Oxidation of diethyl disulfide with atmospheric oxygen in the presence of 1,3-dimethylimidazolium dimethyl phosphate

Article abstract of DOI:10.1007/s11172-020-2859-7

Oxidation of diethyl disulfide with atmospheric oxygen at 363 К in the presence of 1,3-dimethylimidazolium dimethyl phosphate is described. On the basis of the ¹Н, ³¹Р, and ¹³С NMR data, it is shown that oxidation of diethyl disulfide results in ethyl sulfonates. Formation of ethyl alcohol, which is a product of a side reaction of ethyl sulfonate hydrolysis, probably, catalyzed by the 1,3-dimethylimidazolium cation, was detected using HPLC and ^1Н and ¹³С NMR.

Full text of DOI:10.1007/s11172-020-2859-7

Russian Chemical Bulletin, International Edition, Vol. 69, No. 5, pp. 986—989, May, 2020
986
Oxidation of diethyl disulfide with atmospheric oxygen
in the presence of 1,3-dimethylimidazolium dimethyl phosphate*
E. G. Krivoborodov, A. A. Zanin, E. P. Novikova, and Ya. O. Mezhuev
D. Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Fax: +7 (499) 978 8660. E-mail: vv1992@yandex.ru
Oxidation of diethyl disulfide with atmospheric oxygen at 363 К in the presence of 1,3-di-
methylimidazolium dimethyl phosphate is described. On the basis of the 1Н, 31Р, and 13С NMR
data, it is shown that oxidation of diethyl disulfide results in ethyl sulfonates. Formation of
ethyl alcohol, which is a product of a side reaction of ethyl sulfonate hydrolysis, probably,
catalyzed by the 1,3-dimethylimidazolium cation, was detected using HPLC and 1Н and
13С NMR.
Key words: ionic liquids, 1,3-dimethylimidazolium dimethyl phosphate, sulfonate, disulfide.
disulfide with 50% aqueous hydrogen peroxide taken in an amount
of 0.7 mL (ρ = 1.2 g mL–1) containing 0.42 g (1.24•10–2 mol)
of hydrogen peroxide. The reaction was carried out in an excess
of ethanethiol, which made it possible to prevent the secondary
oxidation of diethyl disulfide in the course of its synthesis:
Ionic liquids traditionally attract attention as extract-
ants and solvents often exhibiting a catalytic activity and
providing a significant change in reactivity of solvated
reactants.1,2 Due to the minimal environmental impact,
ionic liquids are considered as "green" solvents, which are
promising for the implementation of various technological
processes.3—5 One of the significant applications of ionic
liquids is the removal of sulfur-containing compounds
from hydrocarbons in the extraction process,6—8 which is
often combined with oxidation of organosulfur compounds
(usually, with hydrogen peroxide).9,10
2 Et—SH + H2O2 = Et—S—S—Et + 2 H2O.
1,3-Dimethylimidazolium dimethyl phosphate (Sigma-
Aldrich), an ionic liquid, in an amount of 0.9 g (4.05•10–3 mol)
was added under stirring to the reaction mixture. Then, the reac-
tion mixture was heated to 363 К for 2 h under vigorous stirring
in a bulb equipped with a direct condenser and a receiver for the
condensate. After the completion of the reaction, the composi-
tion of the obtained condensate was determined by HPLC using
a L-3000 chromatograph equipped with a L-3500 UV-vis detec-
tor (180—250 nm, Rigol, China) and a Luna 5u C 18(2) 100A
column. The HPLC analysis was carried out for mixtures con-
taining 1 L of isopropyl alcohol as an internal standard and 1 L
of the condensate.
The presence of ionic liquids affects the reactivity of
sulfur-containing compounds in a number of reactions.
In particular, oxidation of thiols to disulfides occurs with
an acceptable rate at room temperature already,11 whereas
elemental sulfur is capable to interact with ionic liquids
containing anion of dimethyl phosphate under very soft
conditions.12 Though the reasons for the changes in the
reactivity of organic compounds, including organosulfur
compounds, in the presence of ionic liquids are often not
entirely clear, the need for further study of specificity of
chemical processes in these media is not in doubt. In this
regard, the reported here transformation of diethyl disulfide
into ethyl sulfonates in the presence of 1,3-dimethylimid-
azolium dimethyl phosphate [1,3-MeIm]Me2PO4 under
moderate heating in air appears to be of particular interest.
The 1Н, 31Р, and 13С NMR spectra of starting 1,3-dimethyl-
imidazolium dimethyl phosphate were recorded in methanol.
The 13С NMR spectrum of the reaction mixture was recorded
after the completion of the reaction at 298 К. When the reaction
system was cooled to 283 К, formation of a precipitate was ob-
served, the melting point of which was 298 К. After separation
1
of the precipitate, the Н and 31Р NMR spectra of the residual
mother liquor were recorded. Spectra of all mixtures obtained
after completion of the reaction were recorded in CD3OD with
the use of the equipment of the Center of Collective Usage of
the D. Mendeleev University of Chemical Technology of Russia.
Experimental
Results and Discussion
Ethanethiol (Reachem, RF) in an amount of 3.88 g
(6.25•10–2 mol) was oxidized under stirring for 2 h to diethyl
There are four signals in the 13С NMR spectrum
of 1,3-dimethylimidazolium dimethyl phosphate with
C 139.93, 125.89, 53.85, and 37.57, they correspond to
* Based on the materials of the XXI Mendeleev Congress
on General and Applied Chemistry (September 9—13, 2019,
St. Petersburg).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 0986—0989, May, 2020.
1066-5285/20/6905-0986 © 2020 Springer Science+Business Media LLC

Oxidation of diethyl disulfide
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
987
PriOH
the carbon atoms of the imidazole ring in position 2 and
in equivalent positions 4 and 5 and to carbon atoms of
methyl groups of the 1,3-dimethylimidazolium cation and
dimethyl phosphate anion, respectively (Fig. 1, a). In the
13С NMR spectrum recorded after the completion of the
reaction (Fig. 1, b) these signals were shifted to high field.
The multiplet with C 47—50 that was observed in the
13С NMR spectrum of the starting ionic liquid in metha-
nol and in the spectrum of the reaction mixture in CD3OD
belongs to the methanol carbon atom. The signals with
C 9.26 and 46.08 belong to the methyl and methylene
groups of the ethyl sulfonate anion, respectively. The
observation of the signals with C 17.84 and 57.77 is con-
sistent with the formation of ethanol, which is a product
of a side reaction of the ethyl sulfonate anion hydrolysis,
which can be catalyzed by the 1,3-dimethylimidazo-
lium cation.
EtOH
EtSH
0
1
2
3
4
t/min
Fig. 2. Chromatogram obtained by HPLC for the condensate
formed in the course of removal of the volatile products from the
reaction system at 363 К for 2 h.
Formation of ethanol is corroborated by its detection
in the condensate using HPLC (Fig. 2). A chromatographic
peak with a retention time of 1.28 min corresponds to
propan-2-ol (internal standard), and peaks with retention
times of 3.96 and 1.66 min correspond to ethyl mercaptan
and ethanol, respectively, which are present in the con-
densate in a ratio equal to 1.67 according to the ratio of
their peak areas.
spectrum of the reaction system, which indicates its almost
complete oxidation by atmospheric oxygen. The 1Н NMR
detection of starting diethyl disulfide was possible only
after separation of 1,3-dimethylimidazolium ethyl sulf-
onate by cooling.
Four signals with H 9.29, 7.83, 3.63, and 3.56 are
observed in the 1Н NMR spectrum of 1,3-dimethylimid-
azolium dimethyl phosphate, they belong to protons at
the carbon atoms of the imidazole ring in position 2 and
in equivalent positions 4 and 5 and at the carbon atoms of
the methyl groups of the 1,3-dimethylimidazolium cation
and dimethyl phosphate anion, respectively. The signals
with H 5.19 and 4.09 correspond to the methanol protons
As can be seen from Fig. 1, there are no signals char-
acteristic of the initial diethyl disulfide in the 13С NMR
125.89
a
37.57
1
(Fig. 3, a). In the Н NMR spectrum of the reaction
mixture obtained after precipitation of 1,3-dimethylimid-
azolium ethyl sulfonate by cooling (Fig. 3, b), all signals
of protons of initial ionic liquid are shifted to high field
by 0.5—0.7 ppm. Signals in the region of H 0.8—1.2 cor-
respond to superposition of signals of methyl groups of
several different compounds present in the reaction system.
A multiplet with a chemical shift of about 2.5 ppm, which
corresponds to a methylene group of diethyl disulfide, is
clearly distinguished. Quadruplet with H 3.30 is due to
the methylene group of ethanol. An intensive signal with
H 5.38 corresponds to protons of hydroxyl groups of
hydroxyl-containing compounds present in the mixture,
including water, and indicates that there is a fast proton
exchange in the system. As can be seen, the amount of
unreacted diethyl disulfide is rather small, which indicates
its almost complete oxidation.
53.85
139.93
49.84
140
120
100
80
60
40

124.19
b
36.32
48.68
49.52
57.77
137.42
47.61
46.08
9.26
17.84
Finally, in the 31Р NMR spectrum, a slight shift of the
phosphorus atom signal from 3.32 to 0.6 ppm is observed
(Fig. 4). This is, apparently, related to the change in the
nature of interaction of the dimethyl phosphate anion with
counterions.
140
120
100
80
60
40
20

Fig. 1. 13C NMR spectra of 1,3-dimethylimidazolium dimethyl
phosphate (in MeOH) (a) and the reaction system after the
completion of the reaction (in CD3OD) (b).
The obtained spectroscopic data along with the obser-
vation of precipitation from the reaction mixture at 283 К

Krivoborodov et al.
988
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
a
a
3.32
4.09
3.63
3.58
5.19
7.83
9.29
10
9
8
7
6
5
4

5.38
b
3.05
5
0

3.60
b
0.60
7.25
0.97
3.30
2.50
2.34
8.53
10
8
6
4
2

1
Fig. 3. H NMR spectra of 1,3-dimethylimidazolium dimethyl
phosphate (in MeOH) (a) and the reaction system after the
completion of the reaction (in CD3OD) (b).
after the completion of the reaction indicate that the
process occurring in the reaction mixture follows the se-
quence of reactions shown in Scheme 1.
100
50
0
–50

Fig. 4. 31Р NMR spectra of 1,3-dimethylimidazolium dimethyl
phosphate (in MeOH) (a) and the reaction system after the
completion of the reaction (in CD3OD) (b).
Scheme 1
radicals, which is in favor of the former assumption.
Nevertheless, the intermediate formation of free radicals
cannot be ruled out, especially, if one takes into account
the recently revealed possibility of their solvation by ionic
liquids.14
The formation of a precipitate in the course of cooling
the reaction mixture below room temperature is consistent
with the known melting point of 1-ethyl-3-methylimid-
azolium methyl sulfonate, an isomer of 1,3-dimethylimid-
azolium ethyl sulfonate, equal to 308 К. The formation of
1,3-dimethylimidazolium ethyl sulfonate is a result of an
ion-exchange reaction consisting in the replacement of
the dimethyl phosphate anion of 1,3-dimethylimidazolium
dimethyl phosphate by the ethyl sulfonate anion, which
is formed in the course of oxidation of diethyl disulfide
with atmospheric oxygen.
Reagents and conditions: i. O2/H2O, 363 K, [1,3-Me2Im]Me2PO4.
ii. [1,3-Me2Im]Me2PO4, 283 K. iii. H2O + [1,3-Me2Im]+, 363 K.
Probably, the capability of disulfides to interact with
atmospheric oxygen under the used mild conditions is
determined by solvation of radical-cations formed in one-
electron oxidation of disulfides or by solvation of thioethyl
radicals by the ionic liquids if one assumes in the latter
case that homolysis of the S—S bond precedes the oxida-
tion. It should be expected that the energy of solvation of
radical-cations13 by ionic liquids is higher than that of free
Apparently, the reported reaction can be of certain
technological interest for the purification of hydrocarbon
fuels from sulfur-containing compounds.

Oxidation of diethyl disulfide
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
989
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In conclusion, it is established that diethyl disulfide
can be oxidized with atmospheric oxygen in the presence
of 1,3-dimethylimidazolium dimethyl phosphate at 363 К
for 2 h with formation of ethyl sulfonates. It is shown that
decreasing the temperature of the reaction mixture to
283 К results in precipitation of 1,3-dimethylimidazolium
ethyl sulfonate, which is a product of an ion exchange
involving the replacement of the dimethyl phosphate
anion by the ethyl sulfonate anion. The structures of
1
the obtained compounds are confirmed by 13С, Н, and
31P NMR spectroscopy. Occurrence of a side reaction of
hydrolysis of ethyl sulfonate anion to ethyl alcohol, which
can be catalyzed by the 1,3-dimethylimidazolium cation,
was demonstrated.
12. N. P. Tarasova, A. A. Zanin, P. S. Sobolev, E. G. Krivoborodov,
Dokl. Chem., 2017, 473, 78.
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This work was financially supported by D. Mendeleev
University of Chemical Technology of Russia (Project
No. 036-2018).
14. Y. Akdogan, J. Heller, H. Zimmermann, D. Hinderberger,
Phys. Chem. Chem. Phys., 2010, 12, 7874.
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Received October 1, 2019;
in revised form January 17, 2020;
accepted February 18, 2020