Oxidation of phenols with chlorine dioxide

Article abstract of DOI:10.1007/s11172-005-0114-x

The oxidation of different phenols, viz., phenol, 3-methylphenol, 4-methylphenol, 4-tert-butylphenol, 2-cyclohexylphenol, 2,6-di-tert-butyl-4- methylphenol, and 2,4-dichlorophenol, with chlorine dioxide in acetonitrile was studied spectrophotometrically. The reaction rate is described by a second-order equation w = k[PhOH]? [ClO₂]. The rate constants were measured and activation parameters of oxidation were determined in a temperature interval of 10-60°C. A dependence of the reaction rate constant on the phenol structure was found. The oxidation products were identified, and their yields were established.

Full text of DOI:10.1007/s11172-005-0114-x

Russian Chemical Bulletin, International Edition, Vol. 53, No. 10, pp. 2281—2284, October, 2004
2281
Oxidation of phenols with chlorine dioxide
ꢀ
I. M. Ganiev, E. S. Ganieva, and N. N. Kabal´nova
Institute of Organic Chemistry, Ufa Research Center of the Russian Academy of Sciences,
71 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347 2) 35 6066. Eꢀmail: chemox@anrb.ru
The oxidation of different phenols, viz., phenol, 3ꢀmethylphenol, 4ꢀmethylphenol,
4ꢀtertꢀbutylphenol, 2ꢀcyclohexylphenol, 2,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethylphenol, and 2,4ꢀdichloroꢀ
phenol, with chlorine dioxide in acetonitrile was studied spectrophotometrically. The reaction
rate is described by a secondꢀorder equation w = k[PhОН]•[ClO2]. The rate constants were
measured and activation parameters of oxidation were determined in a temperature interval of
10—60 °С. A dependence of the reaction rate constant on the phenol structure was found. The
oxidation products were identified, and their yields were established.
Key words: oxidation, chlorine dioxide, phenols, rate constants.
We have previously1 studied the kinetics of phenol
oxidation with chlorine dioxide in several solvents. Howꢀ
ever, the influence of the phenol structure on the reaction
rate was not considered. In this work, we studied the
oxidation of phenols with different structures, viz.,
phenol (1), 3ꢀmethylphenol (2), 4ꢀmethylphenol (3),
4ꢀtertꢀbutylphenol (4), 2ꢀcyclohexylphenol (5), 2,6ꢀdiꢀ
tertꢀbutylꢀ4ꢀmethylphenol (6), and 2,4ꢀdichloropheꢀ
nol (7), with chlorine dioxide in acetonitrile.
(300 MHz) using CDCl3 as a solvent and Me4Si as a standard.
Benzene was used as an internal standard in quantitative deterꢀ
mination of the products.
Results and Discussion
Reaction products. The overall conversion of phenols
is observed at the molar ratio of phenol to chlorine diꢀ
oxide equal to 1 : 2. The yields of the identified products
are presented in Table 1. Similar products were obtained
by the oxidation of 2,6ꢀxylenol and mesitol with chlorine
dioxide in water.5 Substances formed from phenol 7 were
not analyzed. Some other products are present in the
reaction mixture along with the indicated products. Howꢀ
Experimental
Chlorine dioxide was synthesized by the reaction of potasꢀ
sium chlorate with oxalic acid in the presence of sulfuric acid.1
Phenols 1—3 and 5 (reagent grade) were twice distilled in argon,
phenols 4 and 7 (reagent grade) were sublimed in vacuo, and
phenol 6 (reagent grade) was recrystallized from EtOH followed
by sublimation in vacuo. Acetonitrile (reagent grade) was puriꢀ
fied by distillation and then distilled above P2O5 to remove
water traces.2
The kinetics of phenol oxidation at 10—60 °С was studied1
spectrophotometrically by a change in the absorbance of chloꢀ
rine dioxide at λ = 400 nm on a Specord M40 instrument (Carl
Zeiss, Jena). At this wavelength, the contribution of absorbances
from phenols and final products to the total absorbance can be
neglected.3,4 The initial concentrations of phenols 1—5, 7 and
chlorine dioxide were varied within (0.1—10.0)•10–2 and
(0.5—1.2)•10–3 mol L–1, respectively. In the case of phenol 6,
the reaction kinetics was studied at equal concentrations of the
reactants ([ClO2]0 = [6]0 = (0.5—1.2)•10–3 mol L–1).
To study the oxygen influence on the kinetic regularities of
the process, dioxygen or an inert gas was passed through the
reaction mixture.
Table 1. Yields of the products of oxidation of phenols 1—6
Phenol
Product
1,4ꢀBenzoquinone
2ꢀChloroꢀ1,4ꢀbenzoquinone
Diphenoquinone
2ꢀMethylꢀ1,4ꢀbenzoquinone
5ꢀChloroꢀ2ꢀmethylꢀ1,4ꢀbenzoquinone
3,3´,5,5´ꢀTetramethyldiphenoquinone
1,4ꢀBenzoquinone
4ꢀMethylꢀpꢀquinol
1,4ꢀBenzoquinone
4ꢀtertꢀButylꢀpꢀquinol
2ꢀCyclohexylꢀ1,4ꢀbenzoquinone
5ꢀChloroꢀ2ꢀcyclohexylꢀ1,4ꢀbenzoquinone
2,2´,6,6´ꢀTetracyclohexyldiphenoquinone
2,6ꢀDiꢀtertꢀbutylꢀ1,4ꢀbenzoquinone
2,6ꢀDiꢀtertꢀbutylꢀ4ꢀmethylꢀpꢀquinol
Yield* (%)
1
50
20
10
40
30
5
30
30
30
30
45
20
5
2
3
4
5
6
35
30
Reaction products were analyzed by GLC on a Chromꢀ5
instrument (column 3.5 m × 3 mm, 5% SEꢀ30 on Chromatone)
and by 1Н NMR spectroscopy on a Bruker AMꢀ300 instrument
* The yield was recalculated to the consumed oxidant.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2184—2187, October, 2004.
1066ꢀ5285/04/5310ꢀ2281 © 2004 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
Ganiev et al.
ln[ClO2] 0.16
–7.5
0.12
[1]0/mol L–1
kapp•103/s–1
For phenol 6, the kinetics was studied at 25 °С under
the condition that the initial concentrations of phenol
and chlorine dioxide were equal. In this case, the kinetic
curves with a high correlation coefficient (0.980—0.999)
are described by an equation of the second order, and the
form of the kinetic equation is similar to that of Eq. (1).
Similar results have been obtained previously6 for the oxiꢀ
dation of phenol 1 in water.
5
2
4
–8.0
The activation parameters of the oxidation process
(excluding those for phenol 6) are presented in Table 2.
Influence of the phenol structure on the reaction rate
constant. The data presented in Table 2 show that the
reactivity of phenols toward chlorine dioxide increases in
the series 7 < 1 < 2 < 5 < 4 < 3 < 6.
A commonly accepted approach to establish a relaꢀ
tionship between the structure and reactivity of organic
compounds is the use of the Hammett equation,7 implyꢀ
ing a linear dependence of the logarithm of a reaction rate
constant on a constant of substituent σ
1
6
3
2
4
–8.5
0
2
4
t•102/s
Fig. 1. Semilogarithmic anamorphoses of the consumption curves
of chlorine dioxide in the reaction with phenol (1) (Т = 40 °С):
[1]0 = 0.05 (1), 0.09 (2), 0.13 (3), and 0.17 mol L–1 (4) and the
apparent reaction rate constant (kapp) as a function of the pheꢀ
nol concentration (5).
ever, attempts of their identification failed, and their
amount was especially large in the case of phenols 3,
4, and 6.
lnk = lnk0 + ρσ,
where k0 and ρ are constants.
Reaction kinetics. For phenols 1—5 and 7, the reꢀ
action kinetics was studied under the condition of
[ClO2]0 << [PhOH]0, where [ClO2]0 and [PhOH]0 are the
initial concentrations of chlorine dioxide and phenol, reꢀ
spectively. For all phenols studied, the kinetic curves with
a high correlation coefficient (0.980—0.999) are deꢀ
scribed by an equation of the pseudoꢀfirst order (Fig. 1).
The apparent rate constants of the pseudoꢀfirst order
kapp = k[PhOH]n (k is a rate constant of the second order)
were calculated from the semilogarithmic anamorphoses
of the kinetic curves. These apparent constants are indeꢀ
pendent of the ClO2 concentration. The dependence of
kapp on [PhOH]0 is linear (see Fig. 1), indicating the first
order with respect to phenol. Thus, the kinetic equation
of the reaction has the form
An advantage of this approach is that the σ values for
diꢀ and trisubstituted organic substances can be calcuꢀ
lated as the sum of constants of all substituents (Σσ). The
use of σ constants is somewhat restricted by the absence
of data for substituents in the orthoꢀpositions of the benꢀ
zene ring, which is related, first of all, to steric hindrances
caused by these substituents.8 It has previously9 been
shown that σ constants of substituents in the paraꢀposiꢀ
tion can be used to describe the influence of substituents
in the orthoꢀposition. We performed similar data processꢀ
ing and obtained a linear dependence with a low correlaꢀ
tion coefficient. When phenols 3 and 6 were excluded
from consideration, the dependence gained a pronounced
linear character (Fig. 2)
–d[ClO2]/dt = k[ClO2][PhOH].
(1)
lnk = –(4.02 0.02) – (3.0 0.1)•Σσ
(r = 0.998).
Table 2. Activation parameters of oxidation of phenols 1—7 with chlorine dioxide (Т = 25 °С,
measurement error ≤10%)
a
b
Phenol
k•101
/L mol–1 s–1
lnA
Ea
∆H≠
∆G≠
∆S≠
/J mol–1 K–1
Σσ
Е1/2
/V
kJ mol–1
1
2
3
4
5
6
7
0.17
0.24
3.53
0.33
0.28
400
13
15
11
15
14
—
42
46
30
46
44
—
40
44
28
43
42
—
83
82
76
81
82
—
–145
–129
–160
–129
–134
—
0.000
–0.069
–0.170
–0.197
–0.150
–0.466
0.600
0.633
0.607
0.543
0.578
0.600
0.381
0.645
0.05
16
53
51
86
–120
a According to the published data.11
b According to the published data.10

Oxidation of phenols with chlorine dioxide
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
2283
lnk
lnk
4
5
2
6
4
0
–4
1
3
–5
7
5
4
1
–4
2
–0.2
0
0.2
0.4 Σσ
7
Fig. 2. Logarithm of the rate constant of oxidation of phenols 1,
2, 4, 5, and 7 with chlorine dioxide (k) as a function of the
substituent constant σ (Т = 25 °С; phenols 3 and 6 are excluded
from correlation).
0.4
0.5
0.6
E1/2/V
Fig. 3. Logarithm of the rate constant of oxidation of
phenols 1—7 with chlorine dioxide (k) as a function of the halfꢀ
wave potential of phenol oxidation (E1/2) (Т = 25 °С).
The negative value of constant ρ indicates that the
introduction of electronꢀreleasing substituents into a pheꢀ
nol molecule enhances the electron density on the reacꢀ
tion center and increases the reactivity. It follows from
this that chlorine dioxide behaves as an electrophilic agent,
and, most likely, the electron transfer from phenol to a
chlorine dioxide molecule is the first (limiting) step of the
oxidation process. A rather high absolute value of ρ indiꢀ
cates a strong sensitivity of the oxidation rate constant to
structural changes in the initial reactants. The calculated
ρ value differs slightly from ρ = –3.2 0.4 obtained for the
oxidation of phenols in water.6
Scheme 1
The use of any measured property of a whole molecule
is an alternative for correlations based on substituent conꢀ
stants. For the oxidation of substituted phenols with chloꢀ
rine dioxide, it seems reasonable to assume a correlation
between k and redox potentials of phenols. For example,
potentials of the oxidation halfꢀwave (Е1/2) are known10
for more than 40 substituted phenols. The obtained poꢀ
tentials of the oxidation halfꢀwave reflect the potential of
elimination of the first electron from a phenol molecule
to form a phenoxyl radical. The E1/2 value depends on the
nature of substituents and can be calculated when the
structure of substituents in a phenol molecule is known.10
The E1/2 values for all phenols studied are presented in
Table 2. The plot of the logarithm of the rate constant vs.
E1/2 is linear (Fig. 3)
Scheme 2
lnk = (16 1) – (33 2)•E1/2
(r = 0.98).
It is seen that the reaction rate decreases with an inꢀ
crease in the halfꢀwave potential.
Taking into account the data obtained, the formation
of the main reaction products can be presented by
Scheme 1, according to which the first (limiting) step is
the electron transfer from a phenol molecule to a chlorine
dioxide molecule to form a phenoxyl radical.
For R2 = Н, the phenoxyl radicals can dimerize
(Scheme 3).
In the case of R2 = Alk, quinols can be formed preꢀ
sumably via the following reaction (Scheme 4).
The presence of chlorosubstituted derivatives in the
oxidation of phenols with chlorine dioxide can be exꢀ
Then the phenoxyl radical is oxidized by another chloꢀ
rine dioxide molecule to form quinone (Scheme 2).

2284
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 10, October, 2004
Ganiev et al.
Scheme 3
phenols 1—7, bubbling of the reaction mixture with oxyꢀ
gen or an inert gas has no effect on the kinetics of chlorine
dioxide consumption.
Thus, in this work, we studied the oxidation of differꢀ
ent phenols with chlorine dioxide, established a relationꢀ
ship between the phenol structure and reactivity, identiꢀ
fied the main oxidation products, and proposed the reacꢀ
tion scheme consistent with the experimental data.
References
Scheme 4
1. I. M. Ganiev, E. S. Suvorkina, and N. N. Kabal´nova, Izv.
Akad. Nauk, Ser. Khim., 2003, 1064 [Russ. Chem. Bull., Int.
Ed., 2003, 52, 1123].
2. B. Keil, V. Herout, M. Hudlicky, I. Ernest, M. Protiva,
J. Gut, R. Komers, and J. Moravek, Laboratorni technika
organicke chemie, Nakladatelstvi eskoslovenske akademie
ved, Praha, 1963, 752 pp.
3. E. Grimley and G. Gordon, J. Inorg. Nucl. Chem., 1973,
35, 2383.
4. J. E. Wajon, D. H. Rosenblatt, and E. P. Burrows, Environ.
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5. B. O. Lindgren and B. Ericsson, Acta Chem. Scand., 1969,
23, 3451.
6. P. G. Tratnyek and J. Hoigne, Water Res., 1994, 28, 57.
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dlya korrelyatsionnogo analiza [Induction Effect of Substituent
Constants for Correlation Analysis], Nauka, Moscow, 1988,
111 pp. (in Russian).
plained by the fact that hypochlorous acid chlorinates
organic compounds11 (Scheme 5).
8. V. A. Pal´m, Osnovy kolichestvennoi teorii organicheskikh
reaktsii [Foundations of the Quantitative Theory of Organic
Reactions], Khimiya, Leningrad, 1977, 360 pp. (in Russian).
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Scheme 5
10. J. C. Suatoni, R. E. Snyder, and R. O. Clark, Anal. Chem.,
1961, 33, 1894.
11. T. A. Tumanova, Fizikoꢀkhimicheskie osnovy otbelki
tsellyulozy [Physicochemical Foundations of Cellulose Bleachꢀ
ing], Lesnaya Promꢀst´, Moscow, 1984, 213 pp. (in Russian).
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13. J. F. Coetzce and F. S. Kolthoff, J. Am. Chem. Soc., 1957,
79, 6111.
It is known12 that phenoxyl radicals react easily with
oxygen, whose content in a solution during kinetic exꢀ
periments is comparable with the chlorine dioxide conꢀ
centration ([ClO2] = (0.5—1.2)•10–3 mol L–1, [O2] =
1.5•10–3 mol L–1).13 It turned out that, in the case of
Received February 14, 2003;
in revised form April 13, 2004