Liquid-phase oxidation of thiols with chlorine dioxide

Article abstract of DOI:10.1023/A:1015023210699

The products and kinetics of the liquid-phase oxidation of 11 aliphatic and aromatic thiols with chlorine dioxide were studied at -10-+70 °C in organic media. The rate constants and activation parameters of the reaction were determined. The influence of the thiol structure on its reactivity was studied. A strong solvent effect on the reaction rate constant was found, and the reaction mechanism was proposed.

Full text of DOI:10.1023/A:1015023210699

2352
Russian Chemical Bulletin, International Edition, Vol. 50, No. 12, pp. 2352—2355, December, 2001
Liquid-phase oxidation of thiols with chlorine dioxide
M. Z. Yakupov,a« V. V. Shereshovets,a †† U. B. Imashev,b and F. R. Ismagilovc
aInstitute of Organic Chemistry of the Ufa Scientific Center, Russian Academy of Sciences,
71 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347 2) 35 6066. E-mail: chemox@anrb.ru
bUfa State Petroleum Technical University,
1 ul. Kosmonavtov, 450062 Ufa, Russian Federation.
cAstrakhan State Technological University,
16 ul. Tatishcheva, 414025 Astrakhan, Russian Federation.
E-mail: ismagilovfr@utis.bashedu.ru
The products and kinetics of the liquid-phase oxidation of 11 aliphatic and aromatic
thiols with chlorine dioxide were studied at –10—+70 °C in organic media. The rate
constants and activation parameters of the reaction were determined. The influence of the
thiol structure on its reactivity was studied. A strong solvent effect on the reaction rate
constant was found, and the reaction mechanism was proposed.
Key words: thiols, chlorine dioxide, oxidation, kinetics, solvent effect.
added. The initial concentrations of ClO2 and thiols in the cell
Chlorine dioxide is an efficient oxidant and acces-
were (1—4)•10 and 2•10 —1•10 mol L–1, respectively.
The reaction products were analyzed by 1H NMR spec-
troscopy using a Bruker AM-300 spectrometer. A solution
–4
–3
–1
sible product widely used for cellulose bleaching and
water decontimination.1 However, the chemistry of chlo-
rine dioxide is poorly studied. The most part of works
are devoted to reactions of chlorine dioxide in an aque-
ous medium.1 Detailed data on reactions of ClO2 with
organosulfur compounds, in particular, thiols (RSH),
are lacking.
–4
containing ClO2 ((1—1.5)•10 mol) was added to a solution
–3
of thiol (1.5•10
mol) in CCl4. After ClO2 was completely
consumed, CDCl3 was added to the solution, and the 1H NMR
spectrum of the reaction products was detected (Me4Si as
standard).
In this work, we studied the chemistry and kinetics
of liquid-phase oxidation of thiols with chlorine dioxide,
measured the activation parameters of the reaction, and
considered the influence of the structure of thiols and
solvents on the rate constants of their reactions with ClO2.
Results and Discussion
Reaction products. At the ratio of initial concentra-
tions [RSH]0 : [ClO2]0 ≥ 10, the corresponding disul-
fides are the main products of the oxidation of thiols.
ClO2
Experimental
RSH
RSSR
R = Et, Prn, HOCH2CH2
Solvents were purified by standard procedures.2 Chlorine
dioxide was prepared as described previously1 and purified
from a Cl2 admixture by adsorption on silica gel. Ethanethiol
(1), propanethiol-1 (2), 2-methylpropanethiol-1 (3), hexa-
decanethiol-1 (4), decanethiol-1 (5), hexadecanethiol-1 (6),
cyclohexanethiol (7), thiophenol (8), 2-methylthiophenol (9),
phenylmethanethiol (10), and 2-mercaptoethanol (11) were
purified by distillation. Purity of thiols was monitored by GLC.
The reaction kinetics was studied spectrophotometrically
by ClO2 consumption at λ = 354—362 nm on an SF-26
spectrophotometer in thermostatted quartz cells with an opti-
cal path length of 1 cm at temperatures from –10 °C to
+70 °C. n-Heptane, 1,4-dioxane, CCl4, benzene, diethyl ether,
ethyl acetate, acetone, and acetonitrile were used as solvents.
A solution of thiol (0.1—0.9 mL) was added to a solvent
(2.0—2.5 mL) in the cell, which was thermostatted at
–10—+70 °C, and a solution of ClO2 (0.1—0.2 mL) was
The oxidation products of thiols 1, 2, and 11 in CCl4
were identified by a previously described procedure.3
Kinetic regularities. At [RSH]0 >> [ClO2]0 in all
solvents in wide ranges of concentrations of the reac-
tants and temperatures with the high correlation coeffi-
cient r > 0.98, the kinetics of ClO2 consumption obeys
the first order equation.
The apparent rate constants k´ (s–1) were calculated
from the semilogarithmic anamorphoses of the kinetic
curves of ClO2 consumption. The apparent constants
k´ = k[RSH]0n (k is the reaction rate constant) linearly
depend on [RSH]0, which indicates the first order with
respect to RSH. A typical anamorphosis of the kinetic
curve of ClO2 consumption in the reaction with thiol 2
and the plot of k´ vs. [PrnSH]0 in 1,4-dioxane are
presented in Fig. 1.
†
Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2244—2247, December, 2001.
1066-5285/01/5012-2352 $25.00 © 2001 Plenum Publishing Corporation

Oxidation of thiols with chlorine dioxide
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001 2353
–1
0.5
0.4
0.3
0.2 [PrnSH]0/mol L
The kinetic equation for the reaction of thiols with
ClO2 has the following form:
ln[ClO2]
–8
k´•102/s
–d[ClO2]/dt = k[ClO2][RSH].
0.4
1
2
The activation parameters for the reaction in CCl4
were calculated from the temperature plots of k for the
compounds studied using the Arrhenius equation
(Tables 1 and 2).
Influence of the thiol structure on the reaction kinet-
ics. It is seen in Table 1 that the thiols can be arranged
by reactivity in the following order: 8 > 9 > 11 > 2 > 3 >
1 > 4 > 6 > 7 > 5 > 10.
0.8
1.2
–9
–10
50
100
150
200
250
t/s
The k value weakly changes with an increase in the
chain length of the hydrocarbon radical R for thiols
1—7. However, the replacement of the alkyl substituent
by phenyl or 2-methylphenyl increases the rate constant
by more than an order of magnitude.
Fig. 1. Semilogarithmic anamorphosis of ClO2 consumption in
the reaction with PrnSH in 1,4-dioxane (1) at [ClO2]0
=
–4
7.8•10 mol L–1, [PrnSH]0 = 3.68•10–1 mol L–1, 22 °C and
the plot of the apparent rate constant k´ vs. [PrnSH]0 (2),
solvent 1,4-dioxane, 22 °C.
Table 1. Kinetic parameters for oxidation of thiols 1—11 (RSH) by chlorine dioxide in CCl4 at 40 °C
Thiol
R
kexp
*
logA
Ea
∆H
∆G
–∆S
/cal mol
–1
–1
Ê
–1
kcal mol
–1
–1
–1
–1
–2
–2
1
2
3
4
5
6
Et
1.0•10
2.1•10
2.0•10
1.0•10
4.1•10
7.4•10
—
—
—
—
—
Prn
6.8±1.8
8.8±1.6
11.9±1.7
8.71±0.9
7.8±3.0
10.7±2.5
13.7±2.32
18.3±2.6
14.5±1.3
12.7±4.5
10.07
13.09
16.58
13.88
12.08
19.33
19.45
19.67
20.41
19.88
29.52
20.37
9.83
20.81
24.95
Bui
C6H13
C10H21
C16H33
–2
7
8
7.3•10
4.2
11.9±1.7
12.4±0.2
18.3±2.6
16.8±0.3
17.68
16.17
19.62
17.39
6.17
3.88
Me
9
3.1
9.2±1.6
12.6±2.2
11.99
17.78
18.54
–2
–1
CH2
10
11
1.5•10
3.2•10
8.3±2.1
6.9±1.0
15.7±3.2
10.6±1.4
15.07
9.98
22.18
19.09
22.66
29.07
HOCH2CH2—
* Error in determination of the constant ≤15%.
Table 2. Kinetic parameters for oxidation of propanethiol-1 (2) and 2-mercaptoethanol (11) by chlorine dioxide in
various solvents at 25 °C
–1 –1
–1
Solvent
ε*
k**/L mol
s
logA
Ea≠/kcal mol
2
11
2
11
2
11
–3
n-Heptane
1,4-Dioxane
CCl4
1.92
2.21
2.23
2.28
4.34
6.02
20.7
36.2
1.7•10
7.2•10
3.9•10
2.6•10
0.8
0.1
3.8
52.7
—
5.5±1.4
14.2±3.0
6.8±1.8
3.1±0.8
8.4±3.0
8.6±2.5
12.4±1.3
12.5±3.5
—
11.3±2.3
21.0±4.5
10.7±2.5
7.7±1.2
11.5±3.8
13.0±3.6
16.1±1.9
14.7±4.4
—
–2
–2
–3
–1
1.1•10
1.4•10
8.3•10
2.8
1.1•10
5.5
7.4±1.4
6.9±4.2
8.3±4.0
4.4±0.8
4.34±2.1
7.7±0.2
8.9±4.1
11.4±4.4
10.6±5.8
11.4±5.6
5.4±0.8
7.25±3.0
9.48±5.17
10.7±5.0
–1
–1
Benzene
Diethyl ether
Ethyl acetate
Acetone
–1
Acetonitrile
11.4
* Dielectric constant according to published data.2
** Error in determination of the constant ≤15%.

2354
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001
Yakupov et al.
logk
ethyl acetate, acetone, and acetonitrile were used as
solvents.
8
0.5
0
9
Note that both thiols 2 and 11 exhibit a strong
dependence of k on the solvent polarity (see Table 2).
For example, for thiol 2, on going from a poorly polar
n-heptane (ε = 1.92) to more polar acetonitrile (ε = 36.2),
the reaction rate constant increases by more than four
orders. This indicates the formation of highly polar
intermediates in the studied reaction. In the case of thiol
11, the rate constant increases by two orders on going
from 1,4-dioxane (ε = 2.21) to acetonitrile. The high
negative activation entropy of the reaction of thiols with
ClO2 (see Table 1) also confirms the polar character of
the activated complex, which is stabilized due to solva-
tion. According to the Leydler—Eyring equation, the
dependence of the reaction rate constant on the dielec-
tric constant of the solvent
11
–0.5
–1.0
3
7
2
4
1
0
0.25
0.50
0.75
σ*
Fig. 2. Rate constants for oxidation of thiols 1, 2, and 8—11
with ClO2 (logk) as functions of the thiol structure in the Taft
coordinates; solvent CCl4, 40 °C, r = 0.90.
We studied (Fig. 2) the influence of the substituents
on the reaction rate constant in the coordinates of the
Taft equation logk = logk0 + ρ*σ* (the σ* values are
taken from Refs. 4 and 5). Found: ρ* = 1.41±0.29,
logk0 = –0.71±0.12, r = 0.94. The positive sign of the
ρ* coefficient indicates that electron-withdrawing sub-
stituents accelerate the reaction.
logk = logk0 + y(ε – 1)/(2ε + 1)
gives for thiols 2 and 11, respectively,
The plot of the k constant vs. pK of thiols (the pK
values are taken from Refs. 6 and 7) shows (Fig. 3)
that k increases with a decrease in pK. On going
logk = (–4.59±0.79) + (11.82±2.4)(ε – 1)/(2ε + 1)
(r = 0.90)
and
from phenylmethanethiol (pK = 11.8) to thiophenol
logk = (–2.18±0.39) + (6.73±1.13)(ε – 1)/(2ε + 1)
(r = 0.95).
–2
(pK = 7.75), the k value increases from 1.5•10
to
–1
4.2 L mol
s
–1. A similar dependence of the oxidation
rate constant on the dissociation constant of thiol was
found for the oxidation of thiols with molecular oxy-
gen.6 This is related to the fact that in this case the key
stage is thiol dissociation to form the thiolate anion,
which is further oxidized to the thiyl radical, and the
The kinetic data were processed with the
Koppel—Palm equation that takes into account both
specific and non-specific solvations,9
0
logk0 = logk0 + yY + pP + eE + bB,
–
limiting step is electron transfer from RS to O2.6
0
Solvent effect on the reaction rate of thiols with
ClO2. The solvent effect on the reaction rate was studied
for the oxidation of thiols 2 and 11. The latter was
chosen because the molecule of 11 contains the OH
group, which is prone, as the SH group, to specific
solvation.8 Analysis of the reaction products showed that
the oxidation of thiol 11 occurs only at the SH group.
n-Heptane, 1,4-dioxane, CCl4, benzene, diethyl ether,
where logk0 is the reaction rate constant in the gas
phase at Y = P = E = B = 0, Y = (ε – 1)/(2ε + 1),
P = (n2 – 1)/(n2 + 2); E and B are the electrophilicity
and nucleophilicity parameters of the solvent; y, p, e,
and b are the coefficients characterizing the sensitivity of
the reaction to the influence of non-specific (polarity,
polarizability) and specific (electrophilicity, nucleophi-
licity) solvations. The parameters obtained are presented
below.
logk
Parameter
2
11
1
0
logk0
–2.36±1.42
8.81±5.60
–5.03±3.59
(1.24±1.16)•10
(1.11±1.16)•10
0.89
–0.06±0.04
5.85±3.75
–5.76±3.24
(–3.23±3.28)•10
(–1.51±2.75)•10
0.95
y
p
e
b
r
8
9
0
–1
–3
–2
–3
11
2
–1
–2
1
The oxidation of thiol 2 illustrates (see Table 3) the
contribution of each component to the reaction kinetics.
It is seen that in almost all solvents the polarity and
polarizability mainly affect the reaction rate and the
influence of specific electrophilic and nucleophilic sol-
vation is comparatively low.
10
8
9
10
11
pK
Fig. 3. Rate constants for oxidation of thiols with ClO2 (logk)
as functions of the dissociation constant of thiols; solvent
CCl4, 40 °C, r = 0.94.
Based on the data obtained for the composition of
the oxidation products, reaction kinetics, and solvent

Oxidation of thiols with chlorine dioxide
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 12, December, 2001 2355
Table 3. Contributions of different types of solvation to the oxidation rate constant of propanethiol-1 (2)
with ClO2 at 25 °C*
Solvent
yY
–pP
eE
bB
logk
Calculation Experiment
n-Heptane
1,4-Dioxane
CCl4
1.68
1.97
1.99
2.03
3.04
3.39
4.10
4.23
1.59
1.70
1.82
1.94
1.48
1.54
1.50
1.44
0
0
–2.28
–1.32
–2.19
–1.96
–0.49
–0.12
0.74
–2.78
–1.15
–1.04
–2.60
–0.09
–0.98
0.57
–1
–1
–1
5.21•10
0.00
2.60•10
0.00
1.98•10
2.60•10
6.45•10
2.62•10
0.00
–2
–1
–1
–1
–1
Benzene
5.30•10
3.09•10
2.00•10
2.48•10
1.77•10
Diethyl ether
Ethyl acetate
Acetone
–1
–1
–1
Acetonitrile
1.25
1.71
* For parameters Y, P, E, and B, see Ref. 9.
effect, we can propose the reaction mechanism for the
case of excess RSH (Scheme 1).
33195à) and the Federal Program "State Support for the
Integration of the High School and Fundamental Sci-
ence in 1997—2000" (State Contract No. 2.1-573).
Scheme 1
References
–
RSH
RS + H+
(1)
(2)
(3)
(4)
(5)
(6)
1. W. J. Masschelein, Chlorine Dioxide: Chemistry and
Enviromental Impact of Oxychlorine Compounds, Ann Arbor
Publishers, Inc., Ann Arbor, 1979.
2. A. J. Gordon and R. A. Ford, The Chemist´s Companion.
A Handbook of Practical Data, Techniques, and References,
J. Wiley and Sons, New York—London—Sydney—Toron-
to, 1972.
3. F. Freeman and C. N. Angeletakis, Org. Magn. Reson.,
1981, 17, 1.
4. E. T. Denisov, Kinetika gomogennykh khimicheskikh reaktsii
[Kinetics of Homogeneous Chemical Reactions], Vysshaya
Shkola, Moscow, 1988, 391 pp. (in Russian).
5. A. N. Vereshchagin, Induktivnyi effekt [Induction Effect],
Nauka, Moscow, 1988, 111 pp. (in Russian).
6. S. Oae, Khimiya organicheskikh soedinenii sery [Chemistry of
Organosulfur Compounds], Khimiya, Moscow, 1975, 512 pp.
(Russ. Transl.).
7. E. N. Prilezhaeva and M. F. Shostakovskii, Usp. Khim.,
1963, 32, 897 [Russ. Chem. Rev., 1963, 32 (Engl. Transl.)].
8. N. M. Emanuel, G. E. Zaikov, and Z. K. Maizus, Rol´ sredy
v radikal´no-tsepnykh reaktsiyakh okisleniya organicheskikh
soedinenii [The Role of Medium in Chain Radical Reactions of
Oxidation of Organic Compounds], Nauka, Moscow, 1973,
279 pp. (in Russian).
•+
ClO2 + H+
HClO2
–
•
RS + ClO2
–
RS + ClO2
•
RS + HOClO
RSH + ClO2
•+
–
HClO2
+ ClO2
HClO2 + ClO2
•
RS + RS
•
RSSR
In Scheme 1 stage (3) seems to be rate-determining.
Stage (4) is of low significance because one-electron
oxidation rather than H atom abstraction is more char-
acteristic of ClO2.1 The increase in the rate of reac-
tion (3) when electron-withdrawing substituents are in-
troduced into the thiol molecule is related to a decrease
in K and facilitation of acidic dissociation.
Since the polar thiolate ion is formed during the
reaction, the increase in the consumption rate of ClO2
with an increase in the solvent polarity is explained by
–
the solvation of the RS ion by the solvent and the
subsequent reaction of the formed solvate with ClO2.
The high reactivity of ClO2 with respect to thiols
allows its use for demercaptanation processes.
9. V. A. Palm, Osnovy kolichestvennoi teorii organicheskikh
reaktsii [Foundations of the Quantitative Theory of Organic
Reactions], Khimiya, Moscow, 1977, 360 pp. (in Russian).
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
Received October 9, 2000;
in revised form May 17, 2001