Kinetics and thermodynamics of oxidation of some meta-substituted anilines by tetrabutylammonium bromochromate in aqueous acetic acid medium

Article abstract of DOI:10.13005/ojc/330134

The tetrabutylammonium bromochromate (TBABC) oxidation of anilines, in an aqueous acetic acid medium in the presence of perchloric acid is described. The reaction is first order with respect to aniline, TBABC and acid. The reaction rate has been determined at different temperatures and activation parameters calculated. The TBABC oxidation of meta-substituted anilines obeys Hammett relationships.

Full text of DOI:10.13005/ojc/330134

ISSN: 0970-020 X
CODEN: OJCHEG
017, Vol. 33, No. (1):
Pg. 288-295
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
2
www.orientjchem.org
Kinetics and Thermodynamics of Oxidation of Some
meta-substituted Anilines by Tetrabutylammonium
Bromochromate in Aqueous Acetic Acid Medium
1
2
3
SHAIK JABIR , BASIM H. ASgHAR and S. SHEIK MANSOOR *
1
Research and Development Centre, Bharathiar University,
Coimbatore – 641 046, Tamil Nadu, India.
Department of Chemistry, Faculty of Applied Sciences,
2
Umm Al-Qura University, P.O. Box: 9569, Makkah, Saudi Arabia.
Department of Chemistry, C. Abdul Hakeem College (Autonomous),
3
Melvisharam - 632 509, Tamil Nadu, India.
*
Corresponding author E-mail: smansoors2000@yahoo.co.in
http://dx.doi.org/10.13005/ojc/330134
(Received: November 30, 2016; Accepted: December 12, 2016)
ABSTRACT
The tetrabutylammonium bromochromate (TBABC) oxidation of anilines, in an aqueous acetic
acid medium in the presence of perchloric acid is described. The reaction is first order with respect
to aniline, TBABC and acid. The reaction rate has been determined at different temperatures and
activation parameters calculated.TheTBABC oxidation of meta-substituted anilines obeys Hammett
relationships.
Keywords: Tetrabutylammonium bromochromate; Aniline; Thermodynamic parameters; Kinetics.
INTRODUCTION
agents. Therefore, a serious effect on human health
over a long period of time is possible, even if aniline
1
Aniline is known to be carcinogenic and also
reacts easily in the blood to convert haemoglobin into
methaemoglobin, preventing oxygen uptake.Aniline
and its derivatives are used as intermediates for the
manufacturing of various organic compounds such
as colorants, agrochemicals and pharmaceutical
is in low concentrations .
Aniline can enter the body either by
inhalation of air containing aniline, ingestion of food
or water containing aniline, or by dermal contact
with aniline. Inhalation of air containing aniline can

289
JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
cause respiratory tract irritation. Exposure to high compounds. Chromium compounds especially
levels can cause a number of adverse health effects Cr(VI) reagents have been versatile reagents and
including breathing difficulties, dizziness, headache, capable of oxidizing almost all the oxidisible organic
irregular heart-beat, methaemoglobinaemia (blood functional groups. The development of newer
disorder), blue colour to the skin, convulsions and chromium (VI) reagents for the oxidation of organic
3
in extreme cases coma and death. Ingestion of substrates continues to be of interest . In recent
aniline can lead to gastrointestinal irritation, nausea, years, many such reagents have been developed
vomiting and diarrhea. Exposure to high levels may with some success. Some new Cr(VI) based
4
lead to effects similar to those for inhalation. Dermal reagents like tetraheptylammonium bromochromate ,
5
contact with aniline can cause skin irritation and tripropyl ammonium fluorochromate , isoquinolinium
6
sensitization.Absorption of large quantities of aniline bromochromate , tetrahexylammonium
7
8
through the skin may cause effects similar to those bromochromate , benzimidazolium fluorochromate
9
for inhalation. There is an increased risk in young and tetrabutylammonium bromochromate have
infants of developing methaemoglobinaemia due to been used to study the oxidation of various organic
2
aniline exposure .
compounds.
Oxidation is an essential reaction for
different organic synthesis. Chromium compounds oxidizing agents have been extensively studied
The oxidation kinetics of anilines by various
10-14
.
have been used in aqueous and non-aqueous Since anilines are very harmful to human health
medium for the oxidation of a variety of organic and the environment, removal of aniline from the
Table 1: Effect of variation of [S], [TBABC] and [H ] on the rate of reaction at 303 Ka
+
03
TBABC]
102
[S]
10 k1 (s )b
4
-1
1
[
(
[H+]
-
3
-3
-3
mol dm ) (mol dm ) (mol dm )
H
m-OCH3 m-OC H5 m-CH3 m-F
m-Cl m-NO2
2
0
1
1
2
2
.6
.0
.6
.0
.6
2.0
2.0
2.0
2.0
2.0
0.16
0.16
0.16
0.16
0.16
10.08
10.16
10.18
10.06
10.20
8.04
8.08
8.14
8.10
8.06
9.80
9.88
9.84
9.90
9.86
11.50
11.52
11.60
11.56
11.54
5.52
5.56
5.58
5.50
5.60
5.10
5.14
5.18
5.16
5.20
2.76
2.78
2.74
2.72
2.80
1
1
1
1
.0
.0
.0
.0
1.0
1.5
2.5
3.0
0.16
0.16
0.16
0.16
5.04
7.30
12.60
15.16
4.12
6.14
10.18
12.20
4.88
7.40
12.42
14.78
5.78
8.66
14.32
17.22
2.82
4.14
6.92
8.38
2.54
3.88
6.44
7.78
1.42
2.12
3.48
4.18
1
1
1
1
1
1
.0
.0
.0
.0
.0
.0
2.0
2.0
2.0
2.0
2.0
2.0
0.10
0.20
0.26
0.30
0.16
0.16
6.40
12.60
16.48
19.00
5.08
10.16
13.22
15.20
6.22
12.38
16.10
18.48
7.22
14.36
18.80
3.44
6.98
9.08
3.22
6.38
8.36
9.66
1.76
3.48
4.52
5.22
21.64 10.46
c
c
c
c
c
c
c
10.10
8.00
9.80
11.58
8.64
5.50
4.22
5.18
2.72
d
d
d
d
d
d
d
7.62
6.08
7.42
3.86
2.08
a
As determined by spectrophotometrically following the disappearance of Cr(VI) at 362 nm;
b
Estimated from pseudo first order plots
c
-3
In the presence of 0.001 mol dm acrylonitrile;
d
-3
In the presence of 0.003 mol dm Mn (II);
Solvent composition = 50% AcOH- 50% H O (v/v)
2

290
JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
environment is the ultimate goal of research today.For
this a deep understanding of the mechanism of the
EXPERIMENTAL
oxidation process of aniline is needed.Furthermore, Materials and reaꢀents
one of the important tools in deciding the mechanism
All the employed chemicals and solvents
of reactions is the study of substituent effects and were of analytical grade. Anilines were used with
thermodynamic parameters.The Hammett equation, substituents H, m-OCH , m-OC H , m-CH , m-F, m-Cl
3
2
5
3
also known as Linear Free Energy Relationships and m-NO . Tetrabutylammonium bromochromate
2
9
(
LFER) have been found useful for correlating (TBABC) was prepared by a reported method and its
reaction rates and equilibrium constants for meta- purity was checked by the iodometric method.Doubly
and para-substituted derivatives of benzene. The distilled water was used for all purposes. The solid
isokinetic relationship is also an important tool for anilines were used as such and the liquid anilines
14
deciding the nature of a mechanism . Keeping this were used after vacuum distillation. Acetic acid was
in view, we report the kinetic features of the oxidation purified by standard method and the fraction distilling
o
of a series of meta-substituted anilines by TBABC in at 118 C was collected.
an aqueous acetic acid medium to propose a suitable
mechanism.
Table 2: Effect of varyinꢀ solvent polarity on the rate of reaction of aniline
m-OCH , m-OC H , m-CH , m-F, m-Cl and m-NO anilines by TBABC at 303 K
3
2
5
3
2
4
-1
1
0 k ( s )
1
%AcOH- Dielectric
H O (v/v) constant
H
m-OCH3 m-OC H5 m-CH3
m-F
m-Cl
m-NO2
2
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
7.80
9.06
10.16
12.20
15.40
6.56
7.24
8.08
9.38
11.46
7.86
8.68
9.88
11.42
13.94
9.26
10.26
11.52
13.38
13.94
4.52
5.00
5.56
6.42
16.30
4.16
4.66
5.14
5.98
7.84
2.26
2.52
2.78
3.24
7.32
2
-3
3
-3
+
-3
1
0 [S] = 2.0 mol dm ; 10 [TBABC] = 1.0 mol dm ; 10 [H ] = 1.6mol dm
Fiꢀ. 1: Showinꢀ plot of 1/D aꢀainst loꢀ k1 showinꢀ effect of solvent
polarity at various temperatures in the presence of oxalic acid

JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
291
Kinetic measurements
The pseudo – first-order conditions were
Data analysis
Correlation analysis were carried out using
Microcal Origin (Version 6.1) computer software.The
goodness of the fit is discussed using the correlation
coefficients and standard deviations.
attained by maintaining a large excess ( x 15 or
more) of aniline over TBABC. The solvent was 50%
acetic acid – 50% water (v/v), unless specified
otherwise.The reactions were followed, at constant
temperatures (± 0.01 K), by monitoring the decrease
in [TBABC] spectrophotometrically at 362 nm using
UV–Vis spectrophotometer, Shimadzu UV-1800
model.
Stoichiometry and Product analysis
The stoichiomety of the reaction was
determined by carrying out several sets of
experiments with varying amounts ofTBABC largely
in excess over aniline. The estimation of unreacted
Table 3: Pseudo-first order rate constants for the oxidation of aniline,
m-OCH , m-OC H , m-CH , m-F, m-Cl and m-NO anilines by TBABC at
3
2
5
3
2
various temperatures in aqueous acetic acid medium
4
-1
1
0 k (s )
1
Substrate
298 K
303 K
308 K
313 K
m-H
m-OCH3
m-OC H5
m-CH3
m-F
7.08
5.52
6.86
8.24
3.74
3.40
1.82
10.16
8.08
9.88
11.52
5.56
5.14
2.78
14.50
11.80
14.24
16.12
8.28
20.40
17.24
20.48
22.58
12.34
11.72
6.52
2
m-Cl
m-NO2
7.76
4.24
2
-3
3
-3
+
-3
1
0 [S] = 2.0 mol dm ; 10 [TBABC] = 1.0 mol dm ; 10 [H ] = 1.6mol dm
Solvent composition = 50% AcOH - 50% H O (v/v)
2
Table 4: Activation parameters and second order rate constants for the oxidation of
aniline, m-OCH , m-OC H , m-CH , m-F, m-Cl and m-NO anilines by TBABC in aqueous
3
2
5
3
2
acetic acid medium
02k (dm mol s )
3
-1 -1
Ea
kJmol
∆H
#
- ∆S
#
∆g
#
1
2
-
1
-1
-1
-1
Substrate
kJmol
JK mol
kJmol
(at 303 K)
2
98K 303K 308K 313K
H
3.54
2.76
5.08
4.04
4.94
5.76
2.78
2.57
1.39
7.25 10.20 54.94
5.90 8.62 58.96
7.12 10.24 56.67
8.06 11.29 52.26
52.30±0.2 96.86±0.6 81.63±0.4
56.48±0.6 85.18±1.8 82.28±1.2
54.18±0.4 91.12±1.2 81.78±0.8
49.78±0.2 104.32±0.6 81.38±0.4
59.35±0.4 78.86±1.2 83.24±0.8
61.65±0.6 71.96±1.8 83.45±1.2
63.56±0.3 70.82±0.9 85.02±0.6
m-OCH3
m-OC H5 3.43
m-CH3
m-F
m-Cl
2
4.12
1.87
1.70
0.91
4.14
3.88
2.12
6.17
5.86
3.26
61.84
62.22
64.05
m-NO2
2
-3
3
-3
+
-3
1
0 [S] = 2.0 mol dm ; 10 [TBABC] = 1.0 mol dm ; 10 [H ] = 1.6mol dm
Solvent composition = 50% AcOH - 50% H O (v/v)
2

292
JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
TBABC showed that 1 mol of TBABC reacts with 1 medium at 303 K, under pseudo first order conditions
mol of aniline.The oxidative products were analysed and the result obtained were discussed in the
using preparative TLC on silica gel, which yields following paragraphs.
o
o
azobenzene m.p 66 C (Lit 68 C) and UV Abs.(EtOH)
at lmax 320 nm.
The values of k were independent of the
1
initial concentration of TBABC (Table 1) suggesting
the reactions were of first order with respect to
RESULTS AND DISCUSSION
TBABC.The reaction was catalysed by hydrogen ions
+
The oxidation of anilines by TBABC has and the order with respect to [H ] was one. Linear
been conducted in 50% acetic acid and 50% water plots of log k versus log [Substrate] with unit slope
1
demonstrate the first-order dependence of the rate
on [Aniline].
Table 5: Reaction constant values for the
oxidation of aniline, m-OCH , m-OC H , m-CH ,
3
2
5
3
m-F, m-Cl and m-NO anilines by TBABC in
aqueous acetic acid medium at different
temperatures
2
The oxidation of aniline in a nitrogen
atmosphere failed to induce the polymerization
of acrylonitrile. Furthermore, the rate of oxidation
decreased with the addition of Mn(II), indicating the
involvement of a two-electron reduction of Cr(VI) to
Cr(IV) (Table 1).Therefore, a one-electron oxidation,
giving rise to free radicals, is unlikely.
Temp.
Reaction Correlation
constant, ρ coefficient
Standard
deviation
2
3
3
3
98K
03K
08K
13K
-0.8088
-0.7648
-0.7214
-0.6696
0.990
0.988
0.991
0.992
0.12
0.14
0.09
0.16
The oxidation of aniline has been studied
in the binary mixture of acetic acid and water as the
solvent medium. The concentration of acetic acid
was varied from 30% to 70% and the rate were
measured. The reaction rate increased remarkably
with the increase in the proportion of acetic acid in
the solvent medium (Table - 2). Positive slope of
2
-3
3
-3
1
1
0 [S] = 2.0 mol dm ; 10 [TBABC] = 1.0 mol dm ;
+
-3
0 [H ] = 1.6mol dm
Solvent composition = 50% AcOH - 50% H O (v/v)
2
Fiꢀ. 2: Showinꢀ Hammett plot of loꢀ k versus substituent constant σ for the oxidation of aniline,
2
p
m-OCH , m-OC H , m-CH , m-F, m-Cl and m-NO anilines by TBABC in aqueous acetic acid medium
3
2
5
3
2
at different temperatures

JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
293
log k versus 1/D plot indicates that the reaction
medium in presence of perchloric acid and the
pseudo-first order rate constants were determined
(Table – 3). Various activation parameters were
calculated and the values were presented in Table -
4. The entropy of activation is negative for anilines.
1
involves a cation-dipole type of interaction in the rate
15
determining step (Fig. 1).
Solvent variations may affect the kinetics
and the energy of the electron transfer processes
in a complex manner, particularly in mixed solvent
media as the physico-chemical properties of mixed
solvent media are often quite different from those
of the pure solvents or of their ideal mixtures . The
dependence of the kinetic parameters for reactions
on the composition of mixed aqueous solvents often
affords complicated patterns.
The effect of structure on reactivity of meta-
substituted anilines were studied. It is interesting to
note that the reactivity decreases in the order m-CH3
> H > m-OC H > m-OCH > m-F > m-Cl > m-NO
2
16
2
5
3
for the substituents.
A linear Hammett’s plot is obtained when
substituent constant (ó) for different substituents were
Rates of oxidation of anilines were
determined at different temperatures between 298
and 313 K in 50% – 50% (v/v) acetic acid – water
plotted against log k for the oxidation of anilines by
TEABC at various temperatures.The value of slope
of Hammett plot is known as reaction constant (p).A
2
O
O
O N(C H )
4 9 4
HO
O
O N(C H )
4 9 4
k1
Cr
Cr
Br
k-1
Br
TBABC
TBABCH+
H
N
H
O N(C H )
4 9 4
HO
O
O N(C H )
k2
O
O
4
9 4
NH2
+
Cr
Cr
Br
k
-2
Br
H
O N(C H )
4 9 4
O N(C H )
4 9 4
O
O
k3
N
H
Cr
NHOH
+
Cr
Br
O
Br
O
NO
+
NHOH
H O
2
NH2
+
ON
N
N
+
H O
2
Azobenzene
Scheme 1: Mechanism of oxidation of aniline by TBABC

2
94
JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
The reaction proceeds with the formation
linear Hammett’s plot is given in Figure 2. Reaction
constant values at different temperatures are given of a complex followed by the loss of hydride on.
in Table 5. According to Hammett, the reaction The complex is formed likely by concerted transfer
with positive p values are accelerated by electron of oxygen from the oxidant to N-atom and electron
withdrawal from benzene ring, whereas those with pair from N-atom to Cr (VI). The negative ñ value
negative p values are retarded by electron withdrawal is indicative of the presence of a positive nitrogen
17
from benzene ring . In this oxidation reactions, the center, which would mean depletion of lone-pair
electron withdrawing groups decreases the rate and electron density, and this can be facilitated only
the electron donating groups increases the rate. when the oxidant forms a complex with the substrate
These observations supporting the negative p values in which the nitrogen lone-pair can be used up
obtained from the Hammett Plot. The negative p in coordinating with an electron-deficient center,
values further confirms the formation of a positively preferably a metal ion.The above mechanism leads
charged transition state.
to the following rate law:
+
Mechanism of oxidation
-d [TBABC] / dt = k k k [Aniline] [TBABC] [H ]
1
2
3
The sequence of reactions for the oxidation
of anilines by TBABC in perchloric acid is shown in
Scheme 1. The oxidation of anilines by TBABC in
acetic acid water medium is remarkably slow, but is
CONCLUSIONS
In this paper we have reported the detailed
catalyzed in the presence of perchloric acid, and the mechanism of oxidation of aniline and some para-
reaction proceeds at a comfortable rate. Catalysis meta substituted anilines by TBABC. The reaction
+
by perchloric acid suggests protonation of TBABC is first order each in [Aniline], [TBABC] and [H ].
species rather than the aniline molecule, which The oxidation of meta-substituted anilines yield
would have resulted in retardation. The reaction did the corresponding azo benzenes. The negative ñ
not promote polymerization of acrylonitrile indicating values obtained from the Hammett plot reveals that
absence of free radicals. However, the addition of a positively charged reactive intermediate is formed
Mn (II), in the form of MnSO , retards the rate of during the oxidation process. Similarly the negative
4
#
oxidation. This indicates the involvement of Cr(IV) value of “S provided support for the formation of
intermediate in the oxidation of anilines by Cr(VI) activated complex in the slow step.
reagent.
REFERENCES
1
2
3
4
.
.
.
.
Bhuvaneshwari, D.S.; Elango, K.P. Z. 7.
Naturforsch. 2006, 61b, 1254 – 1260.
Jin, Q.;Hu, Z.; Jin, Z.;Qiu, L.;Zhong, W.;Pan, 8.
Z. Bioresour. Technol. 2012, 117, 148-154.
Mansoor, S.S.;Asghar, B.H. J.Indian.Chem.
Soc. 2013, 90, 1395 - 1401.
Malik, V. S.; Vannamuthu, I.; Shafi, S. S.;
Mansoor, S. S. Oriental J. Chem. 2015,
Wiberg, W.B.Oxidation in Organic Chemisry, 31, 77-83
Academic Press, New York, 1995
9.
Ghammamy, G.; Mehrani, K.; Afrand, H.;
Hajighahrammani, M.Afr.J.Pure Appl.Chem.
2007, 1, 8 -10.
Ghammamy, G.; Mehrani, K.; Afrand, H.;
Javanshir, Z.; Rezaeibehbahani, G. R.;
Moghimi, A.; Aghbolagh, Z. S. J. Chil. Chem. 10.
Soc. 2009, 54, 491 - 494.
Yogananth, A.; Mansoor, S. S. Oriental J. 11.
Chem. 2015, 31, 17-23
Patwari, S. B.; Khansole, S. V.; Vibhute, Y. B.
J. Iran. Chem. Soc. 2009, 6, 399-404.
Gupta, V.K.React.Catal.Lett. 1985, 27, 207-
211
5
6
.
.
Vibhute, A. Y.; Patwari, S. B.; Khansole, S. 12.
V.;Vibhute,Y. B. Chin. Chem. Lett. 2009, 20,
Mansoor, S. S.; Shafi, S. S. Arab. J. Chem.
2014, 7, 171–176
256 – 260
13.
Bhuvaneswari, D.S.;Elango, K.P.Int.J.Chem.

295
JABIR et al., Orient. J. Chem., Vol. 33(1), 288-295 (2017)
Kinet. 2006, 38, 657–665
1967, 42
1
1
4.
5.
Bhuvaneshwari, D.S.; Elango, K. P. Z. 16.
Naturforsch. 2005, 60b, 1105 – 1111
Amis, E.S.Solvent Effects on Reaction Rates 17.
and Mechanisms.Academic Press, NewYork,
Bhuvaneshwari, D. S.; Elango, K. P. J. Serb.
Chem. Soc. 2008, 73 (7), 735–744
Hammett, L.P. Physical Organic Chemistry,
First Ed. McGraw-Hill, New York, 1940