ce(NO3)4(CH3CN)2]
([CAN]0/d) = (1/K[PEG]0ꢁ) + (1/ꢁ)
[PEG−CAN]
However, the absorbance of CAN and PEG-bound
CAN adduct ([PEG-CAN]) absorb in the same region
significantly; therefore, the observed absorbance (d)
could be written as
Spectrophotometric Determination of
Binding Constants (K) by the
Benesi–Hildebrand Method
The PEG–CAN interaction studies are established,
and binding constants are evaluated using the Benesi-
Hildebrand method [28–30]. The equilibrium for the
molecular complexation is generally written as in the
following equation when donor (D) is assumed to in-
teract with an acceptor (A):
d = dCAN + d[PEG−CAN] ⇒ d[PEG−CAN]
= (d dCAN) = ꢀA
∼
Thus, a plot of ([CAN]0/ꢀA) versus (1/[PEG]) af-
forded a straight line according to the above equa-
tion. Representative Benesi-Hildebrand plots are given
in Figs. 10–13. PEG-CAN binding constants (K) and
corresponding thermodynamic parameters are given
Table V. Free energy of complexation (binding con-
stant) are obtained from van’t Hoff’s isotherm, whereas
enthalpy and entropy of complexation were calculated
from Gibbs–Helmholtz plots as shown in Figs. 14 and
15.
K
D + A ꢀ (D − A) adduct
The equilibrium constant K = [C]/[A] [D], where
[A], [D], and [C] are the equilibrium concentrations of
acceptor, donor, and complex, respectively. If the initial
concentrations of A and D are [A]0 and [D]0, then K =
[C]/([A]0 – [C]) ([D]0 – [C]). Under the condition that
[D]0 ꢀ [A]0, the Benesi-Hildebrand equation becomes
Observed kinetic features together with the posi-
tive test for the formation free radical intermediates
([A]0/d) = (1/K[D]0ꢁ) + (1/ꢁ)
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392
RAJITHA ET AL.
Figure 14 Gibbs–Helmholtz plot for CAN-PEG-300 in-
teractions. [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 12 Benesi–Hildebrand plot for PEG-400. [Color
figure can be viewed at wileyonlinelibrary.com]
Figure 15 Gibbs–Helmholtz plot for CAN-PEG-600 in-
teractions. [Color figure can be viewed at wileyonlineli-
brary.com]
Figure 13 Benesi–Hildebrand plot for PEG-600. [Color
figure can be viewed at wileyonlinelibrary.com]
substantiate the most plausible mechanism as shown
in Scheme 1. The mechanism of the reaction
shows the interaction of PEG-bound Ce(IV) form-
ing a transient complex [PEG-Ce(IV)nitrtate], i.e.,
[H-(OCH2-CH2)n-O(H)-Ce(NO3)4(CH3CN)2], which
decomposes to give substrate radical-ion (bor-
neol radical-ion or isoborneol radical-ion) followed
by the reduction of Ce(IV) species to Ce(III)
species ([PEG-Ce(III)nitrtate], i.e., [H-(OCH2-CH2)n-
O-Ce(NO3)4(CH3CN)2]–, through SET. Substrate
radical-ion upon thus formed is then oxidized by an-
other Ce(IV) species ([PEG-Ce(IV)nitrtate]) to afford
camphor. On the other hand, the kinetic features to-
gether with the detection of free radical intermediates
(as evidenced through the conversion of acrylamide
monomer to polyacrylamide (polymer), the most plau-
sible mechanism for the oxidation of camphor could
be given as in Scheme 2.
Table V PEG–CAN Binding Constants (K) and Corresponding Thermodynamic Parameters
PEG
Temperature (K) K (dm3 /mol) ꢀε (dm3/mol.cm) −ꢀG (k J/mol) ꢀH (k J/mol) ꢀS (J/K/mol)
R2
PEG-200
300
305
310
315
3500
2250
1833
1333
1428
1111
909
20.4
19.6
19.3
18.9
48.5
94.2
0.954
833
PEG-300
PEG-400
PEG-600
300
305
315
7000
3000
1333
1428
1111
833
21.3
20.3
18.9
68.5
116
159
311
70.7
0.995
0.972
0.967
300
305
310
7000
5500
4000
1428
909
833
21.3
20.3
18.9
300
305
310
9000
5000
2000
1111
1000
1250
22.7
21.6
19.6
43.3
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393
Scheme 1 PEG-mediated CAN oxidation of borneol and isoborneol.
The mechanism of the reaction as shown in
Scheme 2 is largely similar to Scheme 1, wherein cam-
phor upon oxidation gives camphoric acid through the
formation of the camphor radical. The general mecha-
nism and rate law for the above schemes are shown in
the following sequential steps:
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394
RAJITHA ET AL.
Scheme 2 PEG-mediated CAN oxidation of camphor.
Substitution of [CAN] in Eq. (2) gives
[PEG − CAN]
+ [PEG − CAN]
K [PEG]
CCAN
=
=
[PEG − CAN] + K [PEG] [PEG − CAN]
K [PEG]
For the above mechanisms, the rate can be written
as
K [PEG] CCAN
1 + K [PEG]
⇒ [PEG − CAN] =
(−d [CAN]/dt) = k [PEG − CAN] [BCT] (1)
Considering the total concentration of (CCAN) as the
Substituting for [PEG-CAN] in Eq. (1)
algebraic sum of free CAN-species and PEG-bound
substrate complex (PEG-CAN) species,
−d [CAN]
dt
kK [PEG] [BCT] [CAN]
1 + K [PEG]
=
(3)
CCAN = [CAN] + [PEG − CAN]
(2)
This rate-law explains first-order dependence on
[CAN], [BCT], and complex order in [PEG]. At con-
stant [PEG], the rate law becomes
From PEG-CAN binding equilibrium,
K = [PEG − CAN] / [PEG] [CAN]
or [CAN] = [PEG − CAN] /K [PEG]
−d [CAN] /dt = kꢁꢁ [CAN] [BCT]
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Binding constants for [PEG-CAN] have been cal-
culated spectrophotometrically as detailed in earlier
sections. Thermodynamic data presented in Table I
show that (PEG-CAN) formation constants (K) are
far greater than unity (very higher magnitudes). The
corresponding free energy values (ꢀG) are nega-
tive, indicating spontaneous nature of the equilibrium
(PEG-CAN) adducts. A further insight into the ac-
tivation parameters presented in Tables I–IV shows
that entropy of activation (ꢀS#) is negative. Nega-
tive entropy of activation (ꢀS#) reorganization of the
transition state and shows the associative mechanism,
whereas positive values for ꢀS# often indicate a disso-
ciative mechanism and suggest that entropy increases
upon achieving the transition state. Observed negative
ꢀS# values in this study indicate that entropy decreases
upon achieving the transition state, depicting on the as-
sociative mechanism [27–29]. These results probably
support the release of proton from PEG and nitrate from
CAN moieties, which readily bring about changes in
the transition state and cause simultaneous association
and dissociation of species causing a greater disor-
derness in the transition state leading to a chemical
reaction. A similar type of trends is recorded in all the
PEGs used in this study.
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In summary, the authors studied oxidation of bicyclic
monoterpenes such as BORN, IBORN, and CAMP in
aqueous acetonitrile medium using a common labora-
tory desktop reagent CAN in catalytic amounts under
mineral acid-free conditions. The reaction underwent
smoothly in the presence of PEG, and followed first
order in both [CAN] and [Bicyclic terpene]. The rate
of oxidation is accelerated with an increase in [PEG]
linearly. The mechanism of oxidation in PEG media
has been explained through the participation of PEG-
bound oxidant (PEG-CAN) and terpene, and PEG-
bound oxidant (PEG-CAN) as more reactive species
than (CAN) itself.
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The authors thank the Principal, Nizam College (Osmania
University) Hyderabad, and the Head, Department of Chem-
istry (Osmania University) for encouragement and facilities.
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