Electronic and steric effects: How do they work in ionic liquids? the case of benzoic acid dissociation

Article abstract of DOI:10.1021/jo100914p

(Figure Presented) The need to have a measure of the strength of some substituted benzoic acids in ionic liquid solution led us to use the protonation equilibrium of sodium p-nitrophenolate as a probe reaction, which was studied by means of spectrophotometric titration at 298 K. In order to evaluate the importance of electronic effect of the substituents present on the aromatic ring, both electron-withdrawing and -donor substituents were taken into account. Furthermore, to have a measure of the importance of the steric effect of the substituents both para- and ortho-substituted benzoic acids were analyzed. The probe reaction was studied in two ionic liquids differing for the ability of the cation to give hydrogen bond and π-π interactions, namely [bm ₂im][NTf₂] and [bmpyrr][NTf₂]. Data collected show that benzoic acids are less dissociated in ionic liquid than in water solution. Furthermore, the equilibrium constant values seem to be significantly affected by both the nature of ionic liquid cation and the structure of the acid. In particular, the ortho-steric effect seems to operate differently in water and in the aromatic ionic liquid, determining in this solvent medium a particular behavior for ortho-substituted benzoic acids.

Full text of DOI:10.1021/jo100914p

pubs.acs.org/joc
Electronic and Steric Effects: How Do They Work in Ionic Liquids? The
Case of Benzoic Acid Dissociation
Francesca D’Anna,* Salvatore Marullo, Paola Vitale, and Renato Noto*
ꢀ
ꢀ
Dipartimento di Chimica Organica “E. Paterno”, Universita degli Studi di Palermo,
Viale delle Scienze-Parco d’Orleans II, 90128 Palermo, Italy
fdanna@unipa.it; rnoto@unipa.it
Received May 10, 2010
The need to have a measure of the strength of some substituted benzoic acids in ionic liquid solution
led us to use the protonation equilibrium of sodium p-nitrophenolate as a probe reaction, which was
studied by means of spectrophotometric titration at 298 K. In order to evaluate the importance of
electronic effect of the substituents present on the aromatic ring, both electron-withdrawing
and -donor substituents were taken into account. Furthermore, to have a measure of the importance
of the steric effect of the substituents both para- and ortho-substituted benzoic acids were analyzed.
The probe reaction was studied in two ionic liquids differing for the ability of the cation to give
hydrogen bond and π-π interactions, namely [bm2im][NTf2] and [bmpyrr][NTf2]. Data collected
show that benzoic acids are less dissociated in ionic liquid than in water solution. Furthermore, the
equilibrium constant values seem to be significantly affected by both the nature of ionic liquid cation
and the structure of the acid. In particular, the ortho-steric effect seems to operate differently in water
and in the aromatic ionic liquid, determining in this solvent medium a particular behavior for ortho-
substituted benzoic acids.
Introduction
an important point: acid or base properties of a solute are
heavily affected by the microenvironment in which they are
determined. Consequently, different reports in the literature
have had, as the main subject, the pKa determination in
nonaqueous solvents and the comparison of the results
obtained with values previously collected in water solu-
tion.4 In particular, as far as benzoic acids are concerned,
the study of the effects due to a different nature and posi-
tion of the substituent on the aromatic ring, together with
The pKa determination of an organic acid or base plays a
significant role in different fields of chemical research. Dur-
ing the years, a plethora of papers on the topic have under-
lined the importance of pKa knowledge in rationalizing
pharmacological effect of organic compounds,1 reaction
mechanisms,2 properties of new organic materials,3 and so
on. However, in all cases, different authors agree in outlining
4828 J. Org. Chem. 2010, 75, 4828–4834
Published on Web 06/18/2010
DOI: 10.1021/jo100914p
r
2010 American Chemical Society

D’Anna et al.
JOCArticle
the effects deriving from solvation phenomena, have
played a significant role in the field of physical organic
chemistry.5
and “specific” solvation has been subjected to criticism,
although it is largely used for practical applications.
Such a kind of analysis may become more complex when
solvent media show peculiar features such as those charac-
terizing ionic liquids (ILs). These new generation solvents
have been frequently highlighted for their environmentally
friendly properties (low vapor pressure and flammability).6
As well as for conventional solvents, ILs have been consi-
dered under different points of views. Indeed, they have been
frequently assimilated to conventional solvents, and their
solvation phenomena have been rationalized on the grounds
of classical solvent parameters, such as R, β, π*, and so on.7
On the other hand, a different picture of these solvent media
describes them as polymeric supramolecular fluids in which a
solute is not solvated but included in their tridimensional
structure.8 Furthermore, they have been also depicted as
molten salts and their different effects rationalized on the
grounds of electrostatic interactions.9 Of course, irrespective
of the viewpoint considered, the acid-base equilibria and the
relevant pKa values, determined in these solvent media, will
be heavily affected by the properties of IL used.
Generally, twodifferentviewpointshave been arguedabout
the essence of solvation phenomena. The first one considers
the extent of solute-solvent interactions as a result of physical
parameters of solvent (such as electrical permittivity and
refraction index) and molecular characteristics of solute.
The second one hypothesizes that solvation phenomena are
determined by specific interactions between solvent and so-
lute. During the years, this distinction between “universal”
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Hardy, T.; Rowlands, M. G.; Aherne, G. W.; Hunter, L. K.; Taylor, K.;
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Taking intoaccount thisinformation, andinthe framework
of our studies on ILs properties,10 we thought it could be
interesting to determine the acid strength of some substituted
benzoic acids in IL solution. Theneedto have a measure of the
acid strength in IL solution has been claimed in order to
control the acidity level in these media11 and to avoid the
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CHART 1. Structure of Benzoic Acids and ILs Used
possible mistakes deriving from the use, in IL solution, of pKa
values collected in aqueous solution.12
Results and Discussion
It has been previously reported13 that in IL solution the
sodium p-nitrophenolate, unlike from p-nitroaniline or 2,4-
dinitrophenolate, is a base able to undergo changes in the
UV-vis spectra that can be correlated to the strength of the
used benzoic acids. We analyzed the behavior of sodium
p-nitrophenolate in [bmim][BF4] andin[bmim][NTf2] solution.
However, in both cases, the indicator is found in protonated
form also in the absence of an acid. Probably, the acidity of H2
of the bmimþ ion was sufficiently high to protonate the base
indicator. On the grounds of this results, we chose as solvent
systems the less acidic [bm2im][NTf2] and [bmpyrr][NTf2].
Regular variations in the UV-vis spectra are obtained as a
function of acid concentration. As an illustrative example,
UV-vis spectra of sodium p-nitrophenolate in the presence
of increasing concentration of p-chlorobenzoic acid, in
[bm2im][NTf2] solution at 298 K, are reported in Figure 1.
Spectra show the absorbance decrease at λmax = 402 nm and
the corresponding increase at λmax = 310 nm. Moreover, the
presence of a neat isosbestic point at λ ∼ 344 nm indicates
that in IL solution a single equilibrium, such as the one
depicted in eq 1, is operating:
We used the protonation equilibrium of sodium p-nitro-
phenolate as probe reaction, which recently allowed us to
have a measure of the acidity of some aliphatic and aromatic
carboxylic acids.13 The reaction was followed by means of
UV-vis spectroscopy at 298 K.
We turned out our attention to some para- and ortho-
substituted benzoic acids (see Chart 1). In particular, the
different nature of the substituents present on the aromatic
ring could allow to evaluate the importance of the electronic
effect in determining the acid strength in IL solution.
Furthermore, it would be possible to verify if the acid
strength may be affected not only by classical electronic
effects, but also by peculiar solute-solvent interactions.
On the other hand, the different position of the substituent
on the aromatic ring (ortho and para position) could allow
for estimation of the importance of its steric effect. We were
not able to study the target reaction for nitro-substituted
benzoic acids because the strong absorption band super-
imposed on the one of the p-nitrophenolate anions did not
allow for a reliable measurement of the equilibrium constant.
The probe reaction was studied in [bm2im][NTf2] and in
[bmpyrr][NTf2] solution [where bm2im =1-butyl-2,3-di-
methylimidazolium, bmpyrr = N-butyl-N-methylpyrrolidi-
mium, NTf2=bis(trifluoromethansulfonyl)imide]. The cho-
sen ILs differ in the cation’s ability to give H-bond and π-π
interactions as well. It is well-known that both these factors
can be important in determining the behavior of a solute in
IL solution. Furthermore, the [NTf2-] anion is a weakly
basic anion, and its use does not probably affect the acidity
scale determination. In our opinion, the present experimen-
tal method can be confidently applied to carboxylic acids
having pKa values ranging from 2.4 to 5.
sK
ArO- Naþ þ HA
ArOH þ NaþA-
ð1Þ
fs
r
The equilibrium constant can be defined as
½ArOHꢀ½NaþA-
ꢀ
K ¼
ð2Þ
½ArO- Naþꢀ½HAꢀ
Considering the mass balances relevant to p-nitropheno-
late and carboxylic acid, the absorbance variation can be
expressed as
2
2
ðΔAbsÞ =Absi ¼ ðAbs0 - AbsiÞ =Absi ¼ ΔεdK½HAꢀ ð3Þ
where Absi and Abs0 are the absorbances, at a suitable
wavelength, of p-nitrophenolate in the presence and in the
absence of acid, respectively, Δε is the variation in molar
absorbitivity as a consequence of the protonation equilib-
rium, K is the equilibrium constant value, and d is the
length of the optical path (for details of the calculation
method see ref 13).
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Chem. B 2006, 110, 8798–8803. (b) Robert, T.; Olivier-Bourbigou, H.;
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J. Electrochem. Soc. 2009, 156, F-115–F-121.
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138, 1077–1101.
It is noteworthy that in water solution equilibrium con-
stant values, calculated by eq 2, equal the ratio between the
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1956.
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D’Anna et al.
JOCArticle
TABLE 1. Equilibrium Constant Values (K ), at 298 K, Corresponding
to Protonation of Sodium p-Nitrophenolate in Water, [bm2im][NTf2], and
[bmpyrr][NTf2] Solution
a
b
b
benzoic acid
KH2O
K[bm2im][NTf2]
K[bmpyrr][NTf2]
1a ( p-OMe)
1b ( p-Me)
1c = 2c (H)
1d ( p-F)
620
790
910
1020
1480
1480
1510
3980
12.5
31.1
20.7
51.6
39.1
74.6
98.8
28.6
204
15.9
18.7
179
55.7
152
1.22
2.71
1.75
5.30
2.01
1.83
3.14
3.21
29.8
3.16
3.00
25.4
10.0
10.7
6.25
6.90
23.2
1e ( p-I)
1c ( p-Cl)
1g ( p-Br)
1h ( p-CN)
1i ( p-CF3)
2a (o-OMe)
2b (o-Me)
2d (o-F)
2e (o-I)
2f (o-Cl)
2g(o-Br)
2h (o-CN)
2i (o-CF3)
1150
1740
7580
20000
17000
20400
11800
33.8
21.3
117
aSee ref 14. bK values are the average between two independent
measurements and are reproducible within (5%.
FIGURE 1. UV-vis spectra of sodium p-nitrophenolate (0.0002 M)
in the presence of increasing concentrations of p-chlorobenzoic acid
(0-0.006 M) at 298 K in [bm2im][NTf2] solution.
those detected in conventional solvents. Indeed, as recently
underlined by Lynden-Bell,16 ILs cannot be assimilated to
continuous media. Rather, they are characterized by dy-
namic heterogeneous environments17 in which, as a conse-
quence of thick network of solvent-solvent interactions, the
reorganization around a solute molecule is slower and
energetically less easy than in ordinary molecular solvents.18
In our case, the feeble but cooperative interactions determin-
ing the supramolecular structure of ILs could cause an
opposition to the solvent reorganization around the benzo-
ate anion, disfavoring the target reaction.
The different solvent behavior of ILs used in this work
with respect water is also testified by the fact that the attempt
to correlate K values calculated in water solution to K values
collected in IL solution gave bad results in both cases (see
Figures 2-6 of the Supporting Information). Furthermore,
no correlation exists between K values collected in
[bm2im][NTf2] and in [bmpyrr][NTf2] (see Figure 7 of the
Supporting Information). Once again, this latter result con-
firms the reports by MacFarlane et al.15 about the peculiarity
of each IL-probe system.
As far as data collected in IL solutions are concerned,
K values decrease on going from the aromatic to the ali-
phatic IL, evidencing a lower proton transfer degree in
[bmpyrr][NTf2] than in [bm2im][NTf2]. In general, the
K[bm2im][NTf2]/K[bmpyrr][NTf2] ratio ranges from 6.8 up to 40.8
for para-substituted benzoic acids and from 3.1 up to 14.2 for
o-substituted isomers. This trend cannot be explained on the
grounds of a different polarity of two ILs. Indeed, the apparent
polarity parameters for [bm2im][NTf2] and [bmpyrr][NTf2]
are quite similar (ETN =0.541 and 0.544, ENR =218.20 and
218.70 kcal/mol, π* = 0.984 and 0.971 for [bm2im][NTf2]
and [bmpyrr][NTf2], respectively).10j,19 On the other hand,
also the H-bond acceptor basicity of solvent, frequently
acid dissociation constant of carboxylic acid and that one
corresponding to p-nitrophenol (pKa=7.15).14 Thus, we cal-
culated the K values for water solution reported in Table 1 by
using literature pKa data.14
In Table 1, the equilibrium constant values (K) determined
in IL solution by eq 3, as a function of benzoic acids, are also
reported. We are aware of the fact that K values in IL
solution could be affected by stability constant values of
two ion pairs (eq 1). However, at a first approximation level
we may assume that the two ion pairs have similar stabilities.
For the sake of clarity, we will discuss our data as a
function of the solvent and the substituent.
Influence of the Solvent. Analysis of data reported in
Table 1 shows that K values decrease along the series
KH2O>K[bm2im][NTf2]>K[bmpyrr][NTf2]. The higher acidity of
benzoic acids in water solution than in IL solution perfectly
agrees with the observation reported by MacFarlane et al.,15
comparing the UV-vis spectra of m-cresol purple and
bromocresol purple in water, in [BF4-] and [PF6-] based
ILs, showing the lower proton transfer degree of the weak
acids in the latter solvent media. This trend recalls the one we
recently detected for carboxylic acids.13 K values can be
considered as a measure of the proton transfer degree.
Obviously, on the basis that the electronic effect, as ac-
counted for σ values, is not dependent on the solvent nature,
the proton transfer degree is strongly favored by the solvent
ability to stabilize the benzoate anion. Thus, our results seem
to indicate that the benzoate anion is less stabilized in IL
solution than in water solution. The less relevance of solva-
tion effects in IL solution has been claimed on different
occasions,10 and in some cases, it allowed to rationalize the
higher reactivity of organic species in these media.10a,c,f
Furthermore, it is noteworthy that the solute-solvent inter-
actions in IL solution could be completely different from
(16) Lynden-Bell, R. M. Phys. Chem. Chem. Phys. 2010, 12, 1733–1740.
(17) Mandal, P.; Sarkar, M.; Samanta, A. J. Phys. Chem. A 2004, 108,
9048–9053.
(18) (a) Kobrak, M. N. J. Chem. Phys. 2006, 125, 064502–11. (b) Yau,
H. M.; Barnes, S. A.; Hook, J. M.; Youngs, T. G. A.; Croft, A. K.; Harper,
J. B. Chem. Commun. 2008, 3567–3578.
(14) Lide, D. R. In CRC Handbook of Chemistry and Physics, 80th ed.;
CRC Press: London, 1999-2000.
(15) (a) MacFarlane, D. R. Forsyth, S. A. In Ionic Liquids as Green Solvents.
Progress and Prospects; ACS Symposium Series 856; American Chemical
Society: Washington, DC, 2003; pp 264-276. (b) MacFarlane, D. R.; Pringle,
J. M.; Johansson, K. M.; Forsyth, S. A.; Forsyth, M. Chem. Commun. 2006,
1905–1917.
(19) Crowhurst, L.; Mawdsley, P. R.; Perez-Arlandis, J. M.; Salter, P. A.;
Welton, T. Phys. Chem. Chem. Phys. 2003, 5, 2790–2794.
J. Org. Chem. Vol. 75, No. 14, 2010 4831

D’Anna et al.
JOCArticle
considered as one of the main factors determining the
strength of benzoic acids in molecular solvents,20 does not
account for the differences in K values collected in IL solu-
tion (β=0.239 and 0.252 for [bm2im][NTf2] and [bmpyrr]-
[NTf2], respectively).19
this light, we hypothesized that steric and electronic para-
meters collected in water solution are unable to describe
quantitatively the substituent effect in IL solution. The lack
of correlation among data in ILs and in water should be a
consequence of fact that enthalpy-entropy compensation
effect is not operating. This is not surprising and seems to
confirm the peculiar solute-solvent and solvent-solvent
interactions present in IL solutions. A similar result was
previously obtained by Chiappe et al. on studying the addi-
tion of trihalides to unsaturated compounds.24
In general, according to data calculated in water solution,
in IL solution the halo-, cyano-, and trifluoromethyl-sub-
stituted benzoic acids show K values higher than those of
unsubstituted benzoic acid. As far as para isomers are
concerned, in both IL solutions the trifluoromethylbenzoic
acid was the strongest acid, despite the fact that the triflu-
oromethyl group is a less able electron-withdrawing substi-
tuent than the cyano group (σp-CF3 = 0.54, σp-CN = 0.66).
In[bm2im][NTf2] solution, the para-substituted benzoic acids
considered above show a further peculiar behavior. Indeed, in
this IL K values strongly depend on the nature of each sub-
stituent as testified for example by the acidity increase induced
by the halogen with respect to benzoic acid (KX/KH=1.1, 1.6,
1.7, and 1.6 for F, Cl, Br, and I in water solution, whereas KX/
KH = 1.9, 2.5, 3.6, and 4.8 for I, F, Cl, and Br in [bm2im][NTf2]
solution). This result confirms the importance of the specific
solute-solvent interactions in IL solution. In [bmpyrr][NTf2]
solution, as a consequence of the above-mentioned easier
reorganization of solvent around the solute molecule, the sub-
stituent discrimination is less marked.
It is well-known that the acid-base equilibria become more
complexes on decreasing the solvating power of the medium,
and poorly solvated anions may form both ion pairs and
hydrogen bond complexes.21 In IL solution, as a consequence
of a less relevance of solvation effects, the species formed at
equilibrium could be more conveniently depicted as a bimo-
lecular species in which the proton is shared by the acid and
the base. This species, having a more extended π contact
surface area, should be able to interact with the aromatic
cation of [bm2im][NTf2] also by means of π-π, cation-π, and
π-quadrupole interactions.22 Hence, on the grounds of these
feeble interactions, a larger stabilization and consequently an
increase in the acid strength should be detected.
A comparison among the data collected in the two ILs
used in this work shows that, particularly for para isomers, K
values collected in [bmpyrr][NTf2] span in a narrower range,
evidencing for this IL a tendency to exert a sort of leveling
effect. A similar behavior was previously detected by us13
and also by Gilbert et al.23 on determining the Hammett
functions for TfOH and HNTf2.
In our opinion, in order to get a better understanding of
the different behavior of the two ILs used in this work, the
nature of solvent-solvent interactions in each solvent med-
ium must be taken into account. Both ILs are obviously
characterized and stabilized by electrostatic cation-anion
interactions. However, as mentioned, the tridimensional
structure of [bm2im][NTf2] is further stabilized by additional
cation-cation interactions, such as π-π interactions. This
further stabilization could induce an increase in the energy
request needed to adapt the solvent cage to the solute
molecule. As a consequence, a faster and easier solvent
reorganization around a solute molecule could be detected
in [bmpyrr][NTf2] than in [bm2im][NTf2]. This induces a
range of different solute-solvent interactions that accounts
for the span of K values in [bmpyrr][NTf2] solution.
Effect of the Substituent. First, in order to identify the
nature of the relationship between the acid strength and the
electronic effect of the substituent, we tried to correlate K
values collected in IL solution with Hammett σ values.
However, in all cases considered only poor correlations were
obtained (see Figures 8 and 9 of the Supporting Informa-
tion). Bad results were also obtained by taking into account
the Es parameter (see Figures 10-13 of the Supporting
Information) or attempting a biparametric correlation. In
As far as electron-donating substituents are concerned,
according to data collected in water solution, the p-methoxy
substituent induces a decrease of benzoic acid strength.
Differently, the p-methyl-substituted benzoic acid in both
IL solutions results a bit stronger species than the unsub-
stituted one.
In the case of ortho-substituted benzoic acids, for each IL, we
first tried to correlate Kvalues with those corresponding to para
isomers (see Figures 14 and 15 of the Supporting Information).
In both cases, no correlation exists, as was unsurprising taking
into account the behavior of para- and ortho-substituted
benzoic acids in water. However, the K value trend (as follow-
ing discussed) seems to indicate that solute-solvent interac-
tions for ortho derivatives are different from those present
for para isomers. In general, the behavior of ortho isomers, as
far as the electronic effect is concerned, seems to recall the one
just discussed for para isomers. In this case, the analysis of K
values cannot be carried out only on the grounds of the
electronic effect. Indeed, it is well-known that the steric or pro-
ximity effect of the substituent heavily affects the dissociation
equilibrium of benzoic acids in conventional solvents, and as
will be discussed later, this effect differently operates also in IL
solution.
(20) Bartnicka, H.; Bojanowska, I.; Kalinowski, M. K. Aust. J. Chem.
1993, 45, 31–36.
(21) Coetzee, J. F. Prog. Phys. Org. Chem. 1967, 4, 45–92.
(22) (a) Hanke, C. G.; Johansson, A.; Harper, J. B.; Lynden-Bell, R. M.
Chem. Phys. Lett. 2003, 374, 85–90. (b) Holbrey, J. D.; Reichert, W. M.;
Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Chem.
Commun. 2003, 476–477. (c) Harper, J. B.; Lynden-Bell, R. M. Mol. Phys.
2004, 102, 85–94. (d) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.;
Sheppard, O.; Soper, A. K. J. Phys. Chem. B 2005, 109, 1573–1598. (e)
Lachwa, J.; Bento, I.; Duarte, M. T.; Lopes, J. N. C.; Rebelo, L. P. N. Chem.
Among ortho isomers, 2d in both IL solutions was the
strongest acid, notwithstanding the fluorine atom is the
smallest substituent. In [bmpyrr][NTf2] solution, more sig-
nificant acidity increases were induced by the presence of
electron-withdrawing substituents with respect to those ones
detected for para isomers in the same solvent medium.
ꢁ
Commun. 2006, 2445–2447. (f ) Lynden-Bell, R. M.; Del Popolo, M. G.;
Youngs, T. G. A.; Kohanoff, J.; Hanke, C. G.; Harper, J. B.; Pinilla, C. C.
Acc. Chem. Res. 2007, 40, 1138–1145. (g) Hardacre, C.; Holbrey, J. D.;
Nieuwenhuyzen, M.; Youngs, T. G. A. Acc. Chem. Res. 2007, 40, 1146–1155.
(23) Thomozeau, C.; Olivier-Barbigou, H.; Magna, L.; Luts, S.; Gilbert,
B. J. Am. Chem. Soc. 2003, 125, 5264–5265.
(24) Chiappe, C.; Pieraccini, D. J. Org. Chem. 2004, 69, 6059–6064.
4832 J. Org. Chem. Vol. 75, No. 14, 2010

D’Anna et al.
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Analysis of data collected as a function of the acid structure
shows once more that the substituent position on the aromatic
ring differently affects the acid strength in aqueous and in IL
solution. Indeed, in aqueous solution, the K values correspond-
ing to ortho-substituted benzoic acids are higher than the ones
corresponding to para isomers. The higher acidity of ortho-
substituted benzoic acids with respect to the corresponding para
isomers has been deeply investigated in aqueous solution in
conventional organic solvents and in their binary mixtures.4a,5c
This behavior has been frequently ascribed to the proximity
between the carboxyl group and the substituent, which induces
an increase in the angle between the benzene ring and the plane
of carboxyl group. This effect, on the whole, causes a decrease in
the conjugation of carboxyl group with the aromatic nucleus. As
a consequence, the -M effect of carboxyl group, responsible for
the relative weakness of benzoic acids, is hampered. Generally,
in water solution the acid strength of the ortho-substituted
benzoic acids increases on increasing the size of the ortho
substituent. Data collected in conventional solvents show that
the extent of this effect is heavily affected by the solvent nature;
in particular, solvent polarity seems to play an important role.
By analogy with that previously detected in water solution, a
deeper analysis of data collected in [bmpyrr][NTf2] shows that,
with the exception of o-trifluoromethylbenzoic acid, all ortho
isomers are stronger acids than the corresponding para isomers.
Conversely, in [bm2im][NTf2] solution a more complex beha-
vior was observed. Indeed, o-F, o-Cl, o-I, and o-OMe benzoic
acids were stronger than the corresponding para isomers. An
opposite trend was detected for Br, CN, CF3, and Me deriva-
tives. Once more, these differences in the behavior of two ILs
cannot be explained on the grounds of a different apparent
solvent polarity or substituent steric hindrance. Probably, above
results are a consequence of the different ability of two ILs to
reorganize around solute molecules. This factor might affect the
twisting of the carboxyl group respect to the plane of the
aromatic ring. On the whole, these data seem to indicate a lower
relevance of the ortho steric effect in [bm2im][NTf2] solution.
These results perfectly agree with those previously collected by
us about a lower significance of steric effects in determining
the reactivity of 1,1,1-tribromo-2,2-bis(2,5-dimethoxyphenyl)-
ethanes in [bmim][BF4].25 It is worth noting that the coplanarity
between the carboxyl group and the ortho substituent could
possibly favor, provided it is possible, the formation of an
intramolecular hydrogen bond, which consequently should
account for a lower tendency of the acid group to share the
proton with the p-nitrophenolate anion.
angle in crystalline 2-substituted benzoic acids (η = 1.3, 13.7,
18.3, and 28 for o-F, o-Cl, o-Br, and o-I, respectively).5c
As noted above, in [bmpyrr][NTf2] solution, with the only
exception of the trifluoromethyl derivative, the ortho iso-
mers show K values higher than those of the para isomers. In
this case, the weaker solvent-solvent interactions and the
less organized tridimensional structure, in which only elec-
trostatic interactions are operative as a consequence of the
aliphatic nature of the cation, probably allow the carboxyl
group twisting, thus determining the strength of acids.
Conclusions
The use of the protonation equilibrium of the p-nitrophe-
nolate anion has allowed us to determine the acid strength of
some substituted benzoic acids in IL solution. In particular,
data collected show that, as a consequence of the peculiar
IL-solute interactions, benzoic acids show a lower proton
transfer degree in IL than in water solution. Furthermore,
the different solvent-solvent interactions operating in the
two ILs give rise to their different ability to discriminate
the benzoic acids on the grounds of the nature and substi-
tuent position. As a matter of fact, for ortho-substituted
benzoic acids, our data evidence the importance of the
specific and peculiar interactions operating on this substrates
series.
Once again, data collected confirm that ILs constitute a
class of solvents for which it is hard to operate a solvent
behavior generalization similar to the one normally made for
conventional solvents. As a matter of fact, bearing in mind
that differences in acid strength are often determined by
changes in entropy values, our data seem to indicate that the
proton transfer degree in IL solution is strongly affected by
variations in this thermodynamic parameter. This should
explain the lack of correlation between K and σ values,
normally found when enthalpy-entropy compensation is
operating. Furthermore, it supports the idea that the solvent
shows a significant structural order degree. Consequently,
the picture of ILs as “polymeric supramolecular fluids” is, in
our opinion, the one that better describes their behavior.
Experimental Section
Materials. [bm2im][NTf2],27 [bmpyrr][NTf2],27 and sodium
p-nitrophenolate28 were prepared according to literature proce-
dure. Methanol and benzoic acids were used as purchased
without further purification. ILs were dissolved before use in
CH2Cl2 and stirred overnight, at room temperature, in the
presence of active charcoal (1% w/w). The solution was filtered
over a neutral aluminum oxide pad. Colorless ILs were dried in
vacuum at 60 °C for 2 h before use, and then they were stored in a
desiccator over calcium choride.
Measurements and Calculations. UV-vis Spectra. The
UV-vis spectra were recorded by using a spectrophotometer
equipped with a temperature controller.
Spectrophotometric Measurements. Samples for a typical
spectrophotometric measurement were prepared by injecting
in a quartz cuvette (light path 0.2 cm) the proper volume of
IL (500 μL), sodium p-nitrophenolate solution (50 μL), and
carboxylic acid solution (25 μL) by means of a microsyringe.
As far as K values relevant to halo-substituted benzoic acids
are concerned, a partial twisting of the carboxyl group seems to
be operating in the presence of smaller substituents (F and
˚
Cl; rv=1.47, 1.75, 1.85, and 1.98 A for F, Cl, Br, and I, respec-
tively),26 as testified by Ko/Kp ratios (Ko/Kp=3.5, 2.0, 0.3, and
1.4 for F, Cl, Br, and I, respectively). These results are probably
a consequence of a higher electron density on F and Cl.
It is noteworthy that the observed trend in IL solution is
completely different from the one detected in water solution.
Indeed, in the latter case, the highest acidity increases were
detected in the presence of the largest substituents, Br and I,
as a consequence of the largest phenyl-carboxyl interplanar
(27) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys.
Chem. Chem. Phys. 2001, 3, 5192–5200.
(28) Bergeron, R. J.; Channing, M. A. J. Am. Chem. Soc. 1979, 101, 2511–
2515.
(25) D’Anna, F.; Frenna, V.; Pace, V.; Noto, R. Tetrahedron 2006, 62,
1690–1698.
(26) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
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D’Anna et al.
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Sodium p-nitrophenolate and carboxylic acids were added as
concentrated solution in methanol.29 The sodium p-nitrophe-
nolate concentration was kept constant (0.0002 M). Each
sample was thermostated at 298 K. A suitable wavelength was
chosen by comparing the sodium p-nitrophenolate solution with
the sample having the highest acid concentration. Each mea-
surement was repeated twice, and K values were reproducible
within 5%. Experimental data were subjected to fitting regres-
sion analysis according to eq 3.
Acknowledgment. We thank MUR (Ministero dell’Uni-
ꢀ
versita
e della Ricerca) for financial support (PRIN
2008KRBX3B). This investigation was also supported by the
University of Palermo (funds for selected research topics).
(29) All UV-vis measurements were carried out by using methanol as
cosolvent (ca. 13% by volume, see the Experimental Section). However, on
the grounds of our previous results,10e we are confident that these binary
mixtures could be compared to neat IL.
Supporting Information Available: Plots of log KIL vs log
H2O, log KIL vs σ values, and log KIL,p vs log KIL,o. This materialis
available free of charge via the Internet at http://pubs.acs.org.
K
4834 J. Org. Chem. Vol. 75, No. 14, 2010