Michael addition and ester aminolysis in w/o AOT-based microemulsions

Article abstract of DOI:10.1039/b507190a

A kinetic study was carried out on the aminolysis of 4-nitrophenylacetate (NPA) by N-methylbenzylamine (NMB) and the Michael addition of piperazine to N-ethylmaleimide (NEM) in AOT-isooctane-water microemulsions. The experimental results show that the rat

Full text of DOI:10.1039/b507190a

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P A P E R
Michael addition and ester aminolysis in w/o AOT-based
microemulsionsw
a
a
a
b
Esperanza Ferna
´
Moises Perez-Lorenzo
´ ´
ndez, Luı
´
a
s Garcı
´
a-Rı
´
o,* J. Ramo
´
n Leis, Juan C. Mejuto and
a
b
Departamento de Quı´mica Fı ´s ica, Facultade de Quı´mica, Universidade de Santiago de
Compostela, 15782 Santiago de Compostela, Spain. E-mail: qflgr3cn@usc.es;
Fax: þ34 981 595 012; Tel: þ34 981 563 100-14280
Departamento de Quı´mica Fı ´s ica, Facultade de Ciencias, Universidade de Vigo,
Campus de Ourense, 32004 Ourense, Spain
Received (in Durham, UK) 23rd May 2005, Accepted 12th October 2005
First published as an Advance Article on the web 28th October 2005
A kinetic study was carried out on the aminolysis of 4-nitrophenylacetate (NPA) by N-methylbenzylamine
(
NMB) and the Michael addition of piperazine to N-ethylmaleimide (NEM) in AOT–isooctane–water
microemulsions. The experimental results show that the rate constant observed for the aminolysis of NPA
increases together with the concentration of the surfactant and the water content of the system. Conversely,
the values of kobs obtained for the Michael addition to NEM decrease as the surfactant concentration
increases. This difference in behavior must be a consequence of the different distribution of the reagents
throughout the different microenvironments of the microemulsion. The application of the formalism of the
pseudophase shows that the aminolysis of NPA takes place solely in the interface of the microemulsion, and
the rate constant in this pseudophase increases along with the water content of the system, due to a greater
hydration of the interface. The different distribution of the reagents in the Michael addition of piperazine to
NEM makes it possible for the reaction to take place simultaneously in the interface and the aqueous
microdroplet, increasing the percentage of the reaction on the interface while the water content of the system
decreases. Thus, the difference in rate observed for the Michael addition between the interface and the
aqueous microdroplet is twelve-fold, due to the smaller polarity of the interface with regard to the aqueous
medium.
place simultaneously in one or many pseudophases. Our
research group has successfully applied the model of the
Introduction
Microemulsions are thermodynamically stable dispersions of
either water-in-oil (w/o) or oil-in-water (o/w), their stability
resulting from the presence of a suitable surfactant. The solutes
can be located in three different compartments: (i) the internal
aqueous core or water pool, (ii) the micellar interface formed
by a monolayer of surfactant molecules with their polar head
groups oriented toward the water pool, and (iii) the external
6
pseudophases to a large number of reactions. Turco Liveri
7
et al. have reached equivalent expressions using the fractions
in volume of each of the pseudophases of the microemulsion.
The model of the pseudophase has been successfully applied to
8
solvolysis reactions, which show the variation in the proper-
ties of the water present in the interior of the microemulsions.
Likewise it has been applied by other authors to explain
1
organic pseudophase. The use of microemulsions as a reaction
9
decarboxylation reactions, reactions of aromatic nucleophilic
medium has gained great importance in recent years. They
have been used to synthesize nanomaterials in their interior,
and in some cases a control of the size of the nanoparticles with
10
substitution and, in a modified version, electronic transfer
reactions.
11
In the present study we have successfully applied the model
of the pseudophase to the aminolysis reactions of p-nitrophe-
nylacetate (NPA) by N-methylbenzylamine (NMB) and to the
Michael addition of piperazine (PIP) to N-ethylmaleimide. In
both cases due to the solubility of the reagents in the different
pseudophases of the microemulsion NPA and NEM are dis-
tributed amongst the three pseudophases of the microemul-
sion. The amines present strongly differentiated solubility:
NMB is practically insoluble in water and is distributed
between the continuous medium and the interface of the
microemulsion, whereas PIP is insoluble in isooctane and
is distributed between the aqueous microdroplet and the
interface.
2
the composition of the microemulsion has been achieved. On
the other hand, due to the structure presented by the micro-
emulsions, compounds of a very different polarity will be able
to come into contact with each other in the interior. Likewise,
it has been observed that the use of microemulsions as a
reaction medium can induce regiospecificity in some organic
3
,4
reactions and constitutes an alternative to phase transfer
5
catalysts.
In order to improve the applications of microemulsions as
new reaction media it is necessary to have models which
explain the kinetic behavior of simple reactions in these media.
These models should take into account the distribution of
different reagents throughout the various pseudophases of
the microemulsion and the possibility of the reaction taking
Experimental
w Electronic supplementary information (ESI) available: Tables S1–S4
and Fig. S1. See DOI: 10.1039/b507190a
AOT (Aldrich) was kept in a vacuum dessicator for two days,
with no further purification. The microemulsions were
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
1
594
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0

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prepared by mixing isooctane (Aldrich), water and a solution
of AOT 1.00 M in isooctane (Aldrich), in appropriate propor-
tions. p-Nitrophenylacetate (Aldrich), N-methylbenzylamine
(
Aldrich), piperazine (Aldrich) and N-ethylmaleimide
(Aldrich) used were of the maximum purity available and did
not undergo further purification.
Scheme 2
The reaction between NPA and N-methylbenzylamine was
studied following the change in absorbance due to the appear-
ance of the reaction products at l ¼ 400 nm, using a spectro-
photometer Hewlett-Packard 8453. The reaction between
piperazine and N-ethylmaleimide was followed at 298 nm using
a spectrophotometer Varian Cary 50. In all the experiments the
behavior is consistent with that found in water and in other
1
polar solvents for ester aminolysis.
2
1. Aminolysis of NPA by NMB
A study has been carried out on the influence of the micro-
emulsion composition on kobs in the aminolysis of NPA by
N-methylbenzylamine (Scheme 2) , keeping constant the total
amine concentration with regard to the total volume of the
system, [NMB] ¼ 0.1 M, and varying the composition of the
microemulsion. For W values studied kobs shows a linear
dependence on amine concentration (see supplementary mate-
rialw)
Fig. 1 shows us by way of example how kobs varies with the
composition of the microemulsion for four values of W. These
results show that kobs increases together with the concentration
of AOT, tending to reach a limit value. Likewise kobs can be
observed to increase together with the water content of the
microemulsion.
ꢁ
5
ester concentration, [NPA] ¼ 5 ꢀ 10 M, is much smaller than
amine, [NMB] ¼ 0.1 M. For the reaction between PIP and
NEM the following concentrations were used: [PIP] ¼ 6.03 ꢀ
ꢁ
3
ꢁ4
1
0
M and [NEM] ¼ 4.21 ꢀ 10 M. In all the experiments
the temperature was kept constant at (25.0 ꢂ 0.1) 1C. The
kinetic absorbance-time data were always adjusted in accor-
dance with the first order integrated equation (r 4 0.999),
giving the values of the first order pseudo rate constants, kobs
which could be reproduced within a margin of 5%.
,
With the aim of determining the distribution constant of
w
NEM between the aqueous pseudophase and the oil, K
solution of NEM in water is prepared ([NEM] ¼ 5.59 ꢀ 10
o
, a
ꢁ
3
w
ꢁ3
M) and another solution in isooctane ([NEM]
o
¼ 5.59 ꢀ 10
M). The two solutions are mixed and shaken for at least 20
minutes. After the separation of the phases at 25 1C the NEM
present in each of the phases is determined. The distribution
constant between the aqueous phase and the organic phase is
This behavior can be explained from a qualitative point of
view: on increasing the concentration of the surfactant and the
water content of the microemulsion, the incorporation of the
reagents into the interface of the microemulsion is favored. In
turn the local concentration of both reagents increases.
w
o
w
o
defined as K
¼ [NEM]
w
/[NEM]
o
, whence K
¼ 0.12.
Results
2. Aminolysis of NEM by PIP
A study was carried out on the influence of the composition of
the microemulsion on the rate constant for both reactions.
Scheme 1 shows the composition of the w/o microemulsions
which were used for this study. The compositions were selected
with the aim of covering a wide interval: the content of AOT
varies between (12–65)% (w/w); that of water between (0.5–
To investigate the influence of the composition of the micro-
emulsion on the aminolysis of NEM by piperazine (Scheme 3),
experiments were carried out in which the composition of the
microemulsion was varied, while keeping the concentration of
ꢁ
4
NEM constant, [NEM] ¼ 4.21 ꢀ 10 M, and also that of PIP,
ꢁ
3
[PIP] ¼ 6.03 ꢀ 10 M, with regard to the total volume of the
4
4)% (w/w) and that of isooctane between (3–87)% (w/w).
system.
These composition intervals allow us to vary W, W ¼ [H O]/
Fig. 2 shows, by way of example, the results obtained for
W ¼ 5, 20 and 35. As we can observe, kobs decreases as the
concentration of the surfactant increases.
This behavior is contrary to that which is observed in the
aminolysis of NPA by NMB (see Fig. 1) and must be a
consequence of the different distribution of the reagents. As
the surfactant concentration increases, so too does the con-
2
[
AOT], between W ¼ 2–40. Hence we used microemulsions
which display a wide variation in properties such as polarity,
viscosity, hydrogen bond donation capacity, saline content,
etc.
In all cases it was found that the observed rate constant, kobs
,
presents a linear dependency on the nucleophilic concentration
when the reaction takes place in the microemulsion. This
Fig. 1 Influence of the composition of the microemulsion on kobs in
the aminolysis of NPA by N-methylbenzylamine. [NMB] ¼ 0.1 M
referred to the total volume of the system. T ¼ 25 1C. (K) W ¼ 2; (J)
W ¼ 7; (’) W ¼ 20 and (&) W ¼ 40.
Scheme 1
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0
1595

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Scheme 3
centration of piperazine in the interface of the microemulsion,
with the resulting decrease in its concentration in the aqueous
microdroplet. This decrease in concentration will cause a
decrease in the reaction rate, given that the rate in the aqueous
microdroplet must be greater than in the interface of the
microemulsion, as a consequence of its different solvation
capacity.
Scheme 4
Discussion
of the reaction rate as the polarity of the medium changes. In
solvents of a low polarity, such as isooctane, the reaction
Kinetic studies of reactions in water in oil (w/o) microemul-
sions can be interpreted in terms of reactivity, only if local
reagent concentrations and intrinsic rate constants in the
various microphases of these organized media can be obtained
from the overall, apparent rate data. To apply the pseudophase
formalism we must consider the microemulsion formed by
three strongly differentiated pseudophases: an aqueous pseu-
dophase (w), a continuous medium formed fundamentally by
the isooctane (o) and an interface formed fundamentally by the
surfactant (i). As is usual on studying reactivity in colloidal
systems (micelles, vesicles and microemulsions) the activity
coefficients of the components in these systems are independent
of their concentrations.
ꢂ
occurs by the formation of an addition intermediate, T , the
decomposition of which is catalyzed by bases in the rate
1
3
determining step of the process. As the polarity of the
medium increases, the slow step becomes the formation of
the intermediate, and consequently kobs shows a linear depen-
dency on the amine concentration.
The distribution of NPA throughout the different pseudo-
phases of the microemulsion (Scheme 4) is determined by the
following equilibrium constants:
½
NPAꢃ
½NPAꢃ
Z
NPA
wi
i
NPA
oi
i
K
¼
W
K
¼
ð1Þ
½
NPAꢃ
, [NPA] , and [NPA]
½NPAꢃo
w
where [NPA]
w
i
o
are the concentrations of
1
.
Aminolysis of NPA by NMB
NPA in the aqueous pseudophase, in the interface and in the
continuous medium, referred to the total volume of the micro-
emulsion. The parameter Z is defined as the molar relationship
Z ¼ [isooctane]/[AOT], by analogy with the parameter W. It is
possible to evaluate the concentrations of NPA in the aqueous
pseudophase and in the interface of the microemulsion con-
sidering that the total concentration of NPA can be expressed
as the sum of the concentration in each of the three pseudo-
phases of the system:
The experimental results obtained in the aminolysis of NPA by
N-methylbenzylamine can be interpreted from Scheme 4. The
low solubility of NMB in water allows us to exclude the
possibility that it is distributed across the three pseudophases
of the microemulsion. In this way NPA and NMB can only
come into contact in the continuous medium and in the inter-
face of the microemulsion. The possibility that the reaction
takes place in the continuous medium can be discarded due to
the existence of a linear dependency between kobs and the
amine concentration when the reaction takes place in micro-
emulsions (not shown). This result contrasts with the behavior
observed in pure isooctane where kobs presents a quadratic
dependency on the amine concentration. This different kinetic
behavior is a consequence of a change in the determining step
NPA
oi
K
NPA NPA
W½NPAꢃ
wi
T
½
NPAꢃw ¼ K
ð2Þ
NPA
NPA
K
þ K
Z þ Koi
W
oi
wi
NPA
ZKwi ½NPAꢃ
T
½
½
NPAꢃ ¼
o
NPA NPA
oi
NPA
wi
NPA
K
K
þ K
Z þ Koi
W
wi
ð3Þ
NPA NPA
K
K
½NPAꢃ
oi
NPA NPA
wi
T
NPAꢃ ¼
i
þ Kwi Z þ KNPAW
NPA
K
K
wi
oi
oi
where the concentrations are referred to the total volume of the
microemulsion. The eqn (3) can be simplified taking into
1
account the values obtained previously for Koi
4
NPA
and
NPA
NPA
NPA
K
wi (Koi
¼ 25.9 and Kwi ¼ 996):
NPA
oi
K
½NPAꢃ
T
½
NPAꢃ ffi
ð4Þ
i
NPA
oi
K
þ Z
From Scheme 4 we can obtain the following expression for the
rate constant observed, kobs
.
NPA
oi
K
NPA
0
kobs ¼ k
ð5Þ
i
K
þ Z
oi
0
where k is the first order rate pseudoconstant referred to the
i
interface. This rate constant can be expressed as a bimolecular
i
rate constant in the interface, k :
2
Fig. 2 Influence of the composition of the microemulsion on kobs in
the aminolysis of NEM by piperazine. [PIP] ¼ 0.1 M referred to the
total volume of the system. T ¼ 25 1C. (J) W ¼ 5; (K) W ¼ 10; (&)
W ¼ 20 and (’) W ¼ 35.
Vtot
Vi
½NMBꢃ
AOT
0
i
2
i
i
i
2
i
2
i
k ¼ k ½NMBꢃ ¼ k
½NMBꢃ ¼ k
ð6Þ
i
i
ꢀ
V
½AOTꢃ
1
596
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0

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i
where [NMB]
the interface of the microemulsion referred to the total volume
of the interface, while [NMB] is the concentration of the amine
i
is the concentration of N-methylbenzylamine in
i
in the interface of the microemulsion referred to the total
volume of the system. As is usual in microemulsions and direct
micelles, the interface has been considered to be consisting
solely of the surfactant, in such a way that the volume of the
interface is given by the volume occupied by AOT. In this way
ꢀ
V
volume of the surfactant, V
represents the total concentration of AOT referred to the total
volume of the microemulsion.
tot/V
i
¼ 1/ Vꢀ AOT[AOT] where VAOT represents the molar
ꢁ
1
ꢀ
¼ 0.34 M
and [AOT]
AOT
Taking into account the distribution equilibrium of N-
methylbenzylamine between the interface and the continuous
medium of the microemulsion, we can obtain the following
expression:
NMB
oi
K
NMB
½
NMBꢃ ¼
½NMBꢃT
ð7Þ
ð8Þ
ð9Þ
i
Fig. 4 Influence of W on the distribution constant of NMB in AOT–
isooctane–water microemulsions at 25 1C.
K
þ Z
oi
Combining eqn (5)–(7) we can write:
2
.
Aminolysis of NEM by PIP
i
2
NPA
NMB
oi
k
K
NPA
K
½NMBꢃT
oi
kobs ¼
The Michael addition of amines to NEM in water and in
15
various solvents is a well known process. To investigate the
ꢀ
þ ZÞ ðK oN iMB þ ZÞ
VAOT½AOTꢃ ðK
oi
influence of microemulsions on the rate of this process it is
necessary to propose a reaction scheme, Scheme 5. The in-
soluble nature of piperazine in isooctane means that piperazine
is found only between the interface and the aqueous pseudo-
phase of the system.
The rate equation can be reordered thus:
NPA
NMB
oi
NPA NMB
½
NMBꢃ
ðKoi þ ZÞðK
þ ZÞ
T
¼
ꢀ
i
kobsVAOT½AOTꢃ
k K
K
oi
2
oi
In the same way as we obtained the NPA concentrations in
the previous section, we can obtain the PIP concentrations:
This expression predicts the existence of a linear and quadratic
ꢀ
dependence of [NMB] /(kobsVAOT[AOT]) on parameter Z.
T
Fig. 3 shows a series of examples in which we can see the
existence of a good fit of the experimental results to the
previous equation.
PIP
wi
K
½PIPꢃ
W½PIPꢃ
wi
T
T
½
PIPꢃ ¼
½PIPꢃw
¼
ð10Þ
i
PIP
wi
PIP
K
þ W
K
þ W
From the adjustments carried out in Fig. 3, and using the
NPA
In accordance with the mechanism of Scheme 5, the reaction
will take place simultaneously in the interface and in the
aqueous microdroplet of the microemulsion. Therefore, on
the basis of equations analogous to [2]–[3] for NEM, we can
propose the following expression for the observed rate con-
value of Koi
distribution constant of N-methylbenzylamine between the
¼ 25.9, we can obtain the values of the
NMB
continuous medium and the interface, Koi , and of the rate
i
constant of the aminolysis in the interface, k . The values
2
NMB
obtained for Koi
us to calculate a mean value of K
are independent of W (see Fig. 4), allowing
¼ 29.1. However, the
stant, kobs
:
NMB
oi
i
0
NEM
oi
NEM
wi
0
NEM
NEM NEM
values obtained for k
2
present a clear variation with W, as
k K
W þ k K
K
w
i
oi
wi
kobs ¼
ð11Þ
shown in Fig. 5. As the water content of the system increases,
there is also an increase in the rate constant of aminolysis in the
interface, due to a progressive increase in the hydration of the
interface.
NEMK
NEM
K
þ K
Z þ Koi
W
oi
wi
0
0
where k
i
and k
w
are the first order rate pseudoconstants
referred to the interface and the aqueous microdroplet, respec-
tively. These rate constants can be expressed as bimolecular
Fig. 3 Representation of the experimental results obtained for the
aminolysis of NPA by N-methylbenzylamine in AOT–isooctane–water
microemulsions in accordance with the eqn (9). [NMB] ¼ 0.1 M
referred to the total volume of the system. T ¼ 25 1C. (K) W ¼ 2;
i
Fig. 5 Influence of W on Logk
methylbenzylamine in AOT–isooctane–water microemulsions. [NMB]
2
for the aminolysis of NPA by N-
(
J) W ¼ 7; (’) W ¼ 20 and (&) W ¼ 40.
¼ 0.1 M referred to the total volume of the system. T ¼ 25 1C
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0
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Scheme 5
i
2
rate constants in the interface, k
w
and in the aqueous micro-
droplet, k
2
, thus:
Fig. 6 Verification of the fit of the eqn (15) for the aminolysis of NEM
by PIP in microemulsions AOT–isooctane–water. [PIP] ¼ 6.03 ꢀ 10
ꢁ
3
Vtot
M referred to the aqueous pseudophase of the system. T ¼ 25 1C. (K)
W ¼ 15; (J) W ¼ 27 and (’) W ¼ 40.
0
i
2
i
i
i
2
k ¼ k ½PIPꢃ ¼ k
½PIPꢃi
i
Vi
i
PIP
k
K
½PIPꢃT
wi
PIP
2
¼
ð12Þ
ð13Þ
values, we can rewrite the eqn (16) as:
ꢀ
VAOT½AOTꢃ ðK
þ WÞ
wi
ð9:5 þ WÞð133 þ WÞ
i
2
PIP NEM
¼
k VH OK
K
wi wi
2
Vtot
w
Intercept
0
w
2
w
w
k
¼ k ½PIPꢃ ¼ k2
½PIPꢃw
w
VH O
2
w
þ k VAOTW
ð17Þ
2
w
2
k
½PIPꢃT
wi
¼
ꢀ
PIP
This expression predicts the existence of a linear dependency
between (9.5 þ W)(133 þ W)/Intercept and the parameter W of
the composition of the microemulsion. Fig. 7 shows the
behavior observed for the aminolysis of NEM by piperazine.
As we can observe, there exists a clear linear dependency for
values of W 4 10, while there is a negative divergence of the
linearity for W o 10. From this linear adjustment, and
VH O½AOTꢃ ðK
þ WÞ
2
i
i
w
w
where [PIP] and [PIP] correspond to the concentrations of
PIP in the interface and in the aqueous microdroplet referred
to the volume of these pseudophases. These concentrations are
referred to the total volume of the system, taking into account
just the volumes of the pseudophases. Combining the eqn (11)–
NEM
wi
NEM
wi
(
13) we can obtain the following rate equation:
considering the values of K
(K
¼ 133) obtained
2 2 2
w
i
w
previously, we can obtain the rate constants k
y k
. The difference in
reactivity between the interface and the aqueous pseudophase
: k
¼
NEM PIP
wi
oN iEMW
½AOTꢃ
K
oN iEMK
K
K
w
ꢁ1 ꢁ1
i
2
ꢁ1 ꢁ1
s
i
2
wi
1
.41 M
s
and k ¼ 0.113 M
k
þ k
Vꢀ AOT½AOTꢃ
2 Vꢀ
H
O
½PIPꢃT
2
kobs ¼
ð14Þ
ð15Þ
NEM
wi
NEM
PIP
Z þ K oN iEMW ðKwi þ WÞ
K
NEMK
þ Kwi
w
2
i
2
oi
of the microemulsion is of k /k ¼ 12.5, this result is compa-
tible with the 17-fold decrease in the rate constant when
passing from the aqueous pseudophase to the interface of the
microemulsion in the aminolysis of NPA with PIP in
This equation can be written thus:
½
PIPꢃ
T
17
¼
NEM NEM
AOT–isooctane–water microemulsions.
ꢀ
ꢀ
kobs½AOTꢃVH OVAOT
2
PIP
wi
NEM
oi
NEM
wi
ðK þ WÞ½K
K
þ K
W þ K
Zꢃ
oi
wi
i
NEM NEM PIP ꢀ
w
NEM ꢀ
k K
K
K
VH O þ k K
VAOTW
2
oi
wi
wi
2
2
oi
T
It predicts the existence of a linear dependency between [PIP] /
ꢀ
kobs[AOT]V
ꢀ
V
and the parameter Z of composition of
H O AOT
2
the microemulsion. Fig. 6 shows, by way of example, the good
fit of the eqn (15) for values of W ¼ 15, 27 and 40.
The ordinates and the slopes of Fig. 6 show a dependency on
W, as predicted by eqn (15). To carry out a quantitative
analysis we will make use of the ordinates in the origin. The
intercepts in Fig. 6, eqn (15), can be rewritten thus:
PIP
NEM
wi
ðKwi þ WÞðK
þ WÞ
i
2
PIP NEM
¼
k VH OK
K
wi wi
2
Intercept
w
2
þ k VAOTW
ð16Þ
6
Previous studies have allowed us to determine the value of
a
the distribution constant of the piperazine between the con-
PIP
PIP
NEM
Fig. 7 Influence of W on (Kwi þ W)(Koi
þ W)/Intercept (eqn (16))
tinuous medium and the interface of the microemulsion Kwi
PIP
,
for the aminolysis of NEM by piperazine in AOT/isooctane/water
microemulsions. [PIP] ¼ 6.03 ꢀ 10 M referred to the total volume of
K
NEM can be determined from the Intercept/slope quotient
wi ¼ 9.5 at 25 1C. The value of the distribution constants of
ꢁ3
1
6
the system. T ¼ 25 1C. The line represents the fit of the eqn (16) to the
experimental results, considering only the values obtained for W 4 10.
NEM
NEM
and amount to Kwi
¼ 133 and Koi
¼ 16. Using these
1
598
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0

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the aminolysis of NPA by NMB. This decrease in the rate
constant in the interface means that the values determined
experimentally diverge negatively from the behavior predicted
by eqn (17). The divergence is more important if we consider
that as the water content of the system decreases, there is an
increase in the percentage of the reaction which takes place in
the interface of the microemulsion (Fig. 8).
Scheme 6
Conclusions
The aminolysis rate constant of NPA by NMB in the interface
of AOT-based microemulsions increases together with W. This
behavior is parallel with that which is observed for the solvo-
lysis reaction of the diphenylmethyl chloride. This increase in
the reactivity when W increases has been attributed to an
increase in the solvation capacity of the partial negative charge
which is generated in the transition state leading to the forma-
ꢂ
tion of the addition intermediate, T . The aminolysis of NEM
by piperazine occurs simultaneously in the aqueous microdro-
plet and the interface, and the percentage of interfacial reaction
increases as W decreases. For values of W o 10 the reaction
occurs mainly in the interface and the interfacial rate constant
decreases along with W, in parallel with the behavior observed
in the aminolysis of NPA by NMB.
Acknowledgements
Fig. 8 Variation of the % of reaction at the interface with the water
content of the AOT-based microemulsion for the Michael addition of
piperazine to N-ethylmaleimide at 25 1C.
Financial support from Ministerio de Ciencia y Tecnologı
´
a
(
Project BQU2002-01184) and Xunta de Galicia (PGIDT03-
PXIC20905PN and PGIDIT04TMT209003PR) is gratefully
acknowledged.
3. Influence of W on the rate constants in the interface and in
the aqueous microdroplet
References
The rate limiting step for aminolysis of substituted phenylace-
tates in aqueous solution is first order in amine concentrations
and leads to the formation of a tetrahedral intermediate
1
2
3
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ꢂ 12,13a
(Scheme 6).
When studying the aminolysis of NPA by NMB in AOT-
(
T )
based microemulsions we found that the rate constant in the
interface increases along with W, Fig. 5. This behavior can be
explained as a consequence of the changes in the properties of
the water as W varies. In the transition state of the rate
determining step there must be a certain degree of partial
negative charge on the oxygen atom of the carbonyl group.
The solvation of this partial negative charge will depend on the
electrophilic character of the of the water molecules: as the
electrophilic character decreases, so too does the reaction rate.
This behavior has also been shown in the study of the
4
5
6
7
8
(a) L. Garcı
Chem., 1993, 97, 3437; (b) L. Garcı
Mejuto, J. Phys. Chem., 1996, 100, 10981.
´
a-Rı
´
o, E. Iglesias, J. R. Leis and M. E. Pen
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8a
´
a-Rıo, J. R. Leis and J. C.
´
solvolysis reaction of the diphenylmethyl chloride in AOT-
based microemulsions. The solvolysis rate constant in the
interface decreases together with W due to the fact that the
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ꢁ
solvation of the salient Cl also decreases. A correlation has
i
been observed between Logk for the aminolysis of NPA by
2
(a) L. Garcı
9
4
´
a-Rı
9, 12318; (b) L. Garcı
55; (c) L. Garcıa-Rıo, J. R. Leis and J. A. Moreira, J. Am. Chem.
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o, J. R. Leis and E. Iglesias, J. Phys. Chem., 1995,
´
a-Rıo and J. R. Leis, Chem. Commun., 2000,
´
i
NMB and Logk for the solvolysis of the diphenylmethyl
´
´
chloride (not shown). The existence of this correlation shows
that the decrease in the aminolysis rate constant of NPA by
decylamine as W decreases is a consequence of the small
capacity of the interfacial water to solvate the development
of the partial negative charge on the carbonylic oxygen atom in
Soc., 2000, 122, 10325; (d) L. Garcı
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´
Mejuto, Langmuir, 2003, 19, 3190.
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9
10 (a) N. M. Correa, E. N. Durantini and J. J. Silber, J. Org. Chem.,
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ꢂ
the transition state for the formation of T .
The results shown in Fig. 5 are consistent with the fact that
the greatest changes in the physical properties of the water of
1
1
P. Lopez-Cornejo, P. Perez, F. Garcıa, R. Vega and F. Sanchez,
´ ´ ´ ´
J. Am. Chem. Soc., 2002, 124, 5154.
the microemulsions are observed for values of W o 15, these
1
2
A. B. Maude and A. Williams, J. Chem. Soc., Perkin Trans. 2,
1997, 179.
i
2
values being where the greatest variation of Logk
with W can
be observed. The results shown in Fig. 7 show that for values of
W 4 15 both the rate constant in the interface and the rate
constant in water scarcely change with W. In this way, we can
obtain a good linear dependence as predicted by eqn (17).
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rate constant in the interface, analogous with that observed for
13 (a) A. C. Satterthwait and W. P. Jencks, J. Am. Chem. Soc., 1974,
9
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Jencks, J. Am. Chem. Soc., 1987, 109, 5790; (d) J. F. Marlier, Acc.
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´ ´ ´
1
4
2004, 28, 988.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0
1599

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1
5
6
(a) A. Lubineau, G. Bouchain and Y. J. Queneau, J. Chem. Soc.,
Perkin Trans. 1, 1995, 2433; (b) D. S. Garwood and D. C.
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Khan, J. Chem. Soc., Perkin Trans. 2, 1985, 891.
that this quotient is independent of W. This result is due to the fact
in such a way as to verify the inequality
NEM
NEM
c Koi
that Koi
NEM
oi
NEM
NEM
NEM
K
c Koi W/Kwi
making Intercept/Slope ¼ Koi
¼ 16.
NEM
Once the value of Koi
distribution constant of NEM between water and isooctane, K
w
is known, and using the value of the
w
o
¼
.
NEM
NEM
NEM
1
From the Intercept/Slope quotient of the representations of Fig. 6
0.120, we can calculate Kwi , considering that Kwi ¼ Koi /K
o
NEM
NEM
NEM
In this way we can obtain the value of Kwi ¼ 133.
and eqn (15) we can obtain Intercept/Slope ¼ Koi
þ (Koi
wi )W which predicts the existence of a linear dependency on the
quotient Intercept/Slope with W. The experimental results show
/
NEM
K
17 L. Garcıa-Rıo, J. C. Mejuto and M. Perez-Lorenzo, Chem.-Eur.
´ ´ ´
J., 2005, 11, 4361.
1
600
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 5 9 4 – 1 6 0 0