Determination of an acidic scale in room temperature ionic liquids

Article abstract of DOI:10.1021/ja0297382

The acidity scale of different Bronsted acids in ionic liquids such as [BMIM][NTf₂], [BMIM][BF₄], and [BMMIM][BF₄] has been investigated by determination of Hammett functions, using a spectrophotometric indicator method. This scale should permit one to correlate the acidity strength of ionic liquid systems with their ability to achieve acid-catalyzed reactions. Copyright

Full text of DOI:10.1021/ja0297382

Published on Web 04/11/2003
Determination of an Acidic Scale in Room Temperature Ionic Liquids
Ce´cile Thomazeau, He´le`ne Olivier-Bourbigou, Lionel Magna, Ste´phane Luts, and Bernard Gilbert*
Institut Franc¸ais du Pe´trole, 1 and 4 AVenue de Bois-Pre´au, 92852 Rueil-Malmaison, France, and
UniVersity of Lie`ge, 4000 Lie`ge, Belgium
Received December 13, 2002; E-mail: lionel.magna@ifp.fr
Ionic liquids are attracting increasing attention as alternative
solvents for a wide range of catalytic and organic reactions.1
Previous works report that acidic chloroaluminates containing
protons may be superacids, as evidenced by the determination of
their Hammett functions (down to -18) and by their use for
different acidic reactions.1,2 Nevertheless, chloroaluminates are very
sensitive to hydrolysis. Traces of water can change the composition
of the melt and the concentration of protons. As a result, it is
difficult to accurately control the acidity of these ionic liquids. Non-
chloroaluminate ionic liquids, air and moisture stable, have hence
been developed and applied for acidic reactions.3 The miscibility
of some ionic liquids with water has been proposed as a guide to
the chemical behavior of Brønsted acids in ionic liquids.4 However,
the determination of a Brønsted acidity scale in such solvents has
Figure 1. Absorption spectra of 2,4-dinitroaniline for various concentrations
not yet been considered.
of HNTf2 in [BMIM][NTf2].
In this paper, we would like to propose an acidity scale based
on Brønsted acids in non-chloroaluminate ionic liquids. The goal
where pK(I)aq is the pKa value of the indicator referred to an aqueous
of this acidity scale is ultimately to find a correlation between the
solution, [IH+]s and [I]s are the molar concentrations of, respectively,
catalytic activities measured in various acidic reactions.
the protonated and unprotonated forms of the indicator in the solvent
The acidity of protons is mainly determined by their solvation.
s, the γ’s are the activity coefficients, and the Γ’s are the transfer
Indeed, weakly solvated protons possess a higher chemical activity.5
activity coefficients from water to the solvent s. Equation 2 shows
In solvents of low dielectric constant, dissociation of ion pairs
clearly that proposing H0 as being equivalent to the pH is not strictly
involving protons should also be taken into account. Finally, the
valid in a thermodynamical sense.7 It first assumes that the indicator
properties of protons depend on both the nature of the solvent and
solutions are diluted enough to consider the activity coefficients
the nature and concentration of the acid.
as unity. If the solvent s is water, the H0 function then becomes
In the present study, we report the behavior observed for different
identical to the pH, at infinite dilution. Second, it assumes that the
Brønsted acids soluble in ionic liquids. Strong acids such as HNTf2
ratio of the transfer activity coefficients of both I and HI+ is unity
(NTf2 ) N(CF3SO2)2) and HOTf (OTf ) CF3SO3) were first
and solvent independent. This simply means that the difference of
evaluated. For comparison purposes, a weak acid such as AcH (AcH
solvation energy of the two I and HI+ indicator forms does not
) CH3CO2H) was also investigated. The ionic liquids chosen for
depend on the solvent. In practice, the latter requirement is more
this application were [BMIM][NTf2], [BMIM][BF4], and [BMMIM]-
difficult to meet. However, if one chooses indicator couples of very
[BF4] ([BMIM] ) 1-butyl-3-methylimidazolium, [BMMIM] )
similar chemical structures, the ratios of the transfer activity
1-butyl-2-methyl-3-methylimidazolium). These ionic liquids were
coefficients, if not unity, can still be considered as constants,
synthesized according to published procedures.6 Subsequent wash-
resulting in a slight but constant shift in the acidity function. This
ing of ionic liquids with a biphasic mixture H2O:CH2Cl2 warranted
is why the Hammett function has proved very useful for semi-
the elimination of sodium and chloride impurities. Water was
quantitative acidity comparison purposes.
removed by vacuum drying.
In the first series of experiments, different concentrations of
The Brønsted acidity was evaluated from the determination of
HNTf2 in [BMIM][NTf2] were prepared. Initial experiments have
the Hammett acidity functions, using UV-visible spectroscopy. In
been made with 4-nitroaniline (pK(I)aq ) 0.99) and 2,5-dichloro-
the present case, this method consists of evaluating the protonation
4-nitroaniline (pK(I)aq ) -1.82). These indicators were in their
extent of uncharged indicator bases (named I) in a solution, in terms
protonated form whatever the HNTf2 concentration involved, which
of the measurable ratio [I]/[IH+]. The chosen indicators belong to
means that the acidity level is too high for such indicators. Hence,
the same chemical family, mainly substituted dinitroanilines.
another indicator of lower pK(I)aq was chosen, where a full variation
In a given solvent (s), assumed as being dissociating, the
could be measured. For each acid concentration, the UV-visible
Hammett function (H0) is defined as
spectrum of the 2,4-dinitroaniline indicator (pK(I)aq ) -4.53) was
then recorded, as shown in Figure 1.
H0 ) pK(I)aq + log([I]s/[IH+]s)
which can also be written as
(1)
As the acid concentration increases, the absorbance of the
unprotonated form of the indicator observed at 340 nm decreases.
In the spectrum, the protonated form of the indicator cannot be
seen due to its small molar absorptivity and its location, probably
below 240 nm. By taking as the initial reference the total
H0 ) -loga(H+aq) - log γ(Ι)/γ(ΗΙ+)- log Γ(Ι)/Γ(ΗΙ+) (2)
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J. AM. CHEM. SOC. 2003, 125, 5264-5265
10.1021/ja0297382 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Calculation of the Hammett Function for Various
Table 3. Comparison of Hammett Functions of HNTf2 in Different
Ionic Liquids
Concentrations of HNTf2 in [BMIM][NTf2]a
1
1
[H+] mmol L-
Amax
[I] (%)
[IH+] (%)
H0
(±
0.05)
[H+] mmol L-
ionic liquid (ppm H2O)
H0 (±0.05)
0
11.9
29.4
62.9
102.1
130.2
209.8
337.2
2.3
100
0
6.5
102.1
105
105
[BMIM][NTf2] (23)
[BMIM][BF4] (141)
[BMMIM][BF4] (16)
-4.55a
-7.00b
-7.05b
2.15
1.76
1.41
1.09
0.94
0.67
0.52
93.5
76.5
61.3
47.4
40.9
29.1
22.6
-3.35
-4.00
-4.35
-4.55
-4.70
-4.90
-5.05
23.5
38.7
52.6
59.1
70.9
77.4
a Indicator: 2,4-dinitroaniline (pK(I)aq ) -4.53). b Indicator: 6-bromo-
2,4-dinitroaniline (pK(I)aq ) -6.7).
The third series of experiments was devoted to the effect of the
ionic liquid nature on the HNTf2 acidity. The influence of the anion
and cation nature of the ionic liquid is reported in Table 3.
In [BMIM][BF4] and [BMMIM][BF4], the Brønsted acidity of
HNTf2 is higher than that in [BMIM][NTf2]. The Hammett functions
of HNTf2 in [BMIM][BF4] and [BMMIM][BF4] are, respectively,
-7.00 and -7.05 for a [H+] concentration of 105 mmol L-1. This
a Water content by Karl Fischer titration: 23 ppm.
Table 2. Comparison of Hammett Functions of Different Acids in
[BMIM][NTf2]a
1
acid
[H+] mmol L-
H0 (±0.05)
HNTf2
HOTf
AcH
175
175
up to 1705
-4.80
-4.60
b
suggests that the BF4 anion is less solvating toward H+ than is
-
-
the NTf2 anion, leading to an increased acidity of the proton.
Nevertheless, the cation ([BMIM] or [BMMIM]) has strictly no
influence on the HNTf2 acidity in these ionic liquids. The presence
of a possible hydrogen bond between BF4- anion and the hydrogen
in position 2 of the imidazolium does not seem to affect the acidic
properties of these systems.
In conclusion, we have shown that in non-chloroaluminate ionic
liquids based on the NTf2- and BF4- anions, to which an acid has
been added, it is possible to reach acidity levels (measured as H0)
ranging from -3.35 to -7.00, depending on the ionic liquid and
on the acid. These media are thus clearly less solvating than water.
Concerning the behavior of water, we have evidenced its basic
character in such ionic liquids. Complementary studies are in
progress to complete the elaboration of acidic scales in room-
temperature ionic liquids which could be further used as a predictive
tool for studies of the acid-catalyzed reactions.
a Indicator: 2,4-dinitroaniline (pK(I)aq ) -4.53). b No protonation of
the indicator.
unprotonated form of the indicator (when no acid is added to the
ionic liquid, spectrum a), we could determine the [I]/[IH+] ratio
from the measured absorbances after each acid concentration
(spectra b-h), and then the Hammett function is calculated (see
Table 1).
In [BMIM][NTf2], the Hammett function of HNTf2 is included
between -3.35 and -5.05, corresponding to concentrations ranging
from 11.9 to 337.2 mmol L-1. The linear H0 correlation with the
proton concentration has been verified. Considering the large errors
for the extreme points, we found that the slope (1.06) is close to
the expected unity value.
Addition of water to [BMIM][NTf2] which already contains
HNTf2 (the molar ratio H2O/HNTf2 being equal to 1) induces an
increase of the absorbance for the unprotonated form of the
indicator. When this H2O/HNTf2 ratio is higher than 1, the
absorbance of the unprotonated form of the indicator increases more.
Hence, the addition of water decreases the acidity of the proton in
the ionic liquid. Water behaves as a base, due to its more
pronounced solvating character toward the proton than that of the
solvent.
Supporting Information Available: Syntheses, characterization of
ionic liquids, and UV-visible experiments (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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In the second series of experiments, we have studied the influence
of added Brønsted acids in [BMIM][NTf2] (Table 2).
In [BMIM][NTf2] ionic liquid, triflic acid (HOTf) exhibits
practically the same level of acidity as HNTf2. For the same H+
concentration, the Hammett functions are, respectively, -4.80 and
-4.60. In water, HNTf2 and HOTf acids are known to be strong
acids with almost the same strength. In anhydrous acetic acid, HOTf
is more acidic (pKa ) 4.2) than HNTf2 (pKa ) 7.0) due to the less
dissociative character of this solvent as compared to that of water.8
As it is observed in an aqueous medium, the leveling effect of the
ionic solvent limits the acidity level which can be reached with
strong acids. Addition of AcH does not lead to a decrease of the
absorption band of the unprotonated indicator. Acetic acid is then
not acidic enough to protonate 2,4-dinitroaniline. As in an aqueous
medium, AcH is a weaker acid than HOTf and HNTf2 in [BMIM]-
[NTf2]. Furthermore, addition of an excess of AcH to a solution
containing HNTf2 results in an increase of the unprotonated
indicator band, which indicates a basic behavior when the acidity
level is very high.
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