Autocatalysis in oxidation reactions of verdazyl radicals with hydroperoxides

Article abstract of DOI:10.1134/S1070363210110125

The reaction of 2,4,6-triphenyl- and 2,3,4,6-tetraphenylverdazyl with tert-butyl hydroperoxide in acetonitrile solution and acetonitrile-water mixture was studied. This reaction was shown to be autocatalytic owing to verdazylium hydroxide formation in the course of the reaction. The main kinetic parameters were determined for the catalytic and non-catalytic reactions. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070363210110125

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 11, pp. 2306–2308. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.I. Buzulukov, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 11, pp. 1843–1845.
Autocatalysis in Oxidation Reactions of Verdazyl Radicals
with Hydroperoxides
V. I. Buzulukov
Ogarev Mordvian State University, ul. Bol’shevistskaya 68, Saransk, 430000 Russia
е-mail: busulukov@mail.ru
Received December 9, 2009
Abstract―The reaction of 2,4,6-triphenyl- and 2,3,4,6-tetraphenylverdazyl with tert-butyl hydroperoxide in
acetonitrile solution and acetonitrile–water mixture was studied. This reaction was shown to be autocatalytic
owing to verdazylium hydroxide formation in the course of the reaction. The main kinetic parameters were
determined for the catalytic and non-catalytic reactions.
DOI: 10.1134/S1070363210110125
It have been revealed earlier [1] that the kinetic
curves of consumption of 2,4,6-triphenylverdazyl
radical I in the reactions with hydroperoxides are
sigmoid: int the initial moment the reaction rate is
close to zero, then it is accelerated, and in the final
stage it is inhibited. The observed dependences are
generally characteristic of autocatalytic reactions. The
final reaction product [1] in benzene solution exerts no
essential influence on the reaction rate. It is
presumable that 2,4,6-triphenylverdazyl hydroxide II
shows the catalytic action in this reaction. Aiming to
confirm this hypothesis we examined some catalytic
regularities of the reaction of radical I with tert-butyl
hydroperoxide III both in the absence and presence of
compound II in acetonitrile and acetonitrile–water
mixture. Compound II is well soluble and relatively
stable in these solvents but not in benzene and
hydrocarbons. The variations in the radical I and
compound II concentrations in the reaction mixture
were registered by the spectrophotometric method.
After mixing the solutions of the starting reagents the
mixture color changed during the reaction progress
from green to violet, and then to pale yellow. If the
initial concentrations are nearly equal the reaction
between compounds I and III proceeds very slowly,
therefore it was carried out using substantial hydro-
peroxide excess: С0(I) = 0.5×10–4–2.0×10–4 mol l–1,
С0(III) = 2.0×10–2–3.0×10–2 mol l–1. Studying this
reaction we observed that it is catalysed with
compound II. In the presence of the preliminary added
compound II the reaction rate increases and induction
time decreases. Compound II was preliminarily pre-
pared in aqueous acetonitrile solution by the reaction
of radical I with hydrogen peroxide at their molar ratio
1:2. It should be noted that in the studied range of
concentration and molar ratios of compounds I and III
the hydroxide II does not react with the hydrperoxide
III in appreciable amounts during the reaction. As it
was found, 2,4,6-triphenylverdazylium hydroxide and
2,4,6-triphenylverdazylium bromide taken in equal
amounts similarly affect the rate of the examined
reaction. It may mean that the catalytic activity
belonged mainly to triphenylverdazylium cation. The
study of dependence of the reaction rate and half-
reaction time of radical I on the concentrations of
compounds I, II, and III shows that the reaction order
by compounds I and II is first; as compound III
concentration increases the half-reaction time for
radical I decreases proportionally to the initial con-
centration of compound III. The obtained results allow
the assumption of the following scheme for auto-
catalytic reaction between compounds I and III:
I + III → II + IV,
I + II ↔ [I···II],
(1)
(2)
(3)
[I···II] + III → 2II + IV,
where IV is the tert-buthoxyl radical (in the ex-
perimental conditions it almost quantitatively trans-
forms into tert-butyl alcohol).
The proposed reaction scheme includes both non-
catalytic (1) and catalytic transformations (2) and (3)
2306

AUTOCATALYSIS IN OXIDATION REACTIONS OF VERDAZYL RADICALS
2307
Kinetic parameters of reaction of tert-butyl hydroperoxide with verdazyl radicals (Т = 293 K)
Ea ± 2.1, kJ mol–1
k
nc×105,
kc,
Run no.
Solventa
εb
log k0nc
log k0c
l mol–1 s–1
l2 mol–2 s–1
Еnc
Еc
1
2
3
4
5
6
7
8
Acetonitrile
37.4
41.9
46.5
50.7
54.1
57.3
37.4
50.7
1.61
1.98
2.56
3.00
3.54
3.89
0.15
0.19
0.65
1.08
1.18
1.37
1.38
1.38
0.014
0.015
52
–
41
–
4.5
–
6.9
–
Aqueous acetonitrile
''
50
–
38
–
4.3
–
6.7
–
''
''
49
49
71
61
34
33
62
57
4.2
4.1
6.8
5.2
6.1
6.0
9.2
8.3
Acetonitrile
Aqueous acetonitrile
In the experiments (1)–(6) radical I was used, and in 7, 8, 2,3,4,6-tetraphenylverdazyl radical. b The value ε in the solvents mixture was
a
calculated basing on the additivity of the dielectric permeability.
of the initial reagents (1) to form compound II.
General equation for the rate of radical I consumption
is as follows:
peroxide bond. The overall reaction rate is satisfac-
torily described with Eq. (5). Some kinetic parameters
of the studied reaction are given in the table. By these
data, the rate of the non-catalytic reaction is con-
siderably lower than the rate of the catalytic reaction.
The change in the medium polarity differently affects
these reactions rates. Thus, the rate constant of the
catalytic reaction increases as the medium polarity
rises to approximately ε = 48, but further increases in
the water content in acetonitrile causes no changes.
The observed dependence can be explained by an
increase in the dissociation degree of hydroxide II as
the water content in acetonitrile rises. As to the rate
constant of the non-catalytic reaction, it rises as the
medium polarity increases. This dependence is
satisfactorily described by the Kirkwood equation:
w = dx/dt = knc(a – x) + kcx(a – x),
(4)
where a is the initial concentration of radical I; x is the
reacted amount of radical I at the moment t, which is
equal to the formed hydroxide II amount; knc and kc are
the rate constants of non-catalytic and catalytic
reactions, respectively.
In this kinetic equation the first term of the sum
corresponds to the non-catalytic reaction rate, and the
second, to the catalytic reaction rate. The latter is
proportional to the catalyst concentration (x). Solution
of the Eq. (4) determines the following time de-
pendence of the reaction rate for the initial time of the
self-acceleration [2], i.е., to the inflection point on the
time dependence curve of radical I consumption rate:
ε − 1
2ε + 1
.
ln knc = 70.34 + 123.5
w = Ae–φt,
(5)
Apparently, the influence of a solvent polarity on
the reaction rate is caused not only by the electrostatic
interaction between the solvent and dissolved sub-
stance, but by the specific reagents solvation, which
occurs in the medium with ε > 20.
where
φ = knc + kca;
kc
knc
1
a
(knc + akc)
+
The possibility of stage (2) mentioned above in the
reaction scheme, i.e. a possibility of the complex
formation between verdazyl radical and triphenyl-
verdazylium hydroxide was established in [3]. The
formation of relatively stable complexes between
compounds I and III, and also between compounds II
and III in acetonitrile and water–acetonitrile mixture
was not detected spectrophotometrically.
.
A =
(kc/knc)2
The slope of the curve in the coordinates ln w–t
equals to φ, and a segment cut off on the ordinate axis
equals to ln A. The values of knc and kc were calculated
from the found numerical values of φ and ln A.
The experimental data conform the given scheme,
where in the limiting stage radical I reacts with hydro-
peroxide III leading to the to homolysis of the
The rate of reaction between compounds I and III
increases as temperature rises. This dependence is
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 11 2010

2308
BUZULUKOV
satisfactorily described by the Arrhenius equation. In
this case the observed dependence of the reaction rate
on the medium polarity as temperature changes
remained that can indicate an invariability of the
reaction mechanism, when the medium polarity and
temperature are changed.
Compounds I and IV were synthesized by
procedure [4], III, by [5]. Acetonitrile was purified by
procedure [6].
REFERENCES
1. Buzulukov, V.I. and Zhivechkova, L.A., Zh. Obshch.
Khim., 1994, vol. 64, no. 2, p. 306.
2. Eremin, E.N., Osnovy khimicheskoi kinetiki (Chemical
Kinetics Fundamentals), Moscow: Vysshaya Shkola, 1976.
3. Buzulukov, V.I., Shchipakin, S.Yu., Zh. Obshch. Khim.,
2009, vol. 79, no. 9, p. 1577.
4. Kuhn, R. and Trischman, H., Monatsh. Chem., 1964,
vol. 95, no. 2, p. 457.
5. Karnozhitskii, V.Zh., Organicheskie perekisi (Organic
Peroxides), Moscow: Inostrannaya Literatura, 1961.
6. Gordon, A.J. and Ford, R.A., The Chemist’s Com-
panion. A Handbook of Practical Data, Techniques and
References, New York: Wiley, 1972.
The reaction of 2,3,4,6-tetraphenylverdazyl radical
(IV) with tert-butyl hydroxide in the mentioned above
solvents is also autocatalytic caused by the
tetraphenylverdazylium accumulation in the reaction
progress. Unlike the reaction of radical I under similar
condition, the reaction of hydroperoxide III with
radical IV proceeds considerably slower (see the
table), that is evidently connected with the decrease in
the radical nucleophilicity and also with the steric
effects occurring at the replacing the hydrogen atom at
the С3 atom of the tetrazine ring carbon by a phenyl
group.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 11 2010