A protic ionic liquid, when entrapped in cationic reverse micelles, can be used as a suitable solvent for a bimolecular nucleophilic substitution reaction

Article abstract of DOI:10.1039/c5ob02664d

In this work, we have explored how the confinement of the protic ionic liquid (IL) ethylammonium nitrate (EAN) inside toluene/benzyl-n-hexadecyldimethylammonium chloride (BHDC) reverse micelles (RMs) affects the Cl^- nucleophilicity on the bimolecular nucleophilic substitution (S_N2) reaction between this anion and dimethyl-4-nitrophenylsulfonium trifluoromethanesulfonate. To the best of our knowledge this is the first report where toluene/BHDC RMs use EAN as a polar component and it is used as a nanoreactor for carrying out kinetic experiments. Dynamic light scattering results reveal the formation of RMs containing the protic IL. The kinetic results show that upon confinement, EAN becomes a suitable solvent for the S_N2 reaction while in homogeneous media it is a bad option. Entrapped in BHDC RMs, due to the strong hydrogen bond interactions, EAN behaves as an aprotic-like IL which cannot deactivate the nucleophilic power of Cl^- and yet increases the substrate solubility. These facts show the versatility of this kind of organized system to alter the polar solvent entrapped and its influence on the reaction rate when it is used as a nanoreactor.

Full text of DOI:10.1039/c5ob02664d

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Protic Ionic Liquid when Entrapped in Cationic Reverse Micelles Can Be Used as
Suitable Solvent for a Bimolecular Nucleophilic Substitution Reaction
Matías A. Crosio, N. Mariano Correa, Juana J. Silber, R. Darío Falcone*
Departamento de Química. Universidad Nacional de Río Cuarto. Agencia Postal # 3.
C.P. X5804BYA Río Cuarto. Argentina.
* Dr. R. Darío Falcone. CorrespondingꢀAuthor, Eꢀmail: rfalcone@exa.unrc.edu.ar
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ABSTRACT
In this work was explored how the confinement of the protic ionic liquid (IL)
ethylammonium nitrate (EAN) inside toluene/benzylꢀnꢀhexadecyldimethylammonium
chloride (BHDC) reverse micelles (RMs) affects the Clꢀ nucleophilicity on the
bimolecular nucleophilic substitution (SN2) reaction between this anion and dimethylꢀ4ꢀ
nitrophenylsulfonium trifluoromethanesulfonate. To the best of our knowledge this is
the first report where toluene/BHDC RMs uses EAN as polar component and is used as
nanoreactor for carrying out kinetic experiments. Dynamic light scattering results reveal
the formation of RMs containing the protic IL. The kinetic results show that upon
confinement EAN becomes a suitable solvent for the SN2 reaction even that in
homogenous media is bad option. Entrapped in BHDC RMs, due to strong hydrogen
bond interactions, EAN behave as an aproticꢀlike IL which cannot deactivate the
nucleophilic power of Clꢀ and yet increasing the substrate solubility. These facts show
the versatility of this kind of organized system to alter the polar solvent entrapped and
its influence on reaction rate when they are used as nanoreactors.
Keywords: protic ionic liquids, reverse micelles, BHDC, ethylammonium nitrate.
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INTRODUCTION
Reverse micelles (RMs) are spatially ordered macromolecular assemblies of
surfactants formed in a nonpolar solvent, in which the polar head groups of the
surfactants point inward and the hydrocarbon chains point toward the nonpolar
medium.1ꢀ3 RMs are an interesting subject due to their broad applications in chemical
reactions, separation science, materials science, and the pharmaceutical industry, among
others.1,4 These nanoscale aggregates are suitable media for processes that involve
hydrophobic and hydrophilic reactants in a variety of chemical and biological
reactions.1,4ꢀ7 There are a wide range of surfactants that form RMs, including anionic,
cationic and nonionic molecules.1ꢀ16 One of them, the cationic surfactant benzylꢀnꢀ
hexadecyldimethylammonium chloride (BHDC, Scheme 1), can make RMs in aromatic
solvents without addition of cosurfactant.1,8,17ꢀ23
Ionic liquids (ILs) have received significant attention as powerful alternatives to
conventional molecular organic solvents.24ꢀ26 According to their proton availability, ILs
can be classified as protic or aprotic.27 Particularly, protic ILs have been used in several
applications.24,28 Ethylammonium nitrate (EAN, Scheme 1) is a widely studied protic IL
and it is known that it has many similarities to water such as high polarity and the
ability to form hydrogen bond.29,30
N
Clꢀ
BHDC
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+
ꢀ
NH3 NO3
EAN
Scheme 1. Molecular structures of BHDC and EAN.
The use of protic ILs (predominantly EAN) in organized systems is usually as a
replacement for water as the polar phase in direct micelles or vesicles. In this sense,
very recently Greaves et al.31 reported in detail the backgrounds in this field. However,
with regard to the formation of RMs containing protic ILs much less information is
available. These RMs can provide potential applications owing to the unique features of
both ILs and RMs. However, the majority of the studies containing protic ILs employed
nonꢀionic surfactants such alkyl oligoethylene oxide (CnEm)32,33 or Brijs34 and they used
normally as nonꢀpolar phase nꢀalkanes or benzene34 and EAN as polar phase. Only a
few works performed by Zech et al.35ꢀ37 have focused on the use of cationic surfactant,
particularly 1ꢀhexadecylꢀ3ꢀmethyl imidazolium chloride (C16mimCl), with decanol as
cosurfactant, EAN as the polar phase, and dodecane as the nonꢀpolar continuous
phase.35,36
With regard to BHDC RMs, although the traditional solvent used as polar component in
this cationic RMs is water,8,17 different aprotic ILs18ꢀ23 have been entrapped in the polar
core but not information about the possibility to use EAN is found. This system is
interesting to explore since it is well known that the physicochemical properties of polar
solvents (water and aprotic ILs) entrapped inside RMs change dramatically from those
of the bulk solvents as a result of specific interactions and confined geometries.1,9,18ꢀ21,23
This can cause, for example, alterations in the reactants and in the course of reactions.
Particularly, nucleophilic substitution reactions provide a familiar avenue for
examination of solvent effects on chemical reactions.23,24,38ꢀ41 As a chemical probe it is
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possible to use the nucleophilic substitution reaction of Clꢀ with dimethylꢀ4ꢀ
nitrophenylsulfonium (S+ in Scheme 2).23,40,41 Previously, we have study the same SN2
reaction in toluene/BHDC RMs but with 1ꢀbutylꢀ3ꢀmethylimidazolium tetrafluoroborate
(bmimBF4) as encapsulated aprotic IL.23 We observed that upon confinement, the ionic
interactions between the cationic surfactant polar head groups and the surfactant
counterion modifies substantially the performance of the aprotic IL as solvent. Taking
into account this background, the aim of the present work is to show that a protic
solvent, EAN, can be used as worthy reaction media when it is entrapped in BHDC
RMs while is a bad option in homogeneous media. Here, it is important to note that the
task of our work is not to try to discover new materials or transformations but to use a
chemical reaction as a model reaction to evaluate the solvent effect in a complex system
such as RMs.
CH3
Clꢀ
S
+
ClꢀCH3
O2N
+
O2N
S
CH3
S+
P
Scheme 2. The SN2 reaction of Clꢀ with dimethylꢀ4ꢀnitrophenylsulfonium (S+) to
produce methylꢀ4ꢀnitrophenyl sulfide (P) and ClꢀCH3.
RESULTS AND DISCUSSION
In order to explore the effect of confinement on EAN in the SN2 reaction studied,
the nucleophilic reactivity of Clꢀ in homogeneous and confined media was evaluated.
Thus, the results obtained are presented in two different sections corresponding to the
studies performed in (i) neat EAN and (ii) toluene/BHDC/EAN RMs at different IL
contents defined as Ws = [EAN]/[BHDC].
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The SN2 reaction was followed by monitoring the absorbance changes at λ = 340
nm corresponding to the formation of the demethylated sulfide product (Scheme 2). All
the experiments were carried out under pseudo first order conditions, were the
sulfonium salt concentration ([S+] = 5 x 10ꢀ5 M) was always smaller than the [Clꢀ].
i) Studies in neat EAN
Absorbance spectra of the mixture of Clꢀ and S+ in neat EAN at different times are
presented in Figure S1. Ammonium chloride (NH4Cl) as source of Clꢀ ion was used and
the variation of the Clꢀ concentration between 1 x 10ꢀ2 M and 0.2 M were performed. As
it can be observed in Figure S1, no absorbance changes using the protic IL as solvent
are detected, even after long time of reaction. Similar results were found by Hallet et.
al.41 for the same reaction performed in water and methanol. The authors explained the
absence of reaction as due to the strong hydrogen bond interaction between the solvents
with the nucleophile, thus reducing its reactivity. In fact, EAN presents a high Kamletꢀ
Taft´s α parameter value (0.8525), which is consistent with the ability of the solvent to
interact through hydrogen bond. In consequence, the Clꢀ anions in EAN are not
available to act as nucleophile because they are associated to the ethylammonium
cation.41
ii) Studies in toluene/BHDC/EAN RMs
Prior to study the SN2 reaction in toluene/BHDC/EAN system and because it is not
known if EAN is truly encapsulated by BHDC to form RMs, Dynamic light scattering
(DLS) experiments were performed. When new RMs are explored, a crucial question
has to be answered: is the polar solvent effectively entrapped by the surfactant creating
a true RMs or is dissolved only in the organic solvent/surfactant mixture without any
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molecular organization (bicontinous structureless microemulsion)?42 DLS normally is
used to assess this matter because if the polar solvent (water for example) is really
encapsulated and interacting with the RM interface, the droplets size must increase as
the water content (W0 = [water]/[surfactant]) increases with a linear tendency (swelling
law of RMs) as it is well established for other RMs.9,17,43 A deviation from the linearity
on the droplets sizes indicates that the RMs droplet – droplet interactions are favored
thus changing the shape of the RMs previous to the phase separation.23,44,45,46,47 On the
other hand, if the polar solvent is not encapsulated by the surfactant the droplets sizes
could remain constants or even decrease with the polar solvent addition.42
In the present work, as the RMs solutions are not at infinite dilution (DLS
experiments were performed at a fixed surfactant concentration equal to 0.2 M) it is
appropriate to introduce an apparent hydrodynamic diameter (dapp) in order to describe
the results.48 Figure 1 shows the dapp values for the toluene/BHDC/EAN system at
different Ws values. As can be seen, there is an increase in the droplet size values when
the IL content increases, showing that EAN is effectively entrapped by the surfactant
layer, yielding RMs. The sizes obtained for toluene/BHDC/EAN RMs are comparable
to the values reported previously for aprotic ILs entrapped in BHDC RMs.18,20,21,23,43
However, the amount of the protic IL that the toluene/BHDC RMs dissolve is quite
small, Ws = 0.3. Similar results by Dai et al.49 in the ternary system cyclohexane/TXꢀ
100/EAN (Ws = 0.4) and by Zech et al.36 in dodecane/decanol/C16mimCl/EAN (Ws =
0.18) have been reported.
Additionally from Figure 1, is it interesting to mention that the droplet size
values increase slightly (only 0.5 nm) with the Ws suggesting that the IL is interacting
weakly with BHDC at the interface of RMs.18,20,21
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3.5
3.0
2.5
2.0
0.05
0.10
0.15
0.20
0.25
Ws
Figure 1. Apparent diameter (dapp) values for toluene/BHDC/EAN RMs obtained at 30
°C varying Ws. [BHDC] = 0.2 M.
As it was mentioned above, we have studied previously23 the same SN2 reaction
in BHDC RMs; particularly in that work we performed kinetic experiments also in neat
toluene and toluene/BHDC RMs at Ws = 0. In both cases, the reaction takes place but
different behaviors were observed.23 In neat toluene, always working below the critical
micellar concentration, the formation of ion pairs between Clꢀ with its counterion
increases with salt concentration, in such a way that the fraction of free Clꢀ available as
nucleophile to react with sulfonium ions decreases and consequently the observed rate
constant also decreases.23 In toluene/BHDC RMs at Ws = 0, the incorporation of S+ into
the RMs is favored (in comparison with toluene) and the presence of Clꢀ at the interface
of the RMs allows the observation of reaction.23
Taking into account these backgrounds and the ability of RMs to alter the
structure of the polar solvent entrapped, in the present contribution we want to explore
if the presence of RMs can affect or not the performance of a bad solvent for
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nucleophilic substitution reactions such as EAN. In this sense, in toluene/BHDC/EAN
RMs the SN2 reaction was studied varying the [BHDC] keeping constants the [S+] and
Ws values. Typical kinetic results in toluene/BHDC/EAN RMs are shown in Figure 2.
Also an example of the absorbance changes at λ = 340 nm as a function of time at
[BHDC] = 0.1 M and Ws = 0.05 are shown in Figure S2. As can be seen in these
Figures and opposite to the results obtained in neat EAN (Figure S1), the reaction in
toluene/BHDC/EAN is observed.
The reaction of the sulfonium S+ with chloride ion presented in Scheme 2 is a
bimolecular reaction, which kinetics follows the rate law descripted by equation 1:
ꢁ ꢃ
rate = ꢀ ꢂ = kꢅ Cl
S
(1)
ꢁ
ꢆꢃꢁ ꢇꢃ
ꢀꢄ
ꢁ
ꢆꢃ
ꢁ
ꢇꢃ
If Cl ≫ S , a pseudo first order behavior for the kinetics of the reaction is assumed
in order to quantify the reaction rate. Thus, the expression for the observable rate
constant (kꢈꢉꢊ) can be defined as equation 2:
ꢁ
ꢆꢃ
kꢈꢉꢊ = kꢅ Cl
(2)
ꢁ
ꢆꢃ
is the total nucleophile
where kꢅ is the second order rate constant and Cl
concentration.
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0.6
3600 s
0.5
0.4
0.3
0.2
0.1
0.0
time
0
300
320
340
360
380
400
Wavelength (nm)
Figure 2. Absorption spectra of the reaction of Clꢀ with S+ in toluene/BHDC/EAN RMs
at Ws = 0.05. [BHDC] = 0.1 M. [S+] = 5 x 10ꢀ5 M. T = 30 ºC. The spectra were recorded
at 60 second intervals.
As in the RMs, the BHDC surfactant was used as the source of Clꢀ ion (See structure in
Scheme 1), the kobs values obtained in toluene/BHDC/EAN RMs at different total
concentration of BHDC ([BHDC]tot) and Ws are shown in Figure 3.
1.0x10-3
8.0x10-4
6.0x10-4
4.0x10-4
2.0x10-4
0.0
0.00
0.04
0.08
0.12
0.16
0.20
[BHDC]tot (M)
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Figure 3. kobs values as a function of total concentration of BHDC ([BHDC]tot) in
toluene/BHDC/EAN at different Ws. Ws: (■) 0.02, (●) 0.05, (▲) 0.11 and (▼) 0.17.
[S+] = 5 x 10ꢀ5 M. T = 30 ºC.
Two facts emerge from Figure 3: (i) the kobs values increase with the amount of BHDC
but not in linear tendency at all the Ws evaluated as could be expected for a simple SN2
reaction; (ii) the kobs values in the RMs are dependent on the EAN content. In Figure 4,
are plotted the kobs values in the toluene/BHDC/EAN RMs as a function of the EAN
content at [BHDC]tot constant.
1.0x10-3
8.0x10-4
6.0x10-4
4.0x10-4
2.0x10-4
0.0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Ws
Figure 4. kobs values as a function of Ws in toluene/BHDC/EAN at different [BHDC]tot.
[BHDC]tot (M): (▲) 0.10, (■) 0.15 and (●) 0.2. [S+] = 5 x 10ꢀ5 M. T = 30 ºC.
As it can be observed, at low Ws (0.02 – 0.11), kobs has similar values in the
toluene/BHDC/EAN RMs and they are independent on the EAN content. Only when the
Ws value is large enough (Ws = 0.17) there is an impact on the rate constant which
decreases dramatically. It should be noted that even at this Ws value the reaction is still
observed. It is evident that the Clꢀ availability is responsible for the changes on rates of
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the reaction.
In order to explain the results obtained in the BHDC RMs performed at different
[BHDC] and Ws, is important to consider which kinetic model is appropriated to
analyse the data.
In view of the S+ chemical structure, it is possible that the substrate solubilize (in
different magnitude) in two regions of the toluene/BHDC/EAN system: the organic and
the RMs pseudophases. Thus, the overall reaction can be descripted as Scheme 3
illustrates.
k´o
Cl-o
S+
+
Product
Product
o
toluene
RMs
Kp
+ BHDCRMs
k´RMs
Cl-RMs
S+
+
RMs
Scheme 3. Schematic representation of the reaction site in toluene/BHDC/EAN RMs.
S+o, Clꢀo, ClꢀRMs and S+RMs are the reactants in the organic and in the RMs pseudophases,
respectively.
As in the present work we used [BHDC] > critical micellar concentration (cmc)
value (0.01 M)50, S+ is incorporated into the RMs (as is expressed by equation 3)
favored by its low solubility in the aromatic solvent.
Sꢈꢇ + BHDCꢋꢌꢊ
Sꢋꢇꢌꢊ
(3)
where Sꢈꢇ and Sꢋꢇꢌꢊ symbolize the substrate in the toluene and in the RMs
pseudophases, respectively. BHDCꢋꢌꢊ represents the cationic RMs. As it is
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conventional for micellar aggregates, the concentration of micellized51 surfactant
ꢁ
ꢃ
ꢁ
ꢃ
ꢁ
ꢃ
can be approximated to BHDC
ꢄꢈꢄ
defined as BHDC
(where BHDC
– cmc)
ꢋꢌꢊ
ꢋꢌꢊ
can be calculated, and if the mass balance for S+ ([S+]tot = [S+]o + [S+]RMs) is considered,
the partition constant of the substrate (Kꢍ) is defined as equation 4:
ꢏ
ꢁ
ꢃ
ꢐꢑꢒ
ꢎ
Kꢍ = ꢁ
(4)
ꢏ
ꢃ ꢁ
ꢔꢕꢖꢗ
ꢃ
ꢐꢑꢒ
ꢎ
ꢓ
Taking into consideration the procedure used previously for the RMs containing other
ILs23, it is possible to obtain the following rate equation (see supporting information
section for a complete rate equation deduction):
ꢐꢑꢒ
ꢣ ꢁꢔꢕꢖꢗꢃ
ꢚ
ꢜ
ꢤ
ꢐꢑꢒ
ꢛ
kꢈꢉꢊ = k ꢘꢈꢙꢉꢊꢘ
+
(5)
ꢞ
ꢝ
ꢥꢇꢣ ꢁꢔꢕꢖꢗꢃ
ꢤ ꢐꢑꢒ
ꢟꢠꢡꢢ
ꢘꢙꢘ
ꢈꢉꢊ
Where k
and kꢋꢅꢌꢊ are the second order rate constants values in the cmc and
ꢁ
ꢃ
ꢋꢌꢊ
[BHDC] > cmc, respectively. BHDC
is the surfactant concentration referred to the
ꢞ
total volume, Vꢔꢕꢖꢗ is the molar volume of BHDC and β is the fraction of surfactant
neutralized charge.23
As Figure 4 shows no dependence on the kobs values at low Ws, the experimental
data obtained for toluene/BHDC/EAN RMs at Ws between 0.02 – 0.11 were analyzed
by equation 5 and for example, in Figure 5 the fitting at Ws = 0.02 is presented.
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1.2x10-3
1.0x10-3
8.0x10-4
6.0x10-4
4.0x10-4
2.0x10-4
0.0
0.00
0.05
0.10
0.15
0.20
[BHDC]RMs (M)
Figure 5. Plot of kobs values as a function of [BHDC]RMs in toluene/BHDC/EAN at Ws
= 0.02 analyzed according equation 5.
ꢞ
From the fitting of the results at Ws values between 0.02 – 0.11 and using Vꢔꢕꢖꢗ = 0.42
Mꢀ1, average values of Kꢍ = 25.1 ± 0.7 Mꢀ1 and kꢋꢅꢌꢊ β = 4.4 ± 0.2 x 10ꢀ4 Mꢀ1 sꢀ1 were
obtained. To estimate kꢋꢅꢌꢊ it is necessary to know the value of β. Previous studies
showed that, independently of the RMs composition,23 β remains constant around 0.96;
thus using this value we have estimated the kꢋꢅꢌꢊ ≈ 4.6 ± 0.2 x 10ꢀ4 Mꢀ1sꢀ1. Additionally,
the Kꢍ value confirms that in the toluene/BHDC/EAN RMs S+ is practically located at
the RMs interface. Moreover, the partition constant value is larger than in the absence of
polar solvent23 (Kꢍ = 22.4 ± 0.6 Mꢀ1 at Ws = 0). Thus, for example at [BHDC] = 0.2 M
the 83% of the substrate is incorporated to the toluene/BHDC/EAN RMs.
If the same analysis is performed to the RMs at the maximum of EAN entrapped
(Ws = 0.17) where the kobs values dramatically decrease in comparison with the values
obtained at Ws values between 0.02ꢀ0.11 (Figure 4), values of kꢋꢅꢌꢊ = 1.7 ± 0.2 x 10ꢀ4
Mꢀ1sꢀ1 and Kꢍ = 18 ± 2 Mꢀ1 are obtained. This drop on the rate constant value (about to 4
times) can be explained considering that in the RMs at the maximum of Ws reached, the
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polar solvent interact much less with the interface than at low Ws, and thus a reduction
in the nucleophilic power of Clꢀ is expected. However, it is important to note that even
at Ws = 0.17 the behavior of EAN confined in BHDC RMs is not equal to the neat EAN,
where the reaction does not take place.
In comparison with the previous results using aprotic IL bmimBF4,23 the
reactivity of Clꢀ ions in toluene/BHDC/EAN RMs is smaller (kꢉꢅꢙꢦꢙꢔꢧꢨ = 2.3 ± 0.1 x 10ꢀ
3
Mꢀ1 sꢀ1).23 In Table 1, the second order rate constant values obtained in both RMs and
homogeneous media are presented. Interestingly, in the EAN case the most notable fact
is that the RMs produce strong alterations in the protic solvent when entrapped in
BHDC RMs which entirely affects the Clꢀ nucleophilicity in comparison with its
behavior in homogeneous media. These alterations are more important than that the
changes observed in the systems using bmimBF4 (Table 1).
Table 1. Second order rate constant (k2) values obtained in neat ILs and entrapped in
BHDC RMs.
System
k2 (M-1s-1)
neat solvent
EAN
N.D.
2.6 ± 0.1 x 10ꢀ3a
bmimBF4
RMs
4.6 ± 0.2 x 10ꢀ4
2.3 ± 0.1 x 10ꢀ3a
toluene/BHDC/EAN
toluene/BHDC/bmimBF4
N.D.: no reaction detected. a Data from reference 23.
Hence, we show evidences that EAN can be consider as a good solvent for the
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SN2 reaction even that in homogenous media is bad option. In order to explain these
results, we propose that the key is the magnitude of the hydrogen bond of EAN when
the solvent is in confinement. In this sense, the ability of ILs to form hydrogen bond
with a solute or between the ions in neat solvent has been described in terms of two
competing equilibria.24,25 In the EAN case (a protic solvent), the cation first interacts by
hydrogen bond with its counterion according to equation 6:
Ethyl − NHꢩꢇ + NOꢆꢩ
Ethyl − NHꢩꢇ ⋯ NOꢩꢆ
(6)
Additionally, the cation Ethyl − NHꢩꢇ can also complex with the Clꢀ present in the
reaction media (equation 7):
Ethyl − NHꢩꢇ + Clꢆ
Ethyl − NHꢩꢇ ⋯ Clꢆ
(7)
Since in the reaction media both equilibria (equations 6 and 7) can be involved24,25, the
results in neat EAN (where not reaction is observed) is possible to deduce that the
amount of free Clꢀ (equation 7) is clearly very low. In contrast, in BHDC RMs we
observe reaction and this is due to the availability of Clꢀ to react because EAN is
entrapped in the organized media. This fact suggests that the amount of ethylammonium
associated to the Clꢀ ions (Ethyl − NHꢩꢇ ⋯ Clꢆ) is smaller than the Ethyl − NHꢩꢇ ⋯ NOꢆꢩ
(equation 6). We believe that this phenomenon is enhanced as a consequence of the ILꢀ
interface interactions present in the BHDC RMs. In this situation, the interactions
between the cationic surfactant and the ions present in the interfacial zone, also have to
be considered. It seems that the cationic surfactant moiety (BHD+) cannot be solvated
ꢀ
by NO3 thus, at the interface the interaction between BHD+…Clꢀ is stronger than
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ꢀ
ꢀ
BHD+…NO3 and ethylammonium only interacts with NO3 . This behavior has been
observed before in BHDC RMs containing other ILs such as 1ꢀbutylꢀ3ꢀ
methylimidazolium trifluoroacetate (bmimTfA).21 In that work, the results suggested
that even that the ILꢀsurfactant interaction was weak (as in the present case), the ion
pairing effect between bmim+ꢀTfAꢀ was substantially enhanced upon confinement,
altering the structure and properties of the entrapped bmimTfA. Thus, the consequence
of the confinement in BHDC RMs, is that EAN is more autoꢀassociated (stronger
hydrogen bond interaction between Ethyl − NHꢩꢇ and NOꢩꢆ) than in bulk solution. These
strongly associated EAN molecules interact weakly with the BHDC interface,
generating only small changes in the droplet sizes of the RMs, as DLS show (Figure 1).
CONCLUSIONS
In the present contribution we have shown how upon confinement EAN can be used as
suitable solvent for a SN2 reaction, situation quite different with the one observed in
homogenous media where EAN acts as any protic solvent complexing the nucleophile.
Additionally, to the best of our knowledge this is the first report where toluene/BHDC
RMs uses EAN as polar component and is used as nanoreactor for carrying out kinetic
experiments. Thus, EAN entrapped in BHDC RMs, strongly autoꢀassociates rendering
an aproticꢀlike IL which is not able to deactivate the nucleophilic power of Clꢀ. Finally,
our results highlight the importance of RMs as nanoreactor media for classical
reactions, where the reactants not only improve their solubilities but increase their
reactivities in comparison with homogeneous media. These facts show how everything
that happens in homogeneous solution has to be reconsidered because the fascinating
effect of the confinement. We hope to stimulate the scientific community to use these
attractive and unique nanoreactors to perform different kind of chemical reactions and
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nanoparticle synthesis.
Supplementary Information. Figure S1: Absorption spectra of the reaction of Clꢀ with
S+ in neat EAN at 30 °C; Figure S2: Absorbance at 340 nm as a function of time in the
reaction shown in Figure 2; Table S1: kobs values obtained in toluene/BHDC/EAN RMs,
varying the [BHDC] at different Ws and, derivation of rate equation are available free of
charge.
EXPERIMENTAL SECTION
Benzylꢀnꢀhexadecyldimethylammonium chloride (BHDC) from Sigma (> 99% purity)
was dried at reduced pressure prior use. Toluene from Sigma (HPLC quality) was used
without prior purification. The IL ethylammonium nitrate (EAN) was synthesized under
anaerobic condition following standard procedures according references 29 and 52.
Thus, equimolar concentration of nitric acid (from Sigma, 70% in water) and
ethylamine (from Sigma, 70 % in water) was used. Nitric acid was added slowly to
ethylamine contained in a roundꢀbottom flask over ice in stirring condition. Excess
water was removed by drying under vacuum. Prior to use EAN was discolored with
activated charcoal and dried at reduced pressure at 60 °C for 4 hours.
The substrate dimethylꢀ4ꢀnitrophenylsulfonium (S+) trifluoromethanesulfonate was
synthesized following the literature procedure.40
Preparation of BHDC RMs solutions: stock solutions of BHDC in toluene were
prepared by mass and volumetric dilution. Aliquots of these stock solutions were used
to make individual reverse micelle solutions with different amount of IL, defined as Ws
= [EAN]/[BHDC]. The incorporation of EAN into each micellar solutions were
performed using calibrated microsyringes. The resulting solutions were clear with a
single phase and they were used in the kinetic experiments. The Ws was varied between
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0ꢀ0.3 for toluene/BHDC/EAN RMs. It was not possible to obtain higher values of Ws
due to turbidity problems. The lowest value for Ws (Ws = 0), corresponds to a system
without the IL addition. It is important to note that, at room temperature EAN is not
soluble in toluene and neither in toluene/BHDC solutions at surfactant concentrations
lower than the critical micelle concentration (around 0.01 M).50
Kinetic procedure: Nucleophilic substitution reactions were followed by monitoring the
UVꢀVis absorbance of the demethylated sulfide product (Scheme 2) using a HP 8452A
spectrophotometer with a thermostated cell holder. In all experiments the wavelength
used for kinetic studies was 340 nm. All the experiments were carried out under pseudo
first order conditions were the sulfonium salt concentration ([S+] = 5 x 10ꢀ5 M) was
always smaller than the [Clꢀ]. All experiments were carried out at 30.0 ± 0.1 ºC.
Kinetics were studied by following the integrated method where the integrated first
order rate equation was fitted to the kinetic data, using a nonlinear regression analysis
program to give the observed rate constants, kobs.23,40,41 The pooled standard deviation of
the kinetic data, using different prepared samples, was less than 5%. Because of
solubility problems in the different media used in this study, we were forced to utilize
the anion trifluoromethanesulfonate as counterion of the cationic sulfonium. No rate
differences were found previously between studies involving the different sulfonium
salts in molecular solvents and ILs,40,41 which is consistent with the view that the
cationic center itself acts as the electrophilic site for reaction.
Kinetic procedure in neat EAN: stock solutions of NH4Cl, as source of Clꢀ, dissolved in
neat EAN were prepared by mass and volumetric dilution. Aliquots of these stock
solutions containing Clꢀ were added to a 1.0 cm path length UV/Vis quartz cuvette
under anaerobic conditions, after which pure EAN was added to a total volume of 2 ml.
After thermostatting for 15 min, the reaction was initiated by injection of a stock
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solution containing the sulfonium electrophile dissolved in EAN. Spectra were recorded
at regular time intervals using a spectrophotometer with a thermostated sample holder.
Kinetic procedure in BHDC RMs: when the reaction was investigated in
toluene/BHDC/EAN RMs, the [Clꢀ] was modified varying only the [BHDC] ([BHDC] >
cmc) at Ws = constant. Thus, the appropriate amount of toluene/BHDC/EAN stock
solution to obtain the desired final [Clꢀ] in the micellar system was transferred into a 1.0
cm path length UV/Vis quartz cuvette to obtain a final volume of 2 ml. After
thermostating for 15 min, the reaction was initiated by injection of a stock solution
containing the sulfonium electrophile dissolved in toluene ([S+] = 5 x 10ꢀ3 M). Spectra
were recorded at regular time intervals using a spectrophotometer with a thermostated
sample holder.
DLS experiments: The apparent diameters of toluene/BHDC/EAN RMs were
determined by dynamic light scattering (DLS, Malvern 4700 with goniometer) with an
argonꢀion laser operating at 488 nm. Cleanliness of the cuvettes used for measurements
was of crucial importance for obtaining reliable and reproducible data.53 Cuvettes were
washed with ethanol, and then with doubly distilled water and dried with acetone. Prior
to use, the samples were filtered three times using an Acrodisc with 0.2 ꢁm PTFE
membrane (Sigma) to avoid dust or particles presents in the original solution. Before
introducing each sample to the cuvette, it was rinsed with pure toluene twice, then with
the 0.2 M surfactant stock solution, and finally with the sample to be analyzed. Prior to
making measurements on a given day, the background signals from air and toluene were
collected to confirm cleanliness of the cuvettes. Prior to data acquisition, samples were
equilibrated in the DLS instrument for 10 min at 30 ºC. To obtain valid results from
DLS measurements requires knowledge of the system’s refractive index and viscosity in
addition to wellꢀdefined conditions. Since we worked with dilute solutions, the
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refractive indices and viscosities for the RM solutions were assumed to be the same as
neat toluene.54,55 Multiple samples at each size were made, and thirty independent size
measurements were made for each individual sample at the scattering angle of 90º. The
instrument was calibrated before and during the course of experiments using several
different size standards. Thus, we are confident that the magnitudes obtained by DLS
measurements can be taken as statistically meaningful for all the systems investigated.
The algorithm used was CONTIN and the DLS experiments show that the polidispersity
of the toluene/BHDC/EAN RMs size is less than 5 %.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support for this work by the Consejo Nacional
de Investigaciones Científicas y Técnicas (CONICET), Agencia Nacional de Promoción
Científica y Técnica and Secretaría de Ciencia y Técnica de la Universidad Nacional de
Río Cuarto. N.M.C., J.J.S. and R.D.F. hold a research position at CONICET. M.A.C.
thanks CONICET for a research fellowship.
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