Kinetic studies on tetrabutylammonium bromochromate oxidation of some mono-and di-substituted benzhydrols

Article abstract of DOI:10.14233/ajchem.2018.21066

The oxidation of 12 mono- and di-substituted benzhydrols (BH) by tetrabutylammonium bromochromate (TBABC) have been studied in aqueous acetic acid medium. 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 tetrabutylammonium bromochromate oxidation of 12 mono- and di-substituted benzhydrols complies with the isokinetic relationship and Hammett relationship. The overall mechanism is proposed to involve a cyclic concerted symmetrical transition state leading to the product.

Full text of DOI:10.14233/ajchem.2018.21066

Asian Journal of Chemistry; Vol. 30, No. 4 (2018), 821-826
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21066
Kinetic Studies on Tetrabutylammonium Bromochromate
Oxidation of Some Mono- and Di-substituted Benzhydrols
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 2017;
Accepted: 5 January 2018;
Published online: 28 February 2018;
AJC-18793
The oxidation of 12 mono- and di-substituted benzhydrols (BH) by tetrabutylammonium bromochromate (TBABC) have been studied in
aqueous acetic acid medium. 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 tetrabutylammonium bromochromate oxidation of 12 mono- and di-substituted
benzhydrols complies with the isokinetic relationship and Hammett relationship. The overall mechanism is proposed to involve a cyclic
concerted symmetrical transition state leading to the product.
Keywords: Tetrabutylammonium bromochromate, Benzhydrol, Thermodynamic parameters, Isokinetic relationship, Kinetics.
p-OC2H5, p-CH3, p-F, p-Cl and p-NO2. Benzhydrols were prepared
by means of the reduction of corresponding benzophenones
with sodium borohydride. The purity of benzhydrols was
checked by m.p. and elemental analysis. Tetrabutylammonium
bromochromate (TBABC) was prepared by a reported method
[9] and its purity was checked by the iodometric method. Doubly
distilled water was used for all purposes. Acetic acid was
purified by standard method and the fraction distilling at 118 °C
was collected.
Deuterated (α-C–D) benzhydrols were prepared by the
method of Shanker and Suresh [21]. α-D-benzhydrols were
prepared by refluxing corresponding benzhydrols (α-C–H) in
D2O for 2-3 h, removing the solvent under a pump and repeating
the process three times to ensure complete exchange of protons.
PMR analysis was conducted to confirm the deuterated (α-C–D)
benzhydrols.
Kinetic measurements: A thermostatic water bath was
used to maintain the desired temperature within 0.1 °C. The
calculated amount of the reactants, i.e., benzhydrol (BH), TBABC,
perchloric acid, acetic acid and water taken in a reaction vessel
which was kept in a thermostatic water bath. After allowing
sufficient time to attain the temperature of the experiment, a
requisite amount of solutions were rapidly pipetted out into
the spectrophotometric cell. The total volume of the reaction
mixture was 5 mL in each case. Progress of the reaction was
followed by measuring the decrease in [TBABC] by spectrophoto-
metrically at 362 nm using UV-visible spectrophotometer,
INTRODUCTION
Chromium(VI) based reagents are widely used in modern
organic synthesis for the oxidation of a variety of compounds
under anhydrous and aprotic conditions, including primary
and secondary alcohol [1]. Extensive work has led to the develo-
pment of a good number of these oxidants such as 4-(dimethyl-
amino)pyridinium chlorochromate [2], tetraethylammonium
chlorochromate [3], tetramethylammonium fluorochromate [4],
2,6-dicarboxypyridinium chlorochromate [5], N-methyl piperi-
dinium chlorochromate [6], tetramethylammonium fluoro-
chromate(VI) [7], N-methylbenzylammonium fluorochromate
(VI) [8] and tetrabutylammonium bromochromate [9]. These
reagents may all be used for the oxidation of alcohols to corres-
ponding aldehydes and ketones.
Literature survey revealed that the studies on the mechanism of
oxidation of benzhydrols by several oxidants such as chloramine-
B [10], tributylammonium chlorochromate [11], N-bromosucci-
nimide [12], Tl(III) [13] and N-bromopthalimide [14] have been
reported. Recently, several researchers reported the oxidation of
some organic substrates by Cr(VI) [15-20]. The present commun-
ication describes the results of oxidation of 12 mono- and di-
substituted benzhydrols by tetrabutylammonium bromochromate.
EXPERIMENTAL
All the employed chemicals and solvents were of analytical
grade. Benzhydrols were used with substituents H, p-OCH3,

822 Hemalatha et al.
Asian J. Chem.
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
0.2
0.1
0.0
-0.1
-0.2
Shimadzu UV-1800 model. The reaction was carried under
pseudo first-order conditions, i.e., [BH] >> [TBABC] in the
presence of perchloric acid in 50 % acetic acid – 50 % water
medium.
Stoichiometric studies and product analysis: Reaction
mixtures, in which [TBABC] was in large excess of [BH], were
kept in the presence of perchloric acid 50 % acetic acid – 50
% water medium. Estimation of unreacted TBABC showed
the following stoichiometric equation (1):
S8
S2
S3
S9
S4
1
k
S10
S1
S5
S6
3(C6H5)2CHOH + 2Cr(VI) → 3(C6H5)2CO + 6H+ + 2Cr(III) (1)
S11
S12
The oxidation of benzhydrol resulted in the formation of
the corresponding benzophenone. The DNP of benzophenone
was determined by a reported method [15].
S7
1.0
1.1
1.2
1.3
1.4
1.5
3 + log [S]
RESULTS AND DISCUSSION
Fig. 1. Order plot for the substrate for the oxidation of benzhydrol (S1), p-
OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F (S5), p-Cl (S6), p-
NO2(S7), p-CH3, p-CH3 (S8), p-OCH3, p-F (S9), p-OCH3, p-Cl (S10),
p-Cl, p-Cl (S11) and p-CH3, p-NO2 (S12) by TBABC at 303 K
The oxidation of 12 mono- and di-substituted benzhydrols
such as p-H (S1), p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-
F (S5), p-Cl (S6), p-NO2 (S7), p-CH3, p-CH3 (S8), p-OCH3, p-
F (S9), p-OCH3, p-Cl (S10), p-Cl, p-Cl (S11) and p-CH3, p-
NO2 (S12) by TBABC have been conducted in 50 % acetic acid
and 50 % water medium at 303 K, under pseudo first order condi-
tions and the result obtained are discussed in the following para-
graphs.
35
S8
30
S2
S3
25
S9
S4
Order of reaction: The values of pseudo first-order rate
constants were unaltered with variation in [TBABC], indicating
the first-order dependence on [TBABC] (Table-1).
20
1
S10
k
15
S1
The rate increases with an increase in [BH] (Table-1). Linear
plots of log k1 vs. log [BH] (Fig. 1) with the slopes (p-H (S1):
slope = 1.02; p-OCH3 (S2): slope = 0.99; p-OC2H5 (S3): slope
= 1.05; p-CH3 (S4): slope = 0.99; p-F (S5): slope = 0.99; p-Cl
(S6): slope = 1.04; p-NO2(S7): slope = 1.01; p-CH3, p-CH3
(S8): slope = 1.07; p-OCH3,p-F (S9): slope = 1.05; p-OCH3,
p-Cl (S10): slope = 1.03; p-Cl, p-Cl (S11): slope = 1.04; p-
CH3, p-NO2 (S12): slope = 1.02) demonstrate first order type
kinetics on [BH]. Further, a plot of k1 versus [BH] is a straight
line passing through origin (Fig. 2), confirming the first-order
dependence on [BH].
S5
10
S6
S11
5
0
S12
S7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
102 [S], M
Fig. 2. Direct plot for the substrate for the oxidation of benzhydrol (S1),
p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F (S5), p-Cl (S6), p-
NO2 (S7), p-CH3, p-CH3 (S8), p-OCH3, p-F (S9), p-OCH3, p-Cl
(S10), p-Cl, p-Cl (S11) and p-CH3, p-NO2 (S12) benzhydrols by
TBABC at 303 K
TABLE-1
RATE CONSTANTS FOR THE OXIDATION OF MONO AND DI SUBSTITUTED
BENZHYDROLS BY TBABC IN AQUEOUS ACETIC ACID MEDIUM AT 303 Ka
(mol dm–3)
104 k1 (s–1)a
103 [Ox] 102 [BH]
[H+]
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
0.28
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
2.63
2.65
2.67
2.68
2.64
1.40
2.00
3.25
3.85
1.11
1.86
3.43
4.14
1.94b
S12
1.88
1.89
1.86
1.85
1.84
1.01
1.40
2.28
2.72
0.80
1.35
2.45
2.95
1.26b
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
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
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
6.06b
17.32
17.25
17.18
17.20
17.28
9.34
13.28
21.10
25.04
7.44
16.28
16.32
16.34
16.38
16.24
8.82
12.54
20.02
23.64
6.90
12.90
12.94
12.88
12.98
12.86
7.06
10.04
15.78
18.76
5.60
7.41
7.35
7.38
7.29
7.30
4.00
5.58
8.96
10.58
3.11
5.18
9.42
11.51
5.28b
4.64
4.59
4.66
4.52
4.55
2.46
3.48
5.64
6.68
1.90
3.22
5.96
7.18
3.42b
1.10
1.12
1.14
1.16
1.10
0.61
0.87
1.35
1.63
0.48
0.80
1.45
1.72
0.84b
21.36
21.40
21.48
21.44
21.47
11.56
16.48
26.19
31.08
9.22
15.10
15.16
15.18
15.22
15.14
8.27
11.63
18.56
22.09
6.44
10.31
10.35
10.43
10.39
10.34
5.59
7.93
12.64
15.02
4.44
12.40
22.12
27.06
11.66
20.88
25.74
9.32
15.36
27.46
33.63
10.74
19.42
23.79
7.32
16.66
20.29
9.08b
13.24
16.22
7.64b
12.66b 11.86b
15.64b 11.24b
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)
bIn the presence of 0.003 mol dm-3Mn (II).

Vol. 30, No. 4 (2018)
Kinetic Studies on TBABC Oxidation of Some Mono- and Di-substituted Benzhydrols 823
in rate constants with varying the concentration of acetic acid
(Table-3). The plot of log k1 versus 1/D (dielectric constant) is
linear with positive slope suggesting the presence of ion-dipole
type of interaction between the oxidant and the substrate [22]
(Fig. 3).
The increase in [HClO4] in the oxidation reaction increases
the rate of the reaction and shows a direct first order depen-
dence on [HClO4] (Table-1). A plot of log k1 against log [H+]
is linear with the slopes [p-H (S1): slope = 1.04; p-OCH3 (S2):
slope = 1.01; p-OC2H5 (S3): slope = 1.03; p-CH3 (S4): slope =
1.06; p-F (S5): slope = 1.04; p-Cl (S6): slope = 0.99; p-NO2
(S7): slope = 0.99; p-CH3,p-CH3 (S8): slope = 1.03;p-OCH3,p-
F (S9): slope = 1.06; p-OCH3,p-Cl (S10): slope = 0.99; p-Cl,
p-Cl (S11): slope = 1.01; p-CH3, p-NO2 (S12) : slope = 0.99]
(the plots are not shown). The experimental data confirms the
first order dependence on [H+]. Further, a plot of k1 versus
[H+] is a straight line passing through origin (the plot is not
shown), confirming the first-order dependence on [H+].
Induced polymerization and effect of added MnSO4:
The oxidation of benzhydrol in a nitrogen atmosphere failed
to induce the polymerization of acrylonitrile. Therefore, a one-
electron oxidation, giving rise to free radicals, is unlikely.
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).
1.6
1.4
S8
1.2
S2
S3
S9
1.0
0.8
0.6
0.4
0.2
0.0
S4
S10
S1
1
k
S5
S6
S11
S12
S7
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
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 (α-C-D) was studied. Results
showed the presence of a substantial primary kinetic isotope
effect (Table-2).
1/D
Fig. 3. Plot of 1/D against log k1 showing effect of solvent polarity of
benzhydrol (S1), p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F (S5),
p-Cl (S6), p-NO2 (S7), p-CH3, p-CH3 (S8), p-OCH3, p-F (S9), p-
OCH3, p-Cl (S10), p-Cl, p-Cl (S11) and p-CH3, p-NO2 (S12)
benzhydrols by TBABC at 303 K
TABLE-2
Structure reactivity correlation: The effect of structure
on reactivity indicates that, the reactivity decreases in the order
p-OCH3 > p-OC2H5 > p-CH3 > p-H > p-F > p-Cl > p-NO2 for
the mono para-substituents. For di para-substituted benzhydrols
the reactivity decreases in the following order: p-CH3, p-CH3 >
p-OCH3, p-F > p-OCH3, p-Cl > p-Cl, p-Cl > p-CH3, p-NO2
The reactivity order is in accordance with the σ-values of
the various substituents at para position (σ (H) = 0.0; σ (OCH3)
= – 0.27; σ (OC2H5) = – 0.25; σ (CH3) = – 0.17; σ (F) = +0.06;
σ(Cl) = +0.23; σ(NO2) = +0.78). The negative σ values enhances
the rate while the positive σ values reduces the rate.
Hammett plot: Hammett equation is a linear free energy
relationship that studies the effect of substituent changes on
reactions [23]. The values of reaction constants (ρ) are deter-
mined from the slope of linear Hammetts plot. The reaction
constant values (ρ) at different temperatures are: 1.1744 (at
298 K), 1.1099 (at 303 K), 1.0693 (at 308 K) and 1.0172 (at
313 K). The negative ρ values obtained from the Hammett
plot is due to decrease in rate by electron withdrawing groups
and to increase in rate by electron donating groups. The posi-
KINETIC ISOTOPE EFFECT ON THE
OXIDATION OF BENZHYDROLBY TBABC
104 k1 (s–1)
Substrate
298 K
5.92
1.11
303 K
8.40
1.48
308 K
11.92
2.05
313 K
16.94
2.75
Benzhydrol
α-C–D
kH/kD
5.33
5.67
5.82
6.14
102 [S] = 2.2 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8
mol dm-3
Effect of solvent polarity on reaction rate: An increase
in solvent polarity accelerates the rates of reactions where a
charge is developed in the activated complex from neutral or
slightly charged reactant. An increase in solvent polarity
decreases the rates of reactions where there is less charge in
the activated complex in comparison to the starting materials.
The concentration of acetic acid was varied from 30 to 70 %
and pseudo-first-order rate constants were estimated for the
oxidation of benzhydrols, with TBABC in the presence of
perchloric acid at a constant ionic strength. There is increase
TABLE-3
EFFECT OF VARYING SOLVENT POLARITY ON THE RATE OF REACTION OF BENZHYDROL (S1),
p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F (S5), p-Cl (S6), p-NO2 (S7), p-CH3,p-CH3 (S8), p-OCH3, p-F (S9),
p-OCH3,p-Cl (S10), p-Cl, p-Cl (S11) AND p-CH3, p-NO2 (S12) BY TBABC AT 303 K
104 k1 (s–1)
% AcOH-
H2O (v/v)
D
S1
S2
S3
S4
S5
5.86
6.60
7.35
8.94
11.28
S6
S7
S8
S9
S10
8.15
9.08
10.34
12.40
15.70
S11
2.14
2.38
2.65
3.21
4.06
S12
1.53
1.70
1.89
2.26
2.92
30-70
40-60
50-50
60-40
70-30
72.0
63.3
56.0
45.5
38.5
6.70
7.56
8.40
10.20
13.00
13.94
15.40
17.25
20.70
26.28
13.19
14.62
16.32
19.90
25.28
10.60
11.60
12.94
15.78
19.80
3.66
4.10
4.59
5.56
7.06
0.92
1.01
1.12
1.36
1.70
17.06
19.20
21.40
26.10
33.40
12.14
13.60
15.16
18.44
23.48
102 [S] = 2.2 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8 mol dm-3

824 Hemalatha et al.
Asian J. Chem.
tively charged transition state also due to the negative ρ values
obtained from the Hammett plot.
log k (at T2) = a + b log k (at T1)
(2)
This method is correct in statistics but has the shortcoming
that it is only applicable to measurements at two temperatures.
Isokinetic temperature obtained is presented in Table-6. At
isokinetic temperature all the compounds of the series react
equally fast. The linear isokinetic correlation implies that all
the mono and di-substituted benzhydrols are oxidized by the
same mechanism and the changes in the rate are governed by
the changes in both the enthalpy and entropy of activation [25].
Activation parameters: The substrates mono- and di-
substituted benzhydrols were subjected to oxidation kinetics
by TBABC at four different temperatures viz., 298, 303, 308 and
313 K in 50 % acetic acid – 50 % water medium in presence of
perchloric acid. The pseudo-first order rate constants at four
different temperatures are given in the Table-4. 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, 308 and 313 K using the Eyring relationship
by the method of least square and presented in Table-5.
TABLE-6
ISOKINETIC TEMPERATURE FOR THE OXIDATION OF
BENZHYDROL (S1), p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F
(S5), p-Cl (S6), p-NO2 (S7), p-CH3, p-CH3 (S8), p-OCH3, p-F (S9),
p-OCH3, p-Cl (S10), p-Cl, p-Cl (S11) AND p-CH3, p-NO2 (S12)
BY TBABC IN AQUEOUS ACETIC ACID MEDIUM
TABLE-4
PSEUDO-FIRST ORDER RATE CONSTANTS FOR THE
OXIDATION OF BENZHYDROL (S1), p-OCH3 (S2), p-OC2H5 (S3),
p-CH3 (S4), p-F (S5), p-Cl (S6), p-NO2 (S7), p-CH3, p-CH3 (S8),
p-OCH3, p-F (S9), p-OCH3, p-Cl (S10), p-Cl, p-Cl (S11) AND p-CH3,
p-NO2 (S12) BY TBABC AT VARIOUS TEMPERATURES IN
AQUEOUS ACETIC ACID MEDIUM
Isokinetic
temperature (β)
422 K
Correlation
coefficient (r)
Temp. (K)
Slope (b)
303 & 298
308 & 298
313 & 298
308 & 303
313 & 303
313 & 308
0.992
0.991
0.998
0.990
0.988
0.997
0.944
0.894
0.857
0.948
0.900
0.948
429 K
448 K
440 K
445 K
104 k1 (s-1)
Substrate
298 K
5.92
12.98
12.10
9.37
303 K
8.40
308 K
11.92
22.75
21.87
17.82
10.56
6.78
313 K
16.94
30.05
29.08
24.64
15.18
9.72
444 K
S1
S2
S3
S4
S5
102 [S] = 2.2 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8
17.25
16.32
12.94
7.35
mol dm-3; Solvent composition = 50 % AcOH - 50 % H2O (v/v)
Mechanism of oxidation: A hydride-ion transfer in the
rate determining step is suggested from the presence of a subs-
tantial kinetic isotope effect. The possibility of a one-electron
oxidation, leading to the formation of free radicals is ruled
out due to the absence of any effect of added acrylonitrile on
the reaction. Product analysis confirmed the formation of
dinitro phenyl hydrazone (DNP) suggesting the oxidation of
benzhydrol to benzophenone. Negative reaction constants have
been used by Banerji as supporting evidence for oxidation
mechanisms involving a hydride-ion transfer in the rate deter-
mining step [26].
5.06
3.17
S6
4.59
S7
0.75
1.12
1.72
2.60
S8
S9
S10
S11
S12
16.41
11.13
7.39
1.78
1.25
21.40
15.16
10.34
2.65
27.81
20.64
14.47
3.92
36.12
28.03
20.24
5.76
1.89
2.82
4.22
102 [S] = 2.2 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8
mol dm-3; Solvent composition = 50 % AcOH - 50 % H2O (v/v)
Isokinetic temperature: Exner suggested a simple method
[24] to evaluate the isokinetic relationship. It consisted of
plotting the logarithms of rate constants at two temperatures
(T2 > T1) against each other according to the eqn. 2:
It is well-established that intrinsically concerted sigmatropic
reactions, characterized by transfer of hydrogen in a cyclic transi-
tion state, are the only truly symmetrical processes involving
TABLE-5
SECOND ORDER RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE OXIDATION OF BENZHYDROL
(S1), p-OCH3 (S2), p-OC2H5 (S3), p-CH3 (S4), p-F (S5), p-Cl (S6), p-NO2(S7), p-CH3, p-CH3 (S8), p-OCH3, p-F (S9), p-OCH3,p-Cl (S10),
p-Cl, p-Cl (S11) AND p-CH3, p-NO2 (S12) BY TBABC AT VARIOUS TEMPERATURES IN AQUEOUS ACETIC ACID MEDIUM
102 k2 (dm3 mol-1 s-1)
∆H#
∆S#
∆G#
Ea
Substrate
(kJ mol–1)
(kJ mol–1)
(J K–1 mol–1)
(kJ mol–1)
298 K
303 K
308 K
313 K
At 303 K
S1
S2
S3
S4
S5
S6
S7
S8
S9
2.69
5.90
5.50
4.26
2.30
1.44
0.34
7.46
5.06
3.36
0.81
0.57
3.82
7.84
7.42
5.88
3.34
2.09
0.51
9.72
6.89
4.70
1.20
0.86
5.42
10.34
9.94
8.10
4.80
3.0
0.78
12.64
9.38
6.58
1.78
1.28
7.70
13.66
13.32
11.20
6.90
84.42
1.18
16.42
12.74
9.20
54.6
43.5
45.6
50.2
56.8
58.4
64.7
41.0
47.8
52.3
60.9
63.0
51.9 0.2
41.0 0.6
43.3 0.4
47.5 0.4
54.4 0.6
55.7 0.2
62.0 0.4
38.3 0.4
45.2 0.6
49.6 0.2
58.4 0.3
60.3 0.2
100.3 0.6
130.9 1.8
123.7 1.2
111.6 1.2
093.8 1.8
092.8 0.6
083.6 1.2
137.8 1.2
117.5 1.8
106.4 0.6
089.0 0.9
085.4 0.6
82.3 0.4
80.7 1.2
80.8 0.8
81.3 0.8
82.8 1.2
83.8 0.4
87.3 0.8
80.1 0.8
80.8 1.2
81.8 0.4
85.4 0.6
86.2 0.4
S10
S11
S12
2.62
1.92
102 [S] = 2.2 mol dm-3; 103 [TBABC] = 1.0 mol dm-3; 10 [H+] = 2.8 mol dm-3; Solvent composition = 50 % AcOH - 50 % H2O (v/v)

Vol. 30, No. 4 (2018)
Kinetic Studies on TBABC Oxidation of Some Mono- and Di-substituted Benzhydrols 825
(C H )
O
O
ON
4
9 4
(C H )
k1
ON
4
9 4
O
Cr
H
+
Cr
Br
k-1
HO
Br
TBABC
C6H5
C6H5
C
OH
Cr
(C H )
ON
4
9 4
O
(C H )
ON
k2
4
9 4
+
OH
Cr
C6H5
C
H
C6H5
O
k-2
HO
Br
Br
H
OH
k3
slow
C6H5
Benzhydrol
C6H5
O
OH
Cr
(C H )
ON
Br
4
9 4
(C H )
ON
4
9 4
fast
C6H5
C
O
Cr
+
+
C
O
H O
+
H
2
H
OH
Br
C6H5
Benzophenone
Cr (IV)
fast
fast
Cr(IV)
Cr(V)
+
+
Cr(VI)
Reductant
2Cr(V)
Product + Cr(III)
Scheme-I: Mechanism of oxidation of benzhydrol by tetrabutylammonium bromochromate (TBABC)
a linear hydrogen transfer [27]. From the above experimental
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In this paper, the detailed kinetic study for the oxidation
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chloric acid is reported. The reaction is first order each in [S],
[TBABC] and [H+]. The oxidation of benzhydrols yield the
corresponding benzophenones. The linear isokinetic corre-
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