Micelle effects of functionalized surfactants, 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium halides, in reactions with p-nitrophenyl p-toluenesulfonate, diethyl p-nitrophenyl phosphate, and ethyl p-nitrophenyl ethylphosphonate

Article abstract of DOI:10.1023/A:1021655813700

1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium chloride and bromide were synthesized for the first time. These compounds are functionalized zwitterionic surfactants which give rise to micelle formation in aqueous solution. Kinetic and thermodynamic analysis of nucleophilic cleavage of p-nitrophenyl p-toluene-sulfonate, diethyl p-nitrophenyl phosphate, and ethyl p-nitrophenyl ethylphosphonate in the presence of 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium halide micelles showed that the latter are powerful nucleophilic reagents whose kinetic behavior can be described in terms of a simple pseudophase distribution model. The efficiency of substrate solubilization with zwitterionic surfactant micelles and the reactivity of the oximate fragment in the micelle phase were estimated on a quantitative level. The observed acceleration of S_N2 reactions with the examined p-nitrophenyl esters relative to analogous reactions of zwitterionic 1-methyl-3-(2-hydroxyiminopropyl)imidazolium halides is, respectively, 12800, 550, and 900 times; it is explained mainly by increased concentration of the reactants in micelles.

Full text of DOI:10.1023/A:1021655813700

Russian Journal of Organic Chemistry, Vol. 38, No. 9, 2002, pp. 1314 1325. Translated from Zhurnal Organicheskoi Khimii, Vol. 38, No. 9, 2002,
pp. 1369 1380.
Original Russian Text Copyright
2002 by Simanenko, Popov, Karpichev, Prokop’eva, Savelova, Bunton.
Micelle Effects of Functionalized Surfactants,
1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium Halides,
in Reactions with p-Nitrophenyl p-Toluenesulfonate,
Diethyl p-Nitrophenyl Phosphate, and Ethyl p-Nitrophenyl
Ethylphosphonate
Yu. S. Simanenko1, A. F. Popov1, E. A. Karpichev1, T. M. Prokop’eva1,
V. A. Savelova1, and C. A. Bunton2
1
Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine,
ul. R. Luxemburg 70, Donetsk, 83114 Ukraine
e-mail: simanenko@infou.donetsk.ua
2
Department of Chemistry and Biochemistry, University of California, Santa Barbara, USA
Received November 7, 2001
Abstract 1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium chloride and bromide were synthesized for the first
time. These compounds are functionalized zwitterionic surfactants which give rise to micelle formation in
aqueous solution. Kinetic and thermodynamic analysis of nucleophilic cleavage of p-nitrophenyl p-toluene-
sulfonate, diethyl p-nitrophenyl phosphate, and ethyl p-nitrophenyl ethylphosphonate in the presence of
1-cetyl-3-(2-hydroxyiminopropyl)imidazolium halide micelles showed that the latter are powerful nucleophilic
reagents whose kinetic behavior can be described in terms of a simple pseudophase distribution model. The
efficiency of substrate solubilization with zwitterionic surfactant micelles and the reactivity of the oximate
fragment in the micelle phase were estimated on a quantitative level. The observed acceleration of SN2
reactions with the examined p-nitrophenyl esters relative to analogous reactions of zwitterionic 1-methyl-
3-(2-hydroxyiminopropyl)imidazolium halides is, respectively, 12800, 550, and 900 times; it is explained
mainly by increased concentration of the reactants in micelles.
Micelle effects represent the most prominent and
extensively studied example of surfactant influence
on the kinetics of organic reactions in water and
aqueous organic mixtures. These systems open wide
prospects for controlling the rate of chemical trans-
formations and the state of equilibria therein [1 3].
Acceleration of organic reactions in micelle solution
is determined mainly by two factors: (1) concentration
of the reactants in the micelle pseudophase and
(2) possible considerable increase in the reaction rate
in going from aqueous solution to surfactant micelles
[4]. Cationic micelles accelerate reactions of anionic
nucleophiles with neutral substrates (including those
classed with ecotoxicants) [5 7]. One of the most
promising ways of modification of cationic surfactant
molecules includes introduction into the head part of
functional groups capable of forming highly reactive
anionic species, e.g., fragments of -nucleophiles,
such as oximate [8 13], hydroxamate [13 17], hydro-
peroxide [18], etc. [19, 20]. An obvious advantage of
such surfactants is that the role of nucleophilic reagent
is played just by the functionalized zwitterionic sur-
factant which gives rise to micelle formation. Here,
the problem of binding of anionic nucleophile with
micelles is eliminated: regardless of the nature of
specific group, even at the lowest possible micelle
concentration, the local concentration of anionic
nucleophilic species in the micelle phase is as high
as possible for a given system [12].
In the present work we performed a detailed kinetic
analysis of nucleophilic cleavage of ecotoxic p-nitro-
phenyl p-toluenesulfonate (I), diethyl p-nitrophenyl
phosphate (II), and ethyl p-nitrophenyl ethylphospho-
nate (III) in the presence of 1-cetyl-3-(2-hydroxy-
iminopropyl)imidazolium halides (IV) which are new
functionalized surfactants whose head part includes
1070-4280/02/3809-1314$27.00 2002 MAIK Nauka/Interperiodica

MICELLE EFFECTS OF FUNCTIONALIZED SURFACTANTS
1315
Hlg = Cl, Br.
an imidazolium ring and hydroxyimino group, as well
as in the presence of 1-methyl analogs V, in water
at 25 C. Our study was aimed at establishing the
nature of factors responsible for the high rates of these
processes, namely (1) change of the reactivity in
going from aqueous to micelle phase and/or (2) effect
of reactant concentration in the micelle phase.
Oximate ions (Ox ) are typical -nucleophiles
which rapidly react with various substrates, such as
carboxylic, phosphoric, phosphonic, and sulfonic acid
esters [21 26]. The kinetic behavior of zwitterionic
species Va, generated from 1-methyl-3-(2-hydroxy-
iminopropyl)imidazolium chloride or bromide accord-
ing to Scheme 1, in reactions with substrates I III
follows analogous general relations. The reaction of
oximate ion Va with esters I III in water (Scheme 2)
includes nucleophilic attack on the electron-deficient
center in the substrate by anionic oxime species to
give the corresponding O-acyl derivatives [25].
phile concentration (Fig. 1a) and pH (pH profile,
Fig. 1b). The rate constant is described by Eq. (1):
kap = k2
[HOx]0 = k2 [Ox ],
(1)
OX
+
where
= Ka/(Ka + aH ) is the fraction of oximate
OX
ion, and Ka is the acid ionization constant of the
oxime. The values of kap were corrected for the
contribution of alkaline hydrolysis, kOH aOH , if it
exceeded 5%. The second-order rate constant k2,
1
l mol 1 s , which characterizes the nucleophilic reac-
tivity of zwitterion Va, was determined from the
linear dependence of kap versus [Ox ]. Zwitterionic
species Va derived from 1-methyl-3-(2-hydroxyimino-
propyl)imidazolium chloride or bromide shows no
anomalies in the kinetic behavior, and its reactivity
conforms to the Bronsted equation for the reaction
/
of oximate ions with esters I III [23].
1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium
chloride and bromide in aqueous solution give rise to
two kinds of reactive micelles: mixed micelles formed
by molecules IV and IVa and those consisting of
only zwitterionic species IVa. The nucleophilic center
in the latter is located in the oximate moiety. This
The dependence of the apparent pseudofirst-order
1
rate constant kap (s ) on the overall oxime concentra-
tion [HOx]0 (M) is typical of processes in which the
reactive species is the major buffer component (Ox ).
The reaction rate increases with rise in both nucleo-
Scheme 1.
IV, Alk = C16H33; V, Alk = Me.
Scheme 2.
X = Ts, (EtO)2P(O), Et(EtO)P(O).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

1316
SIMANENKO et al.
(a) micelles formed by IVa. Therefore, the kinetics of
these processes were studied at pH 12.9 (at pH 12.6
for ester III), i.e., when substrates I III undergo
nucleophilic cleavage only in zwitterionic micelles.
The concentration profiles of these reactions are
more complex (Fig. 3). In the case of esters II and
III, only alkaline hydrolysis* was observed up to
critical micelle concentration; as the micelle concen-
tration rose, the reaction rate considerably increased.
When the substrate is completely bound with micelles
of IVa, the reaction rate no longer depends on the
surfactant concentration, and kap tends to kmax (plateau
in Fig. 3a). It should be noted that at pH 12.9 the con-
centration dependence (as well as the pH dependence)
includes points for 1-cetyl-3-(2-hydroxyiminopropyl)-
imidazolium chloride and bromide. This means that
a common concentration dependence exists, indicating
the absence of inhibitory effect of halide ions on the
reaction under study (cf. [9]). An analogous concen-
tration profile is observed for substrate I (Fig. 3a, 3b).
However, the rate of cleavage of ester I strongly
increases before reaching the critical micelle concen-
(b)
4
tration (2.4 10 M, Fig. 3b), which was determined
by independent experiments using difference UV spec-
troscopy. An analogous CMC value was obtained by
treatment of the kinetic data for reactions of esters II
and III with micelles IVa (Fig. 3b, Table 1). For
example, at a surfactant concentration c0 of 8.06
5
10 M, the contribution of alkaline hydrolysis being
less than 10%, the reaction rate in water is higher by
a factor of 4000 [kap(IVa)/kap(Va)] than the rate of
the reaction with 1-methyl analog. Such increase in
the reaction rate is likely to result either from forma-
tion of highly reactive premicelles or from decrease in
CMC induced by ester I which is more hydrophobic
than II and III [27]. Presumably, the latter factor
should be preferred, for (1) it remains unclear why
premiceles (if they are formed) rapidly react with ester
I but slowly with II and III and (2) the formation
of premicelles should give a distinct maximum on the
concentration profile at c0 < CMC [27], which is not
observed (Fig. 3b). Using the mathematical statistics
methods [28] (see Experimental), the critical micelle
concentration for the reaction of I with IVa was
estimated at 5.40 10 5 M. This value provides a good
agreement between the calculated dependence of kap
on c0 and experimental concentration profile for the
reaction of ester I with micelles IVa. We can con-
clude that 1-cetyl-3-(2-hydroxyiminopropyl)imidazo-
____________
Fig. 1. Nucleophilic cleavage of (1, 2) p-nitrophenyl
p-toluenesulfonate (I), (3) diethyl p-nitrophenyl phosphate
(II), and (4, 5) ethyl p-nitrophenyl ethylphosphonate (III)
with zwitterionic form Va of 1-methyl-3-(2-hydroxyimino-
propyl)imidazolium halides; water, 25 C,
= 1.0 (KCl).
(a) Concentration dependences for the reactions of p-nitro-
phenyl p-toluenesulfonate (I), pH 12.0, and ethyl p-nitro-
phenyl ethylphosphonate (III), pH 10.7, with 1-methyl-3-
(2-hydroxyiminopropyl)imidazolium halides (Va); (b) pH
dependendence for the reaction of diethyl p-nitrophenyl
phosphate (II) with 1-methyl-3-(2-hydroxyiminopropyl)-
imidazolium halides (Va); [HOx]0 = 0.2 M. Point numbers
1, 3, and 4 correspond to the reactions with 1-methyl-3-
(2-hydroxyiminopropyl)imidazolium chloride, and 2 and 5,
with bromide.
follows from the pH profiles for reactions with sub-
strates I and II, shown in Fig. 2. Up to pH 12.5,
the rate of transfer of p-tolylsulfonyl and diethoxy-
phosphinoyl groups to the oximate fragment increases
with rise in pH. This pattern indicates that nucleo-
philic cleavage of the substrates occurs in mixed
micelles IV/IVa. The dependence then comes to
a plateau, and at pH > 12.5 the apparent rate constant
corresponds to the reaction occurring in zwitterionic
*
Undoubtedly, monomeric IVa species also react with esters
I III in water, but the contribution of this pathway is
negligible as compared to alkaline hydrolysis.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

MICELLE EFFECTS OF FUNCTIONALIZED SURFACTANTS
lium halides IV in aqueous solution give rise to only
1317
one kind of reactive species, namely micelles formed
by zwitterions IVa. The observed considerable quali-
tative and quantitative differences in the kinetic
behavior of micelles formed by zwitterionic species
IVa and Va (cf. Fig. 1 and Figs. 2, 3) may be inter-
preted in terms of the simple pseudophase model of
micelle formation [4], according to which substrate S
is rapidly distributed between two phases: micelle
pseudophase (m) and aqueous phase (aq) (Scheme 3).
Scheme 3.
Fig. 2. pH profiles for the reactions of (1) p-nitrophenyl
p-toluenesulfonate (I), c0 = 0.001 M, and (2, 3) diethyl
p-nitrophenyl phosphate (II), c0 = 0.02 M with 1-cetyl-3-
(2-hydroxyiminopropyl)imidazolium (1, 2) chloride and
(3) bromide IVa; water, 25 C.
The equilibrium is controlled by the distribution
coefficient PS [Eq. (2)], and the chemical reaction
per se does not affect the thermodynamics of substrate
distribution.
alkaline hydrolysis. The rates of these processes are
described by Eqs. (6) and (7), respectively.
= km2 [Ox ]m [S]m = k2m (1/V)[S]m = km [S]m; (6)
m
PS = [S]m/[S]aq.
(2)
aq
OH
= kOH [OH ]aq [S]aq = k [S]aq.
(7)
aq
Taking into account the above stated, the current
substrate concentration [S], M, is the sum of substrate
concentrations in the micelle and aqueous phases,
averaged by the overall volume of the reaction system:
The overall rate of the process is described by the
following equation:
=
m c V + aq(1
c V).
(8)
[S] = [S]m c V + [S]aq (1
c V),
(3)
By substituting Eqs. (4) (7) into (8), we obtain
formula (9) for the apparent pseudofirst-order rate
constant:
where V, l/mol, is the partial molar volume of
micelles formed by zwitterionic surfactant, c = c0
CMC, M, is the micelle concentration, and c V and
aq
OH
(1
c V) are the volume fractions of the micelle
km PS c V + k (1
c V)
pseudophase and aqueous phase, respectively. Joint
solution of Eqs. (2) and (3) gives
kap
=
=
,
(9)
[S]
1 + KS c
where KS = (PS 1)V PS V, mol/l. The quantity KS
has a dimensionality of the equilibrium constant for
binding of substrate with zwitterionic surfactant IVa
micelles. Insofar as Eq. (9) was derived without any
simplifying assumptions, it provides the possibility
for analyzing the character of micelle effects with
regard to the reaction conditions, efficiency of binding
of substrates I III, and ratio of the second-order rate
constants in the micelle and aqueous phases.
The situation when a substrate is either weakly
bound or is not bound at all by functionalized surfact-
ant micelles, i.e., when PS 1, is unusual. On the
other hand, it demonstrates the significance of the
PS [S]
[S]m
[S]aq
=
=
,
(4)
(5)
1 + (PS
1)c V
[S]
1 + (PS
.
1)c V
Nucleophilic cleavage of a substrate in aqueous
solutions of 1-cetyl-3-(2-hydroxyiminopropyl)imida-
zolium halides follows two concurrent pathways
shown in Scheme 3: in the micelle pseudophase, SN2
reaction with the oximate fragment of IVa occurs,
while in the aqueous phase the substrate undergoes
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

1318
SIMANENKO et al.
(a) centration c cannot differ from the rate of alkaline
hydrolysis of the substrate, i.e., no acceleration should
be observed. The reason is that the reactants appear in
different phases: the substrate is in the aqueous phase,
and the reagent (oximate ion), in the micelle phase.
Therefore, only hydrolysis of the substrate occurs.
Quite different kinetic relations are observed when
the substrate is well bound by functionalized surfact-
ant micelles. The high efficiency of solubilization of
esters I III with IVa micelles, which follows from
relatively low concentration c corresponding to kmax
(Fig. 3), indicates that PS >> 1 (Table 1) and c V << 1.
In this case, Eq. (9) may be represented as
aq
OH
aq
OH
km KS c + k
1 + KS c
km KS c
k
kap
=
=
+
.
1 + KS c
1 + KS c
(11)
(b)
As follows from Eq. (11), micelles of IVa exert
a dual effect on the apparent reaction rate. On the one
hand, they inhibit alkaline hydrolysis so that its rate
[the second term in Eq. (11)] decreases as c rises.
On the other hand, the rate of substrate cleavage in
micelles of IVa [the first term in Eq. (11)] sharply
increases, which overcompensates the reduced con-
tribution of alkaline hydrolysis. Finally, at KS c >> 1,
almost no alkaline hydrolysis occurs, and the reaction
rate [Eq. (12)] reaches its maximal value (plateau on
the concentration dependence shown in Fig. 3a).
kap = kmax = km = km2
V
.
(12)
1
Therefore, when the substrate is transferred com-
pletely to the micelle pseudophase, the corresponding
value of km, s , which characterizes the reactivity of
the oximate fragment in IVa at [Ox ]m = 1/V, may be
determined from any point on the plateau in the con-
centration dependence for esters I III (Fig. 3a).
The binding constants KS for esters I III charac-
terize the thermodynamics of equilibrium substrate
distribution between the aqueous phase and IVa
micelles, so that they are necessary for kinetic and
thermodynamic description of the reactions under
study. The kinetic method provides a simple, con-
venient, and fairly reliable procedure for quantitative
estimation of KS values. The kinetic data obtained
under the conditions when the fraction of the bound
Fig. 3. Concentration dependences of the apparent pseudo-
first-order rate constants for the reactions of (1, 2) p-nitro-
phenyl p-toluenesulfonate (I), (3, 4) diethyl p-nitrophenyl
phosphate (II), and (5, 6) ethyl p-nitrophenyl ethylphospho-
nate (III) with 1-cetyl-3-(2-hydroxyiminopropyl)imidazo-
lium (1, 3, 5) chloride and (2, 4, 6) bromide (IVa); water,
25 C. (a) p-Nitrophenyl p-toluenesulfonate (I), pH 12.9;
diethyl p-nitrophenyl phosphate (II), pH 12.9; ethyl p-nitro-
phenyl ethylphosphonate (III), pH 12.6. (b) Initial parts of
the kap c0 plots for the reactions of p-nitrophenyl p-tolu-
enesulfonate (I) and diethyl p-nitrophenyl phosphate (II)
with IVa micelles; pH 12.9.
1
effect of reactant concentration as a factor controlling
the rate of micelle reactions. In this case, KS varies
from 0 to V, and Eq. (9) is transformed into (10):
substrate
was within the range 0.1 < S < 0.9 were
S
aq
OH
treated in two ways. According to the first of these,
KS values for esters I III were determined from
Eq. 13; simultaneously, km values were also deter-
mined (Fig. 4a, 4b).
kap = km PS c V + k
.
(10)
Insofar as PS 1 and c V << 1, the first term in
Eq. (10) is negligible. The reaction rate at any con-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

MICELLE EFFECTS OF FUNCTIONALIZED SURFACTANTS
1319
aq
OH
kap
k
(a)
kap = km
.
(13)
KS c
In terms of the second approach, the binding con-
stants KS were estimated using km values calculated by
Eq. (12). For this purpose, Eq. (11) was transformed
into (14):
aq
OH
kap
km
k
= KS c,
(14)
kap
and the constants KS were determined from linear
dependences (14) plotted in Fig. 5. The values of KS
(Table 1), determined for different reaction conditions,
coincided. This indicates the validity of using the
kinetic method for determination of binding constants.
(b)
Table 1 summarizes the calculated values of km and
KS for esters I III and the corresponding critical
micelle concentrations (CMC). A good agreement
between the km values calculated from kmax and those
found using Eq. (13) (Fig. 4) provides a support to
the proposed mechanism of nucleophilic cleavage of
substrates I III (Scheme 3) in zwitterionic micelles
IVa. The values of km and KS given in Table 1 are
apparent quantities. In order to determine the proper
nucleophilicity of the oximate fragment in zwitter-
1
ionic surfactant IVa (km2 , l mol 1 s ), as well as the
distribution coefficients PS, it is necessary to know
the partial molar volume or (more precisely) the
volume of that micelle region in which the substrate
is localized and where the SN2 reaction occurs [Vm;
Eqs. (15), (16)]:
Fig. 4. Plots of the apparent pseudofirst-order rate con-
1
aq
1
stants kap, s , versus (kap
k
)/c, l mol 1 s , for the
OH
reactions of (1, 2) p-nitrophenyl p-toluenesulfonate (I),
(3, 4) diethyl p-nitrophenyl phosphate (II), and (5, 6) ethyl
p-nitrophenyl ethylphosphonate (III) with micelles formed
by (1, 3, 5) 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium
chloride and (2, 4, 6) bromide; water, 25 C. (a) p-Nitro-
phenyl p-toluenesulfonate (I), pH 12.9, and diethyl p-nitro-
phenyl phosphate (II), pH 12.9. (b) Ethyl p-nitrophenyl
ethylphosphonate (III), pH 12.6.
km2 = km Vm ;
PS = 1 + KS/Vm
(15)
(16)
KS/Vm.
In a number of processes involving anionic nucleo-
philes and neutral substrates in cationic surfactant
micelles and co-micelles, the values of km2 are com-
parable to or slightly lower than the second-order rate
constants for the same anionic nucleophiles in water
[2, 3]. The differences often arise from the choice of
Vm. The Vm values for trimethylammonium micelles
are usually assumed to range from 0.14 to 0.37 l/mol
[13]. Taking into account that zwitterionic surfactant
IVa has a bulky head moiety and that one of the
factors determining Vm is just the volume of the head
fragment, the region of IVa micelles where esters
I III are cleaved should be larger than the corre-
sponding region in trimethylammonium micelles. We
selected a Vm value of 0.5 l/mol, for just that value
was used to describe reactions in metal micelles
characterized by a considerable size of their head
fragments [20]. Using Vm = 0.5 l/mol, we calculated
the rate constants km2 and distribution coefficients PS
(Table 1) by Eqs. (15) and (16).
Quantitative estimation of micelle effects produced
by zwitterionic surfactant IVa [ in Eq. (17)] implies
that factors governing the rate of nucleophilic
cleavage of esters I III in the micelle phase (i.e.,
the driving force of the reaction) be known. For this
purpose, let us make use of Eq. (17) which sets as
the ratio of rate constants for the reactions of IVa in
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

1320
SIMANENKO et al.
and is KS/Vm = PS for substrates I III. Such situation
is observed at surfactant concentrations approaching
CMC. As the fraction of the bound substrate rises,
0.1
0.9, the contribution of the reactant con-
S
centration effect PS/(1 + KS c) to remains determin-
ing, though it decreases as the concentration of VIa
micelles increases (Table 2). Finally, at
1 and
S
KS c >> 1 the rate of the reaction with methyl analog
Va is as follows: kap(Va) = (k2/km2 )kap(IVa) for esters
I and III and kap(Va) = kap(IVa) for ester II, and the
concentration of Va becomes equal to the local
concentration of the oximate fragment in zwitterionic
micelles IVa: [Va]0 = [Ox ]m = 1/Vm = 2 M. Taking
into account the change in the reactivity, the reaction
rate in water should be equal to that in the micelle
phase at km2 /k2 [Ox ] = 4, 2, and 3.6 M for substrates
I III, respectively. It is impossible to attain such
a concentration of the methyl analog, so that it cannot
compete with IVa micelles in the reactivity.
Thus the main factor responsible for the micelle
effects of IVa ( ) is the reactant concentration in the
micelle phase, which in turn depends on the distribu-
tion coefficients PS of the substrates: the greater the
distribution coefficient, the greater the observed
increase in the reaction rate (Tables 1, 2). The dif-
ferences in the substrate distribution coefficients
directly indicate how do the standard Gibbs energies
( G0 = G0m G0aq = R T lnPS) change in going from
the aqueous to micelle phase. Transfer of substrates
I III to IVa micelles is accompanied by decrease in
their Gibbs energies, and the maximal change of G0
( 5.0 kcal/mol) is observed for the most hydrophobic
substrate I (see Table 1). Esters I III are polar neutral
organic compounds, and their solubility (see Experi-
mental) increases in the series I < II III. Therefore,
it is reasonable to presume that the main driving force
of the reactant concentration effect is hydrophobic
interactions responsible for substrate transfer.
The universal character of Eq. (17), which takes
into account the effects of reactant concentration and
change in the reactivity, allows us to compare the
nucleophilicities of various functionalized surfactants:
a necessary and sufficient condition is that the values
of k2, km2 , and KS must be known. These requirements
are met in the case of functionalized surfactants of
the pyridinium series containing an oximate fragment,
e.g., 1-alkyl-3-hydroxyiminomethylpyridinium halides
which exhibit high reactivity toward phosphorus acid
esters [9, 10, 22], including substrate II. The kinetic
behavior of 1-dodecyl-3-hydroxyiminomethylpyridi-
nium halide (VI), which at pH 9.3 gives rise to mixed
micelles consisting of the cationic and zwitterionic
surfactant species, in the reaction with ester II was
aq
Fig. 5. Plots of (kap
k
)/(km
kaq) versus c, M, for
OH
the reactions of (1, 2) p-nitrophenyl p-toluenesulfonate (I),
(3) diethyl p-nitrophenyl phosphate (II), and (4, 5) ethyl
p-nitrophenyl ethylphosphonate (III) with micelles formed
by (1, 4) 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium
chloride and (2, 3, 5) bromide; water, 25 C.
the micelle phase [kap(IVa)] and of its methyl analog
Va [kap(Va)] in water at [Va]0 = c. We assume that
the second-order rate constants for compounds IVa
and Va in water are similar or comparable. This
assumption seems to be justified since the inductive
effects of the cetyl and methyl groups on the basicity
of IVa and Va are comparable and the slopes of the
Bronsted relations for the reactions of oximate ions
/
with esters I III and other substrates fall into the
range from 0 to 0.2 [23, 26]. The first multiplier iNn
Eq. (17) shows how does the reaction rate change in
going to the micelle phase. Insofar as km2 /k2 are equal
to 2, 1, and 1.8 for substrates I III, respectively
(Table 1), the difference in the Gibbs energies of
activation for the reactions in the micelle phase ( Gm)
and in water ( Gaq) is negligible. Therefore, the
micelle phase has no specific stabilizing effect on the
transition state. This conclusion is consistent with
numerous data obtained for functionalized surfactants
and mixed micelle systems [2, 3, 29].
kap(IVa)
kap(Va)
km2 [Ox ]m KS c
=
=
k2 [Va]0 (1 + KS c)
k2m
KS c
=
.
(17)
k2
Vm [Va]0 (1 + KS c)
The second multiplier in Eq. (17) reflects the con-
tribution to of the effect of reactant concentration.
When the fraction of the bound substrate
<< 1 and
KS c << 1, this contribution attains its maxSimal value
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

MICELLE EFFECTS OF FUNCTIONALIZED SURFACTANTS
1321
Table 1. Binding constants KS and distribution coefficients PS of p-nitrophenyl p-toluenesulfonate (I), diethyl
p-nitrophenyl phosphate (II), and ethyl p-nitrophenyl ethylphosphonate (III) and second-order rate constants k2 of the
reactions of 1-methyl-3-(2-hydroxyiminopropyl)imidazolium (Va) and 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium
(IVa) halides with esters I III; water, 25 C
Substrate
no.
k2 102,a
kOH 103,
km2 102,
km 102,
KS, l/mol
l/mol
PS,b
CMC 104,c
M
l mol 1 s
1.5 0.2
0.84 0.06
9.0 0.9
l mol 1 s
8.0 0.3
9.6 0.6
150 10
l mol 1 s
s
1
1
1
1
I
3.0 0.1
6.0 0.06d
6.1 0.4e
1.65 0.05d
1.6 0.2e
32 3d
3300 200d
3200 100f
280 30d
270 20f
6500 300 0.540 0.006
( 5.0)
550 60
( 3.6)
520 50
( 3.5)
II
III
0.83 0.02
16 3
2.4 0.2 (S)
2.6 0.4 (K)
2.5 0.2 (S)
260 20d
260 10f
30 5e
a
The pKa value for compound Va is 10.70 0.10 [23].
b
The distribution coefficients PS were calculated by the equation KS = PS Vm; in parentheses are given G0 values calculated by
by the formula G0 = RTlnPS, kcal/mol, which characterize change in the standard Gibbs energy of the substrate in going from
water to the micelle phase.
Critical micelle concentrations were determined by the spectrophotometric (S) or kinetic (K) method.
Calculated by Eq. (13).
Calculated from kmax by Eq. (12).
Calculated by Eq. (14).
c
d
e
f
analyzed in terms of the pseudophase model, and
the following kinetic and thermodynamic parameters
to the model analog of VIIa, heterocylcic tautomer of
2-iodosylbenzoate; also, neither binding constants nor
km2 values were given, which are necessary for taking
into account micelle effects by Eq. (17). Therefore,
the kinetic data of [19] were treated by Eqs. (12) and
were obtained: km2 = 1.06 10 l mol 1 s and KS =
56 l/mol [9]. Taking into account pKa of the oximate
fragment, the nucleophilic reactivity of VI toward
2
1
(13)** to obtain km = (3.7 0.1) 10 s , km2 = 1.85
3
1
2
1
ester II should be km2 = 1.86 10 l mol 1 s . The
corresponding value for the methyl analog, 1-methyl-
3-hydroxyminomethylpyridinium iodide, estimated
3
1
10 l mol 1 s , and KS = 320 30 l/mol. Comparison
of the KS values (see also Table 1) indicates similar
efficiencies in the solubilization of substrate II with
micelles IVa and mixed micelles VIIa VIII, while
in both cases acceleration of the reaction due to effect
of reactant concentration is the same (it cannot exceed
from the Bronsted relation for the reaction of oximate
/
2
1
ions with ester II is k2 = 1 10 l mol 1 s [23].
Calculation of for 1-methyl-3-hydroxyminomethyl-
pyridinium iodide using Eq. (17) and assuming the
KS/Vm = PS). Presumably, the same tendency is
fraction of bound substrate II to be
<< 1 and
=
S
S
typical for the change of km2 /k2
in going to the
***
0.1, 0.5, and 0.9 showed that the increase in the
reaction rate in going from micelles of VIa to its
methyl analog does not exceed a factor of 112, 100,
60, and 12, respectively; these values are considerably
lesser than those found for surfactant IVa (Table 2).
One of the most active functionalized surfactants,
which exhibit really catalytic properties in acyl group
transfer reactions, is 5-[2-cetyl(dimethyl)ammonio-
ethoxy]-2-iodosylbenzoate (VII) (Scheme 4) [19].
In aqueous solution of cetyl(trimethyl)ammonium
chloride (VIII) compound VII exists mainly as
heterocyclic tautomer VIIa which rapidly reacts with
ester II, and the catalytic effect for the mixed micelle
system VIIa VIII is 43600. However, the catalytic
effect was estimated [19] with respect to 5-[2-cetyl-
(dimethyl)ammonioethyl]-2-iodobenzoate rather than
micelle phase of VIIa, km2 /k2(VIIa) 1 (cf. the corre-
sponding km2 /k2 value for IVa), and the maximal
catalytic effect in the nucleophilic cleavage of ester
II with functionalized VIIa micelles is unlikely to
exceed given in Table 2. It should be emphasized
that the reactivity of IVa exceeds that of mixed
VIIa VIII micelles by a factor of 4, i.e., micelles
____________
**
The critical micelle concentration of compound VIII was
reported in [34].
The value of k2 = 1.40 10 l mol 1 s for 2-iodosylbenzo-
ate as a model of monomeric form of VIIa was estimated
***
3
1
from the Bronsted relation for the reaction of inorganic
/
-nucleophiles with diethyl p-nitrophenyl phosphate (II)
3
1
[35]; the value km2 = 1.85 10 l mol 1 s was obtained on
the basis of Vm = 0.5 l/mol [20].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

1322
SIMANENKO et al.
Scheme 4.
formed by zwitterionic surfactant IVa are more
powerful nucleophilic species than mixed micelles
formed by VIIa and VIII.
The above data unambiguously indicate that new
functionalized surfactants IV are superior to pre-
viously known most powerful nucleophilic surfactants
VI and VII [9, 10, 19, 22]. In fact, micelles formed
by zwitterionic species IVa whose head part includes
a covalently linked typical -nucleophile moiety
(oximate group) can be regarded as a unique super-
nucleophilic system ensuring high rates of substrate
cleavage due to reactant concentration effect.
EXPERIMENTAL
p-Nitrophenyl p-toluenesulfonate, diethyl p-nitro-
phenyl phosphate, and ethyl p-nitrophenyl ethylphos-
phonate were synthesized by acylation of p-nitro-
phenol according to the procedure described in [30].
Commercial methyl iodide, cetyl bromide, and chloro-
acetone were purified by standard procedures; bromo-
acetone was prepared as described in [31]. 1-Methyl-
3-(2-hydroxyiminopropyl)imidazolium halides and
1-cetyl-3-(2-hydroxyiminopropyl)imidazolium halides
were synthesized according to Scheme 5, following
the recommendations given in [32].
Table 2. Apparent rate constants for cleavage of p-nitro-
phenyl p-toluenesulfonate (I), diethyl p-nitrophenyl phos-
phate (II), and ethyl p-nitrophenyl ethylphosphonate (III)
with 1-methyl-3-(2-hydroxyiminopropyl)imidazolium (Va)
[kap(Va)] and 1-cetyl-3-(2-hydroxyiminopropyl)imidazo-
lium (IVa) halides [kap(IVa)] and the corresponding values
of
= kap(IVa)/kap(Va); water, 25 C
kap(IVa)
kap(Va)
105,c
s
Substrate
no.
a
d
S
1
1
103,b
s
Scheme 5.
I
13200e
12800
12100
6800
1300
560e
550
0.035
0.107
0.5
2.11
6.53
31
55
0.0165
0.054
0.454
0.9
4.09 0.08
II
0.03
0.1
0.5
0.9
0.508
1.90
8.30
0.0928
0.333
3.0 0.1
25 0.7
510
290
14.9
60
III
940e
900
0.03
0.1
0.5
0.9
9.7
32
160
290
1.10 0.08
3.90 0.09
34.0 0.6
310 9
820
460
94
Initially, imidazole was alkylated with methyl
iodide or cetyl bromide, and carbonyl group was then
introduced into molecules of 1-alkylimidazoles via
reaction with chloro- or bromoacetone. The resulting
ketones were converted into the corresponding oximes
by treatment with hydroxylamine in methanol [24].
After removal of the solvent, oximes V were recrystal-
lized twice or thrice from butanol, and compounds IV,
from ethanol ether. Their melting points and 1H NMR
data are given below.
a
Fraction of the substrate bound with micelles of zwitterionic
surfactant IVa.
Corrected for the contribution of alkaline hydrolysis [Eq. (11)].
The kap(Va) values given without standard deviations were
calculated by Eq. (1), and the others were determined experi-
mentally.
b
c
d
e
Increase in the reaction rate in going from water to micelles
of IVa.
1-Methyl-3-(2-hydroxyiminopropyl)imidazolium
halides. mp 165 168 C (bromide), 170 172 C
The highest possible acceleration
= (km2 /k2)PS.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

MICELLE EFFECTS OF FUNCTIONALIZED SURFACTANTS
1323
Table 3. Apparent pseudofirst-order rate constants kap for cleavage of p-nitrophenyl p-toluenesulfonate (I), diethyl
p-nitrophenyl phosphate (II), and ethyl p-nitrophenyl ethylphosphonate (III) in the presence of 1-cetyl-3-(2-hydroxy-
iminopropyl)imidazolium bromide; water, 25 C
Ester I
Ester II
Ester III
Ester I
Ester II
Ester III
c0 104,
c0 104,
c0 104,
c0 104,
c0 104,
c0 104,
kap
103, s
kap
103, s
kap
103, s
kap
103, s
kap
103, s
kap
103, s
1
1
1
1
1
1
M
M
M
M
M
M
0.04
0.08
0.1
0.63
0.2
0.78
0.85
0.95
1.04
1.28
1.58
1.9
2
2.22
2.4
3
3.6
4.6
5.75
6.8
7.8
8.5
8.9
9.5
9.8
10.5
11.4
0.313
0.625
1.35
2.6
3.28
3.7
4.52
5.1
5.84
6.35
6.8
10
12.6
25
41
50
5.92
6.1
6.1
2.1
2.5
3.57
4
4.2
5.4
6.4
8.0
21.6
25.5
31.0
32.4
33.5
36.4
38.6
42.0
44.8
46.5
49.9
53.0
54.5
55.0
55.0
55.8
57.8
59.0
59.0
60.6
61.0
83
110
150
210
300
320
400
500
12.1
12.8
13.8
14.4
15
15.5
15.4
15.7
300
350
400
500
280
290
290
300
0.695
0.815
0.82
1.13
1.32
1.33
1.78
2.0
2.23
2.73
3.78
5.53
6.65
7.14
9.08
9.15
9.75
0.9
1.8
2.0
3.6
4.38
5.0
5.45
6.0
6.5
7.5
0.117
0.15
0.178
0.20
0.25
0.32
0.53
0.65
0.71
0.806
0.89
0.90
1.0
1.05
1.1
1.25
1.4
1.8
1.88
6.8
12.4
13.5
21.9
25
31.4
36
37.2
56
69
10
11
16
18
24
25
27.8
32
50
10.5
15
21
25
30
33
38
41
50
58
75
120
170
180
210
236
250
260
270
276
75
100
135
175
200
250
110
11.7
160
180
210
13.6
18.7
19
1
(chloride). H NMR spectrum, , ppm: 11.0 s (1H,
NOH), 9.33 s (1H, 2-H, imidazole), 7.78 s and 7.70 s
(2H, 4-H, 5-H, imidazole), 5.0 s (2H, NCH2), 3.91 s
(3H, NCH3), 1.83 s (3H, CCH3).
IV, the ionic strength was not fixed. In all experi-
ments, the required pH value was set using concen-
trated solutions of sodium hydroxide or hydrochloric
acid. The acidity of the medium was checked before
and after each kinetic experiment, using OP-205 and
OP-213 pH-meters (Radelkis, Hungary). If the dif-
ference in pH before and after kinetic experiment
exceeded 0.05 pH units, the results were rejected.
Nucleophilic cleavage of esters I III by reagents
IVa and Va in water at various pH values gives
p-nitrophenoxide ion as one of the products (see
Scheme 1); therefore, the progress of reactions was
monitored following the accumulation of p-nitrophen-
1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium
halides. mp 123 124 C (bromide), 125 127 C
1
(chloride). H NMR spectrum, , ppm: 11.15 s (1H,
NOH), 9.55 s (1H, 2-H, imidazole), 7.85 s and 7.75 s
(2H, 4-H, 5-H, imidazole), 5.05 s (2H, NCH2), 4.25 t
(2H, -CH2 in C16H33), 1.85 s (3H, CCH3), 1.80 t
(2H, -CH2 in C16H33), 1.25 m (26H, C16H33), 0.90 t
(3H, -CH3 in C16H33).
Inorganic reagents of ultrapure or chemically pure
grade were used without additional purification. All
solutions were prepared using doubly distilled water
just before each series of kinetic measurements. The
analytical concentration of oxime V was selected in
such a way that the reactant solutions were simulta-
neously buffer solutions. The ionic strength was
maintained with the aid of 1 M KCl. In solutions of
oxide ion by spectrophotometry at
400 430 nm
(Specord UV-Vis instrument, 25 0.5 C). The ap-
1
parent pseudofirst-order rate constants kap (s ) were
determined from the change in the optical density
using Eq. (18):
ln(D
D ) = ln(D
D0)
kap
.
(18)
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 9 2002

1324
SIMANENKO et al.
10. Lion, C., Despagne, B., Delmas, G., and Fosset, L.,
Here, D0, D , and D are the optical densities at
Bull. Soc. Chim. Belg., 1991, vol. 100, no. 7,
pp. 549 554.
= 0, = , and by the end of the process (Table 3).
In all kinetiic experiments the initial concentration of
substrates I III was much lesser than the initial
nucleophile concentration.
11. Bunton, C.A., Foroudian, H.J., and Gillitt, N.D.,
Langmuir, 1999, vol. 15, no. 4, pp. 1067 1074.
The critical micelle concentrations (CMC) were
determined by difference UV spectroscopy with the
use of Fluorescein ( 507 nm) [33] and from the
kinetic data (Fig. 3a). The CMC value for the reaction
of IVa with ester I was estimated by mathematical
statistics [28]. The kinetic data (more than 40 values
of kap) were treated according to Eq. (13) on variation
12. Simanenko, Yu.S., Karpichev, E.A., and Prokop’-
eva, T.M., Langmuir, 2001, vol. 17, no. 3, pp. 581
582.
13. Bunton, C.A., Hamed, F.H., and Romsted, L.R.,
J. Phys. Chem., 1982, vol. 86, no. 11, pp. 2103
2108.
14. Kunitake, T., Okahata, Y., Tanamachi, S., and
Ando, R., Bull. Chem. Soc. Jpn., 1979, vol. 52,
pp. 1967 1971.
6
of CMC (c = c0 CMC) in the range from 8 10
4
7
to 2 10 M through a step of 1 10 M; the linear
correlation coefficient r was analyzed for statistical
15. Pillersdorf, A. and Katzhendler, J., J. Org. Chem.,
5
significance; a CMC value of 5.38 10 M was
1979, vol. 44, no. 4, pp. 549 554.
obtained. A similar value, 5.42 10 5 M, was obtained
by treatment of nonlinear regression (11) by the quasi-
Newton procedure at fixed km = kmax. The average
16. Ueoka, R., Matsuura, H., Nakahata, S., and
Onkubo, K., Bull. Chem. Soc. Jpn., 1980, vol. 53,
no. 2, pp. 347 351.
5
CMC value, 5.40 10 M (Table 1) was used in
17. Bunton, C.A., Gillitt, N.D., and Foroudian, H.J.,
the calculations of the kinetic and thermodynamic
parameters.
Langmuir, 1998, vol. 14, no. 16, pp. 4415 4421.
18. Moss, R.A. and Alwis, K.W., Tetrahedron Lett.,
The solubilities of esters I III in water were deter-
1980, vol. 21, no. 14, pp. 1303 1306.
5
3
mined as described in [9]: 1 10 (I), 3 10
M
19. Moss, R.A., Kim, K.Y., and Swarup, S., J. Am.
Chem. Soc., 1986, vol. 108, no. 4, pp. 788 793.
(II and III).
20. Scrimin, P., Tecilla, P., Tonellato, U., and
Bunton, C.A., Colloids Surfaces, A, 1998, vol. 144,
pp. 71 79.
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