Oxidation of dimethyl sulfoxide with hydrogen peroxide in the presence of potassium hydroxide

Article abstract of DOI:10.1134/S1070363208110157

Kinetic relationships in oxidation of DMSO to dimethyl sulfone with hydrogen peroxide in the presence of potassium hydroxide were studied. Kinetic parameters of the process were calculated. It was established that the reaction proceeded through the intermediate formation of potassium hydroperoxide.

Full text of DOI:10.1134/S1070363208110157

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 11, pp. 2071–2074. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.F. Choban, I.R. Yurchuk, A.S. Lyavinets, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 11, pp. 1838–1841.
Oxidation of Dimethyl Sulfoxide with Hydrogen Peroxide
in the Presence of Potassium Hydroxide
A. F. Choban, I. R. Yurchuk, and A. S. Lyavinets
Fed’kovich Chernovtsy National University,
ul. Kotsyubinskogo 2, Chernovtsy, 58012 Ukraine
e-mail: liavinetz@chnu.cv.ua
Received April 17, 2008
Abstract—Kinetic relationships in oxidation of DMSO to dimethyl sulfone with hydrogen peroxide in the
presence of potassium hydroxide were studied. Kinetic parameters of the process were calculated. It was
established that the reaction proceeded through the intermediate formation of potassium hydroperoxide.
DOI: 10.1134/S1070363208110157
The superbasic media already long ago became an
effective instrument for intensifying the nucleophilic
reactions [1]. With their aid many of the traditional
reactions were improved and new transformations
were carried out [2–4]. We showed [5–8] that super-
basic media may be used for performing the reactions
of hydroperoxides proceeding with the preservation of
peroxy group as well as with its decomposition. In
particular, it was shown by an example of alkylation
that such media may be used for intensifying the
nucleophilic reactions of hydroperoxides [8].
Typical kinetic curves of consumption of the active
oxygen and the variation in the electroconductivity of
the H2O2–DMSO–KOH system at 293 K are presented
in Fig. 1. Analysis of the kinetic curves of the active
oxygen contsumption shows that they cannot be
described by the formal kinetic equations. This feature
significantly complicates analysis of the processes
proceeding under these conditions. Therefore the kinetic
curve was tentatively divided in two parts: initial stage
and the stage of the developed process. Within the
framework of this study only the initial period was
analyzed basing on the initial rate of the active oxygen
consumption (W0). This value was obtained by graphic
differentiation. Since no other processes involving
Besides, it proved that superbasic media may be
used for performing the oxidative processes [9–11].
Under such conditions high reactivity is exhibited by
the peroxide oxidants because the oxidation proceeds
here according to the heterolytic mechanism.
[–O–O], mol l–1
κ×102, Ω–1 m–1
3.0
2.5
2.0
1.5
1.0
0.4
0.3
0.2
0.1
Results of the investigation presented in this work
show that oxidation of DMSO with hydrogen peroxide
in the presence of KOH belongs to such processes.
Accumulation of dimethyl sulfone at the introduction
of hydrogen peroxide in the superbasic DMSO–KOH
medium was established by GLC. Note that the KOH–
DMSO system is the simplest and most available
superbasic medium [12] where the dipolar hydroxy-
free solvent solvates specifically only cations while the
anions remain bare [1] resulting in significant growth
of reactivity of nucleophilic agents. The aim of this
work was the investigation of kinetic relations of the
oxidation of DMSO with hydrogen peroxide in the
presence of KOH.
2
1
0
20
40
60
t, s
80 100 120 140
Fig. 1. Kinetic curves of consumption of (1) active oxygen
and (2) variation in the specific electroconductivity of the
H2O2–KOH–DMSO system at 293 K. ν(KOH) 2.7 mmol,
[H2O2] 0.38 mol l–1, V(DMSO) 15 ml.
2071

2072
CHOBAN et al.
H2O2 were observed under these conditions W0 can be
regarded as a rate of oxidation of DMSO.
2
1.5
1
1
2
Dependence of the initial rate of the active oxygen
consumption on the amount of alkali is presented in
Fig. 2 (curve 1). As seen, this dependence is described
by the saturation curve, and the ascending part is linear
indicating the first order of the process with respect to
KOH.
0.5
0
0
2
4
6
8
10
12
Dependence of W0 on the initial H2O2 concentra-
tion (Fig. 3, curve 1) also has the character of the
saturation curve. The linear character of this de-
pendence shows the first order of process on H2O2.
From the slope of the straight line the effective
constant was evaluated, k' = 1.72×10–3 s–1.
n(KOH), mmol
Fig. 2. Dependence of the initial rate of consumption of the
active oxygen on the amount of KOH at 293 K. [H2O2]0
0.38 mol l–1, [DMSO]0 0.35 mol l–1, V(S) 15 ml, S: (1) DMSO
and (2) DMF.
First order by KOH and H2O2 and also the satura-
tion character of the concentrational dependences of W0
(Figs. 2, 3) shows that oxidation of DMSO proceeds
through the intermediate formation of KOOH salt.
2
1
KOH + НOOH → KOOН + H2O.
(1)
1.5
Inasmuch as no accumulation of dimethyl sulfone is
observed in the absence of alkali it may be concluded
that just KOOH salt is the key agent of oxidation of
DMSO.
1
2
0.5
0
KООН + (СH3)2SO → KOH + (CH3)2SO2.
(2)
0
0.2
0.4
0.6
0.8
For evaluation of the reaction order with respect to
DMSO which in this case plays the role of reagent and
a solvent dependence of the reaction rate on its
concentration was studied. DMF was chosen as an
inert solvent due to the following considerations.
Firstly, DMF is also a dipolar hydroxy-free solvent.
Besides, H2O2 does not decompose in KOH–DMF
system and does not cause the oxidation of solvent at
293–298 K.
[H2O2], mol l–1
Fig. 3. Dependence of the initial rate of the active oxygen
consumption on the concentration of H2O2 at 293 K.
ν(KOH) 8.9 mmol. (1) V(DMSO) 15.4 ml and (2) [DMSO]0
0.35 mol l–1, V(DMF) 15 ml.
1.6
1.4
1.2
1
Dependence of the initial rate of DMSO oxidation
on its initial concentration is presented in Fig. 4. As
seen, at the DMSO concentration 0.1–7.0 mol l–1 the
rate of the active oxygen consumption does not
materially depend on its concentration (the order on
DMSO is 0.15). This conclusion is confirmed by the
experiments with the deuterated analog DMSO-d6. It
was shown that this change does not practically affect
W0 in the DMSO-d6 concentration range 0.2–7 mol l–1.
Hence, at the above-mentioned DMSO concentrations
in DMSO–DMF–KOH system the oxidative process is
described by the kinetic equation (3). The salt forma-
tion stage is evidently the limiting one [reaction (1)].
0.8
♦ 1
■ 2
0.6
0.4
0.2
0
0
5
10
15
[DMSO], mol l–1
Fig. 4. Dependence of the initial rate of the active oxygen
consumption on the concentration of DMSO, ν(KOH)
0.0025 mol, [H2O2]0 0.58 mol l–1, V(DMF) 15 ml, 293 K,
(1) DMSO and (2) DMSO–d6.
W0 = k1[Н2О2][KОН].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 11 2008
(3)

OXIDATION OF DIMETHYL SULFOXIDE WITH HYDROGEN PEROXIDE
2073
But as seen from Fig. 4 at the DMSO concentra-
tions above 7 mol l–1 the transformation of the order
with respect to DMSO occurred from zero to the first.
At the ascending part the slope gives the value of the
effective constant 1.02×10–4 s–1.
(Fig. 1, curve 2). It is connected with the formation of
KOOH on the surface of KOH crystals that dissolves
with the subsequent dissociation.
(СН3)2S=O···H–O–O–K···O=S(CH3)2
→← (СН3)2S=O···H–O–O– + K+···O=S(CH3)2.
(5)
For establishing the reason of variation in the order
with respect to DMSO at the change in its volume
fraction in the solution and elucidating the role of the
dipolar hydroxy-free solvent in the mechanism of the
process under consideration kinetic rules of this
reaction in presence of DMF as the constituent of the
superbasic medium were studied.
Formation of salt and its dissociation proceeds
considerably easier in DMSO than in DMF (see table).
We believe that it is directly related to the nature of
the solvents used. [1] Both solvents are known to
possess high permittivity (ε > 30) and dipole moments
[μ(DMF) 3.82 D, μ(DMSO) 4.30 D]. Due to high
dipole moments these solvents are easily polarized,
and strong dipolar interaction exists between them and
the dissolved polar substance (KOOH in our case).
Besides, the permittivity of both solvents favors the
dissociation of the dissolved polar substance and the
appearance of hydroperoxy anions in the reaction
medium. At the same time, according to the kinetic
studies, these reactions proceed faster in DMSO that
possesses higher ε and μ values as compared to DMF.
Besides, DMSO has the higher donor number. The
latter according to [13] is the quantitative characteristic
of solvent as the electron pair donor. The value of this
parameter for DMSO DNDMSO = 124.7 kJ mol–1, and
for DMF DNDMF = 111.3 kJ mol–1 [13]. Being the
stronger electron pair donor, and hence the stronger
specific solvating agent DMSO effectively separates
the ion pairs and easily generates “bare” hydroperoxy
anions, the direct DMSO oxidants.
Dependence of W0 on the amount of alkali in the
KOH–DMF medium is presented by the curve 2 in
Fig. 2. As seen, this dependence also has a character of
the saturation curve, and its ascending branch is linear
indicating the first reaction order on KOH also in the
case of using DMF as a solvent.
Dependence of W0 on the initial H2O2 concentration
is presented in Fig. 3 (curve 2). It is also the saturation
curve, and its ascending part is linear. Hence, in the
DMF–KOH medium DMSO oxidation is of the first
order on hydrogen peroxide, the effective constant
value being k" 4.57 10–4 s–1.
First orders on H2O2 and KOH (and therefore on
KOOH) and also on dimethyl sulfoxide show that in
the superbasic DMF–DMSO–KOH medium oxidation
of DMSO is described by the kinetic equation (4).
Oxidation of DMSO [reaction (2)] becomes the rate-
limiting stage.
Hence, dipolar hydroxy-free solvents significantly
affect the stage of salt formation during the oxidation
of DMSO.
W0 = k2[KООН][DMSO].
(4)
Hence, at the transition from DMSO to DMF the
rate-limiting stage of oxidation of DMSO with
hydrogen peroxide in presence of KOH changes. It
was established conductometrically that in both cases
the oxidation proceeds by the ionic pathway. Specific
conductivity of KOH–DMSO (DMF) system is not
high because the solubility of KOH in both systems is
insignificant. It is connected with the ability of the dipolar
hydroxy-free solvent to the specific salvation only of
cations. Besides, the electroconductivity of H2O2–DMSO
(DMF) system is not high showing the sufficiently low
degree of dissociation of hydrogen peroxide in the
above-mentioned solvents. Under these conditions hydro-
gen peroxide exists as the solvate (CH3)2S=O...H–O–
O–H...O=S(CH3)2. At the same time introduction of
H2O2 in the superbasic KOH–DMSO (DMF) system
causes the increase in the specific electroconductivity
In the case of DMF as a solvent this stage is the
rate-limiting one, while in the case of oxidation in
Specific electroconductivity of the compounds under invest-
tigation and the rate of oxidation of DMSO with hydrogen
peroxide
κmах×102, W0×103,
System composition
μ(KOH), g [H2O2], mol l–1
Ω
–1 m–1 mol l–1 s–1
solvent
DMF
–
–
–
0.80
–
–
DMF
1.4
1.4
–
0.96
1.62
0.84
1.17
2.63
DMF
0.33
–
0.68
–
DMSO
DMSO
DMSO
1.4
1.4
–
–
0.33
1.42
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 11 2008

2074
CHOBAN et al.
DMSO the limiting stage is the direct oxidation with
hydroperoxy anion. At the same time it must be
mentioned that the mechanism of oxidation may be
differently interpreted. Considering the electronic
density distribution in the DMSO molecule it may be
suggested that its oxidation proceeds through the
following transition complex.
and the mixture obtained was kept at the required
temperature for 20 min. After that hydrogen peroxide
was added, and the moment of its introduction was
taken as the beginning of the reaction. Reaction
progress was monitored by the iodometric evaluation
of active oxygen in the samples.
All the solvents used were purified according to the
typical procedures [14].
H3C
CH3
S
O_
O
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O
H
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