Non-aqueous reverse micelles media for the SNAr reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

Article abstract of DOI:10.1002/poc.1042

The kinetics of the nucleophilic aromatic substitution (S_NAr) reaction between 1-fluoro-2,4-dinitrobenzene (FDNB) and piperidine (PIP) in ethylene glycol (EG)/ sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n-heptane and dimethylformamide (D

Full text of DOI:10.1002/poc.1042

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 805–812
Published online 22 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1042
Non-aqueous reverse micelles media for the S Ar
N
reaction between 1-fluoro-2,4-dinitrobenzene
y
and piperidine
N. Mariano Correa, Edgardo N. Durantini and Juana J. Silber*
Departamento de Qu ı´ mica, Universidad Nacional de R ı´ o Cuarto, Agencia Postal Nro 3, X5804BYA R ı´ o Cuarto, C o´ rdoba, Argentina
Received 28 October 2002; revised 3 February 2006; accepted 6 February 2006
ABSTRACT: The kinetics of the nucleophilic aromatic substitution (S Ar) reaction between 1-fluoro-2,4-
N
dinitrobenzene (FDNB) and piperidine (PIP) in ethylene glycol (EG)/ sodium bis (2-ethyl-1-hexyl) sulfosuccinate
(
AOT)/n-heptane and dimethylformamide (DMF)/AOT/n-heptane non-aqueous reverse micelle systems is reported.
EG and DMF were used as models for hydrogen bond donor (HBD) and non-hydrogen bond donor (non-HBD) polar
solvents, respectively. The reaction was found not to be base catalyzed in these media. A mechanism to rationalize the
kinetic results is proposed in which both reactants may be distributed between the two environments. The distribution
constants of FDNB between the organic and each micellar pseudophases were determined by an independent
fluorescence method. These results were used to evaluate the amine distribution constant and the intrinsic second-
order rate coefficient of the S Ar reaction in the interface. The reaction was also studied in the pure solvents EG and
N
DMF for comparison. The results in EG/AOT/n-heptane at W ¼ 2 give similar kinetic profiles than in water/AOT/n-
s
hexane at W ¼ 10. With these HBD solvents, the interface saturation by the substrate is reached at around the same
0
value of [AOT] and the intrinsic second-order rate coefficient in the interface, k , has comparable values. On the other
b
hand, when DMF is used as a polar non-HBD solvent, the intrinsic second-order rate constant increases by a factor of
about 200 as compared to the values obtained using HBD solvents as a polar core. It is concluded that higher catalytic
power is obtained when non-HBD solvents are used as polar solvent in the micelle interior. Copyright # 2006 John
Wiley & Sons, Ltd.
KEYWORDS: reverse micelles; AOT; micellar catalysis; waterless microemulsions; aromatic nucleophilic substitution
INTRODUCTION
diminish the energy of the transition state of the reaction
affecting the intrinsic rate constant for bimolecular
reactions and, can provide different places where the
reactants can be located. Solubilization of the reactant in
the same region of the surfactant assembly can lead to
significant acceleration of reaction rates due to a
‘concentration’ effect, while the rates of reactions of
segregated reactants are retarded. When both reactants are
in the polar core, they are concentrated as in a
nanoreactor, and since the size of this reactor is easily
In recent years, attempts have been made to prepare and
study waterless microemulsions. In this sense, polar
solvents having high dielectric constants and low
miscibilities with hydrocarbon solvents have been
employed to replace water. The most common polar
solvents used include formamide (FA), dimethylforma-
mide (DMF), dimethylacetamide (DMA), ethylene glycol
1
2–12
(
of these studies have been focused on phase diagrams,
EG), propylene glycol (PG) and glycerol (GY).
Most
3
varied, the influence of the properties of the micellar
19,20
4
viscosity and conductivity behavior, dynamic light
system is relatively simple to assess.
Moreover,
scattering measurements to determine micellar sizes
Also, dyes absorption
reverse micellar systems are of interest as reaction media
because they are powerful models for biological
21–24
systems.
5,6,8
and intermicellar interactions.
5,9,10,13-15
6,12,16,17
1
or H-
or emission spectra,
and FTIR
spectroscopy have been used to characterize
1
2,16
NMR
microenvironments.
In principle, reverse micellar system can affect the
In spite of the importance of non-aqueous reverse
micellar systems, as potential catalytic media mainly, for
reactions that involve reagents that may react with
18
25
reaction rates by two main processes. They can rise or
water, systematic studies of aromatic nucleophilic
substitution (S Ar) reactions using neutral nucleophiles
N
in these media have not been previously performed. In
previous works, we have been interested in the influence
of cationic and anionic reverse micelles on the
bimolecular S Ar reactions between halo nitro substi-
N
tuted aromatic substrates and aliphatic amines. The
*
Correspondence to: J. J. Silber, Departamento de Qu ´ı mica, Univer-
sidad Nacional de R ´ı o Cuarto, Agencia Postal Nro 3, X5804BYA R ´ı o
Cuarto, C o´ rdoba, Argentina.
E-mail: jsilber@exa.unrc.edu.ar
This article is published as part of the special issue Festschrift for
Norma Nudelman.
y
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

8
06
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
results showed that there is a change in the mechanism
and the reaction is faster in the micellar media than in the
non-polar pure solvent. At this time, we are
interested in investigating how the AOT non-aqueous
amount of the stock solution to obtain a given
concentration of surfactant in the micellar solution was
transferred into the cell. The addition of the polar solvent
to the corresponding solution was performed using a
calibrated microsyringe. The amount of EG or DMF
present in the system is expressed as the molar ratio
between these polar solvents and the surfactant present in
26–29
reverse micelles affect the mechanism of the S Ar
N
reactions.
In this paper, we report for the first time data on the
kinetics of the S Ar reactions between 1-fluoro-2,4-
the reverse micellar solution (W ¼ [polar solvent]/
N
s
dinitrobenzene (FDNB) and piperidine (PIP) in EG/sodium
bis (2-ethyl-1-hexyl)sulfosuccinate (AOT)/n-heptane and
DMF/AOT/n-heptane in non-aqueous reverse micelle
systems. EG and DMF were used as models for hydrogen
bond donor (HBD) and non-hydrogen bond donor (non-
HBD) polar solvents, respectively. The distribution
constant of FDNB between the organic and the micellar
pseudophases were determined by an independent
fluorescence method and used to evaluate the amine
distribution constant and the intrinsic second-order rate
[surfactant]).
Kinetics
Reactions were followed spectrophotometrically by the
increase in the maximum absorption band of the product,
N-(2,4-dinitrophenyl) PIP, at 32.0 ꢀ 0.5 8C. To start a
kinetic run, a stock solution of FDNB was added (10 ml)
into a thermostatized cell containing the PIP and the
reverse micellar solution. The FDNB concentration in
coefficient of the S Ar reaction in the interface. A
N
mechanism to rationalize the kinetic results is proposed.
ꢂ4
the reaction media was 1 ꢁ 10 M. The kinetic runs were
performed following the increase in the absorbance of the
reaction product (lmax ¼ 385 nm). In every case, pseudo-
first-order plots were obtained in excess of PIP. The
pseudo-first-order rate constants (kobs) were obtained by a
non-linear least-squares fit of the experimental data
absorbance versus time, (r > 0.999) by first-order rate
equation. The value of the absorbance at infinite reaction
time was consistent with the value obtained from
authentic samples of the reaction product, within 3%.
The pooled standard deviation of the kinetic data, using
different prepared samples, was less than 5%.
Experimental general
UV-visible spectra were recorded on a Shimadzu UV-
2
401PC using 1 cm path length quartz cells. The kinetics
were recorded on a Hi-Tech Scientific Stopped-Flow
SHU SF-51 (SU-40 spectrophotometer Unit) thermo-
statized at 32.0 ꢀ 0.58C. The HPLC measurements were
performed on a Varian 5000 liquid chromatograph
equipped with a UV-visible variable l detector (Varian
2
550) operating at 250 nm with a Varian MicroPak SI-5
(150 mm ꢁ 4 mm i.d.) column and 1% 2-propanol in
n-hexane as solvent.
Determination of the partition constant (K ) of
s
FDNB in the micellar systems
Materials
The binding constant of FDNB to the reverse micellar
pseudophase was evaluated using the Encinas–Lissi’s
31,32
fluorescence quenching method.
0
FDNB from Aldrich and PIP from Riedel-de Haen were
used without further purification. Sodium bis (2-ethyl-1-
hexyl) sulfosuccinate (AOT) from Sigma (more than 99%)
was dried under reduced pressure and was kept under
vacuum over P O until it was used. The UV-visible of 1-
Tris(2,2 -bipyridi-
ne)ruthenium(II) (Ru(bpy) ) which is quenched by
þ2
3
nitroaromatic compounds was used as the fluorescence
33
probe.
2
5
methyl-8-oxyquinolinium betaine in the presence of AOT
reverse aggregates in n-heptane showed that the surfactant
RESULTS
30
is free of acidic impurities. n-Heptane (Sintorgan, HPLC
quality), DMF and EG, from Aldrich (more than 99%
purity), were used as received. Ultrapure water was
obtained from Labconco equipment Model 90901-01.
As it was previously found in other reverse micellar
systems,
26,27
the reactions of FDNB with PIP in EG and
DMF homogeneous solutions and in reverse micelles,
EG/AOT/n-heptane and DMF/AOT/n-heptane, produce
the corresponding ipso-fluorine substitution product in
quantitative yields as shown by UV-visible spectroscopic
and HPLC analysis of the reaction mixture.
Procedures
Stock solutions of reverse micelles were prepared by
weighing and dilution in n-heptane. Stock solution of
0.5 M surfactant was agitated in a sonicating bath until the
reverse micelle was optically clear. The appropriate
In every case a large excess of PIP was used and the
reactions follow pseudo-first-order kinetic reaction. The
spectra at different reaction progresses show a clear
isosbestic point evidencing the cleanness of the reactions.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

NON-AQUEOUS REVERSE MICELLES MEDIA
807
Table 1. Kinetic and solvents parameters for the reaction of
FDNB with PIP in homogeneous media at 32 8C
0
ꢂ1 ꢂ1 a
00
ꢂ1 -2 a
ꢃb
b
b
Solvent
k (s
M
)
k (s M )
p
a
b
EG
DMF
Water
n-Hexane
13.8 ꢀ 0.7
256 ꢀ 13
2.5 ꢀ 0.1
—
—
—
—
171 ꢀ 8
0.92 0.9 0.75
0.88 0.73
1.09 1.17 0.47
ꢂ0.08
0
c
0
0
a
See Eqn (4).
from Ref. 40.
from Ref. 26.
b
c
When it is attempted to elucidate how the microheter-
ogeneous media affect the S Ar reaction several variables
were investigated as follows.
N
Reaction in neat EG and DMF
The reaction between FDNB and PIP was first studied in
homogeneous solutions of EG and DMF. In both solvents
there is a linear dependence on kobs with the PIP
concentration (See supplementary material, Fig. 1S for
EG). The kinetic results in both solvents are gathered in
0
Table 1, where k values are the second-order rate
constants. Also, for comparison the results previously
obtained for the same reaction in water and n-heptane are
26
included in Table 1. The reaction is faster in DMF than
EG and water.
Figure 1. Dependence of kobs with the AOT concentration,
for the reaction between FDNB and PIP in: (A) EG/AOT/
n-heptane and (B) DMF/AOT/n-heptane. [FDNB] ¼
Reaction in the non-aqueous AOT/n-heptane
microemulsions
ꢂ4
1
.0 ꢁ 10 M, [PIP] ¼ 0.015 M, W ¼ 2. The dashed line
s
shows the fitting by Eqn (11)
Effects of AOT concentration. The kinetics of the
reaction was studied varying AOT concentrations
between 0 and 0.5 M, keeping the other experimental
respectively. In both cases, kobs increases linearly with the
[PIP].
conditions, W , and [PIP] constant. Figure 1A and B
s
Effect of polar solvent dispersed. The effects of
changing the value of W on k keeping AOT and PIP
concentrations constant, is shown in Fig. 3A and B for
EG/AOT/n-heptane and DMF/AOT/n-heptane, respect-
ively. As it can be observed from Fig. 4A and B, the value
shows the kinetic results at W ¼ 2, for EG/AOT/n-
s
s
obs
heptane and DMF/AOT/n-heptane, respectively. As can
be observed from Fig. 1A for EG sequestrated inside the
aggregate, kobs increases on increasing AOT concen-
tration until approximately 0.1 M and, the plots show a
downward curvature. After this AOT concentration value,
kobs change very little with the surfactant concentration.
On the other hand, when DMF is used as the polar solvent
of kobs increases almost linearly with W in the whole
s
range measured. It must be pointed that the maximum Ws
that can be reached is around 4 for DMF and 2.4 for EG at
13,14
28C.
3
(
Fig. 1B) kobs increases on increasing AOT concentration
in the whole range studied. These results are reflecting
that the saturation by FDNB of the micellar interface is
achieved for EG reverse micelles while this is not reached
in the DMF ones at least in the surfactant concentration
range used.
Determination of the partition constant (K ) of
s
FDNB in the micellar systems. A question can be
raised regarding if polar solvents relatively soluble in
n-heptane such as DMF are preferably associated to the
5
micellar pseudophase under all the conditions employed.
However, there are studies that support the assumption
that the sequestrated polar solvent/AOT ratio is indepen-
dent of the AOT concentration at a fixed analytical W
value. Previously we have shown that the spectra of a
probe totally associated to the reverse micelles, change
Effect of amine concentration. To study the effect of
PIP concentration, the reaction was carried out using
s
9
0
results for EG/AOT/n-heptane and DMF/AOT/n-heptane,
.3 M of AOT at W ¼ 2. Figure 2A and B shows the
s
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

8
08
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
Figure 3. Variation of kobs with W , for the reaction between
s
Figure 2. Dependence of kobs with the PIP concentration,
for the reaction between FDNB and PIP in: (A) EG/AOT/
FDNB and PIP in: (A) EG/AOT/n-heptane and (B) DMF/AOT/n-
ꢂ4
h0 .e 3p Mt ane. [FDNB] ¼ 1.0 ꢁ 10 M, [PIP] ¼ 0.015 M, [AOT] ¼
n-heptane and (B) DMF/AOT/n-heptane. [FDNB] ¼ 1.0 ꢁ
ꢂ4
1
the fitting by Eqn (11)
0
M, [AOT] ¼ 0.3 M, W ¼ 2. The dashed line shows
s
with W but not with the AOT concentration (at fixed W )
s
s
Ks
for every solvent considered in the present work. This
indicates that the properties of the micelles, at a given
analytical W value, are not dependent upon the AOT
FNDB þ AOT
Ð FNDBb
(1)
(2)
f
½FDNB ꢄ
s
b
K ¼
s
concentration, implying that most of the polar solvent
must be associated to the micellar pseudophase. More-
½
FDNB ꢄ½AOTꢄ
f
5
over, as Riter et al. have concluded, at small values of W
Where [FDNB] is the analytical concentration of the
b
s
the polar solvent appears to remain preferentially inside
the micelles, even when they are relatively soluble in n-
heptane such as DMF.
substrate incorporated to the micelles, [FDNB] is the
f
concentration of the substrate in the non-polar organic
solvent, and [AOT] is the micellized surfactant (total
surfactant concentration minus the critical micellar
Consequently the distribution of the reactants between
the micelle and the non-polar organic solvent is defined
ꢂ4
concentration ffi10 M). This equation applies at a fixed
34
using the pseudophase model. This model considers the
microaggregates as a distinct pseudophase whose proper-
ties is independent of the AOT concentration and is only
determined by the value of the characteristic parameter
value of W and when [FDNB] ꢅ [AOT].
T
According to the Encinas–Lissi’s fluorescence quench-
the distribution constant can be straight-
forwardly obtained by measuring the quenching of a
31,32
ing method
þ2
W . In this model, only two solubilization sites are
s
fluorophore anchored at the AOT interface. Ru(bpy)
35,36
belongs to this kind of fluorophores
3
and is quenched
considered, that is, the external non-polar solvent and the
micellar interface (i.e., all the surfactant molecules). In
this way, the distribution of FDNB between the micelles
and the external solvent pseudophase defined in Eqn (1) is
33
by FDNB as it was previously found. The intensity of
þ2
the Ru(bpy)3 emission is not affected with the addition
of EG and DMF to the reverse micellar solution (results
expressed in terms of the partition constant K shown in
s
not shown). This method was used to determine K for the
s
Eqn (2):
micellar systems used at W ¼ 2 as it was previously
s
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

NON-AQUEOUS REVERSE MICELLES MEDIA
809
DISCUSSION
Mechanism of reaction
For secondary (and primary) amines as nucleophiles the
general mechanism accepted
37–39
for S Ar reactions
N
involving halogen as leaving groups can be represented
by Eqn (3).
(3)
Where B is the nucleophile or any other base added to the
i
reaction medium. Application of the steady state
hypothesis to this mechanism, and in the limiting
situation when k ꢆ k þ k [B ] Eqn (4) is obtained,
Figure 4. (A) Effect of FDNB addition upon the fluorescence
þ2
intensity from Ru(bpy)3 in EG/AOT/n-heptane reverse
þ2
ꢂ6
micelles at W ¼ 2. [Ru(bpy) ] ¼ 6.85 ꢁ 10 M. lexc
¼
S
3
ꢂ1
2
3
i
4
12 nm. l ¼ 616 nm. [AOT]/M: & 0.05, * 0.10, ~
em
0
00
0
.15, ! 0.20. (B) Analytical concentration of FDNB needed
kobs=½B ꢄ ¼ k ¼ k þ k ½B ꢄ
i
A
i
(4)
to give different I /I values as a function of the AOT concen-
0
tration. I /I values: 1.47 and 1.33
Where kobs is the observed pseudo-first-order constant, kA
0
0
is the second-order rate constant, k ¼ k k /k
and
1
2
ꢂ1
0
0
k ¼ k k /k . In this case the decomposition of Z is
1
3
ꢂ1
rate limiting and base catalysis may be expected. A linear
response to base concentration such as depicted in
Eqn (4) is characteristic of the majority of base-
catalyzed reactions. On the other hand, if k ꢅ k þ
reported for the distribution constant of 9-anthracene-
methanol in the same non-aqueous reverse micelles
14
systems.
ꢂ1
2
Typical results are shown for FDNB in Fig. 4A for EG/
AOT/n-heptane. The observed effect decreases as the AOT
concentration is increased as expected for the distribution
of the molecule between the micellar pseudophase and the
organic bulk. Similar data were obtained in DMF/AOT/n-
heptane (not shown). Figure 4B shows the plots of the
k [B ] or more precisely k ꢆ k , the formation of the
A 1
3
i
2
ꢂ1
intermediate Z is rate limiting and consequently k ¼ k
and a plot of k versus [B ] gives a linear response. There
obs
i
are intermediate situations where curvilinear dependence
3
7–39
of k with amine concentration may be found.
A
In
other words, the intermediate Z stabilization by the
medium is crucial for the reaction pathway. In non-polar
solvents where the Z zwitterionic intermediate is less
stabilized than in polar organic solvents, large value of
analytical FDNB concentration needed to reach a given
þ2
value of I /I ratio (where I and I are the Ru(bpy)
o
o
3
fluorescence intensity, in the absence and in the presence of
FDNB, respectively) against the AOT concentration. The
intercepts of these plots correspond to the FDNB
k
is expected with the consequent base catalysis
ꢂ1
39
observe in this media.
In homogeneous media, as it was previously showed,
the decomposition of Z is rate limiting for the reaction in
2
6
remaining in n-heptane ([FDNB ], while the slope is the
f
FDNB incorporated into the micellar pseudophase per
mole of surfactant ([FDNB ]). In this way, the value of the
b
the non-polar solvent (Table 1). On the other hand, the
mechanism of S Ar reactions can change when polar
distribution constant could be calculated as K ¼ slope/
s
N
31,32
intercept.
The values of K obtained at W ¼ 2 where of
solvents are used the formation of Z being the rate-
26,39
determining step. From Table 1, it can be seen that the
reaction in water, EG, and DMF which are highly polar
s
s
ꢂ1
ꢂ1
10 ꢀ 1 M for EG/AOT/n-heptane and 0.8 ꢀ 0.1 M for
DMF/AOT/n-heptane.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

8
10
N. M. CORREA, E. N. DURANTINI AND J. J. SILBER
ꢃ 40
(
as measured its high values of p ) are not base
A simple mass balance using the distribution constant
K defined in Eqn (2) and the analytical concentration
catalyzed. Moreover, the reaction rates are considerable
faster in non-HBD (with a ¼ 0) solvents than in HBD
ones such as water and EG. This can be explained
considering that in HBD solvents amines are highly
solvated through hydrogen bonding. In non-HBD
solvents, this interaction is not longer present with the
corresponding increase in the amine nucleophilicity.
These results are consistent with the fact that the reaction
in water, which has the higher value of a, is slower than in
the other polar solvents.
S
of FDNB, [FNDB ], allows to calculate the [FDNB ]
T
b
[Eqn (7)].
K ½AOTꢄ½FDNB ꢄ
S
T
½
FDNB ꢄ ¼
(7)
b
ð1þK ½AOTꢄÞ
S
In the same way, using the distribution constant defined
by Eqn (8), [PIP ] can be expressed by Eqn (9)
b
½
PIP ꢄ
b
K ¼
(8)
(9)
A
According to the results showed before and since both
reactants may be distributed between the two environments,
the interface and the bulk organic solvent, a mechanism
summarized Eqn (5) can be proposed.
½PIP ꢄ½AOTꢄ
f
K ½AOTꢄ½PIP ꢄ
A
T
½
PIP ꢄ ¼
b
ð1þK ½AOTꢄÞ
A
If [PIP ] ꢆ [FDNB ] a pseudo-first-order behavior for
T
T
the kinetics of the reaction is assumed. Then, replacing
FDNB ] and [PIP ] in Eqn (6), we can obtain the final
expression for the rate [Eqn (10)] and the observed
[
b
b
pseudo-first-order rate constant kobs [Eqn (11)].
d½Pꢄ
¼
kobs½FDNBTꢄ
(10)
dt
ð5Þ
with
0
b
Where the subscripts and superscripts f and b indicate
the non-polar organic phase and the micellar pseudo-
phase, respectively. AOT represents the micellized
surfactant molecules. The rate coefficients for the reaction
k , k , k , and k were defined above [Eqn (3)]. K and
KA are the distribution constant for FDNB and PIP
between the organic phase and micellar pseudophase,
respectively.
k þðk K K ½PIP ꢄ½AOTꢄ=vÞ
f
S
A
T
kobs
¼
(11)
ð1þK ½AOTꢄÞð1þK ½AOTꢄÞ
S
A
The variation of kobs with the [AOT] can now be
explained from Eqn (11). As can be observed when the
values of the products between the distribution constants
and [AOT] are not negligible respect to unity, kobs would
exhibit a non-linear relationship with the surfactant
concentration being a value of [AOT] where the saturation
1
ꢂ1
2
3
S
As it was previously discussed, the reaction is wholly
base catalyzed in n-heptane while is not in polar solvents.
The micropolarity of the interface in non-aqueous
reversed micelles of EG/AOT/n-heptane and DMF/
AOT/n-heptane is always higher than in the organic
2
6
of the interface is reached. This is the case of EG/AOT/
ꢂ1
n-heptane (Fig. 1A) where K ¼ 10 M . On the other
S
hand, if K and K are small enough, this product are
A
S
almost negligible respect to unity and micellar interface
saturation is not reached as found for DMF/AOT/n-
13
solvent. Thus, it can be assumed that the reactions of
FDNB with PIP are not base catalyzed in the non-aqueous
reverse micelles systems. Consequently, the formation of
ꢂ1
heptane (Fig. 1B) were K ¼ 0.8 M .
S
The intrinsic second-order rate constant of the reaction
in the organic solvent, k , is known from the studies of
these reactions in n-hexane, assuming similar charac-
teristics of this solvent with n-heptane. By fitting the
experimental data with Eqn (11) (Fig. 1A and B) the
the intermediate Z is assumed to be rate limiting.
b
f
26
The rate of the reaction can be expressed by Eqn (6).
d½Pꢄ
dt
½PIP ꢄ½FDNB ꢄ
0
b
b
¼
k ½PIP ꢄ½FDNB ꢄþk
(6)
f
f
f
b
0
v½AOTꢄ
values of k
b
and K
gathered in the Table 2.
A
can be obtained and the results are
Where k represents the intrinsic second-order rate constant
f
On the other hand, linear relationships of kobs with the
nucleophile concentration as shown in Fig. 2A and B, are
indicating the lack of base catalysis at both non-aqueous
micellar pseudophase. These are expected results con-
sidering that the micellar interfaces are more polar than
00
in the non-polar organic solvent (k ¼ k [PIP ]). For
f
f
absolute comparison of reactivity in different media, the
molar reaction volume at the interface, v, should be known.
This can be estimated from the molar volume of AOT in the
ꢂ1 41,42
reverse micelles, which can be taken as v ¼ 0.38 M .
the organic medium with the corresponding Z stabiliz-
13
ation. By introduction of the proper values of K , the
0
Thus, k is the conventional intrinsic second order rate
b
A
0
constant in the interface. The concentrations in square
brackets correspond to the analytical concentration referred
to the total volume of reverse micelle solution.
values of k were recalculated for each system at W ¼ 2
b
s
by fitting a different set of experimental data (Fig. 2A and
B) by Eqn (11). The results are compared in the Table 2.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812

NON-AQUEOUS REVERSE MICELLES MEDIA
811
Table 2. Kinetic parameters and distribution constants for
the reaction of FDNB with PIP in different non-aqueous
reverse micelles media at 328C
was recently found for the partition of 9-anthracene-
methanol and acridine orange base in different non-
aqueous AOT reverse micelles system using polar
14,15
0
ꢂ1 ꢂ1
ꢂ1
solvents such as EG and DMF.
The value of the solvatochromic parameters for PIP,
Medium
k
(s
M
)
K (M
A
)
b
a
a
40
EG/AOT/n-heptane W ¼ 2
20 ꢀ 1
1.6 ꢀ 0.3
a ¼ 0.10 and b ¼ 1.04 show that this amine is a very
s
b
1
9 ꢀ 1
good hydrogen bond acceptor (HBA). Recently it has
been shown that the main driving force in the partition of
c
c
DMF/AOT/n-heptane W ¼ 2
3314 ꢀ 100
0.10 ꢀ 0.02
5.4 ꢀ 0.3
s
d
3
562 ꢀ 100
43
e
amines in aqueous reverse micelles is the HBD ability
of the solute. Consequently, a larger value of KA is
expected when HBD solvents such as water and EG are
used in the polar core of the aggregates as is actually
found.
Water/AOT/n-hexane W ¼ 10
14.3 ꢀ 0.6
0
a
From fitting Fig. 1A by Eqn (11).
from fitting Fig. 2A by Eqn (11).
from fitting Fig. 1B by Eqn (11).
b
c
d
from fitting Fig. 2B by Eqn (11). Parameter values calculated using 0.995
confidence level in non-linear regression.
In summary, the work shows the possibility of using
non-aqueous reverse micelles as catalytic media for S Ar
e
From Ref. 26.
N
reactions when amines are use as nucleophiles. It is
concluded that higher catalytic power is obtained when
non-HBD solvents are used as polar solvent in the micelle
interior.
0
As can be seen good estimates for k within experimental
b
error are obtained by two independent methods.
The results show similar kinetic profiles in EG/AOT/n-
26
heptane at W ¼ 2 than in water/AOT/n-hexane at W¼ 10.
s
With these HBD solvents, the interface saturation by the
substrate is reached at around the same value of [AOT] and
0
Acknowledgements
k has comparable values. These are probably reflecting the
b
fact that EG/AOT/n-heptane at W ¼ 2 has similar proper-
s
We gratefully acknowledge the financial support for this
work by the Consejo Nacional de Investigaciones Cien-
t ´ı ficas (CONICET), Secretar ´ı a de Ciencia y T e´ cnica de la
ties than the reverse micelles made of water/AOT/n-heptane
1
3
at W ¼ 10 as observed with optical probes.
On the other hand, when DMF is used as polar solvent,
0
Universidad Nacional de Rı
´o Cuarto, Fundaci o´ n
there is an increase of around 200 times on k b in
Antorchas and Agencia Nacional de Promoci o´ n Cient ´ı fica
y Tecnol o´ gica. NMC, END and JJS are Scientific Member
of CONICET.
comparison with the values obtained using HBD
solvents as polar core (Table 2). Moreover, the rate of
reaction rates in neat EG (Table 1) and in its
microemulsions is very similar while with DMF there
is an increase of about 15 times in the microheter-
ogeneous medium. The hydrogen bond specific inter-
action between PIP and HBD solvents may reduce the
amine nucleophilicity. On the contrary, the solvation of
the amine through that kind of interaction is not possible
when DMF is used as polar solvent, thus it is expected
that an increase its nucleophilicity. Moreover, previous
studies using 1-methyl-8-oxyquinolinium betaine (QB)
as molecular probe showed that it senses a more polar
environment in DMF/AOT/n-heptane reverse micelle
than in the pure solvent even at the maximum Ws
reached. This indicated that the solvent constrained in
the droplet is peculiarly structured probably by strong
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Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 805–812