Determination of the polarities of some ionic liquids using 2-nitrocyclohexanone as the probe

Full text of DOI:10.1021/jo050288g

Determination of the Polarities of Some
Ionic Liquids Using 2-Nitrocyclohexanone
as the Probe
Guido Angelini,† Cinzia Chiappe,‡ Paolo De Maria,†
Antonella Fontana,† Francesco Gasparrini,§
Daniela Pieraccini,‡ Marco Pierini,*,§ and
Gabriella Siani†
Dipartimento di Scienze del Farmaco, Universita` “G.
d’Annunzio” Via dei Vestini 31, Chieti, Italy, Dipartimento
di Chimica Biorganica e Biofarmacia, Universita` degli
Studi Via Bonanno 33, Pisa, Italy, Dipartimento di Studi di
Chimica e Tecnologia delle Sostanze Biologicamente Attive,
Universita` “La Sapienza” P.le Aldo Moro 5, Roma, Italy
marco.pierini@uniroma1.it
Received February 15, 2005
FIGURE 1. Plot of KT against the permittivity (ꢀ) of the
solvent.
Preliminary kinetic measurements carried out on the
keto-enol interconversion of 2-NCH in aprotic apolar
solvents clearly show that bases (e.g., triethylamine,
TEA, or substituted pyridines) catalyze the tautomeriza-
tion process without affecting the final value of the absor-
bance, A∞, of the enol form. Therefore, catalytic amounts
of TEA (i.e., [TEA]/[2-NCH] ≈ 10-3) were added to speed
up the establishment of the equilibrium. Appreciable
amounts of the nitronic form (the so-called “aci” form)
could not be detected at equilibrium (see Supporting
Information). The absence of the aci form is also shown
by the occurrence of one isosbestic point in the UV spectra
registered at different time intervals after dissolution.
KT values in 5 pure organic solvents (C6H12, CCl4, CHCl3,
CH2Cl2, CH3CN) and 18 binary solvent mixtures span-
ning a wide range (from 2 to 37.5) of permittivity values
Solute-solvent interactions on the keto-enol tautomerism
of 2-nitrocyclohexanone in several organic solvents and
room-temperature ionic liquids (RTILs) have been analyzed
in terms of multiparameter equations. Permittivity and
cohesive pressure values of the RTILs, unavailable by direct
measurements, have been derived.
1
have been measured by UV-visible and H NMR spec-
trometry. There is a good agreement between the values
measured by the two techniques (Table 1 in Supporting
Information). The value of KT in water has been deter-
mined using a previously reported kinetic procedure.3 The
results obtained show a strong dependence of KT on the
permittivity of the solvent (Figure 1).
The transfer of the tautomeric equilibrium to solvents
of increasing polarity is accompanied by a progressive
bathochromic shift of λmax in the UV spectrum of EH. This
positive solvatochromism of EH has been used to assign
a normalized polarity index, TN (see Supporting Informa-
tion), to the investigated solvents (analogous to the well-
known6-8 indexes π*, πa/zo, and ET). The large amount of
experimental data collected allows an in depth regression
analysis of the solvent effects in terms of different
multiparameter equations.9 Various linear combinations
of one, two, and three descriptors have been considered
according to the general eq 1
Nitrocarbonyl compounds have recently attracted con-
siderable interest both from the synthetic1 and com-
mercial2 points of view. Although nitroketones represent
a rare example of monocarbonyl derivatives with ap-
preciable quantities of the enol form, EH, in equilibrium
with the keto form, KH, kinetic3-5 studies of tautomer-
ization and, particularly, studies of solvent effects on the
tautomeric equilibrium constant, KT ) [EH]/[KH], are
very scarce. We have measured the tautomeric equilib-
rium constant of 2-nitrocyclohexanone (2-NCH) in several
organic solvents at 25.0 ( 0.1 °C. Determinations carried
1
out in cyclohexane (C6H12) by a combination of IR, H
NMR, and UV techniques show that, after dissolution of
2-NCH, equilibrium is established between KH and EH.
† Universita` “G. d’Annunzio”.
‡ Universita` degli Studi di Pisa.
§ Universita` “La Sapienza”.
(1) (a) Seebach, D.; Colvin, E. W.; Lehr, F.; Weller, T. Chimia 1979,
33, 1. (b) Rosini, G.; Ballini, R. Synthesis 1988, 833. (c) Ballini, R.
Synlett 1999, 1009.
∆G0x ) ∆G0gas + ΣifiDi
(1)
(2) Fischer, R. H.; Weitz, H. M. Synthesis 1980, 261.
(3) Fontana, A.; De Maria, P.; Siani, G.; Pierini, M.; Cerritelli, S.;
Ballini, R. Eur. J. Org. Chem. 2000, 1641.
(6) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J.-L. M.; Notario,
R. J. Phys. Chem. 1994, 98, 5807.
(4) Bell, R. P.; Robinson, R. R. Proc. R. Soc. A 1962, 270, 411.
(5) Toullec, J. In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley
Sons: New York, 1990; p 323.
(7) Reichardt, C. Chem. Rev. 1994, 94, 2319.
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10.1021/jo050288g CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/03/2005
J. Org. Chem. 2005, 70, 8193-8196
8193

TABLE 1. Tautomeric Constants, KT, for 2-NCH in Different Solvents and Physical Properties of the Solvents
solvent
ꢀ
δ (cal1/2 cm-3/2)
F(ꢀ)
TN
KT
C6H12
2
8.2
0.200
0.03 ( 0.02
0.25 ( 0.02
0.67 ( 0.02
0.67 ( 0.02
0.67 ( 0.02
0.82 ( 0.02
0.77 ( 0.02
0.77 ( 0.02
0.75 ( 0.02
0.77 ( 0.02
6.51 ( 0.57
5.97 ( 0.47
CCl4
2.2
4.8
8.9
35.9
8.6
0.222
CHCl3
9.3
0.358
1.53 ( 0.06
CH2Cl2
9.7
0.420
0.64 ( 0.02
CH3CN
11.8
0.479
0.110 ( 0.006
0.091 ( 0.012
0.138 ( 0.007
0.120 ( 0.007
0.157 ( 0.017
0.156 ( 0.002
1
2
3
4
5
10.0 ( 1.5
22.5 ( 7.4
16.2 ( 3.8
25.7 ( 9.6
29.7 ( 13.3
13.1 ( 0.1
11.8 ( 0.1
12.2 ( 0.1
11.5 ( 0.1
11.5 ( 0.1
0.429 ( 0.010
0.467 ( 0.010
0.455 ( 0.010
0.471 ( 0.009
0.475 ( 0.009
where ∆G0x represents the standard free energy change
associated with the tautomeric equilibrium of 2-NCH in
solvent x, Di represents the i selected descriptors of
solvent polarity, and fi represents the corresponding
regression coefficients. The above correlation analysis has
been initially limited to the five pure organic solvents,
as the corresponding D values are available from the
literature. It turns out that the two best correlations (R2
> 0.999, Table 3 in Supporting Information) are those
obtained from the three-parameter equations that use
Kirkwood’s function F(ꢀ) ) (ꢀ - 1)/(2ꢀ + 1), the cohesive
pressure δ2, and ET or TN [eqs 2 and 3] as the descriptors
of the observed solvent effects.
∆G0x ) ∆G0gas + aδ 2 + bF(ꢀ) + cET
(2)
N
∆G0x ) ∆G0gas + aδ 2 + bF(ꢀ) + cTN )
∆G0gas + ∆G0
FIGURE 2. Comparison between the values of ∆Gx° calcu-
lated from eq 3 (∆Gx°calc) and the experimental values (∆Gx°exp).
(3)
solv
pressure values, δ, due to the absence of volatility, are
available. However, we note that a paper16 has recently
appeared on the estimates of internal energies of vapor-
ization of some RTILs determined on the basis of kinetic
measurements for the reaction of singlet oxygen with 1,4-
dimethylnaphthalene. Empirical polarity scales, devel-
oped using solvatochromic dyes, suggest that imidazoli-
um RTILs are more polar than acetonitrile and less polar
than or of comparable polarity to that of lower alcohols.14
However, polarity is the sum of all possible, nonspecific
It should be considered that these three parameters
express different and complementary properties of the
solvent, namely, the electrostatic shielding of partial or
full charges [F(ꢀ)], the cavitational energy (δ2), and
aspecific as well as lone pair donor-acceptor and hydro-
gen bonding interactions (ET or TN). We have then
extended the regression analysis to the above-mentioned
18 binary solvent mixtures (see Table 4 in Supporting
Information) in terms of eq 3, where TN represents
experimental values, F(ꢀ) has been calculated from
Kirkwood’s theory,10 and δ2 has been calculated from the
known values in the pure solvents taking into consider-
ation the appropriate volume fractions in the examined
binary mixtures.11 The obtained correlation is good
(12) (a) Holbrey, J. D.; Seddon, K. R. Clean Prod. Processes 1999,
1, 223. (b) Earle, M. J.; Seddon, M. K. R. Pure Appl. Chem. 2000, 72,
1391. (c) Welton, T. Chem. Rev. 1999, 99, 2071. (d) Wasserscheid, P.;
Keim M. Angew. Chem., Int. Ed. 2000, 39, 3772. (e) Sheldon, R. Chem.
Commun. 2001, 2399. (f) Olivier-Bourbigou, H.; Magna L. J. Mol.
Catal. A 2002, 182, 419. (g) Dupont, J.; de Souza, R. F.; Suarez, P. A.
Z. Chem. Rev. 2002, 102, 3667. (h) Wilkes J. S. J. Mol. Chem. A 2004,
214, 11. (i) Ionic Liquids in Synthesis Wasserscheid, P., Welton, T.,
Eds.; Wiley-VCH: Weinheim, 2003. (j) Welton T. Coord. Chem Rew.
2004, 248, 2459.
(13) (a) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyana-
sundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (b) Carda-
Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003,
375, 191. (c) Giraud, G.; M. J.; Gordon, C. M.; Dunkin, I. R.; Wynne,
K. J. Chem. Phys. 2003, 119, 464.
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4657. (c) Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. J. Chem. Soc.,
Perkin Trans. 2 2001, 433. (d) Aki, N. V. K.; Brennecke, J. F.; Samanta,
A. Chem. Commun. 2001, 413. (e) Fletcher, K. A.; Storey, I. A.;
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Am. Chem. Soc. 2002, 124, 14247. (b) Abraham, M. H.; Zissimos, A.
M.; Huddleston, J. G.; Willauer, H. D.; Rogers, R. D.; Acree, W. E. Ind.
Eng. Chem. Res. 2003, 42, 413. (c) Ropel, L.; Belve`ze, L. S.; Aki, S. N.
V. K.; Stadtherr, M. A.; Brennecke, J. F. Green Chem. 2005, 7, 83.
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Chem. Commun. 2004, 2178.
(∆G0 ) -3.53 ( 0.12 kcal/mol, R2 ) 0.9815, n ) 23),
gas
and the average difference between the experimental
values of ∆G0 and those estimated from eq 3 is of only
x
0.075 kcal/mol (Figure 2).
Room-temperature ionic liquids (RTILs) are emerging
as convenient, reusable, environmentally benign, and
versatile solvents for a variety of organic reactions.12 An
attractive feature of RTILs is that their physical proper-
ties can be fine-tuned by structural variations within the
cationic or anionic components.13,14a-c Although several
efforts have been made to determine the polarity of RTILs
using solvatochromic dyes and partition methods,14,15
neither direct measurement of permittivities, ꢀ, which
would require a nonconducting medium, nor cohesive
(9) Katritzky, A. R.; Fara, D. C.; Yang, H.; Tamm, K.; Tamm, T.;
Karelson, M. Chem. Rev. 2004, 104, 175.
(10) Wang, P.; Anderko, A. Fluid Phase Equilib. 2001, 186, 103.
(11) Snyder, L. R. In Techniques of Chemistry; Perry, E. S., Weiss-
berger, A., Eds.; Plenum Press: New York, 1978; pp 25-75.
8194 J. Org. Chem., Vol. 70, No. 20, 2005

SCHEME 1. Structures of the Investigated RTILs
and specific, interactions between the solute ions or
molecules and the solvent molecules. These interactions
involve a number of different intermolecular forces with
the consequence that no single probe molecule is capable
of providing a reliable polarity scale. To separate specific
effects of the local electric fields from the hydrogen
bonding effects, Welton has recently determined17 the
polarity of some RTILs applying the classical approach
of Abboud-Kamlet-Taft. Dipolarity/polarizability (π*),
hydrogen bond basicity (â), and hydrogen bond acidity
(R) of some RTILs have been determined by using three
solvatochromic dyes. The data obtained show that all the
examined RTILs are characterized by π* values higher
than those of most nonaqueous molecular solvents.
Depending upon the anion the examined, RTILs can have
a significant hydrogen bond basicity and, depending upon
the cation, a hydrogen bond acidity comparable to or
lower than that of aniline, although the latter parameter
is also affected by the anion.17 More recently, the use of
pentane-2,4-dione as a possible indicator of polarity for
RTILs has been proposed18 by Seddon. Within this
context, we have measured the tautomeric equilibrium
constant, KT, of 2-NCH also in the five RTILs of Scheme
1 to obtain quantitative estimates of the polarities of 1-5.
The values of KT in RTILs 1-5 (Table 1) are generally
close to those measured in solvents of relatively high
polarity such as acetonitrile, where the more polar keto
tautomer, KH, is favored over the less polar enol tau-
tomer, EH, and KT < 1.
FIGURE 3. Plot of Hildebrand’s δ values against free energy
change (∆G0 ) for the tautomeric equilibrium of 2-NCH in the
x
23 organic solvents ([) and RTIL 1 (O).
TABLE 2. Physical Properties of RTILs 1-5
RTIL
δ2 (cal cm-3
)
Vm (cm3 mol-1
)
Uvap (kcal mol-1
)
1
2
3
4
5
171.7 ( 2.4
139.9 ( 2.4
149.3 ( 2.4
132.4 ( 2.4
132.8 ( 2.4
208.9 ( 1.2
256.4 ( 4.2a
299.0 ( 4.9a
287.2 ( 4.7a
317.4 ( 5.2a
35.8 ( 0.4
35.7 ( 1.2
44.5 ( 1.5
38.0 ( 1.2
42.0 ( 1.4
a Estimated by molecular mechanics.
An excellent sigmoidal correlation (Figure 3) exists
between the Hildebrand’s δ values11 of the 23 organic
media and the corresponding ∆G0 associated with the
x
keto-enol tautomerism of 2-NCH. By inserting a previ-
ously calculated20 δ2 value of 182 cal cm-3 (δ ) 13.5) at
25 °C for 1 into the correlation of Figure 3, eq 4 (R2 )
0.9814, n ) 24) can be obtained, which provides the set
of δ of RTILs 1-5 reported in Table 1. The comparison
between the δ data points of Figure 3 and the values of
δ from eq 4 gives a standard deviation of 0.1.
δ ) 0.778(∆G°)3 + 0.258(∆G°)2 + 0.565∆G° + 9.551
x
x
x
(4)
The experimentally determined TN values for RTILs
of Scheme 1 are also of interest. Although the [PF6]-
anion of 1 displays strong interactions with EH (as shown
by the large value of TN), the overall effect of 1 is a
stabilization of KH, as shown by the lowest measured
value of KT. On the other hand, for RTILs 2-5, which
have the [TF2N]- anion in common, the highest formal
polarity, as measured by the corresponding KT value,
should be assigned to 3, despite the larger apolar portion
of its cationic surface. Smaller variations of KT and TN
have been measured for the remaining RTILs 2, 4, and
5, which can be easily accounted for by the different size
of their alkyl groups in position 1.
As the obtained δ values of the investigated RTILs lie
in the interval 13.1 > δ > 11.5, a superficial analysis
would lead to the conclusion that two of them are less
polar than acetonitrile (δ ) 11.8). However, it should be
recalled that δ2 and internal vaporization energy, Uvap
are related by eq 5 as follows
,
δ2 ) ∆Uvap/Vm
(5)
where Vm is the molar volume of the solvent. Using the
literature21 Vm value of 208.9 cm3 mol-1, we calculated a
Uvap of 35.8 kcal mol-1 for 1. The Uvap values for RTILs
2-5 have been estimated from the approximate Vm
values calculated as described in Supporting Information.
The results obtained are reported in Table 2.
The Uvap value calculated for acetonitrile is only 7 kcal
mol-1, and this result is of course in full agreement with
the larger Vms and the extremely low vapor pressures of
RTILs. The values of Kirkwood’s function, F(ꢀ), and
consequently those of ꢀ for 1-5 can be obtained from eq
3 by using the same regression coefficients ∆G0gas, a, b,
The absence of strong acidic hydrogen atoms19 in all
of the investigated RTILs and the fact that the highest
formal polarity should be assigned to 3 (the RTIL lacking
the H atom at C-2) suggest that the solute-solvent
interactions on the keto-enol equilibrium of 2-NCH are
comparable to those observed in aprotic organic solvents.
(17) Crowhurst, L.; Mawdsley, P. R.; Perez-Arlndis, J. M.; Salter,
P. A.; T. Welton, T. Phys. Chem. Chem. Phys. 2003, 5, 2790.
(18) Earle, M. J.; Engel, B. S.; Seddon, K. R. Aust. J. Chem. 2004,
57, 149.
(19) A pKa of 23.0 has been reported for 1,3-dimethylimidazolium
cation in water. Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F.
M.; Toth, K. J. Am. Chem. Soc. 2004, 126, 4366.
(20) Morrow, T. I.; Maginn, E. J. J. Phys. Chem. B 2002, 106, 12807.
(21) Gu, Z.; Brennecke, J. F. J. Chem. Eng. Data 2002, 47, 339.
J. Org. Chem, Vol. 70, No. 20, 2005 8195

and c found with the 23 apolar organic solvents. Only a
few permittivity values, ꢀ, for some imidazolium-based
RTILs have been reported13,22 so far. The ꢀ values,
determined13,22 indirectly by fluorescence experiments,
generally range between 10 and 13. More particularly,
the ꢀ value of 10.0 ((1.5) estimated from eq 3 for 1 is in
good agreement with those of about 10-11 reported for
1 in the previous studies.13,22 The ꢀ values of 1-5 (Table
1) are unexpectedly low, if compared with known ꢀ values
of polar organic solvents generally considered14a similar
to RTILs. It should be recalled though that the permit-
tivity, ꢀ, of a given solvent depends on the dipolar
character of the solvent, which should be absent in ideal
RTIL salts. In real RTIL systems, ion-pair formation,23
with associated dipole moments24 much higher than those
of the molecules of common organic solvents, might well
give rise to substantial ꢀ values. To generalize, the
following four points clearly emerge from the present
study: (i) The tautomeric equilibrium of 2-NCH, which
under vacuum), the water content of RTILs was deter-
mined by the Karl Fisher technique using an apparatus
composed of a stand titrator and a coulometer. A water
content within 120-150 ( 20 ppm was found for ionic
liquids 2-5, whereas a water content of 348 ( 25 ppm
was found for 1.
Determination of the Tautomeric Equilibrium
Constant of 2-NCH. Tautomeric equilibrium constants,
x
KT (NMR), of 2-NCH have been determined in pure organic
1
solvents x by H NMR using peak area ratios for the
acidic hydrogens of the enol and the keto tautomers.
The molar absorptivities of EH in the different solvents
x, ꢀEx H, have been calculated by eqs 6 and 7
AEx H
EH
cyclohexane
ꢀRx )
(6)
(7)
A
ꢀEx H ) ꢀRx ꢀEH
cyclohexane
is in favor of the enol form in the gas phase (∆G0
)
gas
where AEx H is the absorbance at λEH of a solution
obtained by diluting 200 times with somlvaexnt x a standard
solution of 2-NCH in CHCl3 containing about 0.04 M
-3.53 kcal/mol), is displaced toward the keto form upon
transfer to relatively polar organic solvents and RTILs
1-5. This is essentially due to the fact that the cavita-
tional and nonspecific electrostatic contributions by the
solvent, which strongly stabilize KH, more than offset
the specific electrostatic contribution which stabilizes EH.
(ii) RTILs with the [TF2N]- anion and R2dH (Scheme 1)
display solvation effects similar to that of acetonitrile,
albeit with a somewhat higher ability to stabilize the enol
tautomer. (iii) RTILs having a methyl group at C-2 or
[PF6]- as the counteranion are characterized by higher
values of the cohesive pressure and more effectively
stabilize the keto tautomer of 2-NCH. Finally, (iv)
comparison of the polarities of 1-4 with those of 3 and 4
suggests that the anionic portion affects the polarity of
RTILs more than the cationic portion.
trifluoroacetic acid (TFA) in order to stop the intercon-
EH
version of the tautomers. The required ꢀ
value
cyclohexane
has been obtained from a best fit of eq 8 to experimental
KT (NMR) data determined in C6H12, CHCl3, CH2Cl2, CH3-
x
EH
cyclohexane
CN (R ) 0.99, ꢀ
) 8321 ( 86)
1
x
KT (UV)
)
(8)
[2-NCH]ꢀRx ꢀEH
cyclohexane - 1
Aexq
where Aexq is the absorbance of EH at equilibrium in
solvent x. Spectrophotometric equilibrium constants of
x
2-NCH, KT (UV), in the different solvents have been
Interestingly, the values of δ reported by Gordon et
al.16 for 1, 3, and 4 are in good agreement with those of
the present work. This supports the reliability of our
approach and of the values of permettivity (ꢀ) of RTILs
1-5.
x
calculated from eq 8. KT (UV) and ꢀEH are collected in Table
1 and Table 4 of Supporting Information as average
values of three to four determinations.
Determination of Permittivity Values. Permittivity
values of RTILs 1-5 were obtained from a rearranged
form of eq 3 as follows:
Experimental Section
1
Instruments. H NMR spectra were recorded with a
F(ꢀ) ) (∆G0x - ∆G0gas - aδ2 - cTN)/b
ꢀ ) (F(ꢀ) + 1)/(1 - 2F(ꢀ))
Bruker Avance 400 spectrometer. Chemical shifts (δ) in
Figure 1 of Supporting Information are given in parts
per million relative to TMS. IR spectra were recorded on
a Jasco FT/IR-430 spectrophotometer. UV-vis spectra
were recorded with a Varian Cary 1E spectrophotometer
with a spectral resolution set to 0.333 nm.
The uncertainty of the F(ꢀ) values was estimated by the
treatment of propagation of errors. It should be outlined
that error on ꢀ increases exponentially as the value of
F(ꢀ) increases (see Figure 2 in Supporting Information).
Materials. RTILs of Scheme 1 were prepared from the
corresponding halides, following reported procedures.13,25
The purity of imidazolium salts was checked by ESI-MS
and UV spectrophotometry (purified [bmim]+ salts con-
taining [Cl]- or [Br]- < 0.1 ppm have no absorption band
in the 250-300 nm region).26 After drying (2 h at 80 °C
Acknowledgment. We thank MIUR-Rome for the
financial support (PRIN 2003 and 2004).
Supporting Information Available: Spectrophotometric
identification of the tautomers of 2-NCH in cyclohexane;
tautomeric constants for 2-NCH in different solvents; standard
free energy change associated with the tautomeric equilibrium;
calculation of the polarity index TN; and calculation of the
molar volume of RTILs 1-5. This material is available free of
charge via the Internet at http://pubs.acs.org.
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