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New Journal of Chemistry
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DOI: 10.1039/C7NJ03246C
Journal Name
ARTICLE
Iso-Solvation Effects in Mixtures of Ionic Liquids on the Kinetics of
a model SNAr Reaction
J. Alarcón-Espósito,a* R. Contrerasa and P. R. Campodónicob*
Received 00th January 20xx,
Accepted 00th January 20xx
The SNAr reaction between 1-chloro-2,4-dinitrobenzene and morpholine was used as a model system to study solvation
effects in a series of mixtures involving Imidazolium based ionic liquids. Iso- solvation regimes (i.e a solvent composition
regime were the solute is being solvated by approximately the same number of different solvent molecules in the mixture)
is reported for the first time in ionic liquid mixtures, for 4 mixtures including [EMIM][SCN][DCN], [BMIM][DCN][BF4],
[BMIM][BF4][PF6] and [BMIM][PF6][FAP]. The results show that for significant changes in composition, the rate coefficients
remains approximately constant. An additional interesting result is that for the mixture [BMIM][BF4][PF6] (in the
proportion 0.9 in molar fraction of BMIMBF4), the mixture shows a slightly better kinetic performance as compared to the
pure BMIMBF4 and BMIMPF6. Finally, for the [EMIM][SCN][DCN] mixture, an increasing proportion of EMIMSCN respect to
EMIMDCN result in a decrease of the rate coefficient within the range {0.1 - 0.75} in molar fraction of EMIMSCN. This
result may be traced to a competition between the anions [SCN-] and [DCN-] towards the reaction center driven by the
basicity of the reaction medium.
DOI: 10.1039/x0xx00000x
pointing out to a predominant “anion” solvent effects.20
For instance, Rogers et al.21 studied the properties of IL
mixtures as reaction media using H1 and C13 NMR and IR
Introduction
The use of solvent conventional mixtures has largely been
documented in literature.1-5 These studies have been extended to
mixtures involving conventional solvent/ionic liquids6-10 and
mixtures of ionic liquids.11-14 Apart of the solvating properties of
these systems, most of them are focused on the kinetic responses
of several systems. These studies are based mainly on the physico-
chemical properties of the mixtures, namely, polarity, ability to
form hydrogen bonds (HB), density, melting point, viscosity, among
others. However, there are few studies of solvent effects in
mixtures evaluated against the reaction rate, in order to establish
their usefulness in possible catalytic effects exerted by the solvent
mixture.15-17 The main difficulty in mixtures of ionic liquids (IL) is
that the problem implies a three-body problem. Remember that, IL
are generally associated to a very low extent18 and therefore one
has to lead with interactions between the substrate with a non-
associated cation/anion pair.19 Note also, that in the case of IL, the
techniques. The study was based on mixtures of two IL carrying the
-
common cation [EMIM+] with two different anions [OAc-] and [NTF2
], to investigate specific solvation effects. These authors
demonstrated that the electrostatic interactions are very different
from those produced in pure IL respect to their mixtures, thereby
attributing this response to the ability of [OAc-] to form complexes
-
with the [EMIM+] closest to the other anion [NTF2 ]. This effect was
related to effects of basicity and the strength of coulombic forces
leading to high ordered lattices governed by the nature of the
cation/anion. Another determining factor can be, to a lesser extent,
the length and steric hindrance of the substituents in the ions. This
result agrees well with those obtained in a previous work from our
group, regarding the importance of the size, polarizability and
ability to form HB of the anion (which is normally related to their
Lewis basicity).20,22
solvent-solvent interactions also play
a significant role. This
interaction is normally neglected in neutral dipolar solvents. One
way to facilitate the study is to leave one of the components fixed
with respect to the counterions. In this study, the cation of the IL
was fixed, since there are antecedents for the reaction under study,
D`Anna et al.23 have proposed that the three-dimensional
organization in IL mixtures is important, because this packing
defines different domains (micro-environments). These authors
studied the rate constant of a Diels-Alder reaction, finding that in
those IL mixtures that differ only in the size of the anions, there
appear structural variations large enough to compensate the effect
of the viscosity of the mixtures, and consequently the kinetic result
mainly responds to the difference in the structural organization
induced by the medium, thereby emphasizing that the effect of IL
mixtures is strongly substrate- dependent.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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