Kinetics and thermodynamics of oxidation of some para-substituted 4-oxo-4-phenyl butanoic acids by tripropylammonium fluorochromate in aqueous acetic acid medium

Article abstract of DOI:10.14233/ajchem.2017.20625

The kinetics of oxidation of 4-oxo-4-phenyl butanoic acid (4-oxo acid), p-methoxy (p-OCH₃), p-ethoxy (p-OC2H5), p-methyl (p-CH₃), p-chloro (p-Cl), p-bromo (p-Br) and p-acetyl (p-COCH₃) 4-oxo acids by tripropylammonium fluorochromate (TriPAFC) in aqueous acetic acid medium in the presence of perchloric acid have been described. The reaction is first order with respect to 4-oxo acid, TriPAFC and perchloric acid. The order of reactivity among the studied 4-oxoacids is: p-OCH₃ > p-OC₂H5 > p-CH₃ > p-H > p-Cl > p-Br > p-COCH₃. The effect of changes on the electronic nature of the substrate reveals that there is a development of positive charge in the transition state. The activation parameters have been computed from Arrhenius and Eyring plots. Based on the kinetic results, a suitable mechanism has been proposed.

Full text of DOI:10.14233/ajchem.2017.20625

Asian Journal of Chemistry; Vol. 29, No. 7 (2017), 1617-1621
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20625
Kinetics and Thermodynamics of Oxidation of Some para-Substituted 4-Oxo-4-phenyl
Butanoic Acids by Tripropylammonium Fluorochromate in Aqueous Acetic Acid Medium
1
2
3,*
A. YOGANANTH , BASIM H. ASGHAR and S. SHEIK MANSOOR
1
2
3
Research and Development Centre, Bharathiar University, Coimbatore-641 046, India
Department of Chemistry, Faculty of Applied Sciences, Umm Al-Qura University, P.O. Box: 9569, Makkah, Saudi Arabia
Department of Chemistry, C. Abdul Hakeem College (Autonomous), Melvisharam-632 509, India
*
Corresponding author: E-mail: smansoors2000@yahoo.co.in
Received: 4 March 2017; Accepted: 31 March 2017;
Published online: 13 May 2017;
AJC-18405
The kinetics of oxidation of 4-oxo-4-phenyl butanoic acid (4-oxo acid), p-methoxy (p-OCH
chloro (p-Cl), p-bromo (p-Br) and p-acetyl (p-COCH ) 4-oxo acids by tripropylammonium fluorochromate (TriPAFC) in aqueous acetic
acid medium in the presence of perchloric acid have been described. The reaction is first order with respect to 4-oxo acid, TriPAFC and
perchloric acid. The order of reactivity among the studied 4-oxoacids is: p-OCH > p-OC > p-CH > p-H > p-Cl > p-Br > p-COCH
3
), p-ethoxy (p-OC
2
5
H ), p-methyl (p-CH
3
), p-
3
3
2
H
5
3
3
.
The effect of changes on the electronic nature of the substrate reveals that there is a development of positive charge in the transition state.
The activation parameters have been computed from Arrhenius and Eyring plots. Based on the kinetic results, a suitable mechanism has
been proposed.
Keywords: Tripropylammonium fluorochromate, 4-Oxo acid, Thermodynamic parameters, Kinetics.
Even though, numerous oxidants for oxidation of 4-oxo
acids are already reported, as a part of our continuing investi-
gations of the oxidation of organic substrates using Cr(VI)
reagents [19-23], this work reports the kinetic features of the
oxidation of 4-oxo acid by tripropylammonium fluorochromate.
INTRODUCTION
In modern organic synthesis the oxidation of an organic
substrate is important. Therefore, synthetic organic chemists
pay attention to search for new oxidizing reagents [1]. In recent
years, extensive works have led to the development of a good
number of chromium(VI) oxidizing agents such as isoquinolinium
fluorochromate [2], tetramethylammonium fluorochromate
EXPERIMENTAL
[
3], isoquinolinium bromochromate [4], tetrahexylammonium
All the chemicals used were reagent grade products.
Tripropylammonium fluorochromate (TriPAFC) was prepared
by a reported method [9].Acetic acid was purified by standard
method and the fraction distilling at 118 °C was collected.
Preparation of 4-oxo-4-phenyl butanoic acid: The
4-oxo-4-phenyl butanoic acid is synthesized by the Friedel-
Craft’s reaction between succinic anhydride and benzene in
the presence of anhydrous aluminium chloride [12].
Kinetic measurements: Solutions were prepared in
double-distilled water. The reaction mixtures, containing oxo
acid, acetic acid and perchloric acid solutions, were thermally
equilibrated for 1 h at the desired temperature. The reaction
was initiated by the addition of a temperature-equilibrated
TriPAFC solution of the requisite concentration. The rate was
studied spectrophotometrically by monitoring the disappearance
of [TriPAFC] at 362 nm using UV-vis spectrophotometer,
Shimadzu UV-1800 model in 50 % acetic acid - 50 % water
bromochromate [5], tributylammonium chlorochromate [6]
and tetrabutylammonium bromochromate [7] to study the
oxidation of various organic compounds.
Organic chemists must often choose from hundreds of
oxidizing agents and reaction conditions to perform a desired
oxidation without affecting other functional groups present or
causing side reactions. Although there are a broad variety of
oxidants derived from Cr(VI) reported in literature to perform
oxidation reactions tripropylammonium fluorochromate
reported as an efficient oxidizing agent [8-11].
The oxidation kinetics of 4-oxo acids by various oxidizing
agents like permanganate [12], pyridinium fluorochromate
[
bromosaccharin [16], N-chlorobenzamide [17] and benzimi-
dazolium fluorochromate [18] have been extensively studied
by various workers.
13], N-bromosuccinimide [14], N-chlorosaccharin [15], N-

1
618 Yogananth et al.
Asian J. Chem.
(
v/v). All of the reactions were carried out under pseudo-first-
order conditions by keeping an excess (tenfold or greater) of
4-oxo acid] over TriPAFC] i.e. [4-oxo acid] >> [TriPAFC)].
The pseudo-first-order rate constants (k ) were computed from
S1 = H
S2 = p-OCH3
S3 = p-OC H
S4 = p-CH3
S5 = p-Cl
S6 = p-Br
S7 = p-COCH3
S2
S3
S4
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
[
2
5
1
linear plots of log [TriPAFC] against time up to 90 % comp-
letion of the reaction. The precision of the rate constant values
is given in terms of 95 % the confidence limit of the student’s
t test [24] and the values were reproducible to within 5 %.
Product analysis and stoichiometric studies: The
products in the respective oxidation of 4-oxo acids were corres-
ponding benzoic acids and this were confirmed by comparing
the melting point values.
S1
S5
S6
S7
The stoichiometric studies showed that 1 mol of TriPAFC
reacts with 1 mol of 4-oxo-4-phenyl butanic acid.
RESULTS AND DISCUSSION
1.0
1.1
1.2
1.3
1.4
1.5
3
+ log [S]
Order of the reaction: The oxidation of organic comp-
ounds with high selectivity is of extreme importance in synthetic
chemistry. Reactions were carried out under pseudo first-order
conditions with a known excess of [4-oxo acids] over [TriPAFC]
at constant temperature of 303 K in 50 % acetic acid and 50 %
water medium.
Fig. 1. Double logarithmic plot for the oxidation of p-substituted 4-oxo-
4-phenyl-butanoic acids with TriPAFC
S2
S3
S1 = H
S2 = p-OCH3
S3 = p-OC H
S4 = p-CH3
The values of k
1
were independent of the initial concen-
30
S5 = p-Cl
S6 = p-Br
S7 = p-COCH3
tration of TriPAFC (Table-1) suggesting the reactions were of
first order with respect to TriPAFC. The reaction was catalyzed
2
5
S4
S1
25
20
15
10
+
by hydrogen ions and the order with respect to [H ] was one.
The k
of 4-oxo acid for the oxidation show a linear increase (Table-1).
The plot of log k versus log [4-oxo acid] is excellently linear
r = 0.999) with unit slope, indicating that the order of the
1
values measured at various initial concentrations
1
S5
S6
(
reaction with respect to [4-oxo acid] is one (Fig. 1). This is
further evidenced by the excellent linearity observed in the
S7
5
0
plot of k
Fig. 2).
Effect of acrylonitrile and MnSO
1
versus [4-oxo acid], which passes through origin
(
4
: The involvement of
0.0
0.5
1.0
1.5
2.0
2.5
3.0
free-radical intermediates in the present study can be excluded,
as the rate constant is not affected by the addition of acrylo-
nitrile. The addition of Mn(II), indicating the involvement of
a two-electron reduction of Cr(VI) to Cr(IV) (Table-1).
2
1
0 [S], M
Fig. 2. Direct plot for the oxidation of p-substituted 4-oxo-4-phenyl-
butanoic acids with TriPAFC
TABLE-1+
a
EFFECT OF VARIATION OF [S], [TriPAFC] AND [H ] ON THE RATE OF REACTION AT 303 K
3
2
+
4
–1 b
1
0 [TriPAFC]
10 [S]
[H ]
–3
10 k (s )
1
–3
–3
(mol dm )
(mol dm ) (mol dm )
H
p-OCH3
p-OC H5
p-CH3
p-Cl
p-Br
p-COCH3
2
0
1
1
2
2
1
1
1
1
1
1
1
1
1
1
.6
.1
.6
.1
.6
.1
.1
.1
.1
.1
.1
.1
.1
.1
.1
2.0
2.0
2.0
2.0
2.0
1.2
1.6
2.4
2.8
2.0
2.0
2.0
2.0
2.0
2.0
0.26
0.26
0.26
0.26
0.26
0.26
0.26
0.26
0.26
0.10
0.18
0.34
0.42
0.26
0.26
12.18
12.26
12.30
12.22
12.32
7.52
23.52
23.60
23.64
23.66
23.56
14.04
18.78
28.30
33.00
9.04
22.26
22.32
22.40
22.38
22.30
13.30
17.78
26.76
31.16
8.50
18.80
18.88
18.96
18.84
18.92
11.26
15.02
22.60
26.36
7.18
7.88
7.82
7.76
7.90
7.74
4.66
6.18
9.30
10.84
3.00
5.40
10.16
12.66
7.34
7.40
7.42
7.38
7.44
4.38
5.86
8.78
10.30
2.80
5.06
9.56
11.90
3.80
3.88
3.98
3.90
3.84
2.28
3.04
4.60
5.36
1.46
2.66
5.04
6.22c
3.72
9.68
14.86
17.04
4.78
8.30
16.26
30.78
38.02c
23.48
15.36
29.10
36.00c
22.26
13.00
24.60
30.40c
18.72
15.90
19.92c
12.30
c
c
7.78
7.28
d
d
d
d
d
d
d
10.68
19.98
18.92
15.64
b
6.72
6.18
3.16
c
a
As determined by spectrophotometrically following the disappearance of Cr(VI) at 362 nm; Estimated from pseudo first order plots; In the
presence of 0.001 mol dm acrylonitrile; In the presence of 0.003 mol dm Mn(II); Solvent composition = 50% AcOH- 50% H O (v/v)
-3
d
-3
2

Vol. 29, No. 7 (2017)
Oxidation of Some para-Substituted 4-Oxo-4-phenyl ButanoicAcids by Tripropylammonium Fluorochromate 1619
TABLE-2
EFFECT OF VARYING SOLVENT POLARITY ON THE RATE OF REACTION OF 4-OXO ACID,
p-OCH , p-OC H , p-CH , p-Cl, p-Br AND p COCH 4-OXO ACIDS BY TriPAFC AT 303 K
3
2
5
3
3
4
–1
%
AcOH-
Dielectric
constant
10 k (s )
1
H O (v/v)
2
H
p-OCH3
p-OC H5
p-CH3
p-Cl
p-Br
p-COCH3
2
3
4
5
6
7
0-70
0-60
0-50
0-40
0-30
72.0
63.3
56.0
45.5
38.5
9.72
18.66
21.16
23.60
27.32
33.92
17.66
20.04
22.32
26.36
15.12
16.94
18.88
21.96
26.96
6.16
7.04
7.82
9.16
11.16
5.84
6.68
7.40
8.68
10.64
3.08
3.45
3.88
4.52
5.64
11.00
12.26
14.22
17.63
32.66
2
-3
3
-3
+
-3
1
0 [S] = 2.0 mol dm ; 10 [TriPAFC] = 1.1 mol dm ; 10 [H ] = 2.6mol dm
Effect of solvent polarity: In binary mixtures of acetic
different temperatures are collected in Table-3. The second-
order rate constants and activation parameters were calculated
and the values were presented in Table-4.
acid and water as solvent, the rate of the reaction increases
remarkably with an increase in the proportion of acetic acid
in the medium (Table-2). The concentration of acetic acid was
varied from 30 to 70 % and the rate were measured. When the
acetic acid content is increased, the acidity of the medium
increases, while the dielectric constant of the medium decreases.
These two effects cause the rate of the oxidation to increase
TABLE-3
PSEUDO-FIRST ORDER RATE CONSTANTS FOR THE
OXIDATION OF 4-OXO ACID, p-OCH , p-OC H , p-CH , p-Cl,
3
2
5
3
p-Br AND p-COCH 4-OXO ACIDS BY TriPAFC AT VARIOUS
3
TEMPERATURES IN AQUEOUS ACETIC ACID MEDIUM
remarkably.A plot of log k
The linear plot obtained indicates that the involvement of
cation-dipole type of interaction in the rate determining step
1
versus 1/D has been given in Fig. 3.
4
–1
1
0 k (s )
1
Substrate
2
98 K
303 K
12.26
23.60
22.32
18.88
7.82
308 K
313 K
24.30
42.32
41.28
35.92
16.44
15.74
H
8.08
17.60
16.40
13.64
5.36
17.22
31.56
30.34
26.12
11.32
10.80
5.74
[
25].
p-OCH3
p-OC H5
2
p-CH3
p-Cl
p-Br
S1 = H
S2 = p-OCH3
S5 = p-Cl
S2
S3
S4
5.04
2.64
7.40
3.88
S3 = p-OC H5 S6 = p-Br
2
1
1
1
1
0
0
.6 S4 = p-CH3
S7 = p-COCH3
p-COCH
3
8.48
+
2
-3
3
-3
1
0 [S] = 2.0 mol dm ; 10 [TriPAFC] = 1.1 mol dm ; 10 [H ] = 2.6
mol dm ; Solvent composition = 50 % AcOH – 50 % H O (v/v)
-
3
S1
2
.4
.2
.0
.8
.6
Effect of substituents-Hammett plot: Electron-releasing
substituents in the phenyl ring accelerate the rate, while electron-
withdrawing substituents produce the opposite effect [26]. The
S5
S6
S7
log k
2
values show excellent correlation with the σ values.
p
The ρ values calculated from the slopes of the Hammett plots
at the four different temperatures are also included in Table-5.
A negative value of ρ indicates an accumulation of positive
charge at the reaction carbon center.
The effect of structure on reactivity of para-substituted
4
-oxo acids were studied. It is interesting to note that the
reactivity decreases in the order p-OCH > p-OC > p-CH
p-H > p-Cl > p-Br > p-COCH for the substituents.
Mechanism of oxidation: The negative ρ value obtained
from the Hammett plot indicates the formation of a carbocation
in the rate-determining step. The activation parameters are of
the order expected for a bimolecular substitution reaction. The
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
3
2
H
5
3
>
1
/D
Fig. 3. Amis plot for the reaction between para-substituted 4-oxo-4-phenyl-
butanoic acids by TriPAFC between 4 + log k and 1/D
3
1
Activation parameters: The first-order rate constants for
the oxidation of the selected oxo acids with TriPAFC at four
TABLE-4
ACTIVATION PARAMETERS AND SECOND ORDER RATE CONSTANTS FOR THE OXIDATION OF 4-OXO ACID,
p-OCH , p-OC H , p-CH , p-Cl, p-Br AND p-COCH 4-OXO ACIDS BY TriPAFC IN AQUEOUS ACETIC ACID MEDIUM
3
2
5
3
3
2
3
–1 –1
#
#
#
–1
1
0 k (dm mol s )
Ea
∆H
-∆S
∆G (kJ mol )
2
Substrate
–1
–1
–1
2
98 K
303 K
308 K
313 K
(kJ mol )
(kJ mol )
(JK mol)
(at 303 K)
H
4.04
8.80
8.20
6.82
2.68
2.52
6.13
11.80
11.16
9.44
3.91
3.70
1.94
8.61
15.78
15.17
13.06
5.61
5.40
2.87
-3
12.15
21.16
20.64
17.96
8.22
56.07
45.38
47.86
50.16
57.82
58.96
60.69
54.78 ± 0.2
42.88 ± 0.6
45.37 ± 0.4
47.67 ± 0.2
55.33 ± 0.4
56.48 ± 0.6
58.01 ± 0.3
92.07 ± 0.6
120.98 ± 1.8
113.71 ± 1.2
107.20 ± 0.6
69.20 ± 1.2
86.14 ± 1.8
84.98 ± 0.9
82.64 ± 0.4
79.53 ± 1.2
79.82 ± 0.8
80.15 ± 0.4
82.36 ± 0.8
82.58 ± 1.2
83.76 ± 0.6
p-OCH3
p-OC H5
p-CH3
p-Cl
p-Br
2
7.87
4.24
p-COCH3
1.32
-3
2
3
+
-3
10 [S] = 2.0 mol dm ; 10 [TriPAFC] = 1.1 mol dm ; 10 [H ] = 2.6 mol dm ; Solvent composition = 50 % AcOH - 50 % H O (v/v)
2

1
620 Yogananth et al.
Asian J. Chem.
1.4
TABLE-5
S1 = H
S2 = p-OCH3
REACTION CONSTANT VALUES FOR THE OXIDATION OF
S5 = p-Cl
S6 = p-Br
S7 = p-COCH3
4
-OXO ACID, p-OCH , p-OC H , p-CH , p-Cl, p-Br AND p-COCH
1.2
3
2
5
3
3
S3 = p-OC H
S4 = p-CH3
4
-OXO ACIDS BY TriPAFC IN AQUEOUS ACETIC ACID
2
5
MEDIUM AT DIFFERENT TEMPERATURES
1.0
0.8
0.6
Temperature
K)
Reaction
constant (ρ)
Correlation
coefficient
Standard
deviation
(
S2
S3
2
3
3
3
98
03
08
13
-1.0598
-1.0091
-0.9543
-0.8978
0.997
0.992
0.988
0.987
0.18
0.05
0.11
0.12
+
S4
313 K
S1
3
08 K
2
-3
3
-3
0.4
S5
S6
1
0 [S] = 2.0 mol dm ; 10 [TriPAFC] = 1.1 mol dm ; 10 [H ] = 2.6
mol dm ; Solvent composition = 50% AcOH - 50% H O (v/v)
303 K
298 K
-
3
2
0
.2
.0
S7
large negative entropy of activation suggests that the transition
state is more suitably oriented than the reactants as the charge-
separated transition state becomes solvated by a sheath of
solvent molecules. The reaction rate increases linearly with
0
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Substituent constant
p
Fig. 4. Hammett plot of log k versus substituent constant σ for the oxi-
2
+
the increase in [H ] ion and hence protonated TriPAFC is
dation of p-substituted 4-oxo-4-phenyl-butanoic acids with TriPAFC
formed. The rate enhancement observed in the present study
in aqueous acetic acid medium at different temperatures
O
3 7 3
ONH(C H )
HO
O
ONH(C H )
3 7 3
k1
Cr
+
H O
Cr
3
H O
2
+
O
F
F
OH
O
C
CH
2
CH
2
COOH
+
H O
3
C
CH2
CH2
COOH
+
H O
2
+
[
S ]
[S]
OH
C
OH
k2
C
CH2
CH2
COOH
+
H
2
O
CH
CH2
COOH
H O
+ 3
+
[
S ]
[ E ]
OH
C
CH
CH2
COOH
OH
C
k3
CH2
CH2
COOH
H
[
E ]
Slow
O
O
O
O
F
Cr
Cr
F
ONH(C H )
3
7 3
ONH(C H )
3
7 3
[ F ]
OH
O
C
O
CH2
CH2
+
COOH
Fast
COOH
Cr
F
O
H O
2
ONH(C H )
3 7 3
+
Cr
Cr (IV)
F
ONH(C H )
3
7 3
+
Other products
[
F ]
Scheme-I: Mechanism of oxidation of 4-oxo acid by TriPAFC

Vol. 29, No. 7 (2017)
Oxidation of Some para-Substituted 4-Oxo-4-phenyl ButanoicAcids by Tripropylammonium Fluorochromate 1621
5
6
.
.
S.S. Mansoor and B.H. Asghar, J. Indian Chem. Soc., 90, 1395 (2013).
S.S. Mansoor and S.S. Shafi, React. Kinet. Mech. Catal., 100, 21 (2010);
https://doi.org/10.1007/s11144-010-0148-4.
G. Ghammamy, K. Mehrani, H. Afrand and M. Hajighahrammani, Afr. J.
Pure Appl. Chem., 1, 8 (2007).
K. Mahanpour, S. Ghammamy, R. Rahimi, S.Asili, F. Siavoshifar, F. Imani
and A. Hematimoghadam, Asian J. Chem., 21, 4404 (2009).
S. Ghammamy, S. Khorsandtabar, A. Moghimi and H. Sahebalzamani,
J. Mex. Chem. Soc., 53, 41 (2009).
with increase in the acetic acid content of the solvent medium
establishes the involvement of the enol form of the oxo acid
in the reaction, as the enolization of a keto group is facilitated
by a lowering of the dielectric constant of the medium. Based on
these kinetic observations, the following mechanism (Scheme-
I) is proposed.
7
8
9
.
.
.
The above mechanism leads to the following rate law:
+
10. S.S. Mansoor and S.S. Shafi, J. Mol. Liq., 155, 85 (2010);
-
d [TriPAFC]/dt = k
1
k
2
k [4-oxo acid] [TriPAFC] [H ]
3
https://doi.org/10.1016/j.molliq.2010.05.012.
1
1. S.S. Mansoor, E-J. Chem., 8, 643 (2011);
Conclusion
https://doi.org/10.1155/2011/945236.
In this paper, the detailed mechanism of oxidation of 4-
oxo acid and some para-substituted 4-oxo acid by TriPAFC is
reported.The reaction is first order each in [4-oxo acid], [TriPAFC]
12. G. Sikkandar and K.A.B. Ahmed, Indian J. Chem., 38A, 183 (1999).
1
3. S. Kavitha, A. Pandurangan and I. Alphonse, Indian J. Chem., 44A,
15 (2005).
7
1
4. N.A.M. Farook, J. Iranian Chem. Soc., 3, 378 (2006);
+
and [H ]. The oxidation of para-substituted 4-oxo acids yield
https://doi.org/10.1007/BF03245962.
the corresponding benzoic acids. The negative ρ values obtained
from the Hammett plot reveals that a positively charged reactive
intermediate is formed during the oxidation process. Similarly,
the negative value of ∆S provided support for the formation
of activated complex in the slow step.
15. N.A.M. Farook, J. Solution Chem., 36, 345 (2007);
https://doi.org/10.1007/s10953-006-9116-z.
1
6. N.A.M. Farook, S. Manochitra and A.A. Banu, J. Solution Chem., 42,
39 (2013);
https://doi.org/10.1007/s10953-012-9942-0.
2
#
17. N.A.M. Farook and G.A.S. Dameem, E-J. Chem., 8, 561 (2011);
https://doi.org/10.1155/2011/697973.
ACKNOWLEDGEMENTS
18. S.S. Mansoor, S.S. Shafi and S.Z.Ahmed, Arab. J. Chem., 9, S557 (2016);
https://doi.org/10.1016/j.arabjc.2011.06.026.
One of the authors (A.Yogananth) expresses his gratitude
to Research and Development Centre, Bharathiar University,
Coimbatore, India, for the facilities and support. The author
Mansoor is thankful to the Management of C. Abdul Hakeem
College (Autonomous), Melvisharam, India for the support.
19. S.Z.Ahmed, S.S. Shafi and S.S. Mansoor, Asian J. Chem., 25, 8245 (2013);
https://doi.org/10.14233/ajchem.2013.13559.
2
0. S. Hemalatha, B.H.Asghar and S.S. Mansoor, Asian J. Chem., 29, 810 (2017);
https://doi.org/10.14233/ajchem.2017.20318.
2
1. S.S. Mansoor, V.S. Malik, K. Aswin, K. Logaiya and A.M. Hussain, J.
Saudi Chem. Soc., 20, S77 (2016);
https://doi.org/10.1016/j.jscs.2012.09.013.
2
2. V.S. Malik, B.H. Asghar and S.S. Mansoor, J. Taibah Univ. Sci., 10, 131
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