Oxidation of benzhydrol by tetrabutylammoniumbromochromate in presence and absence of picolinic acid and 1,10-phenanthroline: A kinetic study

Article abstract of DOI:10.14233/ajchem.2017.20318

The oxidation of benzhydrol (BH) by tetrabutylammoniumbromochromate (TBABC) was studied in aqueous acetic acid medium. The reaction is first order with respect to tetrabutylammoniumbromochromate, benzhydrol and [H⁺] and the reaction is catalyzed by hydrogen ions. The reaction has been studied in the presence of picolinic acid and 1,10-phenanthroline as promoters. The promoters used in this oxidation reaction, picolinic acid and 1,10-phenanthroline are strong chelating ligands which form complexes with most transition metal ions. Among the two promoters oxidation is much faster with 1,10-phenanthroline. The low dielectric constant of the medium assists the complex formation and enhances the reactivity. The reactions were studied at different temperatures at different acetic acid-water composition and various thermodynamic parameters have been determined. The observed experimental results have been explained by plausible mechanisms.

Full text of DOI:10.14233/ajchem.2017.20318

Asian Journal of Chemistry; Vol. 29, No. 4 (2017), 810-816
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20318
Oxidation of Benzhydrol by Tetrabutylammoniumbromochromate in Presence
and Absence of Picolinic Acid and 1,10-Phenanthroline: A Kinetic Study
1
2
3,*
S. HEMALATHA , 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: 28 September 2016;
Accepted: 27 December 2016;
Published online: 31 January 2017;
AJC-18249
The oxidation of benzhydrol (BH) by tetrabutylammoniumbromochromate (TBABC) was studied in aqueous acetic acid medium. The
reaction is first order with respect to tetrabutylammoniumbromochromate, benzhydrol and [H+] and the reaction is catalyzed by hydrogen
ions. The reaction has been studied in the presence of picolinic acid and 1,10-phenanthroline as promoters. The promoters used in this
oxidation reaction, picolinic acid and 1,10-phenanthroline are strong chelating ligands which form complexes with most transition metal
ions. Among the two promoters oxidation is much faster with 1,10-phenanthroline. The low dielectric constant of the medium assists the
complex formation and enhances the reactivity. The reactions were studied at different temperatures at different acetic acid-water composition
and various thermodynamic parameters have been determined. The observed experimental results have been explained by plausible
mechanisms.
Keywords: Tetrabutylammoniumbromochromate, Benzhydrol, Picolinic acid, 1,10-Phenanthroline.
As a part of our continuing investigations of the oxidation
of benzhydrol by Cr(VI) [29,30], we report the kinetic features
of the oxidation of benzhydrol by TBABC. Literature review
reveal that, no detailed kinetic study of oxidation of benzhydrol
by TBABC in the absence and presence of promoters such as
picolinic acid and 1,10-phenanthroline have so far been
attempted. This prompted us to undertake the present investi-
gation. Here we studied the kinetics of oxidation of benzhydrol
by TBABC in the absence and presence of picolinic acid and
1,10-phenanthroline and observed that 1,10-phenanthroline
is the most suitable promoter of this oxidation. Mechanistic
aspects are also discussed.
INTRODUCTION
A variety of compounds containing chromium(VI) have
proved to be versatile reagents capable of oxidizing almost
every oxidizing functional group [1].A number of new chromium
compounds like pyridinium fluorochromate [2], benzimidazo-
lium fluorochromate [3], tetramethylammonium fluorochromate
[4], tributylammonium chlorochromate [5], tripropylammonium
fluorochromate [6], tetraethylammonium chlorochromate [7]
and tetraheptylammonium bromochromate [8] have been used
to study the kinetics and mechanism of oxidation of various
organic compounds. Recently tetrabutylammoniumbromo-
chromate (TBABC) has been reported as a new and stronger
oxidizing agent [9].
EXPERIMENTAL
Benzhydrols yield benzophenones upon oxidation, which
are useful synthones for fullerenes, bioactive oxygen hetero-
cyclics, dyes and medicines. For this purpose, various oxidizing
agents are used, such as chloramine-B [10], N-bromosucci-
nimide [11,12], Tl(III) [13], N-bromopthalimide [14] and
permanganate [15].
Tetrabutylammonium bromide and chromium trioxide
were obtained from Fluka (Buchs, Switzerland). Benzhydrol
(SRL,AR), picolinic acid (Aldrich) and 1,10-phenanthroline
(Merck, GR) were used after repeated crystallization from
methanol. Acetic acid was purified by standard method and
the fraction distilling at 118 °C was collected.
Among the different chelating agents [16-21] that promote
Cr(VI) oxidation of different types of organic substrate,
picolinic acid and 1,10-phenanthroline are quite important
[22-28].
Preparation of tetrabutylammoniumbromochromate:
Tetrabutylammoniumbromochromate, [(C4H9)4N][CrO3Br]
was prepared by dissolving chromium(VI) oxide (1 g, 10 mmol)
in acetonitrile and addition of this solution to a solution of

Vol. 29, No. 4 (2017)
Oxidation of Benzhydrol by TBABC in Presence and Absence of Picolinic Acid and 1,10-Phenanthroline 811
tetrabutylammonium bromide (3.22 g, 10 mmol) in acetonitrile
under stirring at room temperature until an orange precipitate
was formed. After 2 h stirring, the mixture was filtered. The
solid was washed with hexane and dried under vacuum for
1 h. The TBABC salts are somewhat hygroscopic and best
stored under a layer of hexane, whereas all of the salts are
photosensitive and moisture-sensitive, both in solution and
solids. Tetrabutylammoniumbromochromate was recrystallized
from methylene chloride by addition of hexane [9].
pseudo first order conditions in the absence and presence of
picolinic acid and 1,10-phenanthroline.
Effect of varying TBABC concentration: The concen-
tration of TBABC was varied in the range of 0.5 × 10-3 to 2.5
× 10-3 mol dm-1 at constant [BH], [H+] at 303 K and the rates
were measured (Table-1). The near constancy in the value of
k1 irrespective of the concentration confirms the first order
dependence on TBABC.
TABLE-1
RATE CONSTANTS FOR THE OXIDATION OF
BENZHYDROL BY TBABC IN AQUEOUS
ACETIC ACID MEDIUM AT 303 Ka
C4H9
C4H9
O
N
C4H9
103
102 [BH]
[H+]
104 k1b
(s-1)
102 k2c (dm3
mol-1 s-1)
CrO3
[TBABC]
(mol dm-3)
(mol dm-3)
C4H9
N(C4H9)4Br
(mol dm-3)
Br
MeCN, RT
(1)
0.5
1.0
1.5
2.0
2.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.2
2.2
2.2
2.2
2.2
1.2
1.7
2.7
3.2
2.2
2.2
2.2
2.2
2.2
2.2
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.12
0.20
0.36
0.44
0.24
0.24
8.36
8.40
8.32
8.50
8.46
4.48
6.54
10.24
12.22
3.44
5.92
10.92
13.20
8.46
7.36
3.800
3.820
3.780
3.860
3.840
3.730
3.840
3.790
3.820
0.286
0.296
0.303
0.300
3.840d
3.340e
Cr
O
O
Tetrabutylammonium
bromide
Tetrabutylammonium
bromochromate
Kinetic measurements: The pseudo-first-order conditions
were attained by maintaining a large excess (× 15 or more) of
benzhydrol 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 364 nm
using UV-visible spectrophotometer, Shimadzu UV-1800
model. The pseudo-first-order rate constant k1, was evaluated
from the linear (r = 0.990 to 0.999) plots of log [TBABC] against
time for up to 80 % reaction. The second order rate constant
k2, was obtained from the relation k2 = k1/[BH].
Data analysis: Correlation analyses were carried out using
Microcal origin (version 6) computer software. The goodness
of the fit was discussed using the correlation coefficient (r in
the case of simple linear regression and R in the case of multiple
linear regression) and standard deviation (SD).
Product analysis: Product analysis was carried under
kinetic conditions. In a typical experiment, benzhyrol (1.8 g,
0.01 mol) and TBABC (8.4 g, 0.02 mol) were made up to 100
mL of the 50 % acetic acid-50 % water mixture and kept in the
dark for 24 h to ensure completion of the reaction. The solution
was then treated with an excess (200 cm3) of a saturated
solution of 2,4-dinitrophenylhydrazine in 2 mol dm-3 HCl and
kept overnight in a refrigerator. The solvent was removed and
the precipitated 2,4-dinitrophenylhydrazone (DNP) was
filtered and recrystallized from ethanol. The DNP was found
identical (m.p. and mixed m.p.) with DNP of benzophenone.
(m.p.: 235-237 °C, lit 237 °C).
Stoichiometric studies: The stoichiomety of the reaction
was determined by carrying out several sets of experiments
with varying amounts of TBABC largely in excess over
benzhydrol. The estimation of unreacted TBABC showed that
the following reaction:
aAs determined by a spectrophotometric technique following the
disappearance of oxidant 102 [BH] = 2.2 mol dm-3 ; 103 [TBABC] = 1.0
mol dm-3; [H+] = 0.28 mol dm-3; Solvent composition: 50 % Acetic
acid – 50 % Water (v/v); bEstimated from pseudo-first order plots over
80 % reaction; Individual k2 values estimated as k1/[BH] or k1/[H+];
c
dContained 0.001 mol dm-3 acrylonitrile; eIn the presence of 0.003 mol
dm-3 Mn(II).
Effect of varying concentration of benzhydrol: The
concentration of benzhydrol varied in the range of 1.2 × 10-2
to 3.2 × 10-2 mol dm-1 at 303 K and keeping all other reactant
concentrations as constant and the rates were measured (Table-
1). The rate of oxidation increased progressively on increasing
the concentration of benzhydrol. The plot of log k1 versus log
[BH] gave the slope of 1.02 for benzhydrol (Fig. 1). Under
pseudo-first-order conditions, the plot of k1 versus [BH] is
linear passing through origin. These results confirm the first-
order nature of the reaction with respect to [BH].
Effect of varying perchloric acid concentration: Perchloric
acid has been used as a source of H+ in reaction medium. The
concentration of H+ varied in the range 0.12 to 0.44 mol dm-1
keeping all other reactant concentration as constant at 303 K
and the rates were measured (Table-1). The acid catalyzed
nature of this oxidation is confirmed by an increase in the rate
on the addition of H+. The plot of log k1 versus log [H+] is a
straight line with the slope of 1.04. Therefore, order with
respect to H+ is one for benzhydrol. Tetrabutylammonium-
bromochromate may become protonated in the presence of
acid and the protonated TBABC may function as an effective
oxidant.
3(C6H5)2CHOH + 2Cr(VI) →
3(C6H5)2CO + 6H+ + 2Cr(III)
(2)
RESULTS AND DISCUSSION
Effect of acrylonitrile and MnSO4: The reaction did not
promote polymerization of acrylonitrile indicating the absence
of free radicals (Table-1). However, the addition of Mn(II)
The oxidation of benzhydrol byTBABC has been conducted
in 50 % acetic acid and 50 % water medium at 303 K, under

812 Hemalatha et al.
Asian J. Chem.
TABLE-3
1.4
1.3
1.2
RATE CONSTANTS FOR THE OXIDATION OF
BENZHYDROL BY TBABC IN AQUEOUS ACETIC ACID
MEDIUM IN THE PRESENCE OF PICOLINIC ACID OR
1,10-PHENANTHROLINE AT 303 Ka
6
0
.
1
=
e
p
o
l
S
:
n
e
h
P
1
f
0
o
.
1
e
c
=
n
103
102 [BH]
(mol
[H+]
(mol
dm-3)
103 [Cat]
(mol
e
p
e
104 k1b (s-1)
s
e
o
l
S
r
p
:
e
A
h
2
0
.
1.1
1.0
0.9
0.8
0.7
0.6
[TBABC]
t
P
f
o
n
I
1
(mol dm-3)
dm-3)
dm-3)
=
e
PA
Phen
c
e
n
p
e
o
l
s
e
S
:
t
r
p
s
y
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.5
2.0
2.5
1.0
1.0
1.0
1.0
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
1.2
1.7
2.7
3.2
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.28
0.0
2.0
4.0
6.0
8.0
10.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
8.40
0.48
8.40
e
l
h
t
a
t
n
a
I
c
11.96
16.12
18.00
23.40
29.72
18.10
18.12
18.06
18.04
9.74
o
N
10.50
11.44
12.58
13.76
11.40
11.48
11.42
11.46
6.28
1.0
1.1
1.2
1.3
1.4
1.5
3 + log [BH]
Fig. 1. Showing order plot for the oxidation of oxidation of benzhydrol in
the absence and presence of picolinic acid and 1,10-phenanthroline
by TBABC
9.08
14.44
16.86
14.00
22.16
26.04
aAs determined by a spectrophotometric technique following the
disappearance of oxidant; 102 [MA] = 2.0 mol dm-3 ; 103 [TriPAFC] =
1.2 mol dm-3; 10 [H+] = 2.4 mol dm-3; 103 [Cat] = 6.0 mol dm-3;
Solvent composition: 50 % Acetic acid – 50 % Water (v/v); bEstimated
from pseudo-first order plots over 80 % reaction
(0.003 mol dm-3),in the form of MnSO4 retards the rate of oxida-
tion. This indicates the involvement of Cr(IV) intermediate in
the oxidation of benzhydrol by Cr(VI) reagent and confirms
the two electron transfer process in the reaction. Mn(II) ion
reduces Cr(IV) formed to Cr(III). In the absence of Mn(II)
ion, formed Cr(IV)reduces Cr(VI) to Cr(V) and the oxidation
of benzhydrol by Cr(V) is fast [25]. The decrease in the rate of
Cr(VI) reduction on the addition of Mn(II) has been attributed
to the removal of Cr(IV) by reaction with Mn(II).
rate increases linearly with increasing promoter concentration.
For 1,10-phenanthroline the rate is much faster than picolinic
acid, because of easy decomposition of the ternary complex
of the active oxidant in the presence of 1,10-phenanthroline
and benzhydrol, because of steric crowding.
Effect of acidity: The reaction is catalyzed by hydrogen
ions (Table-1). The acid-catalysis may well be attributed to a
protonation of TBABC to give a stronger oxidant and electro-
phile.
Effect of varyingTBABC and benzhydrol concentration
in the presence of picolinic acid or 1,10-phenanthroline:
The values of k1 were calculated in the presence of 6.0 × 10-3
mol dm-1 picolinic acid or 1,10-phenanthroline for different
concentrations of TBABC by maintaining other parameters
constant. Table-3 shows that the rate constant did not get much
altered with increase in [TBABC]. The constancy of k1 values
at different [TBABC] reveals that the reaction follows a first-
order kinetics with respect to [TBABC]. Further it reveals that
TBABC does not get diminished at our experimental conditions.
Under these experimental conditions, [BH] >> [TBABC],
the concentration of benzhydrol is varied in the range of 1.2 ×
10-2 to 3.2 × 10-2 mol dm-1 at 303 K in the presence of 6.0 ×
10-3 mol dm-1 picolinic acid or 1,10-phenanthroline and
keeping all other reactant concentrations as constant and the
rates were measured (Table-3). The rate of oxidation increased
progressively on increasing the concentration of benzhydrol.
From the plot of k1 versus [BH] it was established that the
reaction shows first-order dependence on [BH]. This result,
coupled with the nearly unit slope value of the double
logarithmic plot between k1 and [BH] confirms the first-order
nature of the reaction with respect to [BH] (Fig. 1).
O2CrFO–NH+(C3H7)3+H+
(OH)OCrFO–NH+(C3H7)3 (2)
The formation of a protonated Cr(VI) species has earlier
been postulated in the reactions of structurally similar PCC
[31] and PFC [32].
Kinetic Isotope Effect: To ascertain the importance of
the cleavage of the α-C–H bond in the rate-determining step,
oxidation of α-deuteriobenzhydrol (DBH) was studied. Results
showed the presence of a substantial primary kinetic isotope
effect (Table-2).
TABLE-2
KINETIC ISOTOPE EFFECT ON THE
OXIDATION OF BENZHYDROL BY TBABC
Substrate
104 × k1 (s-1)
298 K
303 K
308 K
313 K
BH
DBH
kH/kD
5.88
1.10
5.34
8.40
1.52
5.52
11.54
2.02
5.71
15.70
2.66
5.90
Solvent composition = 50 % AcOH-50 % H2O (v/v); 102 [BH] = 2.0
Effect of solvent polarity on reaction rate: The oxidation
of benzhydrol has been studied in the binary mixture of acetic
acid and water as the solvent medium. The reaction rate
increased remarkably with the increase in the proportion of
acetic acid in the solvent medium. These results are presented
in Table-4.
mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8 mol dm-3
Effect of varying picolinic acid and 1,10-phenanthroline
concentration: The concentration of picolinic acid and 1,10-
phenanthroline were varied in the range of 0.0 × 10-3 to 10.0 ×
10-3 mol dm-1 at constant [TBABC], [BH] and [H+] at 303 K
and the rates were measured (Table-3). We observed that the
The effect from solvent composition on the reaction rate
was studied by varying the concentration of acetic acid from

Vol. 29, No. 4 (2017)
Oxidation of Benzhydrol by TBABC in Presence and Absence of Picolinic Acid and 1,10-Phenanthroline 813
TABLE-4
1.45
1.40
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
EFFECT OF VARYING SOLVENT POLARITY ON THE RATE OF
REACTION IN THE PRESENCE AND ABSENCE OF PICOLINIC
ACID AND 1,10-PHENANTHROLINE AT 303 K
104 k1 (s-1)
% AcOH- Dielectric
1/D
No
H2O (v/v)
constant
PA
Phen
promoter
30-70
40-60
50-50
60-40
70-30
72.0
63.3
56.0
45.5
38.5
0.0138
0.0158
0.0178
0.0219
0.0259
6.75
7.48
8.40
9.06
10.22
11.44
14.16
17.16
14.28
15.96
18.00
22.28
27.18
10.40
12.88
102 [BH] = 2.0 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8
mol dm-3; 103 [PA] = 6.0 mol dm-3; 103 [Phen] = 6.0 mol dm-3
30 to 70 %. The pseudo-first-order rate constants were
estimated for the oxidation of benzhydrol with TBABC in the
presence of perchloric acid at a constant ionic strength. The
reaction rate is increases markedly with the increase in the
proportion of acetic acid in the medium (Table-4). When the
acid content increases in the medium, the acidity of the medium
is increased whereas the dielectric constant of the medium is
decreased. These two effects cause the rate of the oxidation to
increase markedly. The enhancement of the reaction rate with
an increase in the amount of acetic acid may be generally attri-
buted to two factors, viz., (i) the increase in acidity occurring
at constant [H+] and (ii) the decrease in the dielectric constant
with an increase in the acetic acid content.
The plot of log k1 versus 1/D (dielectric constant) is linear
with positive slope suggesting the presence of either dipole-
dipole or ion-dipole type of interaction between the oxidant
and the substrate [33] (Fig. 2). Plot of log k1 versus (D – 1)/
(2D + 1) is a curvature indicating the absence of dipole-dipole
interaction in the rate determining step. Positive slope of log
k1 versus 1/D plot indicates that the reaction involves a cation-
dipole type of interaction in the rate determining step.
Amis [34] holds the view that in an ion-dipole reaction
involving a positive ionic reactant, the rate would decrease
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
1/D
Fig. 2. Plot of 1/D against log k1 showing effect of solvent polarity for the
oxidation of oxidation of benzhydrol by TBABC in the absence
and presence of picolinic acid and 1,10-phenanthroline
with increasing dielectric constant of the medium and if the
reactant were to be a negatively charged ion, the rate would
increase with the increasing dielectric constant. In this case
there is a possibility of a positive ionic reactant, as the rate
decreases with the increasing dielectric constant of the medium.
Due to the polar nature of the solvent, transition state is stabi-
lized, i.e., the polar solvent molecules surround the transition
state and result in less disproportion.
Thermodynamic parameters: The kinetics of oxidation
of benzhydrol was studied at four different temperatures viz.,
298, 303, 308 and 313 K at various percentage of acetic acid-
water medium. The kinetics of oxidation of benzhydrol was
also studied in the presence of picolinic acid and 1,10-phenan-
throline. The second order rate constants were calculated
(Table-5). The Arrhenius plot of log k2 versus 1/T is found to
be linear. The enthalpy of activation, entropy of activation and
free energy of activation were calculated from k2 at 298, 303,
TABLE-5
SECOND ORDER RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE OXIDATION OF
BENZHYDROL BY TBABC AT VARIOUS PERCENTAGE OF ACETIC ACID-WATER MEDIUM
IN THE PRESENCE AND ABSENCE OF PICOLINIC ACID AND 1,10 PHENANTHROLINE
102 k2 (dm3 mol-1 s-1)
-∆S#
∆H#
∆G# (kJ mol-1)
% AcOH-H2O
(v/v)
Ea
(kJ mol-1)
(JK-1 mol-1)
(kJ mol-1)
(at 303 K)
298 K
303 K
308 K
313 K
No catalyst
5.77
30-70
40-60
50-50
60-40
70-30
2.14
2.39
2.67
3.35
4.41
3.07
3.40
3.82
4.73
5.85
4.20
4.66
5.25
6.53
8.24
51.12
50.54
50.92
49.58
49.40
113.52 1.8
114.66 2.4
112.75 2.4
114.47 1.2
113.52 1.5
48.64 0.6
48.06 0.8
48.44 0.8
47.28 0.4
46.90 0.5
83.04 1.2
84.24 1.6
83.96 1.6
83.29 0.8
82.29 1.0
6.36
7.14
8.69
11.54
Picolinic acid
30-70
40-60
50-50
60-40
70-30
3.09
3.46
3.82
4.74
5.82
4.12
4.65
5.20
6.44
7.80
5.52
6.21
7.07
8.69
10.51
7.32
8.32
9.18
11.36
44.80
45.56
45.95
45.38
46.72
132.09 1.5
128.84 2.7
127.12 1.5
126.16 1.2
120.60 0.6
42.32 0.5
42.88 0.9
43.27 0.5
43.12 0.4
44.04 0.2
82.34 1.0
81.92 1.8
81.78 1.0
81.34 0.8
80.58 0.4
14.32
1,10-Phenanthroline
10.89
30-70
40-60
50-50
60-40
70-30
4.99
5.63
6.29
7.67
9.43
6.49
7.25
8.18
10.13
12.35
8.33
9.43
10.64
13.36
16.18
40.40
40.21
40.97
42.70
42.30
143.20 1.5
142.42 1.8
139.38 3.0
131.52 2.4
131.14 1.5
37.91 0.5
37.91 0.6
38.28 1.0
38.48 0.8
39.82 0.5
81.10 1.0
81.06 1.2
80.51 2.0
78.33 1.6
79.56 1.0
12.22
13.83
17.47
21.35
102 [BH] = 2.0 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8 mol dm-3; 103 [PA] = 6.0 mol dm-3; 103 [Phen] = 6.0 mol dm-3

814 Hemalatha et al.
Asian J. Chem.
(C H )
O
O
ON
Br
(C H )
ON
4 9 4
4
9 4
O
Cr
H
+
Cr
HO
Br
TBABC
C6H5
C6H5
OH
Cr
(C H )
ON
4
9 4
O
(C H )
ON
4
9 4
K
+
Cr
C6H5
C
H
OH
C6H5
C
H
O
HO
Br
Br
OH
k
slow
Benzhydrol
C6H5
C6H5
O
OH
Cr
(C H )
ON
4
9 4
(C H )
ON
4 9 4
fast
C6H5
C
O
+
+
H2O
C
O
Cr
+
H
Br
H
OH
Br
C6H5
Benzophenone
Cr(IV)
Scheme-I: Mechanism of oxidation of benzhydrol by TBABC
(C H )
O
ONH
(C H )
O NH
4
9 4
4
9 4
O
Cr
H
+
+
Cr
O
Br
HO
Br
TBABC
TBABCH
k1
H
N
H
COOH
N
COOH
k2
C H )
+
O
ONH(
4
9 4
O
+ H O + (C H ) NH + H
Br
2
4
9 4
N
C
Cr
N
COOH
HO
Br
O
H
O
Cr
O
( C1 )
k 3
O
C6H5
C6H5
N
C
O
C6H5
C
H
OH
+
O
N
C
C6H5
C
H
O
Cr
OH
O
O
Cr
O
O
( C2 )
( C1 )
O
C6H5
N
Cr
C
O
k 4
C6H5
N
C
+ H2O
+
O
C6H5
C
H
O
O
C6H5
C
O
Cr
O
OH
O
Benzophenone
Cr (IV)
( C2 )
Scheme-II: Mechanism of oxidation of benzhydrol by TBABC in the presence of picolinic acid

Vol. 29, No. 4 (2017)
Oxidation of Benzhydrol by TBABC in Presence and Absence of Picolinic Acid and 1,10-Phenanthroline 815
308 and 313 K using the Eyring relationship by the method of
least square and presented in Table-5. The entropy of activation
is negative for benzhydrol in the absence and presence of
picolinic acid and 1,10-phenanthroline.
determining step is suggested. Positive slope of log k1 versus
1/D plot indicates that the reaction involves a cation-dipole
type of interaction in the rate determining step. The negative
entropy of activation in conjunction with other experimental
data supports the mechanism outlined in Scheme-I.
Mechanism of oxidation
Reaction mechanism of the oxidation of benzhydrol
by TBABC in the presence of a promoter: The findings with
promoter can be explained by considering the reaction
mechanism outlined in Schemes II and III. Picolinic acid
(Scheme-II) and 1,10-phenanthroline (Scheme-III) readily
form complexes (C1) with Cr(VI) which are active oxidants
[35-38]. In the next step, the complex reacts with the substrate
to form a ternary complex (C2). This ternary complex (C2)
undergoes redox decom-position by two electron transfer within
the cyclic transition state in a rate-determining step involving
simultaneous rupture of C–C and C–H bonds to give a benzo-
phenone and the Cr(IV)-promoter complexes.
Reaction mechanism for TBABC oxidation of benzhy-
drol in the absence of a promoter: From the product analysis,
2,4-dinitrophenylhydrazine was confirmed. Hence, it shows
that under the experimental conditions employed in the present
study, benzhydrol is oxidized to benzophenone. Based on the
above kinetic observations the following mechanism is proposed
for the reaction. Absence of any effect of added acrylonitrile on
the reaction discounts the possibility of a one-electron oxidation,
leading to the formation of free radicals. The presence of a subs-
tantial kinetic isotope effect in the oxidation of α-deutero-
benzhydrol confirms the cleavage of the α-C-H bond in the rate-
determining step. Therefore, a hydride-ion transfer in the rate
(C H )
(C H )
O
ONH
O NH
4
9 4
4
9 4
O
H
Cr
+
Cr
O
Br
HO
Br
TBABC
TBABCH
k1
+
H
N
N
N
HN
C H )
O
ONH(
Br
k 2
4
9 4
+ (C H ) NH + H O
Br
4
9 4
2
+
Cr
N
O
N
HO
Cr
N
HN
O
( C1 )
C6H5
k 3
+
N
N
C6H5
C
H
OH
H
Cr
C
O
O
N
O
N
Cr
OH
C6H5
( C2 )
C6H5
O
( C1 )
k4
C6H5
+ H2O
+
O
C6H5
C
N
N
N
N
H
Cr
Cr
O
Benzophenone
C
O
O
Cr (IV)
OH
C6H5
( C2 )
C6H5
Scheme-III: Mechanism of oxidation of benzhydrol by TBABC in the presence of 1,10-phenanthroline

816 Hemalatha et al.
Asian J. Chem.
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Conclusion
The kinetics of oxidation of benzhydrol by TBABC has
been investigated in aqueous acetic acid medium by
spectrophotometrically at 303 K in the absence and presence
of promoters like picolinic acid and 1,10-phenanthroline. The
oxidation of benzhydrol by TBABC is first order each with
respect to the benzhydrol, TBABC and hydrogen ion. The
oxidation is catalyzed by perchloric acid. The lowering of
dielectric constant of reaction medium increases the reaction
rate significantly. The reaction does not show the polymeri-
zation, which indicates the absence of free radical intermediate
in the oxidation. The reaction becomes faster with each of the
promoters.
https://doi.org/10.1007/s11243-006-0018-0.
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ACKNOWLEDGEMENTS
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https://doi.org/10.1016/j.molcata.2006.10.042.
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Saha, Res. Chem. Intermed., 39, 631 (2013);
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24. S.K. Ghosh, A. Basu, R. Saha, A. Ghosh, K. Mukherjee and B. Saha, J.
Coord. Chem., 65, 1158 (2012);
One of the authors, S. Hemalatha expresses her 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, Melvisharam, India for the support.
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