Kinetics and mechanism of acid catalyzed oxidation of phenol and substituted phenols by isoquinolinium bromochromate

Article abstract of DOI:10.1134/S0036024410130054

Oxidation of phenols by isoquinolinium bromochromate (IQBC) in aqueous acetic acid leads to the formation of corresponding quinones. The reaction is first order with respect to both phenol and IQBC and catalysed by hydrogen ion. The rate of oxidation decreases with increase in dielectric constant of solvent indicating ion-dipole interaction. The rate of oxidation decreases with increase in concentration of KCl, this may be due to the formation of less reactive species by interaction of Cl^- and protonated IQBC. The specific rates of oxidizing phenol reaction correlate with substituents constants affording negative reaction constants. Hammett plot is found to be valid and the correlation between enthalpies and free energies of activation is reasonably linear with an isokinetic temperature of 320 K.

Full text of DOI:10.1134/S0036024410130054

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 13, pp. 2233–2237. © Pleiades Publishing, Ltd., 2010.
CHEMICAL KINETICS
AND CATALYSIS
Kinetics and Mechanism of Acid Catalyzed Oxidation of Phenol
and Substituted Phenols by Isoquinolinium Bromochromate1
S. V. Khansolea, D. D. Gaikwadb, S. D. Gaikwadb, and R. D. Kankariyab
a Indira Gandhi Senior College, Cidco, Nanded (MS)
b Bharatiya Jain Sanghatana College, Wagholi, Puneꢀ412 207
eꢀmail: svkhansole@vahoo.co.in
Received January 5, 2010
Abstract—Oxidation of phenols by isoquinolinium bromochromate (IQBC) in aqueous acetic acid leads to
the formation of corresponding quinones. The reaction is first order with respect to both phenol and IQBC
and catalysed by hydrogen ion. The rate of oxidation decreases with increase in dielectric constant of solvent
indicating ion–dipole interaction. The rate of oxidation decreases with increase in concentration of KCl, this
may be due to the formation of less reactive species by interaction of Cl– and protonated IQBC. The specific
rates of oxidizing phenol reaction correlate with substituents constants affording negative reaction constants.
Hammett plot is found to be valid and the correlation between enthalpies and free energies of activation is
reasonably linear with an isokinetic temperature of 320 K.
DOI: 10.1134/S0036024410130054
INTRODUCTION
Literature survey reveals that no report is available
on the kinetics of oxidation of phenols by isoquinolinꢀ
ium bromochromate (IQBC), hence, here we have
study in detail kinetics and mechanism of oxidation of
phenols by isoquinolinium bromochromate.
Halochromates have been used as mild and selecꢀ
tive oxidizing reagent in synthetic organic chemistry.
Varieties of compounds containing chromium (VI)
have proved to be versatile reagents capable of oxidizꢀ
ing almost every oxidizing functional group. The
kinetics and mechanism of oxidation reaction of Cr
(VI) has been well studied, chromic acid being one of
the most versatile available oxidizing reagents, reactꢀ
ing with diverse substrates [1]. The development of
newer chromium (VI) reagents for the oxidation of
organic substrates continues to be of interest.
EXPERIMENTAL
All the chemicals and reagents were of analytical
grade. The phenols used have the following substituꢀ
ents
mꢀCH3, –H, mꢀCl, pꢀCl, pꢀOH, pꢀNO2. The
solid phenols were used as received while the liquids
were distilled under vacuum before use. All the soluꢀ
tions used in the study were made by using distilled
acetic acid and doubly distilled water. Isoquinolinium
bromochromate was prepared as follows. Chromium
trioxide (10 g : 0.1 mol) was dissolved in water (15 ml)
and cooled to 0°C. To this solution hydrobromic acid
(17 ml : 48%) was added slowly with vigorous stirring
then isoquinoline (13 ml, 0.1 mol) was added drop
wise during 10 min. The reaction mixture was cooled
for 2–3 h and filtered. The resulting yellow orange
needles were dried and crystallized from aqueous ethyl
alcohol. The purity of the reagent was checked by
iodometric method and its' structure was confirmed by
both infrared spectroscopic and elemental analysis.
Infrared spectrum (KBr) gave bands at 945, 868, 767,
and 621 cm–1 characteristics of dichromate ion [11].
A number of new chromium containing compounds
like pyridinum bromochromate [2], quinolinium chloꢀ
rochromate [3], 2,2'ꢀbipyridinium chlorochromate [4],
pyridinium fluorochromate [5], quinolinium fluoroꢀ
chromate [6], quinolinium bromochromate [7], quinoꢀ
linium dichromate [8], pyridinium fluorochromate
[9], imadazolium fluorochromate [10] have been used
to study the kinetics and mechanism of oxidation of
various organic compounds.
However most of these reagents that have been
developed so far suffer from at least one of the drawꢀ
backs such as high acidity, photosensitivity, instability,
hygroscopic, low selectivity, long reaction time and
need for large volume of reagent. To overcome these
disadvantages we synthesized new reagent isoquinoꢀ
linium bromochromate (IQBC) which is mild, effiꢀ
cient, stable and works as both oxidizing as well as broꢀ
minating reagent.
M.P. = 105–106°C,
M.F. = [C9H7N+HCrO–3 Br(iso)].
1
The article is published in the original.
2233

2234
KHANSOLE et al.
Table 1. Dependence of rate constant (
k
×
104 s–1) on [Substrate]: [IQBC] = 0.001 M, [H2SO4] = 1 M,
T
= 308 K, solvent =
ꢀNO2
50% acetic acid (v/v)
[Substrate], M
m
ꢀCH3
ꢀH
m
ꢀCl
p
ꢀCl
p
ꢀOH
p
0.01
0.02
0.03
0.04
14.09 0.10
23.19 0.15
35.84 0.08
49.32 0.07
11.87 0.13
21.74 0.21
32.61 0.00
43.39 0.09
8.72 0.09
15.44 0.11
21.04 0.17
27.98 0.24
6.32 0.00
11.01 0.17
15.09 0.14
19.28 0.30
5.01 0.21
7.98 0.00
11.17 0.11
14.08 0.02
3.12 0.00
4.32 0.03
5.44 0.27
7.58 0.19
Table 2. Dependence of rate constant (
k
×
104, s–1) on [IQBC]: [Substrate] = 0.03 M, [H2SO4] = 1 M,
T
= 308 K, solvent = 50%
acetic acid (v/v)
[IQBC], M
m
ꢀCH3
ꢀH ꢀCl ꢀCl ꢀOH
m
p
p
pꢀNO2
0.001
0.002
0.003
0.004
35.84 0.21
34.19 0.18
35.84 0.08
35.91 0.18
32.61 0.00
32.92 0.26
32.61 0.00
33.14 0.07
21.04 0.12
21.10 0.14
21.04 0.17
21.42 0.13
15.11 0.00
15.49 0.21
15.09 0.14
15.98 0.12
11.17 0.18
11.12 0.02
11.17 0.11
11.92 0.19
5.44 0.00
4.94 0.16
5.44 0.21
5.09 0.03
Kinetic Measurement
Effect of Variation of [IQBC]
The reactions were carried out under pseudo first
order conditions by keeping an excess of substrate over
IQBC. The reactions were followed by monitoring the
decrease in the concentration of IQBC iodometrically
for 80% of the reactions. The rate constants were
determined by least square method, from the linear
plots of log [IQBC] versus time. Replicate runs showed
that the rate constants were reproducible to within
3%.
At constant [substrate] and [H2SO4], the increase
in [IQBC] did not affect the rate of reaction. The first
order plots of log [IQBC] versus time were linear. The
pseudo first order rate constants computed from the
plots remained unaffected by the change in [IQBC],
establishing the first order dependence of the rate on
isoquinolinium bromochromate in all cases (Table 2).
Effect of Variation of [H+]
The reaction is catalyzed by hydrogen ion; the acid
catalysis may well be attributed to the protonated ion
of IQBC to give a stronger oxidant and electrophile.
Stoichiometry and Product Analysis
The stoichiometry of the reaction was determined
by carrying out several sets of experiments with varying
amount of [IQBC] largely in excess over [phenols].
The estimation of unreacted IQBC showed that 1 mol
of phenol reacts with 1 mol of IQBC. The oxidative
products were analyzed using preparative TLC on silꢀ
ica gel, which yields the following product: 1,4ꢀbenzoꢀ
quinone M.P. =112–114°C (Lit 114–116°C), UV
(Et OH)–λmax = 360 nm. IR–νmax, cm–1: 3010, 1650,
The plot of logk
obs versus log[H+] are also straight lines
with unit slope in each case, indicating a first order
dependence on [H+] (Table 3).
Effect of Ionic Strength
The rate of reaction decreases with increase in conꢀ
centration of KCl, this is may be due to the formation
of less reactive species of oxidant [12] by interaction
between Cl– ion and protonated IQBC (Table 4).
1
1317, 1070, 1460, 900 (KBr); H NMR (CDCl3)–
6.7(s, 4H)
RESULTS AND DISCUSSION
Effect of Solvent Composition
The results of oxidation of phenols by IQBC are
presented in Tables 1–6.
At fixed ionic strength and [H+], the rate of oxidaꢀ
tion of phenols with isoquinolinium bromochromate
increases with decrease in polarity of solvent. In other
words decrease in rate with increase in dielectric conꢀ
Effect of Variation of [Substrate]
At constant [IQBC] and [H2SO4], the increase in stant is observed, this is due to polar character of tranꢀ
[substrate] enhances the reaction rate. The plot of sition state as compared to the reactant [13]. The plot
logkobs versus log [substrate] for different initial conꢀ of log
centration of substrate is linear with unit slope indicatꢀ positive slope indicating ion–dipole type of reaction
ing the first order dependence on substrate (Table 1). (Table 5).
kobs versus 1/D (dielectric constant) is linear with
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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KINETICS AND MECHANISM OF ACID CATALYZED OXIDATION OF PHENOL
104, s–1) on [H2SO4]: [Substrate] = 0.03 M, [IQBC] =0.001 M,
2235
Table 3. Dependence of rate constant (
k
×
T
= 308 K, solꢀ
vent = 50% acetic acid (v/v)
[H2SO4], M
m
ꢀCH3
–H
m
ꢀCl
p
ꢀCl
p
ꢀOH
pꢀNO2
0.5
0.75
1
18.92 0.19
27.38 0.14
35.84 0.03
40.04 0.00
17.32 0.14
24.95 0.13
32.61 0.00
40.61 0.11
11.94 0.21
16.94 0.00
21.04 0.17
25.59 0.21
8.54 0.00
11.82 0.13
15.09 0.14
18.29 0.20
6.52 0.17
8.84 0.13
11.17 0.11
13.48 0.14
2.72 0.10
4.48 0.21
5.44 0.27
6.80 0.14
1.25
H#
is the isokinetic temperature, the isokinetic
=
Δ
H0
+
βΔS#
,
(1)
Effect of Temperature
Δ
The oxidation of the phenols were carried out at
different temperature between the range of 293 to
323 K at constant concentrations of substrate and the
oxidant. The rate constants are given in Table 6. The
where
β
temperature for the reactions between phenols and
IQBC in aqueous acetic acid is which is greater than
experimental temperature. The values of entropy of
activation also suggest that the reaction is entropy as
plots of logkobs versus 1/T are linear. Activation
parameters are presented in (Table 6). The negative
values of entropy of activation reflect that the transiꢀ
tion state is more rigid than initial state [14]. The
well as enthalpy controlled. The values of free energies
≠
of activation ( G ) of the reaction were found to be
Δ
nearly constant
nism is operative for the oxidation of phenols (Table 7).
The values of
H# #, and G# are calculated by
using following formulae
ΔG value indicates that similar mechaꢀ
nearly the same. These trends also suggest that identiꢀ
cal reaction mechanism is being followed in these
reactions [15]. The linear relationship in Exner plot
Δ
,
Δ
S
Δ
[16, 17] at 3 + logk303 K and 3 + logk308 K observed in
(a)
Δ
H#
– 10.576 – log
G# H# S#
.
= Ea – RT,
present study also supports the conclusion drawn from
isokinetic temperature. The isokinetic temperature is
the temperature at which all the compounds of series
react equally fast. Also, at the isokinetic temperature
the variation of substituent has no influence on the free
energy of activation. Based on above experimental
observations a probable mechanism of oxidation of
phenol by isoquinolinium bromochromate is in the
scheme below:
(b)
ΔS
# = 19.16(log
k
T + Ea/19.16T),
(c)
Δ
=
Δ
–
TΔ
Energy–Entropy Relationship
The entropy of activation and heat of reaction are
correlated by the equation
O
O
−OQH+
Br
−OQH+
O
k
..
1
+
Cr
Cr
OH
O +
k
–1
O−
Br
H
−OQH+
Br
O
−OQH+
Br
O
..
k
2
+
+
Cr
O
Cr
O
.. +
HO
slow
HO
−O
−OQH+
HO
−OQH+
Br
+
H
,
Cr
Cr
HO
Br
HO
O
O
H
H
k
3
+
+ 2H+
.
O
slow
O
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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2236
KHANSOLE et al.
Table 4. Dependence of rate constant (
k
×
104, s–1) on [KCl]: [Substrate] = 0.03 M, [H2SO4] = 1 M, [IQBC] = 0.001 M,
T
= 308 K, solvent = 50% acetic acid (v/v)
[KCl], M
m
ꢀCH3
–H
m
ꢀCl
pꢀCl
0.02
0.01
0.005
0
26.08 0.00
28.97 0.04
32.21 0.18
35.84 0.08
24.99 0.00
27.28 0.08
29.81 0.09
32.61 0.00
17.08 0.07
18.28 0.21
19.59 0.00
21.04 0.17
12.09 0.00
13.65 0.14
14.34 0.00
15.09 0.14
Table 5. Dependence of rate constant (
k
×
104, s–1) on solvent composition (acetic acid (
c
)ꢀwater) [Substrate] = 0.03 M,
[H2SO4] = 1 M, [IQBC] = 0.001 M, = 308 K, solvent = 50%
T
c
, %
1/
D
m
ꢀCH3
–H
m
ꢀCl
pꢀCl
30
40
50
60
0.01798
0.02238
0.02604
0.03170
28.69 0.10
31.90 0.18
35.49 0.00
39.50 0.14
24.68 0.21
26.95 0.20
29.12 0.18
34.09 0.19
19.60 0.16
20.69 0.19
21.04 0.00
22.14 0.21
13.12 0.08
14.34 0.19
15.09 0.12
16.81 0.00
Table 6. Rate constants (
k
104, s–1) of oxidation of phenols by isoquinolinium bromochromate in aqueous acetic acid at
different temperatures, [Phenols] = 0.03 M, [H2SO4] = 1 M, [IQBC] = 0.001 M
×
Sr. no.
Substrate
ꢀCreasol
Phenol
ꢀChlorophenol
293 K
303 K
308 K
313 K
323 K
1
2
3
4
5
6
m
28.86 0.01
26.83 0.08
14.51 0.00
8.84 0.09
6.58 0.07
2.49 0.00
33.33 0.14
29.56 0.19
18.55 0.14
12.84 0.09
9.27 0.12
4.60 0.14
35.84 0.08
32.61 0.00
21.04 0.17
15.09 0.14
11.17 0.11
5.44 0.27
38.54 0.00
35.97 0.03
23.89 0.10
17.61 0.11
13.55 0.13
6.33 0.08
44.62 0.00
42.87 0.11
30.94 0.10
24.47 0.00
20.33 0.01
9.55 0.0
m
p
p
p
ꢀChlorophenol
ꢀHydroxyphenol
ꢀNitrophenol
Table 7. Activation parameters at [Phenols] = 0.03 M, [IQBC] = 0.001 M, [H2SO4] = 1 M,
T = 308 K
Name of Substrate
ꢀCreasol
Phenol
ꢀChlorophenol
k
×
104, s–1
E
a, kJ mol–1
Δ
H#, kJ mol–1
–
Δ
S#, J mol–1 K–1
Δ
G#, kJ mol–1
m
35.84
32.61
21.04
15.01
11.17
5.44
14.40
18.34
25.89
31.63
39.40
43.42
11.84
15.78
23.33
29.07
36.84
40.86
250.73
238.10
216.85
199.63
178.19
170.72
88.84
89.11
90.11
90.55
91.72
93.44
m
p
p
p
ꢀChlorophenol
ꢀHydroxyphenol
ꢀNitrophenol
A linear free energy relationship is attempted by
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