Nucleophilic oxidizing systems based on hydrogen peroxide for decomposition of ecotoxicants

Article abstract of DOI:10.1134/S1070428011070013

The kinetics of oxidation of methylsulfanylbenzene and nucleophilic decomposition of diethyl 4-nitrophenyl phosphate with hydrogen peroxide in the presence of ammonium hydrogen carbonate or boric acid in aqueous, aqueous-alcoholic, micellar, and microemulsion media were studied. Quantitative parameters of the examined processes were determined, and the possibility of using hydrogen peroxide for the design of oxidative nucleophilic decontaminating systems was demonstrated.

Full text of DOI:10.1134/S1070428011070013

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 7, pp. 965–973. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © L.N. Vakhitova, K.V. Matvienko, N.A. Taran, N.V. Lakhtarenko, A.F. Popov, 2011, published in Zhurnal Organicheskoi Khimii,
2011, Vol. 47, No. 7, pp. 951–960.
Nucleophilic Oxidizing Systems Based on Hydrogen Peroxide
for Decomposition of Ecotoxicants
L. N. Vakhitova, K. V. Matvienko, N. A. Taran, N. V. Lakhtarenko, and A. F. Popov
Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine,
ul. R. Lyuksemburg 70, Donetsk, 83114 Ukraine
e-mail: lu5005@mail.ru
Received November 20, 2010
Abstract—The kinetics of oxidation of methylsulfanylbenzene and nucleophilic decomposition of diethyl
4-nitrophenyl phosphate with hydrogen peroxide in the presence of ammonium hydrogen carbonate or boric
acid in aqueous, aqueous–alcoholic, micellar, and microemulsion media were studied. Quantitative param-
eters of the examined processes were determined, and the possibility of using hydrogen peroxide for the
design of oxidative nucleophilic decontaminating systems was demonstrated.
DOI: 10.1134/S1070428011070013
Taking into account huge worldwide stocks of
chemical weapons [1–6], the problem implying utiliza-
tion of highly toxic chemical agents has recently
become especially important. The most common
chemical weapons may be divided into three main
groups: (1) phosphorus acid esters (neurotoxins and
structurally similar pesticides, e.g., GB, GD, and
Metaphos); (2) organosulfur compounds (blister
agents, e.g., Yperite or HD); and (3) compounds with
combined (neuroparalytic) action (VX).
The optimal system for decontamination of toxic
chemical agents should ensure their solubilization and
simultaneously high rate of decomposition. Therefore,
organized nanosize systems such as micellar solutions
and microemulsions are promising from the viewpoint
of development of ecologically safe decontaminating
compositions [11, 12, 15–21]; these systems provide
increased reagent concentration at the phase boundary
between micelles (oil drop) and water and create favor-
able conditions for nucleophilic attack on electrophilic
centers of toxic substrates [21, 22].
S
P
Me
O
While searching for chemically active components
capable of rapidly and irreversibly decomposing toxic
chemicals, specific attention is now given to the design
of universal oxidative nucleophilic systems for degra-
dation of chemical agents of different natures. For
example, nucleophilic reagents are efficient toward
phosphorus acid esters and halides (GB) [23, 24], di-
alkyl sulfides like HD are decomposed by the action of
oxidants [16, 25, 26], whereas oxidative nucleophilic
systems containing an HOX–OX– couple (X = OH,
Hlg, etc.) are preferred for decontamination of chem-
ical agents like VX or mixtures of compounds belong-
ing to the above three types (GB, HD, VX) [20, 27]. In
this respect, hydrogen peroxide due to its dual nature
(it is an effective oxidant toward HD analogs [15, 25]
and reactive α-nucleophile toward phosphorus acid
esters [1, 24, 28]) may be regarded as universal agent
in the design of mild and ecologically safe decontami-
O
P
O
P
Bu-t
MeO
MeO
OPr-i
Me
Me
F
F
NO2
GB
GD
Metaphos
Pr-i
O
P
Cl
Cl
N
Me
OEt
S
i-Pr
S
HD (Yperite)
VX
Procedures for chemical degradation of organo-
phosphorus compounds are based on hydrolysis in
aqueous alkali, oxidative chlorination with a mixture
of chlorinated lime and calcium hypochlorite, and
alcoholysis with 2-aminoethanol or potassium butox-
ide, followed by thermal bituminization and stocking
of salt concentrates [7–10]. Organosulfur compounds
are decomposed by selective oxidation [11–14].
965

VAKHITOVA et al.
966
nating systems. The problem related to simultaneous
occurrence of oxidative and nucleophilic processes, es-
pecially in the presence of activators, implies addition-
al studies aimed at determining the optimal pH value,
i.e., that ensuring maximal rates of both oxidation and
nucleophilic substitution.
The goal of the present work was to find optimal
and universal oxidative nucleophilic decontaminating
systems by studying the reactivity of hydrogen perox-
ide and its derivatives toward electrophilic substrates
in microorganized catalytic media with a high solubi-
lizing ability with respect to hydrophobic compounds.
As model substrates we used diethyl 4-nitrophenyl
phosphonate (I) (Paraoxon), which is a structural
analog of organophosphorus neuroparalytic agents and
pesticides [1, 20, 21, 23], and methylsulfanylbenzene
(II, thioanisole) as analog of HD in both reactivity and
hydrophobic properties [17, 29–34].
ways, perhydrolysis by the action of HOO– ion gen-
erated according to the reaction H2O2 + HO– ↔ H2O +
HOO– and alkaline hydrolysis by the action of hydrox-
ide ion HO– (Scheme 1). The contribution of the latter
process is minimal; it does not exceed 1–5% of the
overall rate of substrate consumption [40]. By special
experiments we found that no neutral hydrolysis or
alcoholysis of I or oxidation of 4-nitrophenoxide ion
with hydrogen peroxide occurred in our kinetic experi-
ments or the rates of these reactions were very low.
The pKa value of hydrogen peroxide in water is 11.6,
which implies an appreciable concentration of HOO–
ions at high pH values.
Scheme 1.
O2N
O
HOO– or HO–
O
P
OEt
OEt
O
O2N
I
O2N
SMe
O
P
+
(EtO)2P(O)OH
OEt
OEt
O
According to the GLC data, the only product of
oxidation of sulfide II with hydrogen peroxide under
the conditions of kinetic measurements ([II] = 2.0×
10–3 M, [H2O2] = 1.1 M, [NH4HCO3] ≤ 0.5 M) was
methyl phenyl sulfoxide [31] (Scheme 2). No sub-
sequent oxidation of methyl phenyl sulfoxide to the
corresponding more toxic sulfone [27] was observed,
at least during 24 h after oxidation of II started.
I
II
As the main decontaminating agent we examined
hydrogen peroxide which is a mild and ecologically
safe oxidant toward HD analogs, and its anionic form
HOO– shows supernucleophilic properties with respect
to organophosphorus compounds. In order to enhance
the oxidizing power of H2O2, it was activated using
ammonium hydrogen carbonate NH4HCO3 or boric
acid B(OH)3; the latter are known to convert hydrogen
peroxide into peroxo acids which are efficient oxidants
[15, 19, 25, 29, 35]. Nucleophilic activity of peroxo
anions was studied very poorly [1, 36], though it
attracts strong practical interest from the viewpoint of
creation of oxidative nucleophilic decontaminating
systems.
Scheme 2.
H2O2
+
PhSMe
PhS(O)Me + H2O
II
The activating effect of hydrogen carbonate ion in
the oxidation of sulfides with hydrogen peroxide is
related to generation in the system H2O2–NH4HCO3
of hydrogen peroxycarbonate ion HCO4– which is
a stronger oxidant than hydrogen peroxide [15, 25, 30,
41] (Scheme 3).
With a view to overcome mutual insolubility of
substrate and decontaminating agent, decomposition of
compounds I and II was performed in nanosize multi-
component systems such as microemulsions, micellar
solutions, and aqueous–alcoholic mixtures in the pres-
ence or in the absence of surfactant. As the latter we
used cetyl(trimethyl)ammonium bromide and Triton
X-100 [4-(1,1,3,3-tetramethylbutyl)phenyl-poly-
ethylene glycol], taking into account that anionic
surfactants were shown previously [37–39] to inhibit
processes in the above systems.
Scheme 3.
K
(1)
HCO3
+
H2O2
HCO4
Ph
+
H2O
O
O
S
+
+
HCO3
PhSMe
HO
O
O–
Me
II
(2)
Transformations of ester I in the presence of H2O2–
Equilibrium (1) at pH 8–9 establishes relatively
rapidly (in ~5–30 min) [25]. Within this pH range
HO– in all the examined media take two main path-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

NUCLEOPHILIC OXIDIZING SYSTEMS BASED ON HYDROGEN PEROXIDE
967
acid–base dissociation of HCO3– (pKa = 10.3 [25]) and
of pH also leads to reduction of the concentration of
the active oxidant species as a result of dissociation of
HCO4– ions to CO42–; like HO2–, CO42– is a weaker
oxidant than its protonated form [32, 36].
HCO4– ions may be neglected [31].
Peroxoborates are mainly formed in the system
H2O2–B(OH)3 [29, 33] (Scheme 4). The ratio of
peroxoborates depends on the initial concentrations of
B(OH)3 and H2O2 and pH [33]. At relatively low con-
centrations of B(OH)3 and H2O2 (<1 M), pH 6–14, the
main products are mono- and diperoxoborate ions
B(OH)3(OOH)– and B(OH)2(OOH)2–; whereas at high
reactant concentrations polyperoxoborates like
B2(O2)2(O2H)n(OH)4–n (n = 0, 2, 4) predominate [35].
Therefore, distinct separation of oxidation pathways
with determination of the reactivity of peroxoborates
generated in the system is often difficult; nevertheless,
the reaction of sulfides with H2O2 is accelerated in the
presence of B(OH)3, as compared with oxidation of the
same substrate with hydrogen peroxide in the absence
of an activator [29, 35].
The presence of B(OH)3 in the system containing
hydrogen peroxide gives rise to extremal dependence
of the apparent rate constants for the oxidation of
sulfide II versus pH of aqueous solution, the maximum
being located at pH ~10 (Fig. 1e, 2). This pattern is
consistent with the corresponding variation of the con-
centration of the active species, B(OH)3(OOH)– and
B(OH)2(OOH)2– (Fig. 2), calculated according to
equilibria (3) and (4) [35].
The pH dependences of the apparent rate constants
for decomposition of substrate I with peroxide ion in
the presence and in the absence of activators in
aqueous and aqueous–alcoholic media are traditional.
A more or less significant nucleophilic substitution
pathway appears at pH ≥10, and the reaction rate
further increases in parallel with pH (Fig. 1, 1). It is
interesting that the rate of nucleophilic process in-
creases almost twofold upon activation of hydrogen
peroxide with B(OH)3 (Fig. 1e; cf. curves 1 and 3) and
by a factor of 1.3 in the presence of NH4HCO3
(Fig. 1c, 1) as compared to the reaction rate in the
absence of an activator (Fig. 1c, 3). The only reason-
able explanation is participation of peroxo anions
HCO4–, B(OH)3(OOH)–, B(OH)2(OOH)2–, etc. in nucleo-
philic substitution. In other words, peroxycarbonate
and peroxoborate ions are characterized by dual
oxidative–nucleophilic nature, which may be regarded
as an additional preference (apart from economical and
ecological) for using hydrogen carbonate and borate
catalysis in the design of universal decontaminating
compositions.
Scheme 4.
B(OH)3
+
H2O
H2O2
H2O2
B(OH)4
+
H+
+
B(OH)4
+
B(OH)3(OOH)
H2O
(3)
B(OH)3(OOH)
+
B(OH)2(OOH)2
+
H2O (4)
There are no published data on nucleophilic reac-
tivity of peroxoborates, but peroxo anions HCO4– and
CO42– are known [36] to act as typical nucleophiles.
The second-order rate constants for the reactions of
HCO4– and CO42– with ester I are equal to 0.01 and
0.105 l mol–1 s–1, respectively. It might be expected
that activation of hydrogen peroxide with borates and
hydrogen carbonates would give rise to additional
paths of decomposition of organophosphorus com-
pounds according to nucleophilic mechanism with
participation of peroxo anions generated in the systems
B(OH)3–H2O2 and NH4HCO3–H2O2.
The given data unambiguously indicate that the
main problem in the development of oxidative nucleo-
philic systems on the basis of hydrogen peroxide is
proper choice of the activating agent and acidity of the
medium to ensure maximal rate of decomposition of
ester I and of oxidation of sulfide II.
Figure 1 shows the pH dependences of the apparent
rate constants for decomposition of substrates I and II
with hydrogen peroxide in the presence and in the
absence of activators. The general trend is sharp reduc-
tion of the rate of oxidation of sulfide II with hydrogen
peroxide (Fig. 1a, 1b; curve 2) and hydrogen peroxy
carbonate HCO4– (Fig. 1c, 1d; curve 2) with rise in pH
(pH >9.5). The observed pH dependences may be
rationalized as follows (on a qualitative level): the
oxidation process involves both neutral hydrogen per-
oxide and its anion HO2–; the concentration of the latter
increases as pH rises, whereas its oxidizing power is
weaker than that of neutral species [30, 35]. Increase
Figure 3 shows the dependence of the apparent rate
constant for decomposition of ester I in the system
H2O2–HO– in the presence of cetyl(trimethyl)ammo-
nium bromide (CTAB) upon concentration of the latter
in the pH range from 8.5 to 12.5 (micellar solution).
These data clearly demonstrate that the absolute value
of the observed micellar effect increases more than
twofold as pH decreases from 12.5 to 8.5. Change of
pKa value of hydrogen peroxide in going to micellar
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

VAKHITOVA et al.
968
kap ×102, s–1
(a)
kap ×103, s–1
kap ×103, s–1
(b)
kap ×104, s–1
3
1.6
4.52
8
2
2
4.50
4.48
4.46
4.44
1.2
0.8
0.4
6
4
2
2
1
1
1
0
7.5
0
8
0
4.42
pH
8.5
9.5
9
pH
kap ×102, s–1
(c)
kap ×103, s–1
(d)
kap ×103, s–1
kap ×103, s–1
2
10
2
1
1
2.0
4
8
40
20
6
1.5
2
3
4
2
3
0
7.5
0
7.5
1.0
pH
8.5
9.5
pH
8.5
9.5
kap ×102, s–1
(e)
kap ×103, s–1
3.0
2
1.4
1
2.0
1.0
1.0
3
0.0
6
0.6
8
10 pH
Fig. 1. Dependences of the apparent rate constants upon pH for (1, 3) nucleophilic replacement in diethyl 4-nitrophenyl phosphate (I)
and (2) oxidation of methylsulfanylbenzene (II) with hydrogen peroxide in the absence and in the presence of activators at 25°C:
(a) water, phosphate buffer, (1) [H2O2]0 = 1.12, (2) 1.12 M; (b) water–propan-2-ol (74.8:25.2, wt %), (1) [H2O2]0 = 1, (2) [H2O2]0 =
1.13 M; (c) water, (1) [H2O2]0 = 1 M, [NH4HCO3] = 0.1 M; (2) [H2O2]0 = 1.13 M, [NH4HCO3] = 0.066 M; (3) [H2O2]0 = 1 M;
(d) water–propan-2-ol (74.8:25.2, wt %), (1) [H2O2]0 = 1 M, [NH4HCO3] = 0.066 M; (2) [H2O2]0 = 1.13 M, [NH4HCO3] = 0.066 M;
(3) [H2O2]0 = 1.13 M; (e) water, (1) [H2O2]0 = 1 M, [B(OH)3] = 0.1 M, (2) [H2O2]0 = 1 M, [B(OH)3] = 0.1 M; (3) [H2O2]0 = 1 M.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

NUCLEOPHILIC OXIDIZING SYSTEMS BASED ON HYDROGEN PEROXIDE
969
c, M
solution [42] (which is especially appreciable in
2
1
weakly alkaline medium) in combination with micellar
catalytic effect (up to 15-fold) makes the rate of de-
composition according to nucleophilic mechanism at
pH 8.5 comparable to the rate of analogous process
at pH ~10.2 at a constant concentration of hydrogen
peroxide.
0.06
0.04
0.02
0.02
The results of studying decomposition of com-
pounds I and II in micellar systems H2O2–CTAB
(Fig. 4a), H2O2–NH4HCO3–CTAB (Fig. 4b), and
H2O2–B(OH)3 (Fig. 4c) allowed us to draw some pre-
liminary conclusions. In particular, micellar catalysis is
effective in nucleophilic and redox reactions involving
anionic species [HOO–, HCO4–, B(OH)3(OOH)–,
B(OH)2(OOH)–2] in the presence of cationic surfactant
(CTAB) in weakly alkaline medium (pH 8.5–9.5),
whereas the rate of oxidation of sulfide II with
hydrogen peroxide sharply decreases in the presence of
CTAB (Fig. 4a); activation of hydrogen peroxide with
B(OH)3 in the reaction with ester I increases the
micellar effect by a factor of ~1.5 (Fig. 4c; cf. curves 1
and 3), whereas the catalytic effect of CTAB in
the presence of NH4HCO3 is weaker (Fig. 4b; cf.
curves 1 and 3).
6
8
10
pH
12
14
Fig. 2. Dependences of the concentration of (1) mono-
peroxoborate B(OH)3(OOH)– and (2) diperoxoborate
B(OH)2(OOH)2– on pH of aqueous solution, 25°C; water,
[H2O2]0 = 1 M, [B(OH)3] = 0.1 M.
kap, s–1
0.010
0.005
0.005
1
2
3
Table 1 contains the second-order apparent rate
constants for the oxidation of sulfide II (k2ap,ox,) and
nucleophilic replacement in ester I (k2ap,nu) with hydro-
gen peroxide in different media in the absence and in
the presence of NH4HCO3 and B(OH)3 as activators at
pH 9. The rate constants were calculated from the
overall concentration of H2O2 (oxidation) and HOO–
(nucleophilic replacement) with no account taken of
their distribution over pseudophases (surfactant
micelles or oil drops) and contributions of pathways
involving peroxo anions generated according to reac-
tions (1), (3), and (4). This approach seems to be more
informative for comparison of the rates of substrate
decomposition in dynamic media that are prone to
phase and aggregation transitions and are characterized
by different solubilizing effects on hydrophobic sub-
4
5
0
0.01
0.02
0.03
0.04
[CTAB], M
Fig. 3. Dependences of the rate of decomposition of diethyl
4-nitrophenyl phosphate upon concentration of cetyl(tri-
methyl)ammonium bromide in the system H2O2–HO– at pH
(1) 8.5, (2) 9.5, (3) 10.5, (4) 11.5, and (5) 12.0; [HOO–] =
0.002 M, 25°C.
The data in Table 1 indicate the absence of strict
relation between properties of the reaction medium and
reactivity of the main decontaminating species in the
absence (H2O2, HOO–) and in the presence of activator
[H2O2, HOO–, HCO4–, B(OH)3(OOH)–, B(OH)2(OOH)2–]
in both oxidation of sulfide II and nucleophilic decom-
position of ester I. Nevertheless, some general trends
in variation of the efficiency of hydrogen peroxide
systems in going from aqueous to aqueous–alcoholic
and organized media may be noted, and the reactivities
of the examined decontaminating systems may be
compared. Activation of hydrogen peroxide with am-
strate. In addition, the ratios iox (inu) of k2ap,ox (k2ap, nu
)
for all the examined systems and the apparent rate
constants in water in the absence of activator are given
(Table 1, no. 1). The iox/nu values were assumed to
characterize the efficiency of oxidative (nucleophilic)
system in going from aqueous to nanosized and/or
activated media, and the product iox inu was assumed to
quantitatively characterize the efficiency of universal
system for simultaneous oxidation of sulfide II and
decomposition of ester I.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

VAKHITOVA et al.
970
Table 1. Apparent second-order rate constants k2ap (l mol–1 s–1) for the oxidation of sulfide II and nucleophilic substitution in
ester I with H2O2 in different media
Oxidation
Nucleophilic replacement
No.
Reaction mediuma
H2O (1)
Activator
iox inu
k2ap, ox ×104,
k2ap, nu ×102,
b
c
iox
inu
l mol–1 s–1
l mol–1 s–1
01
02
03
04
05
06
07
08
09
11
–
14.30
85.80
170.000
04.00
20.50
02.00
10.60
08.32
40.60
50.10
01.00
06.00
11.90
00.30
01.40
00.14
00.74
00.60
02.80
03.50
53.0
95.6
132.00
41.0
35.1
43.4
38.8
32.0
30.1
68.5
1.00
1.80
2.50
0.80
0.70
0.80
0.73
0.60
0.60
1.30
1.00
10.8
30.0
0.24
1.00
0.10
0.54
0.36
1.70
4.60
H2O (1)
NH4HCO3
B(OH)3
–
H2O (1)
H2O–propan-2-ol (2)
H2O–propan-2-ol (2)
H2O–butan-2-ol (3)
H2O–butan-2-ol (3)
H2O–ethylene glycol (4)
H2O–ethylene glycol (4)
NH4HCO3
–
NH4HCO3
–
NH4HCO3
NH4HCO3
H2O–propan-2-ol–CTAB
(5)
12
13
14
15
H2O–ethylene glycol–
CTAB (6)
–
NH4HCO3
B(OH)3
–
20.20
94.40
67.00
12.80
01.40
06.60
04.70
00.90
86.4
100.00
250.00
35.2
1.63
1.90
4.70
0.70
2.30
12.500
22.000
0.63
H2O–ethylene glycol–
CTAB (6)
H2O–ethylene glycol–
CTAB (6)
H2O–ethylene glycol–
X-100 (7)
16
17
18
19
20
21
22
H2O–CTAB (8)
–
NH4HCO3
B(OH)3
–
06.77
75.50
165.000
12.30
04.72
06.23
04.68
00.47
05.30
11.50
00.86
00.33
00.44
00.33
602.00
142.00
721.00
116.00
74.0
11.400
2.70
5.40
14.300
1560.000
1.90
H2O–CTAB (8)
H2O–CTAB (8)
13.600
2.20
H2O–X-100 (9)
Microemulsion (10)
Microemulsion (10)
Microemulsion (10)
–
1.40
0.46
NH4HCO3
B(OH)3
34.1
0.64
0.30
21.2
0.40
0.13
a
b
c
In parentheses is given the number of the reaction medium according to Table 3.
iox is the ratio of the oxidation rate constant k2ap,ox to the corresponding constant in water.
inu is the ratio of the rate constant k2ap,nu for nucleophilic substitution to the corresponding constant in water.
monium hydrogen carbonate and boric acid increases
the rate of oxidation of sulfide II by at least an order of
magnitude, and the rate of decomposition of ester I, by
a factor of 1.5–2 (Table 1, entry nos. 2, 3, 13, 14, 18).
The use of B(OH)3 as activator is more advantageous
as compared to NH4HCO3; unlike peroxycarbonate ion
active at pH 7–9, the maximal oxidizing power of
peroxoborates is retained at pH 10, the accelerations of
both processes being comparable.
Comparison of the iox inu values for entry nos. 2 and
17, 3 and 18, 5 and 11, and 9 and 13 (Table 1) showed
that the presence of cationic surfactant (CTAB) in
activated media enhances its decontaminating efficien-
cy. In addition, micellar and aqueous–alcoholic surfac-
tant mixtures, especially in the presence of salt activa-
tors, considerably improve the solubility of substrates
(Table 2), which is one of the main advantages of these
media from the viewpoint of development of oxidative
nucleophilic compositions for decontamination of
toxic chemical agents. The most effective universal
decontaminating system on the basis of hydrogen per-
oxide is a micellar solution of CTAB in the presence of
B(OH)3 (Table 1, no. 18); it ensures the highest reac-
tivity of anions in oxidative nucleophilic processes
with high constants KS (Table 2, no. 3) for binding of
both substrates I and II by CTAB micelles.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 7 2011

NUCLEOPHILIC OXIDIZING SYSTEMS BASED ON HYDROGEN PEROXIDE
971
kap ×103, s–1
(a)
kap ×103, s–1
(b)
kap ×103, s–1
4.5
3
1
4
3
2
1
8
7
6
3.5
2.5
1.5
2
1
2
0.5
0
0
0
5
0.03
0.01
0.02
0.03
0.01
0.02
[CTAB], M
[CTAB], M
kap ×102, s–1
(c)
kap ×103, s–1
1
3
4
3
2
1
3
2
1
2
0
0
0
0.04
0.02
[CTAB], M
Fig. 4. Dependences of the apparent rate constants upon concentration of cetyl(trimethyl)ammonium bromide for (1, 3) nucleophilic
replacement in diethyl 4-nitrophenyl phosphate (I) and (2) oxidation of methylsulfanylbenzene (II) with hydrogen peroxide in the
absence and in the presence of activators at 25°C: (a) water–CTAB, [H2O2]0 = 1 M, (1) pH 8.5, (2) pH 8.2; (b) water–CTAB,
(1) [H2O2]0 = 1 M, [NH4HCO3] = 0.5 M, pH 8.5; (2) [H2O2]0 = 1.14 M, [NH4HCO3] = 0.083 M, pH 8.5; (3) [H2O2]0 = 1 M; (c) water–
CTAB, (1) [H2O2]0 = 1 M, [B(OH)3] = 0.1 M, pH 9.5; (2) [H2O2]0 = 1 M, [B(OH)3] = 0.1 M, pH 9.5; (3) [H2O2]0 = 1 M, pH 9.5.
Despite obvious appeal of microemulsions as
strongly solubilizing media (Table 2, no. 6), carrying
out oxidative nucleophilic processes in these media is
not free from some disadvantages. First, microemul-
sions are as a rule multi-component systems neces-
sarily containing an oil (i.e., toxic organic solvent),
which considerably reduces their ecological safety.
Second, the data in Table 1(nos. 20–22) show that both
oxidative and nucleophilic processes in microemul-
sions are characterized by lower rates relative to analo-
gous reactions in aqueous, aqueous–alcoholic, and
micellar systems, the KS values being comparable for
both substrates (Table 2, cf. nos. 1 and 6).
and enhance the reactivity of oxidants and nucleo-
philes (as compared to aqueous solutions); in addition,
such systems are simple to prepare and are ecologi-
Table 2. Binding constants (KS, l mol–1) of substrates I and
II in different media
KS, l/mol
No.
Reaction medium
H2O–CTAB
I
II
1
2
3
4
5
350 [42] 400 [38]
700 [38] 900 [38]
H2O–CTAB–NH4HCO3
H2O–CTAB–B(OH)3
500
300
150
250
H2O–ethylene glycol–CTAB 050
Thus our results showed that weakly alkaline
(pH 8–9) aqueous and aqueous–alcoholic mixtures
containing hydrogen peroxide, cationic surfactant, and
activator ensure high degree of substrate solubilization
H2O–ethylene glycol–CTAB– 090
NH4HCO3
6
Microemulsion
250
300
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VAKHITOVA et al.
972
Table 3. Compositions of the reaction media (wt %)
No.
Reaction medium
Water
Water
CTAB Propan-2-ol X-100 Butan-1-ol Ethylene glycol Hexane
01
02
03
04
05
06
07
08
09
10
100.00
74.8
74.7
67.7
70.0
68.8
70.0
Water–alcohol
25.2
25.3a
32.3
Water–alcohol–surfactant
5.0
1.8
25.0
29.4
29.9
0.1
0.1
Micellar solution
Microemulsion
99.7–99.5 0.3–0.5
99.9
88.0
5.0
5.0
2.0
a
Butan-2-ol.
cally safe. Therefore, they may be regarded as prom-
ising for decontamination of toxic chemicals according
to oxidative nucleophilic mechanism.
determined concentrations. The concentrations of
oxidants and nucleophiles in the mixture were cal-
culated on the overall volume of the alcohol–water–
surfactant mixture.
EXPERIMENTAL
The concentration of surfactants in micellar solu-
tions (Table 3, nos. 8–9) was varied by dilution of
a stock solution containing components whose concen-
tration remained constant in a given series of experi-
ments.
Commercial diethyl 4-nitrophenyl phosphate
(Aldrich), ammonium hydrogen carbonate of chemi-
cally pure grade, sodium hydrogen phosphate
Na2HPO4·2H2O of chemically pure grade, boric acid
of chemically pure grade, cetyl(trimethyl)ammonium
bromide and Triton X-100 (Aldrich), and potassium
hydroxide (Lachema) were used without preliminary
purification. Propan-2-ol, butan-1-ol, butan-2-ol, and
ethylene glycol were purified by standard methods
[43]. Solutions were prepared using doubly distilled
water. Methylsulfanylbenzene was synthesized accord-
ing to the procedure described in [44]. Hydrogen
peroxide of analytical grade (33% aqueous solution)
was preliminarily distilled under reduced pressure
(5 mm), and its concentration was monitored by titra-
tion with potassium permanganate.
The compositions of the examined reaction media
are given in Table 3. Solutions of reactants were
prepared just before kinetic measurements. Aqueous–
alcoholic solutions (Table 3, nos. 2–4) were prepared
by mixing required volumes of alcohol and aqueous
solutions of reactants with accurately determined con-
centrations and pH values. The reactant concentrations
were calculated on the overall volume of aqueous–
alcoholic mixture. Mixtures alcohol–water–surfactant
(Table 3, nos. 5–7) with different reactant ratios (by
weight) were prepared by mixing all components. In
doing so, the aqueous phase was a solution of potas-
sium hydroxide and hydrogen peroxide with accurately
Butan-1-ol–water–surfactant–hexane microemul-
sion (Table 3, no. 10) was prepared by mixing the
components in the following order: aqueous solution
of reactants with required concentrations, surfactant,
alcohol, hexane. Mixing and subsequent shaking over
a period of no longer than 1 min gave a transparent
microemulsion. Peroxide–hydrogen carbonate solu-
tions were kept for at least 30 min for equilibration
[Eq. (1)].
The acidity of solutions was monitored using
a Metrohm-827 pH-meter (Switzerland) with an ac-
curacy of ±0.05 pH unit. Solutions were adjusted to
a required pH value by adding a concentrated solution
of potassium hydroxide. Special experiments showed
that no side decomposition of hydrogen peroxide
occurred at high pH values over a period of 5 h (this
time was sufficient to perform kinetic measurements
for one series of experiments). No oxidation of
4-nitrophenol liberated in the course of nucleophilic
substitution process was observed.
The progress of oxidation and nucleophilic substi-
tution was monitored by spectrophotometry according
to the procedure described previously [31, 38]. Nu-
cleophilic substitution was monitored by measuring the
absorbance at λmax 405 nm (4-nitrophenoxide ion) at
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NUCLEOPHILIC OXIDIZING SYSTEMS BASED ON HYDROGEN PEROXIDE
973
[HOO–] >> [I]. Oxidation reactions were examined
under the condition [II]0 << [NH4HCO3]0 < [H2O2]0;
the concentration of II did not exceed 2×10–3 M.
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