D’Anna et al.
JOCArticle
We have been particularly interested in the last topic, and
we have taken into consideration some classical organic
reactions that are affected by base catalysis as probe reac-
tions.3 We have frequently used aliphatic amines as base
catalysts, and in several cases we have observed a lack in
correlation between our kinetic data in ionic liquid (IL)
solution and pKa values collected for these catalysts in water
solution.3d,g In agreement with a referee’s comment,4 we
believe that lack of pKa values determined in IL solution
could represent a limitation to the full understanding of data
collected in these solvent systems. A recent paper on this
issue has pointed out that particular caution is needed in
regard to possible mistakes deriving from the use, in IL
solution, of pKa values collected in aqueous solution.5
Indeed, it is well-known that thermodynamic parameters
of a solute are strongly affected by solvent properties.
In our opinion, the determination of amines basicity
in these solvent media could prove interesting also because
the analysis of the papers published in the past few
years about ILs shows that aliphatic amines have been
frequently used both as nucleophiles and as bases.3b,d,g,6 In
particular, in different cases the results obtained evidenced
higher basicity of aliphatic amines in IL solution than in
conventional organic solvents, outlining the significant dif-
ferences in the solvation processes occurring in these solvent
media.3a,c-e,6b
CHART 1. Structures of Used Amines and Ionic Liquids
In this light, and bearing in mind some preliminary results
recently obtained by us,4 we chose the equilibrium of amine/
p-nitrophenol ion pair formation as a suitable probe to
have a measurement of the basic strength of aliphatic amines
in IL solution (Chart 1). By analogy with basicity measure-
ments carried out in organic solvents, such as benzene,7
1,4-dioxane,8 or tetrahydrofuran,9 we determined the stabi-
lity constants by spectrophotometric titration at 298 K.
We took into account ILs differing both in anion and cation
properties, such as [bmim][BF4], [bmim][PF6], [bmim]-
[NTf2], [bm2im][NTf2], and [bmpyrr][NTf2] (Chart 1) [where
bmim = 1-butyl-3-methylimidazolium; bm2im=1-butyl-2,3-di-
methylimidazolium; bmpyrr=N-butyl-N-methylpyrrolidinium;
and NTf2=bis(trifluoromethansulfonyl)imide]. Chosen ILs
differ in the anion size, symmetry, and coordination ability,
but also in the cation ability to give hydrogen bond, π-π,
and π-cation interactions.
It is well-known that all these features significantly con-
tribute to determine the properties of ILs. Indeed, changes in
the cation-anion combination give rise to significant varia-
tions in classical solvent parameters such as R, β, π*, etc.10
However, they could also affect the tridimensional structure
of these media that are frequently depicted as polymeric
supramolecular fluids in which constituent ions interact by
means of Coulomb’s interactions, hydrogen bond, and π-π
interactions.11
As far as amines are concerned, they were chosen on the
grounds of their basicity (see later) in aqueous solution,
structure, and steric hindrance (Chart 1). Some different
primary, secondary, and tertiary aliphatic amines were used.
In particular, for secondary amines both acyclic and cyclic
structures with different ring size and conformational free-
dom were taken into account. All these factors could affect
not only the equilibrium position but also the ability of the
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