Characterization of different reverse micelle interfaces using the reaction of 4-fluoro-3-nitrobenzoate with piperidine

Article abstract of DOI:10.1002/poc.876

The characterization of different reverse micellar interfaces, benzene-sodium 1,4-bis-2-ethylhexylsul-fosuccinate (AOT)-water and benzene-benzyl-rt-hexadecyldimethyJammonium chloride (BHDC)-water, were studied using an aromatic nucleophilic substitution reaction, S_NAr, between acid 4-fluoro-3-nitrobenzoic (FNBA) and piperidine. The reaction was also studied in homogeneous media, water and benzene, varying the ionic strength in the aqueous solution. The kinetic profiles were investigated as a function of variables such as surfactant, amine concentration and the amount of water dispersed in the reverse micelles, W₀ = [water]/[surfactant]. In the pure solvents, water and benzene, there is no genuine base-catalyzed reaction. In water the reaction is faster than in benzene, owing to the better stabilization of the intermediate Z. Also, the reaction rate in water is accelerated by the addition of salt. In both micellar systems FNBA is totally incorporated in the interface of the aggregate because, under the working conditions of excess of amine, it is ionized to carboxylate, The reactions kinetics in these media can be quantitatively explained taking into account the distribution of the nucleophile between the bulk solvent and the micelle interfaces. In the AOT reverse micelles at W₀= 10, the anionic substrate seems to be located in the water side of the interface whereas in BHDC it resides in the oil side of the interface. The results were used to evaluate the intrinsic second-order rate coefficient of the S_NAr reaction at the interface. From these data, properties such as micropolarity and ionic strength of this environment can be deduced. Hence, as expected, the micropolarity of the reverse micelles systems is higher than that of pure benzene and increases upon water addition. In the AOT system and owing to the location of the probe, the average ionic strength of the AOT reverse micelles could be estimated and a value of around 4.5 M was found. Copyright

Full text of DOI:10.1002/poc.876

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2005; 18: 121–127
Published online 5 October 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.876
Characterization of different reverse micelle interfaces
using the reaction of 4-fluoro-3-nitrobenzoate
with piperidiney
N. Mariano Correa, Edgardo N. Durantini and Juana J. Silber*
Departamento de Quı´mica, Universidad Nacional de Rı´o Cuarto, Agencia Postal Nro 3, 5800 R´ıo Cuarto, Argentina
Received 3 November 2003; revised 2 March 2004; accepted 24 March 2004
ABSTRACT: The characterization of different reverse micellar interfaces, benzene–sodium 1,4-bis-2-ethylhexylsul-
fosuccinate (AOT)–water and benzene–benzyl-n-hexadecyldimethylammonium chloride (BHDC)–water, were stu-
died using an aromatic nucleophilic substitution reaction, SNAr, between acid 4-fluoro-3-nitrobenzoic (FNBA) and
piperidine. The reaction was also studied in homogeneous media, water and benzene, varying the ionic strength in the
aqueous solution. The kinetic profiles were investigated as a function of variables such as surfactant, amine
concentration and the amount of water dispersed in the reverse micelles, W0 ¼ [water]/[surfactant]. In the pure
solvents, water and benzene, there is no genuine base-catalyzed reaction. In water the reaction is faster than in
benzene, owing to the better stabilization of the intermediate Z. Also, the reaction rate in water is accelerated by the
addition of salt. In both micellar systems FNBA is totally incorporated in the interface of the aggregate because, under
the working conditions of excess of amine, it is ionized to carboxylate, The reactions kinetics in these media can be
quantitatively explained taking into account the distribution of the nucleophile between the bulk solvent and
the micelle interfaces. In the AOT reverse micelles at W0 ¼ 10, the anionic substrate seems to be located in the water
side of the interface whereas in BHDC it resides in the oil side of the interface. The results were used to evaluate the
intrinsic second-order rate coefficient of the SNAr reaction at the interface. From these data, properties such as
micropolarity and ionic strength of this environment can be deduced. Hence, as expected, the micropolarity of the
reverse micelles systems is higher than that of pure benzene and increases upon water addition. In the AOT system and
owing to the location of the probe, the average ionic strength of the AOT reverse micelles could be estimated and a
value of around 4.5 M was found. Copyright # 2004 John Wiley & Sons, Ltd.
KEYWORDS: reverse micelles; AOT; aromatic nucleophilic substitution reaction; ionic strength
INTRODUCTION
properties affected by its high ionic strength and by the
electronic influence of the charged surfactant heads.5–8
Two differentiated aqueous and organic pseudophases
present in reversed micelles allow compartmentalization
of solubilized species at the microscopic level. Solubili-
zation of a reactant in the same region of the surfactant
assembly can lead to significant acceleration of reaction
rates, whereas the rates of reactions of segregated reac-
tants are retarded. When reactants are in the water
droplet, they are concentrated as in a nanoreactor, and
since the size of this reactor is easily varied, the influence
of the properties of the micellar system is relatively easy
to assess.9,10 On the other hand, reverse micellar systems
are of interest as reaction media because they are power-
ful models for biological systems.11–13
Chemical reactivity in reverse micelles systems is an
attractive object of study because these media are macro-
scopically homogeneous and isotropic, although they are
heterogeneous on the molecular scale. These systems
contain aqueous microdroplets entrapped in a film
of surfactants and dispersed in a low-polarity bulk
solvent.1–6 It is also known that the physical character-
istics of the water in the microdroplets differ from those
of bulk water, mainly as regards mobility, polarity and
´
*Correspondence to: J. J. Silber, Departamento de Quımica, Universi-
dad Nacional de Rıo Cuarto, Agencia Postal Nro 3, 5800 Rıo Cuarto,
´
´
Argentina.
E-mail: jsilber@exa.unrc.edu.ar
Systematic studies using neutral nucleophiles in this
kind of system are scarce. In previous work, we have
been interested in the influence of cationic and anionic
reverse micelles on the bimolecular aromatic nucleophi-
lic substitution (SNAr) reactions between halo nitro-
substituted aromatic substrates and aliphatic amines.
The results showed that the reactions are faster in the
ySelected article presented at the Seventh Latin American Conference
on Physical Organic Chemistry (CLAFQO-7), 21–26 September 2003,
´
Florianopolis, Brazil.
Contract/grant sponsors: Consejo Nacional de Investigaciones Cientı-
´
´
ficas (CONICET); Secretarıa de Ciencia y Tecnica de la Universidad
Nacional de Rıo Cuarto; Fundacion Antorchas; Agencia Nacional de
´
´
´
´ ´ ´
Promocion Cientıfica y Tecnologica.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

122
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
micellar media than in the pure solvent, being base
catalyzed in the organic solvents but not in the reverse
micelles.14–16
The aim of this work was to characterize two anionic and
cationic reverse micelles interfaces, namely benzene–
sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)–water
ð1Þ
and
benzene–benzyl-n-hexadecyldimethylammonium
chloride (BHDC)–water, using the SNAr reaction be-
tween acid 4-fluoro-3-nitrobenzoic (FNBA) and piperi-
dine (PIP). Substrates containing carboxylic groups can
be conveniently anchored in the reverse micelle systems.
These types of substrate were used to evaluate the
effect of the micellar orientation in direct micelles
systems on the SNAr reaction with hydroxide ion.17,18
These studies discriminate between substrates with reac-
tion center at the micellar water interface, where it is
better situated for the substitution, and those with the
reaction center that are more deeply buried in the micellar
interior.
It should be noted that because the reactions are
performed in excess of amine, FNBA is always present
as carboxylate [the pirimidinium ion is not shown in Eqn
(1) for the sake of clarity].
All the reactions follow pseudo-first-order kinetics.
The spectra at different times of reaction show a clear
isosbestic point, indicating the cleanness of the reactions.
However, considering that hydroxide ion may result from
amine hydrolysis in the water pool, the possibility of
the reaction between FNBA and this ion was examined,
but no reaction was detected under these experimental
conditions.
It is also known that in SNAr reactions with electri-
cally neutral nucleophilic reagents, e.g. amines, specific
salt effects enhance the reaction rate.19,20 Bimolecular
reactions studied in micelles can be used to estimate the
local concentration of ions at the interface, by using the
rate constants in the micellar pseudophase. The major-
ity of these studies were performed in direct micelles,
In trying to elucidate the mechanism of the SNAr
reaction and to compare the homogeneous and micro-
heterogeneous media, the influences of several variables
were investigated as follows.
21,22
where it was shown that the interfacial ion con-
centration is different from that in the bulk. On the other
hand, the literature in this respect for reverse micelles is
scarce. For instance, Das et al.,23 based on a modified
phenyl cation-trapping protocol, showed that the local
molar concentrations of bromide counter ions inside the
water pool of cetyltrimethylammonium bromide
(CTAB)–isooctane–n-hexanol–water reverse micelles
decrease from 1.9 to 0.29 M as W0 increases from 12
to 44, with a concomitant increase in the local water
pool concentration of water from 43.8 to 55.4 M. Also,
Cuccovia et al.,24 by using chemical trapping, deter-
mined in reverse micelles of CTAB in n-dodecane–
CHCl3 and isooctane–n-hexanol, the bound bromide
concentration, which decreases with W0 and its value
depends on the organic solvent and co-solvent. In this
paper, it is shown that the kinetic results of the SNAr
reaction between amines and FNBA, which is totally
incorporated at the aggregate interfaces, can be used
to estimate the ionic strength of reverse micellar
pseudophases.
Reaction in benzene and water
The kinetic results are shown in Fig. 1(A). As can be
inferred, the reaction rates increase with increasing amine
concentration, being faster in water than benzene. In both
cases a linear behavior was observed. It must be men-
tioned that the results in water are almost identical either
considering the analytical PIP concentration or
calculating it taking into account the hydrolysis basic-
equilibrium (pKb ¼ 2.88).25 This negligible protonation
effect is due to the high amine concentration used in these
experiments.
The effect of the ionic strength on this reaction was
analyzed in water at [PIP] ¼ 0.1 M, for [NaNO3] in the
range 0–5 M. The results are showed in Fig. 1(B). As it
RESULTS
The reaction of FNBA with PIP in homogeneous media,
benzene and water, and in reverse micelles, benzene–
AOT–water and benzene–BHDC–water, produces the
corresponding ipso-fluorine substitution product [Eqn
(1)] in quantitative yields, as shown by UV–visible
spectroscopic analysis of the reaction mixture.
Figure 1. Dependence of kobs on PIP concentration for the
reaction between FNBA and PIP in (A) water (&) and
benzene (*) [FNBA] ¼ 1.5 ꢀ 10ꢁ4 M. (B) Dependence of
kobs on NaNO3 concentration. [FNBA] ¼ 1.5 ꢀ 10ꢁ4 M;
[PIP] ¼ 0.1 M
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

CHARACTERIZATION OF REVERSE MICELLE INTERFACES
123
Figure 4. Variation of kobs with W0 in water–AOT–benzene
reverse micelles for the reaction of FNBA with PIP.
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [PIP] ¼ 0.1 M; [AOT] ¼ 0.2 M
Figure 2. Dependence of kobs on AOTconcentration for the
reaction between FNBA and PIP in water–AOT–benzene
reverse micelles at (A) W0 ¼ 0 and (B) W0 ¼ 10.
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [PIP] ¼ 0.1 M
can be observed, the reaction rate increases with increas-
ing ionic strength, owing to the stabilization of the
transition state, as was observed previously for similar
SNAr reactions.19,20
Effect of water dispersed
The effects of changing the value of W0 on kobs keeping
the AOT and PIP concentrations constant is shown in
Fig. 4. As can be observed, kobs increases as W0 increases
until it reaches a constant value at W0 ꢂ 10.
Reaction in the AOT–benzene system
Reaction in benzene–BHDC–water
Effects of AOT concentration
reverse micelles
The kinetics of the reaction was studied varying [AOT]
between 0 and 0.4 M, keeping the other experimental
conditions fixed. Figure 2(A) shows the kinetic results
at W0 ¼ 0. As can be observed, there is a sharp decrease in
kobs with increase in AOT concentration, reaching a
constant value beyond [AOT] ꢂ 0.05 M. On the other
hand, at W0 ¼ 10, in the same surfactant range, there is
a very rapid increase in kobs with increase in [AOT]
[Fig. 2(B)]. At [AOT] > 0.05 M, there is practically no
variation of the reaction rate.
Effect of BHDC concentration
Typical kinetic results with varying BHDC concentration
are shown in Fig. 5 for W0 ¼ 0 and 10. As can be
observed, at W0 ¼ 0, kobs decreases sharply with increas-
ing BHDC concentration, reaching a constant value at
[BHDC] > 0.05 M. On the other hand, at W0 ¼ 10 only a
small increase in kobs is observed, and also the reaction
rate is constant at [BHDC] > 0.05 M.
Effect of amine concentration
Effect of amine concentration
Figure 6 shows the variation of kobs with PIP concentra-
tion in the micelle at [BHDC] ¼ 0.2 M, at W0 ¼ 0 and 10.
At both values of W0, there is a linear dependence
between kobs and the amine concentration. As was
found previously in a similar SNAr reaction with PIP as
nucleophile, the reaction is faster at higher water
content.14
To study the effect of amine concentration, the reaction
was carried out using 0.2 M AOT at W0 ¼ 0 and 10. The
results are shown in Fig. 3(A) and (B), respectively. In
both cases, kobs increases linearly with increase in PIP
concentration. The results show that the reaction is faster
at high water content, as was observed by varying [AOT].
Figure 3. Variation of kobs with PIP concentration for the
reaction between FNBA and PIP in water–AOT–benzene
reverse micelles at (A) W0 ¼ 0 and (B) W0 ¼ 10.
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [AOT] ¼ 0.2 M. The dotted line shows
the fitting by Eqn (8)
Figure 5. Dependence of kobs on BHDC concentration for
the reaction between FNBA and PIP in water–BHDC–ben-
zene reverse micelles at W0 ¼ 0 (&) and W0 ¼ 10 (*).
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [PIP] ¼ 0.1 M
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

124
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
Figure 6. Variation of kobs with PIP concentration for the
reaction between FNBA and PIP in water–BHDC–benzene
reverse micelles at W0 ¼ 0 (&) and W0 ¼ 10 (*).
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [BHDC] ¼ 0.2 M. The dotted line
shows the fitting by Eqn (8)
Figure 7. Variation of kobs with W0 in water–BHDC–
benzene reverse micelles for the reaction of FNBA with PIP.
[FNBA] ¼ 1.5 ꢀ 10ꢁ4 M; [PIP] ¼ 0.1 M; [BHDC] ¼ 0.2 M
Bi
reactions. On the other hand, if kꢁ1 << k2 þ k3 [Bi] or,
more precisely, k2 >> kꢁ1, the formation of the intermedi-
ate Z is rate limiting and consequently kA ¼ k1 and a plot
of kobs vs [Bi] gives a linear response.20,26,27
Effect of the water dispersed
The effect of increasing the water content is shown in
Fig. 7. As can be observed, this tendency is similar to that
found in the AOT system (Fig. 4). The rate of reaction is
faster at higher W0, reaching a constant value at W0 > 10.
Figure 1 shows a linear dependence between kobs and
[PIP] in benzene and water. Therefore, according to
Eqn (3), there is no genuine base-catalyzed reaction in
both pure solvents. From the linear regression (Fig. 1,
dotted lines) the valꢁue1s of (6.6 ꢄ 0.2) ꢀ 10ꢁ3 and
(1.1 ꢄ 0.1) ꢀ 10ꢁ2 sꢁ1
M
were found for k0 in benzene
DISCUSSION
and water, respectively. Thus, the formation of the Z
intermediate is the rate-limiting reaction in both solvents.
It must be noted that the reaction between 1-fluoro-2,4-
dinitrobenzene with PIP in benzene is wholly base
catalyzed.28 On the other hand, in the same system but
using 1-fluoro-2-nitrobenzene as substrate, the reaction is
not base catalyzed.20 Therefore, it can be inferred that the
carboxylate group in FNBA is a poor electron-withdrawing
substituent compared with the nitro group, the formation of
the Z intermediate being the rate-determining step. As
expected, enhancing the polarity of the solvent produces
an increase in the reaction rate (k0water/k0benzene ꢂ 2) owing
to the better stabilization of the transition state leading to
this intermediate. It could be argued that the differences
with benzene are not large, but it should be taken into
account that in water the nucleophilicity of the amines is
diminished by solvation of the nucleophile,15 counteracting
the effect of the intermediate stabilization.
For primary or secondary amines as nucleophiles, the
accepted general mechanism20,26,27 for SNAr reactions,
involving halogen or nitrite as leaving groups, can be
represented by Eqn (2).
ð2Þ
According to the results and considering that only two
pseudophases, the interface and the bulk organic solvent,
are mainly present at W0 ¼ 0 and 10 and taking into
account the amine distribution between these two envir-
onments, a mechanism summarized in Scheme 1 can be
proposed:2,5,14,15
where R0 could be H or an alkyl group, X is the leaving
group, G represents electron-withdrawing substituents
and Bi is the nucleophile or any other base added to the
reaction medium. Application of the steady-state hypoth-
esis to this mechanism, and in the limiting situation when
Bi
k
ꢁ1 >> k2 þ k3 [Bi], the following equation is obtained:
kobs=½Biꢃ ¼ kA ¼ k0 þ k00½Biꢃ
where kobs is the observed pseudo-first-order rate con-
ð3Þ
stant, kA is the second-order rate constant, k0 ¼ k1k2/k
ꢁ1
Bi
and k00 ¼ k1k3 /kꢁ1. In this case the decomposition of Z is
rate limiting and base catalysis may be expected. A linear
response to base concentration such as depicted in Eqn
(3) is characteristic of the majority of base-catalyzed
Scheme 1
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

CHARACTERIZATION OF REVERSE MICELLE INTERFACES
125
where the subscripts f and b indicate the organic phase
and the micellar pseudophase, respectively, and Surf
represents the total surfactant concentration. The rate
[FNBA] and [PIPb] in Eqn (4), we can obtain the final
expression for the rate [Eqn (7)] and kobs [Eqn (8)]:
coefficients of the reaction k1, k and k2, were defined
d½Pꢃ
dt
d½FNBAꢃ
dt
ꢁ1
¼ ꢁ
¼ kobs½FNBAꢃ
ð7Þ
in Eqn (1). KA is the distribution constant for PIP between
the organic phase and micellar pseudophase. As men-
tioned previously, under the experimental conditions the
carboxylic acid group of the substrate is ionized, forming
an ion pair with the amine. In this way, the salt is mainly
dissolved in a polar microenvironment (the interface)17
and its distribution constant is not taken into account in
Scheme 1. Moreover, this implies that the reaction occurs
only in the micellar interior.
The micropolarity of the interface in reversed micelles
of benzene–AOTor BHDC–water is always higher that in
benzene.8,29 Assuming that the reactions of FNBA with
PIP are not base catalyzed in either the or the cationic
anionic micellar interface, the formation of the inter-
mediate Zb is rate limiting. The linear relationships of
kobs with the nucleophile concentration (Figs 3 and 6)
indicate this lack of base catalysis at the micellar pseu-
dophase. These are expected results considering that the
micellar interfaces are more polar than the organic
medium,29 where the reactions are not base catalyzed
either.
with
ðk0 KA½PIP ꢃ=vÞ
b
T
kobs
¼
ð8Þ
ð1 þ KA½SurfꢃÞ
This equation is valid only when the surfactant con-
centration is high enough in order to ensure that all the
substrate is anchored to the micellar pseudophase. From
Figs 2 and 5, it can be seen that for [Surf] > 0.02 M, kobs is
practically independent of surfactant concentration.
These results confirm whole substrate incorporation in
the anionic and cationic micellar interfaces at both W0
values as assumed in Scheme 1. Hence the values of k0b
were calculated using Eqn (8) for [AOT] > 0.02 M and the
results are given in Table 1. The molar volumes of BHDC
ꢁ1
and AOT were estimated as 0.44 and 0.38 M respec-
tively.15,30 The values of KA were calculated in previous
work.14,15
Also, as shown previously,14,15 there is no evidence of
amine protonation in the reverse micellar systems used in
this work, and this possibility is considered negligible.
The rate of the reaction can be expressed by
By introduction of suitable values of KA, the values of
k0b were recalculated for each system at W0 ¼ 0 and 10 by
fitting a different set of experimental data by Eqn (8). The
results are compared in Table 1. As can be seen, good
estimates for k0b within experimental error are obtained
by two independent methods.
It must be noted that the kinetic profiles obtained at
W0 ¼ 0 for both systems [Figs 2(A) and (5)] show a sharp
decrease in the rate constant with increasing surfactant
concentration. These results can be explained by con-
sidering that at low surfactant concentration, both reac-
tants are unevenly distributed between both pseudophases
whereas FNBA is incorporated in the micelles where the
amine concentration in this region is very low (see the
d½Pꢃ
dt
d½FNBAꢃ
dt
½PIPbꢃ½FNBAꢃ
ꢀv½Surfꢃ
¼ ꢁ
¼ kb0
ð4Þ
For absolute comparison of reactivities in different
media, the molar reaction volume at the interface, ꢀv
should be known. This can be estimated from the molar
volume of AOT and BHDC in the reverse micelles, which
can be taken as ꢀv.15 Hence k0b is the conventional intrinsic
second-order rate constant in the interface. The concen-
trations in square brackets refer to the total volume of the
reverse micelle.
Table 1. Kinetic parameters and distribution constants for
the reaction of FNBA with PIP in different media at 25 ꢅCe
The distribution constant of PIP can be expressed by
the equation 5.5
ꢁ1
ꢁ1 a
)
System
k0b (sꢁ1
M
)
KA (M
½PIPbꢃ
ꢁ3 b
ꢁ3 c
ꢁ2 b
ꢁ2 c
ꢁ3 d
AOT–benzene, W0 ¼ 0
(3.4 ꢄ 0.2) ꢀ 10
(3.8 ꢄ 0.2) ꢀ 10
(4.0 ꢄ 0.3) ꢀ 10
(3.4 ꢄ 0.2) ꢀ 10
0.09 ꢄ 0.04
0.10 ꢄ 0.04
0.3 ꢄ 0.1
KA ¼
ð5Þ
½PIPfꢃ½Surfꢃ
AOT–benzene, W0 ¼ 10
A simple mass balance using the distribution constant
KA and the analytical concentration of PIP, [PIPT], allows
the calculation of [PIPb]:
BHDC–benzene, W0 ¼ 0 (1.8 ꢄ 0.2) ꢀ 10
(1.9 ꢄ 0.2) ꢀ 10ꢁ3
BHDC–benzene, W0 ¼ 10 (7.8 ꢄ 0.3) ꢀ 10ꢁ3 d
(6.9 ꢄ 0.3) ꢀ 10ꢁ3
0.3 ꢄ 0.1
a From Ref. 15.
KA½Surfꢃ½PIPTꢃ
½PIPbꢃ ¼
ð6Þ
b
From Fig. 2, calculated by Eqn (8) for [AOT] > 0.02 M.
ð1 þ KA½SurfꢃÞ
c From fitting of Fig. 3 by Eqn (8).
d
From Fig. 5, calculated by Eqn (8) for [BHDC] > 0.02 M.
From fitting Fig. 6 by Eqn (8). Parameter values calculated using 0.995
confidence level in non-linear regression.
e
If [PIPT] >> [FNBA], a pseudo-first-order behavior for
the kinetics of the reaction is assumed. Then, replacing
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

126
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
value of KA in Table 1). For both systems, at
[Surf] > 0.02 M the reaction takes place only at the inter-
face of the aggregate.
surfactant concentration, reflecting the different location
of the substrate and the nucleophile. However, at W0 ¼ 10
the reaction rates present a different situation. In the
BHDC system, the reaction rate increases in comparison
with W0 ¼ 0 reaching a similar value to that in benzene. It
seems that the Z intermediate is anchored at the oil side
of the cationic interface not sensing the water. On the
other hand, the reaction in the AOT system shows an
important increase in the reaction rate, being 10 times
faster than in benzene and even faster than in pure water.
Under this condition the reaction rate is similar to that
obtained in water at an ionic strength of 4.7. In this way,
and taking into account that the Z intermediate is located
in the water region of the interface, it can be deduced that
the average ionic strength corresponding to interface of
AOT–benzene aggregates is of the order of 4–5 M. This
interesting result shows that the average ionic strength of
AOT–benzene reverse micelles is similar to that obtained
in isooctane or hexane–AOT–water systems.
On the other hand, the value of k0b in AOT–benzene is
twice that BHDC–benzene (Table 1), reflecting the higher
micropolarity of the AOT than the BHDC interface.29
Surprisingly, the reaction is slower at these interfaces
than in benzene and water because it may take place near
the oil side of the reverse micelle interface, away from the
residual water and the counter ions.
The kinetic behavior obtained at W0 ¼ 10 [Figs 2(B)
and 5] in both micellar systems is different, showing the
different influences that the anionic and cationic inter-
faces have in the reaction. There is an important incre-
ment in the reaction rate with increasing AOT
concentration. At [AOT] > 0.02 M, the values obtained
are higher than that found in pure water (Table 1). Since
FNBA is negatively charged, it may be pushed towards
the water side of the interface, by the anionic heads of
AOT and where the counterions are located. Considering
that this rate enhancement could be an effect of the ionic
strength of the interface, the value of the reaction rate was
correlated with the corresponding values in water at
different ionic strength [Fig. 1(B)]. From this comparison
it can be inferred that the average ionic strength inside the
reverse micelles is around 4.7 M. Similar values were
assumed for AOT–isooctane reverse micelle systems.31 It
seems that the negative charged intermediate Zb is
located at the water side of the interface sensing the
average ionic strength of the microenvironment. In con-
trast, with increase in [BHDC] a small effect on the rate
constant is observed similar to that obtained in pure
benzene (Table 1). In this case, the Zb intermediate could
be anchored to the cationic interface and the reaction
occurs mostly at the oil side.
EXPERIMENTAL
General. UV–visible spectra were recorded on a Hewlett-
Packard HP 8453 spectrophotometer. Proton nuclear
magnetic resonance (1H NMR) spectra and mass spectra
were recorded on a Bruker 200 MHz spectrometer and
Micromass ZAB-SEQ instrument, respectively. Silica gel
thin-layer chromatographic (TLC) plates (250 mm) from
Aldrich were used.
Materials. 4-Fluoro-3-nitrobenzoic acid (FNBA) from
¨
Aldrich and piperidine (PIP) from Riedel-de Haen were
used without further purification. Sodium bis(2-ethylhex-
yl)sulfosuccinate (AOT) from Sigma (purity > 99%) was
used as received. The UV–visible spectra of 1-methyl-8-
oxyquinolinium betaine in the presence of AOT reverse
aggregates in benzene showed that the surfactant was free
of acidic impurities.32 It was kept under vacuum over
P2O5 until it was used. Benzyl-n-hexadecyldimethylam-
monium chloride (BHDC) from Sigma was recrystallized
twice from ethyl acetate and dried under vacuum over
P2O5.29 Benzene (Sintorgan, HPLC quality) was used as
received. Ultrapure water was obtained from a Labonco
Model 90901-01 system.
In both systems, the addition of water (W0) produces an
increase in the reaction rate (Figs 4 and 7), reflecting the
enhancement of the interface micropolarity by the addi-
tion of water.29
CONCLUSIONS
The reaction of FNBA with PIP is not base catalyzed
either in homogeneous media or in the micellar systems
studied. In water, the reaction is faster than in benzene,
probably owing to the better stabilization of the Z
intermediate transition state in the more polar solvent.
Also, the reaction rate in water is accelerated by the
addition of salt. These properties are used to characterize
the micellar systems studied, benzene–AOT–water and
benzene–BHDC–water. The reaction rates are practically
independent of the surfactant concentration above 0.02 M,
indicating the complete incorporation of FNBA as car-
boxylate in the micellar pseudophase where the reaction
takes place. When water is not added, i.e. W0 ¼ 0, there is
a sharp decrease in the reaction rate with increasing
Procedures. Stock solutions of surfactant reverse micelles
were prepared by weighing and dilution in benzene.
Stock solutions of 0.5 M surfactant were agitated in a
sonicating bath until the reverse micelle was optically
clear. The appropriate amount of stock solution to obtain
a given concentration of surfactant in the micellar media
was transferred into the cell. The addition of water to the
corresponding solution was performed using a calibrated
microsyringe. The amount of water present in the system
is expressed as the molar ratio between water and the
surfactant present in the reverse micelle (W0 ¼ [H2O]/
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127

CHARACTERIZATION OF REVERSE MICELLE INTERFACES
127
[surfactant]). The lowest value of W0 (W0 ¼ 0) corre-
REFERENCES
sponds to a system with no addition of water and the
presence of water corresponds to the intrinsic humidity of
the system (W0 ꢂ 0.3).
1. Fendler JH. Membrane Mimetic Chemistry. Wiley: New York,
1982.
2. Pileni MP. Structure and Reactivity of Reverse Micelles. Elsevier:
New York, 1989.
3. De TK, Maitra A. Adv. Colloid Interface Sci. 1995; 59: 95–193.
4. Moulik SP, Paul BK. Adv. Colloid Interface Sci. 1998; 78: 99–
195.
5. Silber JJ, Biasutti MA, Abuin E, Lissi E. Adv. Colloid. Interface
Sci. 1999; 82: 189–252.
Kinetics. Reactions were followed spectrophotometrically
by measuring the increase in the maximum absorption
band of the product, N-(2-nitro-4-carboxyphenyl)
piperidine, at 25.0 ꢄ 0.5 ꢅC. To start a kinetic run, a stock
solution of FNBA was added (10 ml) to a thermostated
cell containing the PIP and the reverse micelle solution.
The FNBA concentration in the reaction media was
1.5 ꢀ 10ꢁ4 M. The kinetic runs were performed by fol-
lowing the increase in the absorbance of the product of
the reaction (ꢀmax ¼ 434 nm). In every case, pseudo-first-
order plots were obtained in excess of nucleophile. The
pseudo-first-order rate constants (kobs) were obtained by a
non-linear least-squares fit of the experimental data of
absorbance vs time, (r > 0.999) by a first-order rate
equation. The value of the absorbance at infinite reaction
time was consistent with the value obtained from authen-
tic samples of the reaction product, within 3%. The
pooled standard deviation of the kinetic data, using
different prepared samples, was < 5%.
´
´
6. Garcıa-Rıo L, Leis JR, Pen˜a E, Iglesias E. J. Phys. Chem. 1993;
97: 3437–3442.
7. Falcone RD, Correa NM, Biasutti MA, Silber JJ. Langmuir 2002;
18: 2039–2047.
8. Biasutti MA, Correa NM, Silber JJ. Molecular Interactions in
Reverse Micelles: Current Topics in Colloid and Interface Re-
search of Research Trends, vol. 3, Gon˜i FM, Hoffmann H (eds).
Research Trends: India, 1999; 35–51.
´
´
9. Garcıa-Rıo L, Leis JR, Iglesias E. J. Phys. Chem. 1995; 99:
12318–12326.
10. Pileni MP. J. Phys. Chem. 1993; 97: 6961–6973.
11. Luisi PL, Straub B. Reverse Micelles (8th edn). Plenum Press:
London, 1984.
12. Ballesteros A, Bornscheuer U, Capewel A, Combes D, Condoret
JS, Koenig K, Kolisis FN, Marty A, Mengue U, Scheper T,
Stamatis H, Xenakism A. Biocatal. Biotransform. 1995; 13: 1–42.
13. Liou JY, Huang TM, Chang GG. J. Chem. Soc., Perkin Trans. 2
1999; 10: 2171–2176.
14. Correa NM, Durantini EN, Silber JJ. J. Org. Chem. 1999; 64:
5757–5763.
15. Correa NM, Durantini EN, Silber JJ. J. Org. Chem. 2000; 65:
6427–6433.
Synthesis of the product. FNBA (57 mg, 0.308 mmol)
was stirred with PIP (500 ml, 5.88 mmol) in tetrahydro-
furan (5 ml) for 24 h at room temperature. The reaction
mixture was partitioned between water and dichloro-
methane–methanol (10%) and neutralized with HCl.
The volatiles of the organic phase were evaporated under
vacuum and afforded 74 mg (96%) of pure product. TLC
(silica gel), RF (dichloromethane–methanol, 10%) ¼
16. (a) Zingaretti L, Boscatto L, Chiacchiera SM, Silber JJ. ARKI-
VOC 2003; 189–200; (b) Correa NM, Durantini EN, Silber JJ.
Atual. Fisicoquim. Org. Brasil. 2000; 12: 96–118.
17. Broxton TJ, Christie JR, Chung RP-T. J. Org. Chem. 1988; 53:
3081–3084.
18. Broxton TJ, Christie JR, Sango X. J. Org. Chem. 1989; 54: 1919–
1922.
19. Loupy A, Tchoubar B. Salt Effects in Organic and Organome-
tallic Chemistry. VCH: New York, 1992.
20. Bernasconi CF. MTP Int. Rev. Sci. Org. Chem., Ser. 1 1973; 3:
33–63.
21. Bunton CA, Quina FH, Romsted LS. Acc. Chem. Res. 1991; 24:
357–364.
22. Cuccovia IM, Romsted LS, Chaimovich H. J. Colloid Interface
Sci. 1999; 220: 96–102.
23. Das PK, Srilakshimi GV, Chaudhuri A. Langmuir 1999; 15: 981–
987.
1
0.42. H NMR (DMSO-d6, TMS), ꢁ (ppm): 1.48 (m,
6H, CH2); 3.01 (t, 4H, CH2, J ¼ 5.0 Hz); 7.29 (d, 1H, Ar,
J ¼ 9.1 Hz); 7.98 (dd, 1H, Ar, J ¼ 2.2, 8.8 Hz); 8.25 (d,
1H, Ar, J ¼ 2.2 Hz); 9.80 (br, 1H, COOH). FAB-MS:
m/z 250 (Mþ ) (250.0954 calculated for C12H14N2O4).
24. Cuccovia IM, Dias LG, Maximiano FA, Chaimovich H. Langmuir
2001; 17: 1060–1068.
25. Iglesias E, Leis JR, Pen˜a ME. Langmuir 1994; 10: 662–669.
26. Terrier F. Nucleophilic Aromatic Displacement. VCH: New York,
1991.
Acknowledgments
27. Nudelman NS. In The Chemistry of Amino, Nitroso, Nitro and
Related Groups, Patai S (ed). Wiley: New York, 1996; Chap. 26.
28. Bernasconi CF, Rossi RH. J. Org. Chem. 1976; 41: 44–49.
29. Correa NM, Biasutti MA, Silber JJ. J. Colloid Interface Sci. 1996;
184: 570–578.
We gratefully acknowledge the financial support of this
´
work by the Consejo Nacional de Investigaciones Cientı-
ficas (CONICET), Secretarıa de Ciencia y Tecnica de la
´
´
´
´
Universidad Nacional de Rıo Cuarto, Fundacion An-
torchas and Agencia Nacional de Promocion Cientıfica
30. Lang J, Jada A, Malliaris A. J. Phys. Chem. 1988; 92: 1946–1953.
´
´
31. Garcıa-Rıo L, Leis JR, Pen˜a E, Iglesias E. J. Phys. Chem. 1993;
97: 3437–3442.
´
´
´
y Tecnologica. All authors are Scientific Members of
CONICET.
32. Correa NM, Biasutti MA, Silber JJ. J. Colloid Interface Sci. 1995;
172: 71–76.
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 121–127