pubs.acs.org/joc
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,3
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Aliphatic amines and pyridines have been used as
nucleophiles or bases in organic reactions performed in
ILs. In some cases, rate constants are higher than those
Basicity of Pyridine and Some Substituted Pyridines
in Ionic Liquids
expected on the basis of the pK values in water, suggesting a
a
higher nucleophilicity/basicity of amines in ILs than in
conventional organic solvents.
†
Guido Angelini, Paolo De Maria, Cinzia Chiappe,
†
‡
†
Antonella Fontana, Marco Pierini, and Gabriella Siani*
§
,†
†
Dipartimento di Scienze del Farmaco, Universit aꢀ “G.
d’Annunzio” Via dei Vestini 31, Chieti, Italy, Dipartimento di
It is well-known that the acid-base strength of amines
depends on the solvent and it is reasonable to expect that
‡
5
Chimica e Chimica Industriale, Universit aꢀ degli Studi,
Via Risorgimento 35, Pisa, Italy, and Dipartimento di
they can interact differently with a molecular solvent and an
ionic solvent.
§
Chimica e Tecnologia del Farmaco, Universit aꢀ “La Sapienza”,
P.le Aldo Moro, Roma, Italy
According to McFarlane, an uncharged nitrogen base will
remain in its base form and be poorly solvated in ILs as, for
example, the interaction of the nitrogen lone pair with a
proton at C2, the most acidic proton of imidazolium ILs, is
6
weaker than that with the protons of water. Hence, parti-
cular caution is necessary if pK values determined in water
Received March 25, 2010
a
7
are used to rationalize quantitative results obtained in ILs.
Unfortunately quantitative data on the basicity of amines
2
,8,9
in ILs are quite scarce.
It is largely accepted that proton
transfer reactions generally proceed through hydrogen-
bonded complexes and ion pairs which eventually dissociate
into free ions, depending on the permittivity, ε, of the
1
0
solvent. In water the equilibrium constant for a general
proton-transfer reaction of the following charge type
HA þ B a A þ BHþ
-
refers to the formation of free ions. In less polar media, such
1
1
as the investigated ILs (10< ε < 29.7), due to extensive ion
pairing, the measured equilibrium constants, K , refer to ion
ip
1
2
pair rather than free ion formation. Recently, D’Anna and
Noto measured the equilibrium constants of ion pair forma-
tion for a number of aliphatic amines and considered these
The equilibrium constants for ion pair formation of some
pyridines have been evaluated by spectrophotometric
titration with trifluoroacetic acid in different ionic li-
quids. The basicity order is the same in ionic liquids and
in water. The substituent effect on the equilibrium con-
stant has been discussed in terms of the Hammett equa-
tion. Pyridine basicity appears to be less sensitive to the
substituent effect in ionic liquids than in water.
2,9
values as reliable indicators of their basic strength in ILs.
In the present work the basicity of pyridine (Pyr) and some
substituted pyridines has been estimated quantitatively from
the aptitude of the base to take a proton from trifluoroacetic
acid (TFA). The values of K have been measured in the four
ip
ILs of Scheme 1 by spectrophotometric titration, adding
-4
increasing amounts of TFA to a 2 ꢀ 10 M solution of
pyridine. A typical example is reported in Figure 1.
The absorbance at λ = 255 nm of pyridine increases upon
addition of increasing amounts of TFA until a constant value
is reached.
Ionic liquids (ILs) are low-temperature molten organic
salts, with melting points lying often below room tempera-
ture. Due to their peculiar properties, such as low volatility,
low flammability, and high stability, they have been widely
used in the last two decades as alternative reaction media to
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912 J. Org. Chem. 2010, 75, 3912–3915
Published on Web 05/03/2010
DOI: 10.1021/jo100570x
r 2010 American Chemical Society