Cr(VI) Oxidation of Cholesterol—A Kinetic Study Using N-Cetylpicolinium Dichromates, A Class of Novel Phase Transfer Oxidants

Article abstract of DOI:10.1134/S0023158419020083

Abstract: Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichromates (CPDC), a class of phase transfer oxidants, in acetic acid medium under first order conditions with respect to oxidant. Rate constants were calculated in the temperature range 290–300 K. The kinetics was followed spectrophotometrically; cholest-5-en-3-one is found to be the only oxidation product. Unlike the previously reported lipopathic oxidant containing cetyltrimethylammonium ions, these oxidants show a direct variation of rate with the oxidant concentration ruling out any reversed micellar organization of the oxidant molecules. From the experimental data formation of an unstable cyclic transition state followed by intra-molecular proton transfer has been proposed. Solvent isotope effect for the cholesterol oxidation (Formula Presented.) indicated a carbon-hydrogen cleavage rather than a carbon-carbon cleavage. Variation of solvent polarity is found to impose a remarkable impact on the rate of oxidation: more polar reaction environment favours the oxidation by β-CPDC oxidant to a higher extent, compared to the other two oxidant isomers, α-CPDC and γ-CPDC.

Full text of DOI:10.1134/S0023158419020083

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 2, pp. 147–154. © Pleiades Publishing, Ltd., 2019.
Cr(VI) Oxidation of Cholesterol—A Kinetic Study Using
N-Cetylpicolinium Dichromates, A Class of Novel Phase
Transfer Oxidants
a
a
a
a,
S. Priyadarshini , P. S. Guru , R. DS , and S. Dash *
a
Department of Chemistry, Veer Surendra Sai University of Technology, Burla, 768018 India
*e-mail: sukalyan_dash@yahoo.com
Received January 5, 2018; revised June 1, 2018; accepted November 1, 2018
Abstract—Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichro-
mates (CPDC), a class of phase transfer oxidants, in acetic acid medium under first order conditions with
respect to oxidant. Rate constants were calculated in the temperature range 290–300 K. The kinetics was fol-
lowed spectrophotometrically; cholest-5-en-3-one is found to be the only oxidation product. Unlike the pre-
viously reported lipopathic oxidant containing cetyltrimethylammonium ions, these oxidants show a direct
variation of rate with the oxidant concentration ruling out any reversed micellar organization of the oxidant
molecules. From the experimental data formation of an unstable cyclic transition state followed by intra-mo-
lecular proton transfer has been proposed. Solvent isotope effect for the cholesterol oxidation
(
kCHCl3 kCDCCl3 = 1.52) indicated a carbon-hydrogen cleavage rather than a carbon-carbon cleavage. Varia-
tion of solvent polarity is found to impose a remarkable impact on the rate of oxidation: more polar reaction
environment favours the oxidation by β-CPDC oxidant to a higher extent, compared to the other two oxidant
isomers, α-CPDC and γ-CPDC.
Keywords: N-cetylpicolinium dichromates (CPDC), cholesterol, phase transfer oxidation, solvent polarity,
solvent isotope effect
DOI: 10.1134/S0023158419020083
INTRODUCTION
(CTACN) [21, 22]. Besides, we have also examined the
solubility as well as oxidizing behaviour of CTAP in aque-
ous medium assisted by β-cyclodextrine [23–25].
Cr(VI) is a versatile oxidant for many types of
organic or inorganic substrates and has been proved to
be versatile reagents capable of oxidizing almost every
oxidizable functional group [1]. Numerous number of
chromium(VI) containing compounds such as pyri-
dinium chlorochromate [2], quinolinium fluorochro-
mate [3], quinolinium bromochromate [4], quinolin-
ium dichromate [5], pyridinium fluorochromate [6],
imidazolium fluorochromate [7], isoquinolinium bro-
mochromate [8], triphenylmethylposphonium chlo-
rochromate [9], tributylammonium chlorochromate
Recently, we synthesized some N-cetylpicolinium
dichromate (CPDC) compounds and have studied
their aqueous solubility using β-cyclodextrine [26].
However, we observed that the oxidant becomes too
mild in the β-cyclodextrine assisted aqueous medium
to study the kinetics of oxidation of various substrates.
In the present work we have reported the oxidation
kinetics of cholesterol in organic solvent media using
the CPDC oxidants.
[
10], prolinium chlorochromate [11], tripropylammo-
nium fluorochromate [12], tetraethyl ammonium bro-
Earlier, cholesterol oxidation using chromium
mochromate [13], triethylammonium chlorochromate based oxidants has been reported by various scientists
[
14], and tetraethylammonium chlorochromate [15] [27–30]. In organic synthesis, cholesterol has been
have been used to study the kinetics and mechanism of oxidized by pyridinium dichromate or pyridinium
various organic compounds. The process of phase- chlorochromate [31–33] producing cholest-4-en-3,6-
transfer catalysis is a method introduced in the last dione that has a maximum absorption of 250 nm and
4
century [16] to utilize the possibility of extracting molar extinction coefficient of about 1.3 × 10 . How-
anions or cations into an organic layer. In our efforts in ever, the reactions require a time period of 6–24 h for
exploring some phase transferring oxidants to oxidize completion. Jones reagent has been utilized to oxi-
organic substrates in organic solvents, we have earlier dize cholesterol to the above product, however,
reported the oxidation behaviour of cetyltrimethylam- within a less time period compared to above reactions
monium permanganate (CTAP) [17–20] and cerate [34, 35]. Using cetyltrimethylamminium dichromate
147

1
48
PRIYADARSHINI et al.
8
7
6
5
4
3
2
0
0
0
0
0
0
0
Kinetic Measurements
Solutions having appropriate concentrations of the
CPDC, cholesterol and acetic acid were transferred to
the sample cuvette of a pair of matched quartz cells to
measure the absorbance (λ ) by a thermostated
max
Hitachi U-3010 double beam UV-vis spectrophotom-
eter at a wavelength (λ ) of 354 nm (for α- and
max
10
γ-CPDC) or 358 nm (for β-CPDC). Measurements
are done at three different temperatures [38]: 17 ±
0
1000
2000
Wavenumber, cm
3000
4000
5000
1
°C, 22 ± 1°C and 27 ± 1°C for 90 min in each case
–1
with 5 min time interval and the first-order rate con-
stants (k ) were calculated using the Eq. (1).
obs
Fig. 1. IR specrum of oxidation product of cholesterol
cholest-5-en-3-one).
(
2
.303
t
 A
0

kobs
=
log
,
(1)
(
)


At 

1
900
700
500
300
100
where A is the optical absorption at an initial time,
1
1
1
1
0
A is the optical absorption at final time. The results
t
reported are an average of three runs with an error of
±
6.0ꢀ.
9
7
5
3
00
00
00
00
Second-order rate constants (k ) were calculated by
s
dividing the first-order rate constants with the sub-
strate concentration, using the Eq. (2) [39]
kobs
100
ks =
.
(2)
–
100
[substrate]
1
4
12 10
8
6
4
2
0
–2 –4
≠
≠
Enthalpy (ΔH ), entropy (ΔS ) and free energy
δ, ppm
≠
(
ΔG ) of activation have been calculated using the
well-known Arrhenius and Eyring equations.
1
Fig. 2. H NMR spectrum of oxidation product of choles-
terol (cholest-5-en-3-one).
Product Analysis
(
CTADC) oxidant in organic media, cholesterol
Chol) was oxidized where the reaction has been pro-
The product analysis was performed under kinetic
(
conditions. The reaction mixture of α-CPDC
posed to be occurring in reversed micellar system, akin
to an enzymatic environment, since CTADC forms
reversed micellar arrangement in organic solvent
media [36].
The CPDC oxidants used in the present case, on
the contrary, do not undergo any reversed micelliza-
tion phenomena in organic media and carry out a mild
oxidation of cholesterol in a measurable time period
producing cholest-5-en-3-one.
(
0.1 mol), cholesterol (0.2 mol), and acetic acid
AcOH) (0.5 mol) were made up to 100 ml in DCM
(
was kept for 24 h followed by the reduction of the vol-
ume of the reaction mixture to 20ꢀ under vacuum.
The mixture was kept in refrigerator for 24 h. A white
crystal separating out from the solution was recovered
and recrystallized from absolute ethanol. The IR and
NMR spectral analyses and the melting point of the
product agreed well with those of cholest-5-en-3-one.
IR spectral data of important peaks at various frequen-
–1
EXPERIMENTAL
cies (cm ): 1718 (unconjugated ketone), 1468 (alkene
1
‒
(
1
C=C– str.) (Fig. 1). H NMR (CDCl ), δ, ppm: 0.52
Materials and Methods
3
CH , d), 0.67 (CH , d), 0.89 (CH , dd), 1.05 (CH , m),
3 3 3 3
α-, β-, and γ-CPDC oxidants were synthesized as
reported earlier [37]. Dichloromethane (DCM), chlo-
roform, 1,4-dioxane, toluene, benzene, chloroben-
zene, and ethyl acetate (Merck, India) have been dis-
tilled just before use. Cholesterol (Spectrochem) was
recrystallized from ethanol.
.25 (CH , s), 1.33 (CH , d), 1.49 (cyclopentane CH,
3 2
m), 1.61 (methane CH, s), 1.76 (cyclopentane CH2,
d), 1.99 (cyclohexene, m), 2.29 (α-CH to C=O, m),
2
3
.52 (β-CH to C=C, s), 5.36 (cyclohexene H, s)
2
(Fig. 2). M. p. = 120°C (reported value 127°C).
KINETICS AND CATALYSIS
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No. 2
2019

Cr(VI) OXIDATION OF CHOLESTEROL
149
Table 1. Dependence of observed first- and second-order rate constants of oxidation of cholesterol by CPDC in the pres-
ence of acid at 295 K in DCM medium
CPDC isomer
[
CPDC] × 105 [Chol] × 103
α
β
γ
5
–1
–1 –1
5
–1
–1 -1
5
–1
–1 –1
mol/L
kobs × 10 , s k , L mol s kobs × 10 , s k , L mol s kobs × 10 , s k , L mol s
s
s
s
4
5
6
.0
.0
.0
0.10
0.10
0.10
0.10
0.10
0.10
0.06
0.14
0.18
0.22
0.10
0.10
0.10
0.39
0.41
0.45
0.49
0.52
0.58
0.31
0.62
0.86
1.20
0.10
0.34
0.79
0.039
0.041
0.045
0.049
0.052
0.058
0.052
0.044
0.046
0.054
0.010
0.034
0.079
0.83
1.04
1.21
1.40
1.53
1.65
1.01
1.66
1.91
2.54
0.37
0.78
1.72
0.083
0.104
0.121
0.140
0.153
0.165
0.168
0.118
0.106
0.115
0.037
0.078
0.172
0.51
0.62
0.86
1.15
1.34
1.52
0.56
1.17
0.051
0.062
0.086
0.115
0.134
0.152
0.093
0.083
0.087
0.092
0.010
0.045
0.130
7.0
8.0
9.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
1.57
2.02
0.10
0.45
1.30
RESULTS AND DISCUSSION
Kinetic Analysis
The oxidation of cholesterol by N-cetylpicolinium
dichromates (CPDC) is studied spectrophotometri-
cally in dichloromethane medium at three tempera-
tures in the range 290–300 K. Under experimental
conditions, the rate of disappearance of Cr(VI) species
has been monitored at 354 nm. First-order rate con-
Stoichiometric Analysis
Stoichiometric experiments were carried out by
conducting the reactions at 298 K under the condi-
tions of [CPDC] @ [Chol] at constant acid concentra-
tions of 5 M. After 1 h, the concentration of residual
Cr(VI) was estimated by monitoring the absorbance at
stants k were calculated for the variation of oxidant
obs
and substrate concentrations at each temperature
3
54 nm. Limiting logarithmic method has been (Table 1).
adopted to calculate the stoichiometric ratio of Chol
and CPDC [40]. In this technique, the experiments
Rate constants were calculated after varying the
concentrations of oxidant and substrate in presence of
were done in two different ways. In the first set of acetic acid (Table 1). The reaction process is found to
experiment, the Chol concentration was varied (0.6 × be first order with respect to the oxidant and the sub-
–
5
–5
strate. No reaction occurs in the absence of AcOH,
signifying the catalytic action of acetic acid. Hence,
direct oxidation of cholesterol by CPDC leads to the
presumption of a mechanism involving the acid-cata-
10
(
to 1.4 × 10 M) at a fixed CPDC concentration
–4
6.0 × 10 M) and in the second set of experiment the
CPDC concentration was varied (5.0 × 10 to 7.0 ×
–4
–4
–5
1
0 M) at a fixed Chol concentration (1.0 × 10 M). lysed formation of an ester intermediate. In the mech-
anistic assay showed below, the protonated CPDC
species is assumed to be the only oxidising agent pres-
ent in the reaction process.
The absorbance of Cr(VI) at 354 nm were recorded
after 1 h of addition of reactants. The logarithms of the
obtained absorbance were plotted against log [Chol]
and log [CPDC] in the first and the second set of
experiments, respectively. The slopes of the fitting line
in both sets of experiments were calculated and found
to be 0.176 and 1.263, respectively. Thus, the molar
reactivity of the reaction is 0.176/1.263, i.e. the reac-
tion proceeds in the ratio of around 0.14:1 with respect
to Chol and CPDC.
Second-order rate constant k was calculated exper-
s
imentally due to the use of larger excess of substrate over
oxidant in the kinetic runs using Eq. (3), where [Chol]
0
stands for the initially fixed common concentration of
each cholesterol in all kinetic runs [41]
k
obs
ks =
.
(3)
[Chol]0
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

1
50
PRIYADARSHINI et al.
0.30
(a)
(b)
0
0
0
0
0
.18
.16
.14
.12
.10
α-CPDC
β-CPDC
γ-CPDC
0
.25
0.20
.15
0.10
α-CPDC
β-CPDC
γ-CPDC
0
0.08
0
.06
.04
0
0.05
0.02
0
0
4
6
8
10
0.05
0.10
0.15
0.20
0.25
5
3
[
CPDC] × 10 , M
[Chol] × 10 , M
–
3
Fig. 3. Plots of k vs. [CPDC] of different isomers at a constant cholesterol concentration (0.1 × 10 M) (a) and kobs vs. [Chol]
obs
–5
at a constant concentration of CPDC different isomers (6 × 10 M) (b). T = 295 K, DCM medium.
Moreover, the rate constant values increase with 0.12 × 10–5 L mol s ) and (a = 0.21 × 10 s , b =
substrate as well as with oxidant concentrations
Figs. 3a, 3b), in contrast to the previous reports,
–1
–1
–5 –1
–5
–1 –1
0
.17 × 10 L mol s ), respectively. CPDC may
(
become protonated in the presence of acetic acid
which may function as an efficient oxidant.
where the increase in the concentration of oxidant
resulted in the decrease in the rate constant values in
all the amphipathic oxidants such as CTAP [17–20],
CTADC [36] and CTACN [21, 22] due to their reverse
micellar organization in non-aqueous media. The
reverse micellar aggregation in case of CPDC is ruled
out due to the presence of a rigid and bulkier picolin-
ium moiety. Steric crowding inhibits the CPDC mol-
ecules to assemble in reverse micellar organization in
contrast to the other quaternary ammonium ions.
The mechanism of oxidation reaction, from the
above findings, has been proposed through reactions
I)–(III). The observed hydrogen ion dependence can
be explained by assuming a rapid reversible protona-
tion of CPDC molecule in the acid medium followed
by the transfer of α-proton in the dichromate ester,
leading to the formation of cholest-5-en-3-one and
unstable inorganic by-product Cr(IV) species in rate
determining step.
(
Effect of Varying Acid
The reaction was catalysed by hydrogen ions. The
hydrogen ion dependence has the form, k = a +
+
k1
+
⎯ ⎯ ⎯→
CPDC + H
CPDCH
(I)
←
⎯ ⎯ ⎯
k−1
Protonated
oxidant
obs
+
b[H ] [42]. The values for a and b at 295 K for α-, β-
–
5
–1
and γ-CPDC were found to be (a = 0.21 × 10 s ,
+
k2
CPDCH + Cholesterol ⎯ ⎯ ⎯→ Complex (II)
b = 0.17 × 10–5 L mol s ), (a = 0.22 × 10 s , b =
–1
–1
–5 –1
Cr(VI) ester
k3
Complex ⎯ ⎯ ⎯→ Cholest-5-en-3-one + Cr(IV)
(III)
(4)
Oxidised
product
Reduced
product
Cr(IV) (obtained as the inorganic by-product) dis-
proportionates to Cr(III) (reactions (IV) and (V)), the
existence of which has been confirmed from the
absorption at 580 nm [43].
+
k k [CPDC][Cholesterol][H]
1
2
Rate =
.
k + k [Cholesterol]
−1
2
If k–1 is not neglected in the Eq. (4), the rate
depends upon the concentrations of oxidant and sub-
strate, which is actually the case as observed in the
Cr(IV) + Cr(VI) → 2Cr(V),
Cr(V) → Cr(III) – 2e.
(IV)
(V)
From the reactions (I) and (III), the complete rate previous discussions. Hence the detailed mechanism
law for overall oxidation reaction is given by Eq. (4):
is proposed in the Scheme 1.
KINETICS AND CATALYSIS
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2019

Cr(VI) OXIDATION OF CHOLESTEROL
151
+
−
+−
CP OO CrO
O
O
k
1
CP OO CrO
O
O
2
2
+ H+
+ CP+
Cr
Cr
Cr
k
−1
+
−
CP O
HO
CP O−
+
H
+−
+−
CP OO CrO
O
O
CP OO CrO
2
k
2
2
+
Cr
O
O
+
−
+−
CP O
CP O
H O
+
−
+−
CP O
H
k
3
CP OO CrO
OH
OH
2
+−
+
Cr
CP OO CrO
2
+
−
Cr
O
O
CP O
+
−
CP O
O
+
Effect of Oxidant Geometry
and α-isomers of CPDC oxidize with a lower rate of
about 12–45 and 52–73%, respectively, and with
respect of γ-CPDC, the α-isomer oxidizes with a
lower rate of about 23–61%. Hence out of all the three
isomers of CPDC oxidant, the β-isomer behaves as
the strongest oxidant.
The rates of the reactions show a remarkable varia-
tion with the variation of oxidant geometry (Table 1).
The role of the carrier (i.e. cetylpicolinium ion) is to
solubilise and stabilize the dichromate ion in organic
medium, where it remains in the form of ion pair [44].
The tightness of the ion pair in non-polar organic
medium gives stability to the dichromate ion. In the
light of the molecular structure of the CPDC moieties
in three dimensional space (α-, β-, and γ-CPDC),
each one differs from the other with respect to the
position of methyl substituent present in the picolin-
ium nucleus. Two cetylpicolinium ions counter bal-
anced by a divalent dichromate ion constitute a unit of
CPDC moiety.
Thermodynamics
For each of the oxidant the rate constant becomes
nearly double with 10°C rises in temperature. Ther-
modynamic studies suggest the oxidation processes to
be endothermic owing to the positive values of the
change in enthalpy of the reactions and the reactions
with each oxidant are followed by a decrease in the
In case of the α-CPDC molecule, the methyl sub- entropy values (Table 2).
stituent present at α-position might be diminishing
≠
≠
With β-CPDC oxidant, the ΔH and ΔS values
are the highest among all the CPDC oxidants, whereas
with α-CPDC these values are the lowest. On the
the flexibility of the cetyl chain due to its pushing
effect resulting in the maintenance of the tightness of
the ion pair. However, in addition, it sequesters the
dichromate ion and its access to the cholesterol mole-
cule becomes hindered, thereby decreasing the rate of
oxidation. On the other hand, in case of γ-isomer, the
position of the methyl substituent does not allow it to
sequester the dichromate ion, thereby leaving the cetyl
chain flexible and decreasing the tightness of the ion
pair, compared to the α-isomer. In case of the β-isomer
the molecular geometry is an intermediate between both
α- and γ-counterparts. The methyl group at β-position
helps to maintain the tightness of ion pair by its pushing
effect and also provides more room for the substrate to be
localized, facilitating increased oxidation compared to
the α and γ counterparts.
≠
other hand, a reverse trend in the values of ΔG at cor-
responding temperatures is observed. However, a large
≠
negative value of entropy of activation ΔS in each case
suggests slow bimolecular reaction in the rate deter-
mining steps, besides the rate determining transition
state being less disorderly than the reactants, i.e. a rigid
transition state by an associative process [45]. This
transition state complex, a chromate diester, is unsta-
ble and breaks to form the product.
≠
≠
2
The plot of ΔH vs. ΔS is found to be linear (R =
0
.999) with an isokinetic temperature 259.5 K (Fig. 4).
At this temperature, all the CPDC oxidants oxidize
cholesterol with a common mechanism. The excellent
As a result, there occurs a difference in the oxida- linear compensation of enthalpy and entropy changes
tion efficiency of the oxidants in the order: β-CPDC > predicts the existence of a simple mechanism of the
γ-CPDC > α-CPDC. With respect to β-CPDC, the γ- oxidation reactions.
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

1
52
PRIYADARSHINI et al.
Table 2. Temperature effect on the oxidation of cholesterol by CPDC*
–
1 –1
#
#
–1
–1
#
CPDC isomer
K , mol L s
T, °C
ΔH , kJ/mol
ΔS , J K mol
ΔG , kJ/mol
s
α
0.037
17
22
27
17
22
27
17
22
27
46.0
–162.4
93.1
93.9
94.7
91.6
92.2
92.8
92.1
92.8
93.5
0.045
0
.070
.058
0
β
γ
0.121
.130
.050
0.086
.104
58.7
53.1
–113.8
–134.5
0
0
0
*
Reaction conditions: [CPDC] = 6 × 10–5 M, [Chol] = 0.1 × 10–3 M, [AcOH] = 5 M, DCM medium.
Solvent Isotope Effect
[46, 47] and other systems [48–52] and on dielectric
relaxation [53]. The solvent isotope effect in the pres-
ent work thus envisages the importance of medium
during the oxidation process.
A noticeable decrease in the rate of the reaction is
observed when chloroform (CHCl ) (k = 0.068 mol
3
s
–
1
–1
L
s ) is replaced with deuteriated chloroform
–1
–1
(
CDCl ) (k = 0.045 mol L s ). A solvent isotope
3
s
Influence of Solvent Polarity
effect (k
kCDCl3 ) of 1.52 has been found when
CHCl3
Efforts have been made to throw some light on the
impact of solvent on the oxidation of cholesterol by all
the three oxidant type (α-, β-, and γ-CPDC) in seven
different solvents (benzene, toluene, chloroform,
dichloromethane, chlorobenzene, ethylacetate, and
dioxane) of variant polarity. Mathematical regression
analysis has been carried out with five different solvent
parameters (dipole moment μ, dielectric constant ε,
Kemlet–Taft polarity π, Dimroth–Reichardt polarity
studied with β-CPDC and Chol, suggesting the
occurrence of a carbon-hydrogen cleavage during the
oxidation process as suggested in the Scheme 1.
Besides, it can also be concluded that CHCl solvo-
3
lyzes the activated state more than CDCl does. Ear-
3
lier, such deuterium isotope effect (i.e., slowing down)
has been observed for solvation dynamics in bulk water
index E (30), and water-octanol partition coefficient
T
59
57
55
53
logP) to understand their contribution on overall oxi-
dation process (Eqs. (5)–(7)).
ks,α-CPDC = 0.39 + 0.26π – 0.010ET
( )
30
+
0.058μ – 0.022ε – 0.004logP,
(5)
2
R = 0.972, SD = 0.014, F = 7.15, n = 07;
51
ks,β-CPDC = 0.11+ 0.38π – 0.002E (30)
T
–
0.035μ – 0.015ε – 0.032logP,
(6)
(7)
4
9
2
R = 0.993, SD = 0.11, F = 28.3, n = 07;
47
ks,γ-CPDC = 0.91+ 1.4π – 0.041E (30)
T
4
5
+
0.19μ – 0.065ε + 0.013logP,
–
0.18
–0.16
–0.14
–0.12
–0.10
2
ΔS , kJ K mol–1
#
–1
R = 0.843, SD = 1.39, F = 1.08, n = 07,
where SD is the standard deviation, F is the confi-
dence level, n is the number of data points.
The specific reaction rate of β-CPDC oxidant
(Eq. (6)) is found to be influenced much by electronic
≠
≠
Fig. 4. ΔH vs. ΔS compensation plot for the oxidation of
cholesterol with α-, β-, and γ-CPDC oxidants in DCM
–5
–3
medium. [CPDC] = 6 × 10 M, [Chol] = 0.1 × 10 M,
AcOH] = 5 M. Slope = 259.5, R = 0.999.
2
[
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Cr(VI) OXIDATION OF CHOLESTEROL
153
(a)
(b)
0
0
0
0
.16
.14
.12
.10
0.45
0.40
0.35
0.30
0.25
0.20
0.08
0
.06
.04
0
.15
.10
0
0
0.02
0.05
0
0.1
0.2
0
0.1
ks,β-CPDC, mol L s–1
0.2
ks,β-CPDC, mol L s–1
–
1
–1
Fig. 5. Plots of ks, β-CPDC vs. ks, α-CPDC (a) and ks, γ-CPDC (b). [CPDC] = 6 × 10–5 M, [Chol] = 0.1 × 10–3 M, [AcOH] = 5 M,
T = 22°C.
transition energy (E (30)) and dielectric constant(ε)
REFERENCES
T
whereas that of γ-CPDC (Eq. (7)) is more influenced
by the nonspecific part of van der Waals interactions
between solvents and solutes [54], corroborating the
fact that the reaction environment of the former is
1
. Wiberg, K.B., Oxidation in Organic Chemistry, Part A,
New York: Academic Press/Wiley, 1965, p. 69.
2. Corey, E.J., and Boger, D.L., Tetrahedron Lett., 1978,
vol. 28, p. 2461
more polar than the latter. Moreover, plots of k
s,β-CPDC
3. Dave, I., Sharma, V., and Banerji, K.K., J. Indian
vs. k
and k
at a comparable oxidant,
Chem. Soc., 2002, vol. 79, p. 347.
s,α-CPDC
s,γ-CPDC
substrate, acetic acid concentrations and temperature
condition produce scattered plots (Fig. 5), showing a
clear difference in their oxidation capabilities in differ-
ent solvents.
4. Nalawaya, N., Jain, A., and Hiran, B.L., J. Indian
Chem. Soc., 2002, vol. 79, p. 587.
5
6
7
8
9
. Medien, H.A.A., Z. Naturforsch., 2003, vol. 58b,
p. 1201.
. Kavitha, S., Pandurangan, A., and Alphonse, I., Indian
J. Chem., 2005, vol. 44A, p. 715.
. Pandurangan, A., Rajkumar, G.A., Arbindoo, B., and
Murugesan, V., Indian J. Chem., 1999, vol. 38B, p. 99.
. Vibhute, A.Y., Patwari, S.B., Khansole, S.V., and
Vibhute, Y.B., Chin. Chem. Lett., 2009, vol. 20, p. 256.
. Hajipour, A.R., Khazdooz, L., and Ruoho, A.E., J.
Iran. Chem. Soc., 2005, vol. 2, p. 315.
CONCLUSIONS
N-cetylpicolinium dichromates act as a class of
mild oxidants in acid medium. Oxidation of choles-
terol by these phase transfer oxidants is fractional
order with respect to both oxidant and substrate; how-
ever, the reactions are overall second order. Geometri-
cal variation of the oxidant structure brings about a
change in the rate of oxidation. The oxidation process
does not involve any reversed micellar organization of
oxidants, which is a striking difference from the earlier
reported alkyl onium oxidants. Cr(III) species, the
inorganic by-product produced during the oxidation
process, is not environmentally hazardous. The
CPDC oxidants can find further use in various oxida-
tive transformation reactions under mild conditions.
1
0. Mansoor, S.S., and Shafi, S.S., React. Kinet. Mech.
Catal., 2010, vol. 100, p. 21.
1
1. Mamaghani, M., Shirini, F., and Parsa, F., Russ. J. Org.
Chem., 2002, vol. 38, p. 1113.
1
2. Mansoor, S.S., and Shafi, S.S., J. Mol. Liq., 2010,
vol. 155, p. 85; Mansoor, S.S., E.-J. Chem., 2011, vol. 8,
no. 2, p. 643.
1
3. Mansoor, S.S., and Shafi, S.S., React. Kinet. Mech.
Catal., 2010, vol. 100, p. 21.
1
4. Mansoor, S.S., and Shafi, S.S., Arabian J. Chem., 2010,
vol. 7, p. 171.
1
5. Mansoor, S.S., and Shafi, S.S., J. Mol. Liq., 2010,
ACKNOWLEDGMENTS
vol. 155, p. 85.
S. Dash acknowledges the financial help extended 16. Starks, C.M., and Owens, R.M., J. Am. Chem. Soc.,
by All India Council of Technical Education
1973, vol. 95, p. 3613.
(
AICTE), New Delhi, through a RPS project to set up 17. Dash, S., and Mishra, B.K., Int. J. Chem. Kinet., 1995,
the laboratory.
vol. 27, p. 627.
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

1
54
PRIYADARSHINI et al.
1
8. Dash, S., and Mishra, B.K., Bull. Chem. Soc. Jpn., 37. Chidambaram, M., Sonavane, S.U., de la Zerda, J.,
1
994, vol. 67, p. 3289. and Sasson, Y., Tetrahedron, 2007, vol. 63, p. 7696.
1
2
9. Dash, S., and Mishra, B.K., Indian J. Chem., 1997, 38. Patel, S., and Mishra, B.K., J. Org. Chem., 2006,
vol. 36A, p. 662.
vol. 71, p. 6759.
0. Dash, S., Nayak, B.B., and Mishra, B.K., Indian J. 39. Bhuvaneshwari, D.S., Elango, K.P., Int. J. Chem.
Chem., 2001, vol. 159, p. 40.
Kinet., 2006, vol. 38, p. 657.
21. Nayak, B., Panigrahi, B.M., Patel, S., Dash S., and
4
0. Hegde, R.N., Shetti, N.P., Nandibewoor, S.T., Ind.
Mishra, B.K., Indian J. Chem., 2006, vol. 45A, p. 2229.
Eng. Chem. Res. 2009, vol. 48, p. 7025.
2
2
2
2
2
2
2
2. Nayak, B.B., Sahu, S., Patel, S., Dash, S., and
41. Kabilan, S., Girija, R., Reis, J.C.R., Segurado, M.A.P.,
Mishra, B.K., Indian J. Chem., 2008, vol. 47A, p. 1486.
and Gomes de Oliveira, J.D., J. Chem. Soc., Perkin
Trans. 2, 2002, p. 1151.
3. Bank, S.P., Guru, P.S., and Dash, S., Carbohydr.
Polym., 2014, vol. 111, p. 806.
4
4
2. Mansoor, S.S., Malik, V.S., Aswin, K., Logaiya, K.,
and Hussain, A.M., J. Saudi Chem. Soc., 2016, vol. 20,
p. 77.
3. Cotton, F.A., Wilkinson, G. Murillo, C.A., and Boch-
mann, M., Advanced Inorganic Chemistry, 6th ed., New
York: John Wiley and Sons, 1999, vol. 747.
4. Bank, S.P., Guru, P.S., and Dash, S., Spectrochim.
Acta, Part A, 2015, vol. 142, p. 34.
5. Bank, S.P., Guru, P.S., and Dash, S., Ind. Eng. Chem.
Res., 2015, vol. 54, p. 9142.
6. Priyadarshini, S., Guru, P.S., and Dash, S., Carbohydr.
Polym., 2017, vol. 171, p. 122.
7. Dauben, W.G., Lorber, M.E., and Fullerton, D.S., J.
4
4. Brandstrom, A., Adv. Phys. Org. Chem., 1978, vol. 15,
p. 267.
Org. Chem., 1969, vol. 34, p. 3587.
4
5. Glasstone, S., Laidler, K.J., and Erying, H., The The-
ory of Rate Process, New York: McGraw-Hill, 1941,
Chapters III and IV.
8. Fousteris, M.A., Koutsourea, A.I., Nikolaropoulos, S.S.,
Riahi, A., and Muzart, J., J. Mol. Catal. A: Chem.,
2006, vol. 250, p. 70.
46. Zolotov, B., Gan, A., Feinberg, B.D., and Huppert, D.,
2
9. Pearson, A.J., Chen, Y.S., Han, G.R., Hsu, S.Y., and
Ray, T., J. Chem. Soc. Perkin Trans., 1985, vol. 1,
p. 267.
Chem. Phys. Lett., 1997, vol. 265, p. 418.
4
7. Castner, E.W., Chang, Y.J., Chu, Y.C., and Walfaren, G.E.,
J. Chem. Phys., 1995, vol. 102, p. 653.
3
0. Dong, J., Chen, W., Wang, S., Zhang, J., Li, H., Guo, H.,
Man, Y., and Chen, B., J. Chromatography B, 2007,
vol. 858, p. 239.
4
4
5
8. Pant, D., and Levinger, N.E., J. Phys. Chem., 1999,
vol. 103, p. 7846.
9. Shirota, H., Pal, H., Tominaga, K., and Yosihara, K.,
31. Hector, M., Hartmannn, R.W., and Njar, V.C.O.,
J. Phys. Chem., 1996, vol. 100, p. 14575.
Synth. Commun., 1996, vol. 26, p. 1075.
0. Reid, P.J., and Barabara, P.F., J. Phys. Chem., 1995,
32. Nangia, A., and Anthony, A., Synth. Commun., 1996,
vol. 99, p. 3554.
vol. 26, p. 225.
51. Shirota, H., Tamato, Y., and Segawa, H., J. Phys.
33. Li, S., and Li, T., Steroids, 1998, vol. 63, p. 76.
Chem. A, 2004, vol. 108, p. 3244.
3
4. Silva, M.R., Beja, A.M., and Paixao, J.A., Powder
Diffr., 2003, vol. 18, p. 306.
52. Das, S., Datta, A., and Bhattacharyya, K., J. Phys.
Chem. A, 1997, vol. 101, p. 3299.
35. Hunter, A.C., and Priest, S., Steroids, 2006, vol. 71,
p. 30.
53. Kaatze, U., Chem. Phys. Lett., 1993, vol. 203, p. 1.
3
6. Patel, S., and Mishra, B.K., J. Org. Chem., 2006, 54. Marcus, Y., Wiley Series in Solution Chemistry, 1998,
vol. 71, p. 3522.
vol. 4.
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