Oxidation of some methoxy anilines by tetrabutylammoniumbromochromate in aqueous acetic acid medium: A kinetics study

Article abstract of DOI:10.14233/ajchem.2017.20656

The oxidation of aniline (A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA) by tetrabutylammoniumbromochromate (TBABC) have been studied in 50 % acetic acid – 50 % water medium in the presence of perchloric acid. The oxidation leads to the formation of the corresponding azobenzenes. The reaction is first order with respect to [TBABC], [S] and [H⁺]. The reaction has been found to be catalyzed by H⁺ ions. The reactions were studied at four different temperatures and thermodynamic parameters were calculated. The rates decreased in the order: 2,4,6-TMA > 2,6-DMA > 2,4-DMA > 3,4,5-TMA > aniline.

Full text of DOI:10.14233/ajchem.2017.20656

Asian Journal of Chemistry; Vol. 29, No. 8 (2017), 1806-1810
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2017.20656
Oxidation of Some Methoxy Anilines by Tetrabutylammoniumbromochromate in
Aqueous Acetic Acid Medium: A Kinetics Study
1
2
3,*
SHAIK JABIR , BASIM H. ASGHAR and S. SHEIK MANSOOR
1Research and Development Centre, Bharathiar University, Coimbatore-641 046, India
2Department of Chemistry, Faculty of Applied Sciences, Umm Al-Qura University, P.O. Box: 9569, Makkah, Saudi Arabia
3Department of Chemistry, C. Abdul Hakeem College (Autonomous), Melvisharam-632 509, India
*Corresponding author: E-mail: smansoors2000@yahoo.co.in
Received: 18 March 2017;
Accepted: 12 April 2017;
Published online: 12 June 2017;
AJC-18446
The oxidation of aniline (A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-
TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA) by tetrabutylammoniumbromochromate (TBABC) have been studied in 50 % acetic
acid – 50 % water medium in the presence of perchloric acid. The oxidation leads to the formation of the corresponding azobenzenes. The
reaction is first order with respect to [TBABC], [S] and [H+]. The reaction has been found to be catalyzed by H+ ions. The reactions were
studied at four different temperatures and thermodynamic parameters were calculated. The rates decreased in the order: 2,4,6-TMA >
2,6-DMA > 2,4-DMA > 3,4,5-TMA > aniline.
Keywords: Tetrabutylammoniumbromochromate, Methoxy aniline, Thermodynamic parameters, Kinetics.
was subsequently used by various workers without analyzing
the structure of the oxidant. Corey in his novel attempt to
establish pyridiniumchlorochromate [3] as a versatile oxidant,
revisited Sarett’s reagent and discovered it to be pyridinium
dichromate [4]. Significant improvements are achieved in the
development of new Cr(VI) based oxidizing agents, such as
pyridiniumfluorochromate [5], benzimidazoliumfluoro-
chromate [6], tetrahexylammoniumbromochromate [6], triethyl-
ammoniumchlorochromate [7], tetraethyl ammonium bromo-
chromate [9], tetraethylammoniumchlorochromate [10] and
tetrabutylammoniumbromochromate [11].
INTRODUCTION
Aniline is one of the hazardous chemicals that are environ-
mentally persistent and cannot be degraded by traditional
treatment processes. It causes metheglobinemia and hemolysis;
these changes can be detected by blood tests or by the colour
and appearance of the blood. Children may be more vulnerable
to loss of effectiveness of hemoglobin because of their relative
anemia, higher metabolic rate and greater sensitivity to hypoxia
compared to adults. The elderly are more vulnerable due to
limited cardiovascular reserves.
Inhalation of aniline can cause respiratory tract irritation
with cough, or difficulty in breathing. Persons exposed to aniline
may have chronic effects due to the persistence of acutely
produced damage to the brain, heart and kidneys. Wastewaters
contaminated with aniline from pesticides, dyestuffs, anti-
oxidants and pharmaceuticals can pose adverse impacts on
receiving waters due to its biorefractory and highly toxic pro-
perties [1].
The oxidation of aromatic amines by different oxidants have
been the subject of study by various workers due to the complex
behaviour of their mode of oxidation due to the formation of
polymeric products, many of which find application in drug and
dyestuff industries [12-16]. Aniline on oxidation gives azoben-
zenes which are important reagents in organic synthesis and are
widely used in the synthesis of organic dyes, food additives,
indicators and also in drugs [17].
Chromium(VI) compounds are widely used as oxidation
agents. Water-soluble potassium and sodium dichromates with
strong acids were used as oxidants and in most of the cases
the products were non-specific. The first attempt to make a
mild reagent of substituted chromates was reported by Sarett
and co-workers [2]. They used pyridine to form a salt with
CrO3 in order to oxidize some steroidal alcohols. This reagent
The present study focuses on the study of kinetics and
mechanism of oxidation of aniline, 2,6-dimethoxy aniline, 2,4-
dimethoxy aniline, 2,4,6-trimethoxyaniline and 3,4,5-
trimethoxy aniline by tetrabutylammoniumbromochromate
(TBABC) in aqueous acetic acid media. Anilines (aromatic
amines) are the most widespread and principal contaminants
of industrial wastewaters. Better understanding of the

Vol. 29, No. 8 (2017) Oxidation of Some MethoxyAnilines by Tetrabutylammoniumbromochromate inAqueousAceticAcid Medium 1807
mechanism of oxidation of such compounds/contaminants to
harmless products is the important goal for basic research and
industrial applications.
The rate and other experimental data were obtained for aniline
(A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-dimethoxy aniline
(2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-TMA) and 3,4,5-
trimethoxy aniline (3,4,5-TMA). The pseudo-first-order rate
constants are given in Table-1. The results are summarized
here.
EXPERIMENTAL
Aniline, 2,6-dimethoxy aniline, 2,4-dimethoxy aniline,
2,4,6-trimethoxy aniline and 3,4,5-trimethoxy aniline were
used as substrates. All the chemicals used were reagent grade
products. The oxidant TBABC was prepared from chromium(VI)
oxide in MeCN and addition of this solution to a solution of
tetrabutylammonium bromide as reported in the literature [11].
Acetic acid was purified by standard method and the fraction
distilling at 118 °C was collected.
Order of reaction: The oxidation of organic compounds
with high selectivity is of extreme importance in synthetic
chemistry. Reactions were carried out under pseudo first-order
conditions with a known excess of [aniline] over [TBABC] at
constant temperature of 303 K in 50 % acetic acid and 50 %
water medium.
The values of k1 were independent of the initial concen-
tration of TBABC (Table-1) suggesting the reactions were of
first order with respect to TBABC. The reaction was catalyzed
by hydrogen ions and the order with respect to [H+] was one.
The k1 values measured at various initial concentrations
of aniline for the oxidation show a linear increase (Table-1).
The plot of log k1 versus log [aniline] is excellently linear (r =
0.999) with unit slope, indicating that the order of the reaction
with respect to [aniline] is one (Fig. 1). This is further evi-
Kinetic measurements: Solutions were prepared in
double-distilled water. The reaction mixtures, containing ani-
line, 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
TBABC solution of the requisite concentration. The rate was
studied spectrophotometrically by monitoring the disappea-
rance of [TBABC] at 364 nm using UV-visible spectrophoto-
meter, Shimadzu UV-1800 model in 50 % acetic acid – 50 %
water (v/v).All of the reactions were carried out under pseudo-
first-order conditions by keeping an excess (tenfold or greater)
of [Aniline] over [TBABC] i.e. [Aniline] >> [TBABC)]. The
pseudo-first-order rate constants (k1) were computed from
linear plots of log [TBABC] against time up to 90 % completion
of the reaction. The precision of rate constant values is given
in terms of 95 % the confidence limit of the student’s t-test
[18] and the values were reproducible to within 5 %.
2.1
2.0
2,4,6-TMA
1.9
2,6-DMA
1.8
1.7
2,4-DMA
1.6
1.5
1.4
3,4,5-TMA
1.3
1.2
Stoichiometry and product analysis: The products in
the respective oxidation of methoxyanilines were corre-
sponding azobenzenes and this were confirmed by analysing
using preparative TLC on silica gel. The stoichiometric studies
showed that 1 mol of TBABC reacts with 1 mol of aniline.
Aniline
1.1
1.0
0.9
0.8
0.7
1.0
1.1
1.2
1.3
1.4
1.5
RESULTS AND DISCUSSION
3 + log [S]
Fig. 1. The order plot of aniline (A), 2,4-dimethoxy aniline (2,4-DMA),
2,6-dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-
TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA)
The reaction was studied in 50 % acetic acid - 50 % water
medium at 303 K, under the pseudo-first-order conditions.
TABLE-1
EFFECT OF VARIATION OF [S], [TBABC] AND [H+] ON THE RATE OF REACTION AT 303 Ka
104 k1 (s–1)b
103 [TBABC]
(mol dm–3)
102 [S]
(mol dm–3)
[H+]
(mol dm–3)
H
2,4-DMA
35.50
35.56
35.58
35.60
35.54
17.68
26.58
44.40
53.26
22.22
44.52
57.82
66.72
35.38c
28.42d
2,6-DMA
42.68
42.76
42.70
42.74
42.72
21.30
32.00
53.36
64.08
26.80
53.52
69.56
80.26
42.56c
2,4,6-TMA
70.80
3,4,5-TMA
10.80
10.88
10.94
10.86
10.82
5.40
0.6
1.0
1.6
2.0
2.6
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
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
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.16
0.10
0.20
0.26
0.30
0.16
0.16
10.08
10.16
10.18
10.06
10.20
5.04
70.84
70.76
70.72
70.86
33.30
7.50
53.06
8.08
12.60
15.16
6.40
88.48
13.52
16.24
6.82
106.16
44.34
12.60
16.48
19.00
10.04c
7.98d
88.64
13.56
17.58
20.32
10.72c
115.18
132.88
70.56c
56.64d
35.24d
8.32d
b
c
aAs determined by spectrophotometrically following the disappearance of Cr(VI) at 364 nm; Estimated from pseudo first order plots; In the
presence of 0.001 mol dm-3 acrylonitrile; dIn the presence of 0.003 mol dm-3 Mn(II); Solvent composition = 50 % AcOH- 50 % H2O (v/v)

1808 Jabir et al.
Asian J. Chem.
2,4,6-TMA
denced by the excellent linearity observed in the plot of k1
versus [aniline], which passes through origin (Fig. 2).
2.0
1.8
1.6
1.4
1.2
1.0
0.8
2,6-DMA
120
2,4,6-TMA
2,4-DMA
100
80
2,6-DMA
3,4,5-TMA
Aniline
60
2,4-DMA
40
3,4,5-TMA
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
20
1/D
Aniline
Fig. 3. Plot of 1/D against log k1 showing effect of solvent polarity for the
oxidation of aniline (A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-
dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-
TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA) by TBABC
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
102 [S], M
Fig. 2. Direct plot of aniline (A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-
dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-
TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA)
Activation parameters: The first-order rate constants for
the oxidation of aniline, 2,6-DMA, 2,4-DMA, 2,4,6-TMA and
3,4,5-TMA with TBABC at four different temperatures were
collected in Table-3. A linear Arrhenius plot of log k2 versus
1/T is obtained. From the Eyring relationship, various thermo-
dynamic parameters were calculated and the values were pre-
sented in Table-4.
Effect of acrylonitrile and MnSO4: The involvement of
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).
Effect of solvent polarity on reaction rate: In binary
mixtures of acetic 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
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
remarkably. A plot of log k1 versus 1/D has been given in Fig. 3.
The linear plot obtained indicates that the involvement of cation-
dipole type of interaction in the rate determining step [19].
Structure reactivity correlation: The effect of structure
on reactivity indicates that, the reactivity decreases in the order:
2,4,6-TMA > 2,6-DMA > 2,4-DMA > 3,4,5-TMA > aniline
for the substituents. Methoxy group at ortho [σ(OCH3) =
– 0.39] and para [σ(OCH3) = – 0.27] to aniline enhances the
rate while at meta [σ(OCH3) = + 0.12] reduces the rate.
Thus for 2,4-DMA [σ(OCH3) = – 0.66] < 2,6-DMA
[σ(OCH3) = – 0.78] and also for 3,4,5 - TMA [σ(OCH3) =
– 0.03] < 2,4,6 - TMA [σ(OCH3) = – 1.05].
TABLE-3
PSEUDO-FIRST ORDER RATE CONSTANTSFOR THE
OXIDATION OFANILINE, 2,4-DMA, 2,6-DMA, 2,4,6-TMA
AND 3,4,5-TMA BY TBABC AT VARIOUS TEMPERATURES
IN AQUEOUS ACETIC ACID MEDIUM
104 k1 (s–1)
Substrate
298 K
303 K
308 K
313 K
H
7.08
26.92
32.88
55.30
7.78
10.16
35.56
42.76
70.84
10.88
14.50
46.88
55.56
90.52
15.24
20.40
61.92
72.24
115.88
21.32
2,4-DMA
2,6-DMA
2,4,6-TMA
3,4,5-TMA
102 [S] = 2.0 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 1.6 mol
dm-3; Solvent composition = 50 % AcOH - 50 % H2O (v/v)
Isokinetic temperature: An Exner’s plot [20] between
log k2 at 298 K and at 303 K was linear (Fig. 4). The value of
isokinetic temperature evaluated from the Exner’s plot is 448
K. From the linear isokinetic relationship, it is implied that
same mechanism is applicable for the oxidation of all the
methoxy anilines [21].
TABLE-2
EFFECT OF VARYING SOLVENT POLARITY ON THE RATE OF REACTION OF
ANILINE, 2,4-DMA, 2,6-DMA, 2,4,6-TMA AND 3,4,5-TMA BY TBABC AT 303 K
104 k1 (s–1)
% AcOH-H2O
(v/v)
Dielectric
constant
H
2,4-DMA
28.16
2,6-DMA
33.86
2,4,6-TMA
56.10
3,4,5-TMA
8.56
30-70
40-60
50-50
60-40
70-30
72.0
63.3
56.0
45.5
38.5
7.80
9.06
31.28
37.62
62.34
9.52
10.16
12.20
15.40
35.56
42.76
70.84
10.88
12.56
15.32
41.20
49.60
82.12
50.32
60.50
100.22
102 [S] = 2.0 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 1.6 mol dm-3

Vol. 29, No. 8 (2017) Oxidation of Some MethoxyAnilines by Tetrabutylammoniumbromochromate inAqueousAceticAcid Medium 1809
TABLE-4
ACTIVATION PARAMETERS AND SECOND ORDER RATE CONSTANTS FOR THE OXIDATION OFANILINE,
2,4-DMA, 2,6-DMA, 2,4,6-TMA AND 3,4,5-TMA BY TBABC IN AQUEOUS ACETIC ACID MEDIUM
102 k2 (dm3 mol–1 s–1)
∆H#
(kJ mol–1)
∆S#
(JK–1 mol)
∆G# (kJ mol–1)
at 303 K
Ea
Substrate
(kJ mol–1)
298 K
303 K
308 K
313 K
H
3.54
13.46
16.44
27.65
3.89
5.08
17.78
21.38
35.42
5.44
7.25
23.44
27.78
45.26
7.62
10.20
30.96
36.12
57.94
10.66
54.94
43.26
40.78
38.29
52.26
52.30 0.2
40.58 0.6
38.29 0.4
35.80 0.2
49.78 0.3
96.86 0.6
125.19 1.8
131.51 1.2
135.53 0.6
104.90 0.9
81.63 0.4
78.51 1.2
78.14 0.8
76.86 0.4
81.56 0.6
2,4-DMA
2,6-DMA
2,4,6-TMA
3,4,5-DMA
102 [S] = 2.0 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 1.6 mol dm-3 ; Solvent composition = 50 % AcOH - 50 % H2O (v/v)
Mechanism: The reaction rate increases linearly with the
increase in [H+] ion and hence protonated TBABC is formed.
The rate enhancement is observed in the present study with
increase in the acetic acid content of the solvent medium. The
corresponding methoxyazobenzenes were obtained as products
for the oxidation of methoxy anilines. The involvement of free-
radical intermediates in the present study can be excluded, as
the rate constant is not affected by the addition of acrylonitrile.
The addition of Mn(II) in the form of MnSO4, retards the rate
of oxidation, indicating the involvement of a two-electron
reduction of Cr(VI) to Cr(IV).A linear plot of log k1 versus
1/D with positive slope indicates that the involvement of a
cation-dipole type of interaction in the rate determining step.
The product formation is in accordance with the literature
report [22]. The sequence of the oxidation is similar to that
reported in the literature [23]. The negative entropy of acti-
vation in conjunction with other kinetic observations supports
the mechanism (Scheme-I).
1.6
1.4
1.2
1.0
0.8
0.6
2,4,6-TMA
2,6-DMA
2,4-DMA
3,4,5-TMA
Aniline
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2 + log k at 298 K
2
Fig. 4. Exner’s plot for the oxidation of aniline (A), 2,4-dimethoxy aniline
(2,4-DMA), 2,6-dimethoxy aniline (2,6-DMA), 2,4,6-trimethoxy
aniline (2,4,6-TMA) and 3,4,5-trimethoxy aniline (3,4,5-TMA)by
TBABC between 2 + log k2 (303 K) and 2 + log k2 (298 K)
O
O
O N(C4H9)4
Br
HO
O
O N(C4H9)4
Br
k1
Cr
Cr
k-1
TBABC
TBABCH+
H
N
H
O N(C4H9)4
O
O
HO
O N(C4H9)4
k2
k-2
Cr
Br
Cr
+
NH2
O
Br
H
N
H
O N(C4H9)4
O N(C4H9)4
Cr
Br
O
k3
+
NHOH
NO
Cr
Br
O
O
O
+
H2O
NHOH
NH2
+
ON
N
N
+
H2O
Azobenzene
Scheme-I: Mechanism of oxidation of aniline by tetrabutylammoniumbromochromate (TBABC)

1810 Jabir et al.
Asian J. Chem.
6. S.S. Mansoor, Asian J. Chem., 22, 7591 (2010).
Conclusion
7. S.S. Mansoor and B.H. Asghar, J. Indian Chem. Soc., 90, 1395 (2013).
8. S. Ghammamy and S. Dastpeyman, J. Chin. Chem. Soc. (Taipei), 55,
229 (2008);
In this work, the detailed kinetics of oxidation of aniline
(A), 2,4-dimethoxy aniline (2,4-DMA), 2,6-dimethoxy aniline
(2,6-DMA), 2,4,6-trimethoxy aniline (2,4,6-TMA) and 3,4,5-
trimethoxy aniline (3,4,5-TMA) in acetic acid–water medium
has been studied by spectrophotometrically at 303 K using
TBABC as an oxidant. Methoxy group at ortho [σ(OCH3) =
– 0.39] and para [σ(OCH3) = – 0.27] to aniline enhances the
rate while at meta [σ(OCH3) = + 0.12] reduces the rate. Higher
rate is observed for 2,4,6-trimethoxyaniline and lower rate for
aniline.
https://doi.org/10.1002/jccs.200800034.
9. S.S. Mansoor and S.S. Shafi, Z. Phys. Chem., 225, 249 (2011);
https://doi.org/10.1524/zpch.2011.0044.
10. P. Swami, D.Yajurvedi, P. Mishra and P.K. Sharma, Int. J. Chem. Kinet.,
42, 50 (2010);
https://doi.org/10.1002/kin.20466.
11. G. Ghammamy, K. Mehrani, H. Afrand and M. Hajighahrammani, Afr.
J. Pure Appl. Chem., 1, 8 (2007).
12. S.B. Patwari, S.V. Khansole andY.B. Vibhute, J. Iran. Chem. Soc, 6, 399
(2009);
https://doi.org/10.1007/BF03245850.
13. V.K. Gupta, React. Catal. Lett., 27, 207 (1985);
https://doi.org/10.1007/BF02064488.
ACKNOWLEDGEMENTS
14. S.S. Mansoor and S.S. Shafi, Arab. J. Chem., 7, 171 (2014);
https://doi.org/10.1016/j.arabjc.2010.10.020.
15. D.S. Bhuvaneswari and K.P. Elango, Int. J. Chem. Kinet., 38, 657 (2006);
https://doi.org/10.1002/kin.20199.
One of the authors, Shaik Jabir expresses his gratitude to
the Research and Development Centre, Bharathiar University,
Coimbatore, India for the facilities and support.Another author
S.S. Mansoor is thankful to the Management of C. Abdul
HakeemCollege(Autonomous),Melvisharam, India for the support.
16. D.S. Bhuvaneshwari and K.P. Elango, Z. Naturforsch., 60b, 1105 (2005).
17. C.A.H. Aguilar, J. Narayanan, N. Singh and P. Thangarasu, J. Phys. Org.
Chem., 27, 440 (2014);
https://doi.org/10.1002/poc.3281.
REFERENCES
18. C. Srinivasan, S. Rajagopal andA. Chellamani, J. Chem. Soc. Perkin Trans.
II, 1839 (1990);
https://doi.org/10.1039/p29900001839.
1. N. Boonrattanakij, M.C. Lu and J. Anotai, J. Hazard. Mater., 172, 952
(2009);
19. E.S.Amis, Solvent Effects on Reaction Rates and Mechanisms,Academic
Press, New York, p. 42 (1967).
https://doi.org/10.1016/j.jhazmat.2009.07.079.
2. G.I. Poos, G.E. Arth, R.E. Beyler and L.H. Sarett, J. Am. Chem. Soc.,
75, 422 (1953);
20. O. Exner, J.R. Streitwiser and R.W. Talt, Progress in Physical Organic
Chemistry, John Wiley, New York, p. 41 (1973).
21. J.F. Leffler and E. Grunwald, Rates and Equilibrium of Organic Reactions,
Wiley, New York (1963).
https://doi.org/10.1021/ja01098a049.
3. E.J. Corey and J.W. Suggs, Tetrahedron Lett., 16, 2647 (1975);
https://doi.org/10.1016/S0040-4039(00)75204-X.
4. E.J. Corey and G. Schmidt, Tetrahedron Lett., 20, 399 (1979);
https://doi.org/10.1016/S0040-4039(01)93515-4.
5. S.Z. Ahmed, S.S. Shafi and S.S. Mansoor, Asian J. Chem., 25, 8245
(2013);
22. T. Alam, H. Tarannum, S.R. Ali and Kamaluddin, J. Colloid Interface
Sci., 245, 251 (2002);
https://doi.org/10.1006/jcis.2001.7968.
23. S. Jabir, B.H.Asghar and S.S. Mansoor, Orient. J. Chem., 33, 288 (2017);
https://doi.org/10.13005/ojc/330134.
https://doi.org/10.14233/ajchem.2013.13559.