Kinetics and mechanism of oxidation of diclofenac sodium by Keggin type 12-tungstocobalt(III) in aqueous medium

Article abstract of DOI:10.14233/ajchem.2018.20973

The kinetics of electron transfer reaction of diclofenac sodium with 12-tungstocobaltate (III) complex has been studied spectrophotometrically over the range 2.0 × 10^-3 ≤ [diclofenac sodium] ≤ 6.0 × 10^-3 mol/L, 6.03 ≤ pH ≤ 8.0 and at 293 ≤ T ≤ 308 K in aqueous medium at constant ionic strength I (0.5 mol/L sodium perchlorate). The electron transfer reaction showed pseudo-first order dependence in [diclofenac sodium] and [12-tungstocobaltate(III)] and less than unit order in [OH^–]_T. The activation parameters calculated for the electron transfer reaction favoured the formation of a precursor complex between the reactants. The product is characterized by FTIR and NMR spectra and is found to be [2-(2,6-dichloro phenylamino)phenyl]methanol.

Full text of DOI:10.14233/ajchem.2018.20973

Asian Journal of Chemistry; Vol. 30, No. 1 (2018), 179-182
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.20973
Kinetics and Mechanism of Oxidation of Diclofenac Sodium by
Keggin Type 12-Tungstocobalt(III) in Aqueous Medium
1,*
2
2
3
M. SANJANA , A.K. PATNAIK , P. MOHANTY and S.K. BADAMALI
1
2
3
Department of Chemistry, Christ College, Cuttack-753 008, India
Post Graduate Department of Chemistry, Ravenshaw University, Cuttack-753 001, India
Post Graduate Department of Chemistry, Utkal University, Bhubaneswar-751 004, India
*
Corresponding author: E-mail: msanjanaa@gmail.com
Received: 14 August 2017; Accepted: 29 September 2017;
Published online: 30 November 2017;
AJC-18672
The kinetics of electron transfer reaction of diclofenac sodium with 12-tungstocobaltate (III) complex has been studied spectrophotometrically
-3
-3
over the range 2.0 × 10 ≤ [diclofenac sodium] ≤ 6.0 × 10 mol/L, 6.03 ≤ pH ≤ 8.0 and at 293 ≤ T ≤ 308 K in aqueous medium at constant
ionic strength I (0.5 mol/L sodium perchlorate). The electron transfer reaction showed pseudo-first order dependence in [diclofenac sodium]
–
and [12-tungstocobaltate(III)] and less than unit order in [OH ]
T
. The activation parameters calculated for the electron transfer reaction
favoured the formation of a precursor complex between the reactants. The product is characterized by FTIR and NMR spectra and is
found to be [2-(2,6-dichloro phenylamino)phenyl]methanol.
Keywords: Kinetic, Oxidation, Diclofenac sodium, 12-Tungstocobaltate(III), Keggin Type.
examine the oxidative degradation of diclofenac sodium, its
oxidation reaction with the above oxidant has been studied.
INTRODUCTION
Non-steroidal anti-inflammatory drugs (NSAIDs) are of
great significance [1] due to their large pharmaceutical impor-
tance. Diclofenac [2-(2,6-dichloranilino)phenylacetic acid]
is the most potent member of this class of drugs. It works by
reducing the production of prostaglandins [2], the chemical
that causes pain, fever and inflammation. Diclofenac blocks
the enzyme (cyclooxygenase) that produces prostaglandin,
resulting in decrease in production of prostaglandin and hence
subsequent relief from pain. The drug is used in the form of
its sodium salt due to its low solubility in water. This analgesic
drug is used for the treatment of osteoarthritis, rheumatoid
arthritis and ankylo spondylitis [3,4]. This drug has also many
side effects such as nausea, heartburn, diarrhoea, constipation,
gastritis, headache, drowsiness and dizziness. Its presence in
wastewater is harmful for ecological environment. The oxida-
tive degradation of the drug has large significance [5] because
it may through light how to reduce the diclofenac content in
waste water.
EXPERIMENTAL
III
5-
Co W12
O
40 was prepared as reported by McAuley et al.
[
9]. It was characterized spectrophotometrically [16] at 388
-1
-1
-1
nm (ε388 = 1150 ± 2 L mol cm ).A.R. grade chemicals were
used and conductivity water was used for preparing different
solutions. The redox reaction was studied at pH = 6.03 to 8.0
using Systronics (India) digital pH meter which was standardized
by suitable buffers. The phosphate buffer has been used to
maintain pH of different solutions.
The CECIL CE 7200 UV-visible spectrophotometer
equipped with CE 2024 thermoelectric controller was used for
measuring absorbance for the kinetic studies.
Kinetic measurements: In order to follow the kinetics,
pseudo first order conditions were maintained. The maximum
mole ratio of reductant/oxidant was 12:1. The reaction was
initiated by mixing previously thermostated solutions of
5-
III
5-
III
5-
diclofenac sodium and CoW12O40 and conductivity water. The
The oxidant is 12-tungstocobalt(III) (Co W12O40 or Co W )
redox reaction was followed at 388 nm. The absorbance due
to oxidant decreased with time. The pseudo-first order rate
constant (kobs) were determined from the slope of the linear
is a Keggin type cluster and its redox potential is 1.0V [6]. It
is a well known outer sphere oxidant [7,8] and a mild oxidant.
Its electron transfer reaction with ascorbic acid [9] glutathione
plot of ln (A
t
- A
∞
) versus t (s) using Excel program.
[
1]0, citric acid [11], DL-methionine [12], L-cysteine [13],
NADH [14] and L-cystine [15] have been studied. In order to
ln (A
t
- A ) = ln (A - A
∞
0
∞
) – kobs.t
(1)

180 Sanjana et al.
Asian J. Chem.
–
–
–
–
–
3
3
3
3
3
where A
t
and A
∞
are the absorbance of the reaction mixture at
3.00×10
time ‘t’and at equilibrium respectively. The reaction was studied
up to 80 % completion and rate constants were reproducible
within ± 5 %. The correlation coefficients of the plots which were
used to determine kobs were found to be 0.99 in most of the cases.
2
93 K
98 K
303 K
08 K
2.50×10
2.00×10
1.50×10
1.00×10
5.00×10
2
3
RESULTS AND DISCUSSION
UV-visible spectral scan of the reaction mixture was studied
spectrophotometrically at different time intervals (Fig. 1). The
decrease in absorbance at λmax 388 nm with time was observed
on mixing the diclofenac sodium with 12-tungstocobaltate(III)
and simultaneously increase of a new peak at 624 nm appeared
due to the formation of Co W12
–4
0
5
.5
6.0
6.5
7.0
pH
7.5
8.0
8.5
II
O40. The redox reaction was
III
5-
-4
Fig. 3. Plot of kobs versus pH at fixed [Co W12
O
40 ] (5.0 × 10 mol/L),
monitored at λmax 388 nm under pseudo-first order conditions
-3
[
diclofenac sodium] 3.0 × 10 mol/L, pH varied from 6.03 to 8.0
and temperature was varied from 293 to 308 K
III
5-
-4
keeping [Co W12
fenac sodium] from 2.0 × 10 to 6.0 × 10 mol/L and pH from
.03 to 8.0. The pseudo-first order rate constants were found
O40 ] = 5.0 × 10 mol/L and varying [diclo-
-3
-3
III
5-
6
Effect of diclofenac sodium: At a fixed [Co W O
(5 × 10 mol/L), ionic strength I = 0.3 mol/L, pH = 6.03,when
]
12
40
-4
to increase with the increase in the concentration of reductant
showing first order dependence of rate with [diclofenac sodium]
-3
-3
[diclofenac sodium] was changed from 2.0 × 10 to 6.0 × 10
4
-1
(
Fig. 2).
mol/L, 10 k changed from 4.63 to 7.79 s at 298 K. The
obs
pseudo first order rate constants were found to increase with
the increase in the concentration of diclofenac sodium, showing
first order dependence of rate with [diclofenac sodium]. The
plot of kobs versus [dicofenac sodium] at different temperature
and at fixed pH is shown in Fig. 2.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Effect of variation of pH on rate: The electron transfer
reaction has been carried out in pH range 6.03 to 8.0. At 298
III
5-
-4
K, when [Co W12
O
40 ] = 5.0 × 10 mol/L, [diclofenac sodium]
-3
=
3.0 × 10 mol/L with change in pH from 6.03 to 8.00, the
0 kobs varied from 5.50 to 13.38 s . The plot of kobs versus pH
4
-1
1
at different concentration of diclofenac sodium at 298K is
shown in Fig. 3. This plot shows that the electron transfer reaction
is pH dependent. By increasing pH, the concentration of conju-
gate base of diclofenac sodium increases which is a better
reductant, hence the rate of reaction increases.
Effect of ionic strength: The effect of ionic strength on
the redox reaction was studied by increasing the ionic strength
from 0.3 to 1.0 mol/L. This pseudo first order rate constant
was found to be unaffected indicating a charged and a neutral
species were involved in the rate determining step.
Wavelength (nm)
Fig. 1. UV-visible spectral scan of the reaction mixture at different time
III
5–
-4
intervals. [Co W12
O
40 ] = 5.0 × 10 mol/L, [diclofenac sodium] =
-3
2.0 × 10 mol/L, pH 6.03 and temperature 298 K
–3
–3
–3
–3
–3
–4
–4
–4
–4
1
.80×10
.60×10
.40×10
.20×10
.00×10
.00×10
.00×10
.00×10
.00×10
1
2
93 K
1
298 K
303 K
308 K
1
1
8
Effect of temperature:The pseudo first order rate constants
were determined by varying the temperature from 293 to 308 K.
The oxidant concentration was kept constant and reductant
6
4
-3
-3
2
concentration was varied from 2.0 × 10 to 6.0 × 10 mol/L at
constant pH and varying temperature. The kobs was found to
increase with increase in temperature. The plot of kobs versus
0
0
0.001 0.002 0.003 0.004 0.005– 0.006 0.007
3
[
Diclofenac sodium] (mol dm )
[
diclofenac sodium] at constant pH and varying temperature
Fig. 2. Plot of kobs versus [diclofenac sodium] at different temperatures.
III
5-
40
is shown in Fig. 2. Basing on the above experimental facts the
mechanism of the reaction is delineated as follows.
Temperature was varied from 293 to 308 K at fixed pH. [Co W12
O
]
-4
=
5.0 × 10 mol/L ionic strength I = 0.3 mol/L, pH = 6.03, [diclofenac
-3 -3
sodium] was varied from 2.0 × 10 to 6.0 × 10 mol/L
Mechanism
COOH
COO-
The pseudo first order rate constants were measured by
varying the pH from 6.08 to 8.0 at four different temperatures,
K
1
293 to 308 K keeping all other parameters constants. The plots
+
H
+
NH
NH
of kobs versus pH at four different temperatures (Fig. 3) shows
that the electron transfer reaction could not be studied at higher
pH because the oxidants, Co(III) clusters will undergo base
hydrolysis.
Cl
Cl
Cl
Cl
S-
SH

Vol. 30, No. 1 (2018)
Kinetics and Mechanism of Oxidation of Diclofenac Sodium by Keggin Type 12-Tungstocobalt(III) 181
−
III
5−
k
1
1
S +[Co W O ] → Product
1
2
40
Slope =
, Intercept =
−
+
k
1
K
1
k
1
[
S ] [H ]
e
K =
1
[SH]e
Intercept
Slope
1
=
K1,
= k1
Intercept
K [SH]
−
1
e
[S ] =
e
+
(1)
(2)
K
1
, k
1
, activation enthalpy and activation entropy are collected
[
H ]
in Table-1.
−
III
5−
12 40
Rate = k [S ][Co W O ]
1
Substituting [S]
e
in eqn. 2 we have
TABLE-1
ELECTRON TRANSFER RATE CONSTANTS AT DIFFERENT
TEMPERATURES AND ACTIVATION PARAMETERS
AND THERMODYNAMIC PARAMETERS FOR THE
REDOX REACTIONS OF 12-TUNGSTOCOBALTATE(III)
WITH DICLOFENAC SODIUM
III
5−
k K [SH] [Co W O ]
1
1
e
12 40
Rate =
+
(3)
[
H ]
−
[SH] = [SH] +[S ]
T
e
e
Temperature (K)
Parameters
K [SH]
1
e
293
0.57
–
–
–
298
0.64
5.33 × 10
42.87
303
1.16
–
–
–
308
1.27
–
–
–
=
=
[SH] +
e
+
-1
[H ]
k (s )
1
-7
K (mol/L)
∆H (kJ mol )
∆S (J K mol )
1


K

#
-1
1
+
[SH] 1+
e

#
-1
-1
[
H ]
-103.6


+

[H ]+ K 
1
=
[SH]e

+

Product analysis: The product [2-(2,6-dichlorophenyl-
[
H ]


amino)phenyl]methanol is confirmed by its characteristic data
-
1
+
of IR (Fig. 5). The broad peak at 3324 cm is due to hydrogen
bonded –OH group fused with secondary amine –NH stret-
ching frequency and C–H stretching frequency in the aromatic

[H ]

[
SH] = [SH]
e
T

+

(4)
[
H ]+ K1


-1
Putting the value of [SH]
e
from eqn. 4 in eqn. 3, we have
group. The peaks in 1694-1453 cm region are due to C=C
aromatic group and –NH bending. 1283 cm is due to –OH
bending vibration. CO stretching at 1725-1700 cm region is
missing in the product. All the above data [17] corresponds to
the formation of the product [2-(2,6-dichloro phenylamino)-
phenyl]methanol (Fig. 6)
-1
III
5−
40
k K [SH] [Co W O ]
1
1
T
+
12
-1
Rate =
(5)
(6)
[
H ]+ K
1
III
5−
Rate = kobs[Co W O ]
Comparing eqns. 5 and 6:
k K [SH]
T
12
40 T
22
1
1
+
kobs
=
20
18
(7)
[
H ]+ K1
1
14
6
+
+
1
[H ]+ K
1
[H ]
1
=
=
+
(
(
8)
9)
1
1
2
0
8
6
4
2
0
2
kobs k K [SH]
k1[SH]T kK [SH]T
1
1
T
1
+
[
SH]T
1
[H ]
=
+
kobs
k1
k1K
+
1
[
SH]
T
/kobs is plotted against [H ] (Fig. 4).
-
5
4
4
3
3
2
2
1
1
0
.0
.5
.0
.5
.0
.5
.0
.5
.0
.5
0
-4
-
6
-
8
4000
3500
3000
2500
Wavenumber (cm )
2000–1 1500
1000
500
Fig. 5
Conclusion
The oxidation of diclofenac sodium by 12-tungstocobalt(III)
in aqueous medium was studied. The redox reaction showed
first order dependence in [diclofenac sodium] and [12-tungsto-
–
cobaltate(III)] and fractional order in [OH ]
T
. The activation
#
-1
#
-1
-1
parameters ∆H (kJ mol ) and ∆S (JK mol ) values are found
to be 42.87 and -103.6, respectively. The negative value of
0
0.0000002 0.0000004 0.0000006 0.0000008 0.0000010
#
+
∆S indicates an ordered transition state for the electron transfer
Fig. 4. Plot of variation of [Diclofenac]/kobs versus [H ] at 298 K.
-
3
III
5-
-4
reaction. The product is found to be [2-(2,6-dichloro phenylamino)-
phenyl]methanol.
[
[
Diclofenac] = 2 × 10 , [Co W12O40 ] = 5.0 × 10 , I = 0.5 mol/L.
+ -7 -8
H ] was varied from 9.33 × 10 to 1.0 × 10 mol/L

182 Sanjana et al.
Asian J. Chem.
CH
Cl
2
OH
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