Hammett Acidity Scale in Ionic Liquids :
H
N
Et
An Indication of Their Weak Dissociating Character
?
N
Robert T.*, Magna L.**, Olivier-Bourbigou H.**, Gilbert B*.
NTf2
T.Robert@ulg.ac.be; *University of Liège, Sart-Tilman, B6c, B-4000 Liège,
Belgium F
N
** Institut Français du Pétrole, BP3-69360 Solaize, France
F
F
B
F
Introduction
Solvating power, for instance towards the proton, is a fundamental property of solvents. Therefore, we are interested to investigate the acid-base properties
in ionic liquids (ILs) in order to ultimately establish a correlation with the acidic catalysis activity. To determine the accessible acidity levels in these new
“green” ionic solvents, we suggest the colorimetric Hammett acidity function H0.1 Following the Hammett proposition, this UV-Vis spectroscopic method
is based on the protonation equilibrium for a family of coloured indicator (CI) with pKa’s assumed as solvent independent,.
Hammett Acidity Function
Basic form, I
1.25
Absorbance
Experimental
Theory
1.50
Acid concentration increases
1.00
Acidic form,
HI+
0.75
0.50
H0  pKaI,H 2O  log
H 0  pHH 2O  log
0.25
 Because the ILs and the acids (HOTf and HNTf2) are very
hygroscopic and/or moisture sensitive, storage, preparation of
all solutions and UV-cells filling are performed in a glove box.
[I]IL
[HI ]IL
t (I)IL
H 2O
Γ t (i)IL
H 2O
t (HI ) IL
H 2O
Batch solution
of indicator:
IL + I
Acidic batch
solution of
indicator:
IL + acid + HI+
+ acid
Hammett assumption
400
450
500
550
Wavelength,  (nm)
H 0  pKaI,H 2O  log
Figure 1: Evolution of the 2,4-dichloro 6-nitroaniline spectrum in BMImNTf2
with the addition of HNTf2
Intermediate
solutions:
IL + acid + HI+/I
In practice
X IL
100 - X IL
with X, the ratio of the unprotonated form
of the indicator determined by computer
Use of 2 Indicators …
 Spectroscopic measurements are made with a Perkin Elmer
Lamda 14P spectrometer in quartz cuvettes. References are the
same IL with the same acid concentration but without indicator.
Results and Discussion
H0 in Various ILs
-2.0
For a same concentration of a same added acid; a difference
of H0 is observed when two different CI are used in a given
IL (▼, ○, △). This is typical of a non totally dissociating
media. Then, the Hammett acidity function becomes an
apparent function underestimating the real acidity level.
The Hammett acidity function is evaluated in several ILs to study the effect
of the ILs nature towards the proton
acidity. Very acidic level, -2 to -8, can
be reached in common ILs.
IC
IC IC
-3
-3 -4
.3
.3 .5
2
2 3
350
-2.5
-4
.5
3
300
-3.0
IC
0.00
250
-3.5
unitary slope
HOTf vs. HNTf2
The comparison of two common acids, HOTf and HNTf2,
is investigated in BMImOTf, BMImNTf2 and BMImBF4.
In a given IL, the comparison is made with the same CI.
IL autoprotolysis: BMIm+A- ⇌ BMIm• + HA
CATION EFFECT
-4.0
-4.5
-5.0
-5.5
the proton linked on the IL
cation has no influence on the
accessible acidity because its
acidity is masked by HNTf2
-6.5
-7.0
BMImNTf2
HOTf (●) is weaker than HNTf2 (○) (CI – 4.53)
HOTf + BMImNTF2
HNTf2 + BMImOTf
-8.0
-0.5
HNTf2 ≠ HOTF
high acidity level
BMIm+ = BMImH+ = HNEt3+
-6.0
BMImOTf
no difference of H0 (CI -3,32: ▼,▽)  HNTf2 is strong
HNTf2 + BMImOTf → HOTf + BMImNTf2 (leveling effect)
BMImBF4
HNTf2 (◇) is stronger than HOTf (◆) but both are weak acids
BMImBF4 + HNTf2
HBF4 + BMImNTf2
BMImBF4 + HOTf
HBF4 + BMImOTf
The cation effect is investigated in
NTf2- based ILs with HNTf2 as
spiking acid. No significant difference
is observed for a same CI (○,□,∆):
-6
.7
 log(CHX )
-2
.5
X
IC
formation constant of IH+X-

IC
(H0 )app  pKaI,H 2O  logKfHI
Apparent acidity function (H0)app
Analytical Chemistry and Electrochemistry
Et
Et
-7.5
ANION EFFECT
0.0
0.5
1.0
1.5
2.0
2.5
3.0
- log (Cacid)
Figure 2: Comparison of the apparent
Hammett acidity with addition of (filled
symbol) HOTf or (empty symbols) HNTf2 in
(▼▽) BMImOTf, (●○) BMImNTf2, (▲△)
HNEt3 NTf2, (∎) BHImNTf2, (◆◇) BMImBF4
and ()BMImPF6.
In this case, the IL anion nature plays a
dominant role on the accessible acidity.
The accessible acidity level follows :
OTf- < NTf2- < BF4- < PF6A higher acidity corresponds to a lower
solvating power of the IL anion towards
the proton.
Conclusions
 The acidity increases when acids are added in ILs and high acidity levels can be reached. The Hammett acidity function allows the evaluation of the acidity
level in our ILs which ranges from -2 to -8. Nevertheless, because the solvent affects the dissociation equilibrium, the Hammett function must be
understood as an apparent function, underestimating the real acidity.
 Changing the IL cation does not affect the accessible acidity level: BMIm = BHIm = HNEt3.
On the contrary, clear variations of solvating power towards the proton are observed between
H2O
OTfNTf2BF4PF6several IL anions. Accessible acidity levels can be classified with the following anion order:
 Differences of acidity are observed between HOTf and HNTf2 in BMImNTf2 and much more in BMImBF4.
This is in agreement with high acidity levels and the proposed acidity order: the higher is the acidity level, the more strong acids can be differentiated.
 All these conclusions are confirmed by our electrochemical measurements with the hydrogen electrode. 2
References: (1) T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, J. Electrochem. Soc., 156, 115 (2009)
(2) C. Malherbe, T. Robert, L. Magna, H. Olivier-Bourbigou and B. Gilbert, ECS Trans., 16, 3 (2009)