Solvent effects on chemical processes: New solvents designed on the basis of the molecular-microscopic properties of (molecular solvent + 1,3-dialkylimidazolium) binary mixtures

Article abstract of DOI:10.1002/poc.1227

The purpose of this work was to analyze the microscopic feature of binary solvent systems formed by a molecular solvent (acetonitrile or dimethylformamide or methanol) and an ionic liquid (IL) cosolvent [1-(1-butyl)-3- methylimidazolium tetrafluoroborate

Full text of DOI:10.1002/poc.1227

Research Article
Received: 3 May 2007,
Accepted: 17 May 2007,
Published online in Wiley InterScience: 13 December 2007
(www.interscience.wiley.com) DOI 10.1002/poc.1227
Solvent effects on chemical processes: new
solvents designed on the basis of the
molecular–microscopic properties of
(
molecular solvent þ 1,3-dialkylimidazolium)
binary mixtures
P. M. Mancini *, G. G. Fortunato , C. G. Adam and L. R. Vottero
a
a
ay
a
The purpose of this work was to analyze the microscopic feature of binary solvent systems formed by a molecular
solvent (acetonitrile or dimethylformamide or methanol) and an ionic liquid (IL) cosolvent [1-(1-butyl)-3-methyli-
midazolium tetrafluoroborate or 1-(1-butyl)-3-methylimidazolium hexafluorophosphate]. The empirical solvatochro-
mic solvent parameters E
T
(30), p*, a, and b were determined from the solvatochromic shifts of adequate indicators.
The behavior of the solvent systems was analyzed according to their deviation from ideality. The study focused on the
identification of solvent mixtures with relevant solvating properties in order to select mixed solvents with particular
characteristics. The comparison of the molecular–microscopic solvent parameters corresponding to the selected
binary mixtures with both ILs considered at similar mixed-solvent composition revealed that the difference is
centered on the basic character of them. A kinetic study of a nucleophilic aromatic substitution reaction between
1-fluoro-2,4-dinitrobenzene (FDNB) and 1-butylamine (BU) developed in (acetonitrile or dimethylformamide R IL)
solvent mixtures is presented in order to investigate and compare the solvent effects on a chemical process. For the
explored reactive systems the solvation behavior is dominated by both the dipolarity/polarizability and the basicity
of the media, contributing these solvent properties to accelerating the chemical process. Copyright ß 2007 John
Wiley & Sons, Ltd.
Keywords: molecular solvent; ionic liquid; binary mixtures; molecular–microscopic properties; solvatochromic parameters;
aromatic nucleophilic substitution
solvent properties. In most cases only one indicator is used to
INTRODUCTION
T
build the respective scale. Among them, the E (30) parameter
[1]
Molecular solvents are the usual media where physical and
was proposed by Dimroth and Reichardt to measure solvent
dipolarity/polarizability, though it is also sensitive to the solvent’s
hydrogen-bond donor (HBD) capacity. On the other hand, the
[1]
chemical processes in solution take place. In the last decade,
the attention has been drawn to the use of room-temperature
ionic liquids (ILs) as solvents for different applications, the interest
being mainly based on their potential as solvents for clean
[12–14]
p*, b, and a (Kamlet, Abboud, and Taft)
multiparametric
approach assigns different parameters to each particular solvent
characteristics (dipolarity/polarizability, hydrogen-bond acceptor
(HBA) basicity, and HBD acidity).
[4–11]
technologies.
Because of the wide range of employable
cations and anions, many new ionic solvents can be prepared.
From the combination of pure solvents in binary mixtures, the
availability and diversity of mediums increased considerably.
Over the last few years, designing solvents with desired
properties has become an important task for chemists in order
to modulate processes in solution. The challenge consists in
developing a ‘desired solvent’ with a particular use in mind.
To evaluate the behavior of a liquid as a solvent, the
understanding of the solvation interactions at a molecular level
is necessary. In this direction, it is of interest to quantify its most
relevant molecular–microscopic solvent properties, which deter-
mine how it will interact with potential solutes. An appropriate
method to study solute–solvent interactions is the use of
solvatochromic indicators that reflect the specific and nonspecific
solute–solvent interactions on the UV–Vis spectral band shifts. In
this sense, a number of empirical solvatochromic parameters
have been proposed to quantify the molecular–microscopic
While the use of pure ILs as reaction media was investigated in
[4–11]
a variety of chemical processes,
reactions carried out in their
binary mixtures with molecular solvents have been less
investigated. We have recently studied the kinetics of nucleo-
philic aromatic substitution (S Ar) reactions carried out in some
N
´
´
*
Departamento de Qu ´ı mica, Area Qu ´ı mica Organica, Facultad de Ingenier ´ı a
Qu ´ı mica, Universidad Nacional del Litoral, Santiago del Estero 2829, (3000)
Santa Fe Rep u´ blica Argentina.
E-mail: pmancini@fiqus.unl.edu.ar
a
P. M. Mancini, G. G. Fortunato, C. G. Adam, L. R. Vottero
´
´
Departamento de Qu ´ı mica, Area Qu ´ı mica Organica, Facultad de Ingenier ´ı a
Qu ´ı mica, Universidad Nacional del Litoral, Santiago del Estero 2829, (3000)
Santa Fe Rep u´ blica Argentina
y
Researcher from CONICET.
J. Phys. Org. Chem. 2008, 21 87–95
Copyright ß 2007 John Wiley & Sons, Ltd.

P. M. MANCINI ET AL.
molecular solvent/IL systems. The reaction between 1-fluoro-
,4-dinitrobenzene (FDNB) and 1-butylamine (BU) or piperidine
performed in binary mixtures of the type (molecular sol-
vent þ alkylammonium salts) was particularly analyzed (Mancini,
Fortunato, Adam, unpublished work).
anion is parallel to the value of acetonitrile. The higher proton
2
acceptor ability of [BF
employing other solutes as solvatochromic probes.
4 6
] with respect to [PF ] has been confirmed
[21]
The molecular solvents included in the analysis are acetonitrile
N
(AN, E ¼ 0.46, p* ¼ 0.75, a ¼ 0.19, b ¼ 0.40), N,N-dimethyl-
T
N
At this point, the main purpose of this work is to investigate
solvent systems resulting from the basis of binary mixtures
formed by a molecular solvent (with different structures) and an
IL cosolvent. With respect to the ionic cosolvent, we are
particularly interested in ILs characterized by slighter HBD acidity
than other ILs such as the alkylammonium type. In this direction,
the selected liquids are based on 1,3-dialkylimidazolium cations.
They exhibit a wide spectrum of physicochemical properties. It is
demonstrated that their water content, density, viscosity, surface
tension, melting point, and thermal stability are affected by the
formamide (DMF, E ¼ 0.40, p* ¼ 0.89, a ¼ 0, b ¼ 0.71), and
T
N
T
[1,22]
methanol (MeOH, E ¼ 0.76, p* ¼ 0.60, a ¼ 0.98, b ¼ 0.66).
The selection was done on the basis of their miscibility with the IL
and their microscopic properties. AN is a polar HBA/HBD solvent,
DMF is a polar aprotic HBA solvent, and MeOH is a protic solvent.
It is important to remark that the microscopic solvent parameters
for (MeOH þ [bmim][BF
4
]) mixtures have been reported
[23]
recently.
Characterization of binary solvent mixtures. Determination
of the molecular–microscopic solvent parameters
[
7,9]
length of the alkyl chain and by the nature of the anion.
The attention is given to the microenvironment of solutes in
order to investigate the solvation capability of this type of binary
mixtures. So far, the aims are: (i) to characterize the molecu-
lar–microscopic solvent properties measuring a set of empirical
solvatochromic parameters for molecular solvent/IL systems; (ii)
to analyze the response patterns property versus solvent
composition focusing on the changes in the ability to develop
the most relevant interactions (hydrogen-bond acidity, hydro-
gen-bond basicity, and polarity); (iii) to identify relevant mixtures
paying particular attention to pronounced changes in the
microscopic solvent feature in order to select ‘new solvents’; (iv)
N
The E , p*, a, and b solvatochromic parameters (which were
T
determined UV–Vis spectroscopically by means of the adequate
reference compounds) are listed in Table 1. Figure 1 shows the
response patterns of the molecular–microscopic solvent proper-
ties on the overall IL mole fraction range. It can be observed that,
in general, the binary mixtures with [bmim][PF
dipolarity/polarizability, slightly higher acidity, and lower basicity
except at the lowest concentration of IL) than mixtures with
bmim][BF ]. The shape of the property versus solvent composition
curves reflects the experimental behavior patterns of the
explored mixtures to the chemical properties of the reference
probes being analyzed according to their deviation from ideality.
6
] exhibit similar
(
[
4
N
to carry out a kinetic study of a S Ar reaction developed in those
binary mixtures which present these relevant microscopic
properties in order to investigate and compare the solvent
effects on a chemical process with relation to the pure molecular
solvent.
Polarity ENT
N
T
On the one hand, as it has been mentioned above, the E values
for the [bmim] ILs lie in the range corresponding to polar HBD
[
1,24]
solvents (such as short-chain alcohols).
On the other hand, it
N
RESULTS AND DISCUSSION
is known that the ET scale is particularly sensitive to the HBD
[25]
ability of the cation of ILs.
In connection with this, the
The selected ILs are composed of 1-(1-butyl)-3-methyl-
imidazolium [bmim] as the cation and tetrafluoroborate and
experimental data reflect the expected results considering that
both ILs contain the same cation: the parameter values and the
N
hexafluorophosphate as the anions: [bmim][BF
4
] (E ¼ 0.67,
T
shapes of the curves are similar for the mixtures with both ILs. The
N
p* ¼ 1.05, a ¼ 0.63, b ¼ 0.38); [bmim][PF ] (E ¼ 0.67, p* ¼ 1.03,
6
T
N
T
E
values exhibit, in all cases, positive deviation from the ideal
[5]
a ¼ 0.63, b ¼ 0.21). The [bmim][BF
4 6
] and [bmim][PF ] were
behavior. Moreover, whereas in the mixtures with AN, a slight
synergetic effect takes place at high IL mole fractions, in the
mixtures with MeOH this effect is clearly marked at all
compositions. The prominence of the synergetic effect in
chosen taking to account that they are the most currently used
ILs and they are nonprotic solvents. They comprise the same
cation and a different anion. The anion mainly determines the
water miscibility. With respect to their physical properties,
N
mixtures with MeOH can be related to the fact that the ET
[bmim][PF
6
], exhibits lower melting point, higher viscosity, higher
N
values of both pure solvents are nearly equal. Synergism on ET
density, and lower conductivity than [bmim][BF ]. While the latter
4
values was reported for the [bmim][PF
6
] þ ethanol solvent
one displays a complete miscibility with water at 25 8C, the first
[26,27]
system.
[7,8,9]
one shows a very low miscibility with this solvent.
It is important to remark that the agreement between the ETN
Taking into account their molecular–microscopic properties,
values for MeOH þ [bmim][BF
4
] mixtures here determined and
N
while E , p*, and a values are similar, the b value of [bmim][BF
] is
]. The ENT
values lie in the range associated with polar HBD solvents such as
[23]
4
N
T
the recently reported values
is excellent (DE ꢀ 0.02).
T
near twofold the value corresponding to [bmim][PF
6
Dipolarity/polarizability p*
[1,16,17]
short- chain alcohols.
with nonaqueous molecular solvents and both the cation and the
anion affect this parameter.
The p* values are high in comparison
The plots presented in Fig. 1 show that p* values exhibit positive
deviations from the ideal behavior, which are more marked when
[15]
The a values are largely
determined by the nature of the cation; in this sense, it is
known that 1-methyl-3-alkylimidazolium cations can act as weak
HBDs because of the weak acidic C(2)—H hydrogen atom at the
heterocyclic ring (which is the most acidic of all imidazolium
the difference between the molecular–microscopic property of
both pure solvents is larger. The p* values of mixtures with both
ILs are similar. For the mixtures with AN, the pattern shows an S
type curve, whereas in mixtures with DMF the behavior is nearly
ideal. Additionally, the results for MeOH þ IL mixtures clearly
reflect that the dipolarity/polarizability for these mixtures is
[18–20]
H-atoms).
The b values are dominated by the nature of the
] as
anion. The hydrogen-bond acceptor basicity of the IL with [BF
4
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 87–95

DIALKYLIMIDAZOLIUM IONIC LIQUIDS
N
Table 1. Solvatochromic parameters E , p*, b, and a for binary mixtures of acetonitrile or dimethylformamide or metha-
T
nol þ [bmim][BF
4 6
] or [bmim][PF ], measured at 25 8C
Ionic liquid mole fraction
Parameter
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
AN þ [bmim][BF
4
]
N
E
0.62
0.86
0.41
0.58
0.64
0.88
0.49
0.61
0.66
0.93
0.48
0.61
0.67
0.97
0.47
0.61
0.68
0.99
0.46
0.62
0.69
1.01
0.45
0.62
0.70
1.01
0.45
0.63
0.70
1.00
0.45
0.64
0.71
1.01
0.45
0.65
0.72
1.01
0.45
0.66
T
p*
b
a
DMF þ [bmim][BF ]
4
N
E
0.56
0.95
0.68
0.33
0.63
0.98
0.56
0.40
0.65
1.00
0.48
0.53
0.67
1.00
0.42
0.59
0.67
1.02
0.38
0.63
T
p*
b
a
MeOH þ [bmim][BF
4
]
N
E
0.84
0.95
0.46
0.97
0.83
1.00
0.47
0.92
0.80
1.03
0.46
0.86
0.78
1.04
0.43
0.81
0.77
1.04
0.41
0.77
T
p*
b
a
AN þ [bmim][PF
6
]
N
E
0.62
0.87
0.40
0.59
0.64
0.89
0.35
0.62
0.65
0.91
0.30
0.64
0.66
0.93
0.27
0.65
0.67
0.94
0.24
0.66
0.68
0.96
0.22
0.66
0.68
0.97
0.20
0.66
0.69
0.98
0.19
0.67
0.69
0.98
0.17
0.67
T
p*
b
a
DMF þ [bmim][PF
6
]
N
E
0.56
0.94
0.71
0.34
0.61
0.95
0.64
0.40
0.65
0.95
0.57
0.46
0.65
0.95
0.48
0.53
0.65
0.95
0.39
0.59
0.66
0.96
0.32
0.60
0.66
0.97
0.25
0.61
0.67
0.99
0.22
0.62
0.67
1.00
0.18
0.63
T
p*
b
a
MeOH þ [bmim][PF
6
]
N
E
0.84
1.00
0.45
0.93
0.85
1.01
0.26
0.98
0.83
1.01
0.17
0.96
0.80
1.03
0.18
0.87
0.74
1.00
0.20
0.77
T
p*
b
a
0.49
dominated by the IL showing the largest preferential solvation.
These results can be related to the ion indicator of nonspecific
interactions which control the solvation pattern. It can be pointed
hand, the calculated basicity of MeOH þ [bmim][BF ] mixtures is
[23]
4
lower than previously reported data (0.09 ꢀ Db ꢀ 0.23).
out that the here determined p* values of MeOH þ [bmim][BF ]
4
mixtures are, in all cases, higher than those reported previously
Acidity a
[23]
(
0.02 ꢀ Dp* ꢀ 0.16).
The plots presented in Fig. 1 show that a values exhibit positive
deviations from the ideal behavior. Two different response
patterns can be observed, independently of the IL which
takes part in the mixtures: (i) for AN and DMF the parameter
values abruptly increase with small increases in cosolvent
content, this effect being controlled by the IL; and (ii) the acidity
decreases continuously when the IL is added to MeOH exhibiting
a positive deviation from the ideal behavior. This last solvation
pattern is controlled by the protic solvent. It can be noted that the
Basicity b
The plots of b versus solvent composition presented in Fig. 1 reveal
that in all cases the basicity values of the mixtures with
[bmim][BF
4
] are higher than those of the mixtures with
[bmim][PF ]. The general trend shows a negative deviation of
6
b from the ideal behavior, excluding AN þ [bmim][BF
4
] mixtures.
This latter solvent system exhibits a synergetic effect for this
property. In connection with this, it is important to notice that the
calculated a values of MeOH þ [bmim][BF ] mixtures are lower
4
[23]
basicity of [bmim][BF ] matches that of AN. The response patterns
than those previously reported (0.10 ꢀ Da ꢀ 0.18).
4
show that the influence of the IL is strong except at the lowest IL
concentration, suggesting that the property values are domi-
nated by the anion. These results can be related to the prefer-
ential solvation of the solutes by the ILs through the anions, with
These results can be ascribed to two reasons: on the one hand,
Reichard t´ s dye has
strongly hydrogen-bond accepting
phenolate oxygen which interacts with the cation of the IL,
a
T
and, on the other hand, the E (30) values are the key in the
4 6
[BF ] being a better solvating species than [PF ]. On the other
measurements of acidity a.
J. Phys. Org. Chem. 2008, 21 87–95
Copyright ß 2007 John Wiley & Sons, Ltd.
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P. M. MANCINI ET AL.
Figure 1. molecular–microscopic solvent properties versus mixed-solvent composition
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 87–95

DIALKYLIMIDAZOLIUM IONIC LIQUIDS
Table 2. Selected binary solvent mixtures and their molecular–microscopic solvent properties
N
T
Solvent
Composition
E
p*
b
a
Liquid 1
Liquid 2
Liquid 3
Liquid 4
Liquid 5
Liquid 6
Liquid 7
Liquid 8
X
AN 0.9/X[bmim][BF4] 0.1
0.64
0.62
0.69
0.67
0.56
0.56
0.80
0.83
0.88
0.87
1.01
0.94
0.95
0.94
1.03
1.01
0.49
0.40
0.45
0.24
0.68
0.71
0.46
0.17
0.61
0.59
0.62
0.66
0.33
0.34
0.86
0.96
XAN 0.9/X[bmim][PF6] 0.1
X
X
AN 0.5/X[bmim][BF4] 0.5
AN 0.5/X[bmim][PF6] 0.5
XDMF 0.9/X[bmim][BF4] 0.1
X
X
DMF 0.9/X[bmim][PF6] 0.1
MeOH 0.5/X[bmim][BF4] 0.5
XMeOH 0.5/X[bmim][PF6] 0.5
It is worth noticing that the MeOH þ [bmim][PF
6
] mixtures
The changes observed in solvents 5 and 6 with respect to DMF
exhibit a higher HBD ability than the MeOH þ [bmim][BF ]
are similar to those observed in solvents 1 and 2, the increases in
a values being smaller than those for AN. Both liquids have
practically the same solvation pattern.
Solvents 7 and 8 show dipolarity/polarizability and basicity
values comparable to those of the pure ILs, the acidity values
being greatly increased with respect to the values of both ILs.
From the comparison of their properties, it can be observed that
the main divergence between them lies in the HBA ability which
is higher for liquid 7.
4
mixtures. Considering that these solvents systems differ only in
the nature of the anion, this behavior can be connected with an
anion effect on the acidity: the HBD ability is controlled by the
HBA ability of the anion. The more basic the anion, the greater the
effect and this is particularly significant in a protic solvent such as
MeOH.
Selection of ‘new solvents’
As it is known, binary mixtures of solvents can be deliberately
used to modify particular chemical characteristics of a reaction
media. In connection with this, we have focused our attention on
those binary mixtures as part of the solvent systems explored
in this work, with particular molecular–microscopic solvent
properties. In this sense, we selected individual solvent mixtures
with the objective of providing ‘new liquids’ with particular
solvating properties. It should be remarked that, in order to com-
Solvent effects on chemical processes: Aromatic nucleophilic
substitution reactions
In order to investigate this type of binary solvent systems as
media to carry out chemical reactions, a kinetic study of
the S
performed in some selected binary mixtures with AN and DMF, at
5 8C. Moreover, it is of interest to evaluate the behavior of the
N
Ar reaction between FDNB and the primary amine BU was
2
individual binary mixtures selected as ‘new solvents’.
6 4
pare the behavior of [bmim][PF ] with respect to [bmim][BF ], the
selected mixtures are of similar concentration for both ILs. The
proposed ‘new solvents’ are presented in Table 2.
The reactions were carried out at pseudo-first-order conditions:
they yielded the expected product [N-(2,4-dinitrophenyl)1-
butylamine] in quantitative yield and proved to be first order
in the corresponding substrate. The second-order rate constants
General trends
k
A
were calculated from the experimental pseudo-first-order
rate constants k and the appropriate amine concentration.
w
Solvents 1 and 2 exhibit similar changes with respect to AN. On
the one hand, the basicity remains constant or is slightly
increased, the dipolarity/polarizability being moderately
increased. On the other hand, the acidity is markedly increased.
These new liquids are also comparable with respect to their HBD
ability and dipolarity/polarizability but differ in the HBA ability
which is slightly higher for liquid 1.
Solvent 3 exhibits a slightly higher dipolarity/polarizability than
solvent 4, whereas this liquid is a slightly better HBD solvent. In
spite of this, it is the basicity that clearly characterizes the
solvation behavior of each liquid, solvent 3 being nearly two
times more basic than solvent 4.
At this point it is important to remark that the reactions
carried out in MeOH þ IL mixtures do not generate N-(2,4-
dinitrophenyl)1-butylamine in a quantitative way.
We assume that the reaction proceeds by the classical two-
step mechanism shown in the Scheme 1. Application of
the steady-state approximation for the concentration of ZH,
d[ZH]/d[t] ¼ 0, yields Eqn (1) for the second-order reaction rate
A
constant k .
À
Á
B
k1 k2 þ k ½Bꢁ
3
kA ¼
(1)
B
3
kꢂ1 þ k2k ½Bꢁ
Scheme 1.
J. Phys. Org. Chem. 2008, 21 87–95
Copyright ß 2007 John Wiley & Sons, Ltd.
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P. M. MANCINI ET AL.
ꢂ1 ꢂ1 ꢂ1
s ) and pseudo-first-order rate constants kobs (s ) for the reaction between
-fluoro-2,4-dinitrobenzene (4 ꢄ 10 M) and 1-butylamine in (AN or DMF þ IL) binary mixtures measured at 25 8C (including the
Table 3. Second-order rate constants k
1
data in the pure molecular solvents)
A
(l mol
ꢂ5
[1-BU]/M
Solvent
X
IL
0.002
0.004
0.006
0.008
k
A
AN þ [bmim][BF
4
]
a
0
0
0.0150
0.0316
0.0344
0.0360
0.03650
0.0370
0.0359
0.0398
0.0400
0.0430
0.0304
0.0623
0.0670
0.0660
0.0710
0.0740
0.0743
0.073
0.0454
0.0904
0.0950
0.1000
0.1040
0.1074
0.108
0.1043
0.100
0.100
0.0600
0.1185
0.1260
0.1342
0.1400
0.1440
0.1420
0.135
7.60
14.4
.1
.2
.3
.4
.5
.6
.7
.8
.9
0
0
0
0
0
0
0
0
15.1
16.4
17.2
17.7
17.6
16.0
13.4
12.9
0.0733
0.0720
0.121
0.12
AN þ [bmim][PF6]
DMF þ [bmim][PF6]
0
0
0
0
0
.1
.2
.3
.4
.5
0.0140
0.0161
0.0168
0.0163
0.0168
0.0297
0.0322
0.0320
0.0330
0.0329
0.0476
0.0500
0.0470
0.0526
0.0578
0.0681
0.0710
0.0625
0.0733
0.0761
7.60
8.98
9.01
9.49
10.1
b
0
0.0714
0.0336
0.0400
0.0545
0.0590
0.1492
0.1180
0.1200
0.1150
0.1270
0.230
0.2344
0.183
0.1560
0.1510
0.313
38.9
32.9
28.4
25.8
22.1
0.1
0.2
0.3
0.4
0.2144
0.2080
0.2130
0.198
a
Ref. 31.
Ref. 28.
b
B
For the condition kꢂ1 ꢃ k
2
þ k [B] Eqn (1) is reduced to k
A
¼ k
1
,
not operative in S
nucleophiles.
In order to analyze the changes in the second-order rate
constant when the IL is added to the molecular solvent, the plots
A
k versus solvent composition are shown in Fig. 2. According to
N
Ar reactions with primary amines as
3
the formation of the intermediate is rate limiting, and the reaction
is not base catalyzed. If the condition is not obeyed, the
decomposition of the intermediate is rate limiting, and base
catalysis is observed, the kinetic form of the catalysis depending
B
on the relative magnitudes of k and k þ k [B].
the kinetic response of the reactive systems, the following
observations can be made:
ꢂ1
2
3
Table 3 presents the experimental pseudo-first-order rate
constants k
performed in AN þ IL solvent systems. It can be pointed out that
in mixtures with [bmim][BF ] the reaction was explored at the
whole range of solvent composition, while in mixtures with
w A
and the second-order rate constants k for reactions
(i) Undoubtedly, a synergetic effect on the reaction rate occurs
4
in the case of AN þ [bmim][BF
values are higher than those in pure AN and in the (XAN
0.1/X 0.9) mixture. It can be observed that k values exper-
4
]: in some binary mixtures the
k
A
[
bmim][PF ] it was explored just from pure AN up to X ¼ 0.5. This
6
IL
IL
A
can be related with the fact that at high IL concentrations the
medium viscosity produces a disturbing trend in the results
iment a remarkable enhance when small amounts of IL are
added to the molecular solvent. Focusing the attention on
the proposed new solvents, the k values increase 90 and
A
[9]
(h
[bmim][BF4] ¼ 154(20) cP; h[bmim][PF6] ¼ 371(20) cP). The kinetic
data for the reaction carried out in DMF þ [bmim][PF ] mixtures
133% (referred to the value in AN) at X ¼ 0.1 (solvent 1) and
6
IL
from pure DMF up to XIL ¼ 0.5 are additionally presented.
Moreover, with respect to the reactions carried out in both pure
ILs, the formation of a colored ‘complex’ is evidenced as the
substrate is dissolved. The kinetic results presented in Table 3
show that no acceleration in rate is produced for an increasing
amount of amine, so the reactions do not exhibit base catalysis
and the formation of the intermediate is rate determining. This
trend is reasonable considering that base catalysis, in general, is
at XIL ¼ 0.5 (solvent 3), respectively. This special kinetic syner-
N
getic behavior was manifested in S Ar reactions between
FDNB and secondary amines developed in some binary
mixtures of molecular solvents, in which base catalysis is
operative. In this case it is concluded that this effect is
concerned with a complex combination of factors related
to the variation of the influence of base catalysis (which
changes with solvent composition) and to specific solvent
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 87–95

DIALKYLIMIDAZOLIUM IONIC LIQUIDS
aid in the interpretation of the solvent effects on the explored
reaction.
Correlation analysis
The often linear correlation between empirical solvent
parameters and other solvent-dependent properties (e.g.,
logarithms of rate, equilibrium constants) can be considered as
manifestations of general free energy relationships (LFER). In
order to interpret quantitatively the influence of the solvent
effects on the explored reaction we have correlated the reported
kinetic data and the quantified molecular–microscopic solvent
properties by means of the multiparametric approach developed
[14]
by Kamlet et al.
[Eqn (2)]:
ꢅ
log kA ¼ Y þ sp þ aa þ bb
(2)
where s, a, and b measure the relative susceptibilities of the
reactive system with respect to the solvent properties (dipolarity/
polarizability, HBD, and HBA ability). The results are presented in
Table 4. As can be seen, the correlations are good. The signs and
values of the s, a, and b correlation coefficients reveal the
following: (i) the incidences of the solvation effects ascribed to
the dipolarity/polarizability and HBA character of the solvent are
more important than that attributed to the HBD solvent ability; (ii)
the dipolarity/polarizability and HBA solvent character manifest
Figure 2. Plots of k
AN or DMF þ [bmim][BF
data in the pure molecular solvents)
A
versus XIL for the reaction of 2,4-DNFB and 1-BU in
(
4
] or [bmim][PF ]) binary mixtures (including the
6
[29]
effects, particularly hydrogen-bond interactions.
can be remarked that the synergism is manifested in non-
catalyzed reactions.
Here, it
positive influence on k values whereas, in contrast, the incidence
A
of solvation effects originated on the solvent HBD capability is
negative. The data presented in Table 4 reveal that, in general, an
accelerating effect on the reaction rate can be attributed to a
combination of the dipolarity/polarizability and HBA ability of the
solvent. Moreover, it can be pointed out that the observed kinetic
synergetic effect on kA values can be clearly related with the
solvent basicity.
(
ii) The
k
A
values experiment
a slight enhancement in
AN þ [bmim][PF
6
] mixtures. As it is shown the reaction rate
remains unchanged at X ¼ 0.1 (solvent 2) while the secon-
IL
d-order rate constant undergoes a percentage increase of
3
3% at XIL ¼ 0.5 (solvent 4).
(
iii) If the kinetic data in mixtures with AN are compared between
The results derived from the correlation analysis reveal that: (i)
the reaction rate increases by the increasing solvent dipolarity/
polarizability since the zwitterionic intermediate is more polar
than the reactants and the activated complex leading to ZH is
favored by increasing solvent dipolarity/polarizability; (ii) the
reaction rate increases by the increasing solvent HBA ability for
the reason that the zwitterionic intermediate and the corre-
sponding transition state have HBD character owing to the extra
hydrogen of the primary amine, and (iii) the reaction rate
decreases with the increasing HBD ability of the solvent for the
reason that the amine can be stabilized via hydrogen-bonding
interactions.
them the ratios corresponding to the new solvents (liquid 1/
liquid 2 and liquid 3/liquid 4) are kA[bmim][BF4]/kA[bmim][PF6]
¼
1.9 and 1.75 respectively, indicating that the system is near
4
twofold more reactive in mixtures with [bmim][BF ] than in
mixtures with [bmim][PF
iv) With respect to the reaction performed in DMF þ
bmim][PF ] mixtures, the shape of the curve reveals that
6
].
(
[
6
A
k values decrease continuously with the increase in the IL
concentration.
At this point, it is of interest to relate the kinetic data with the
molecular–microscopic solvent properties with the purpose to
2
Table 4. Correlation coefficients (r and r ), standard deviation (SD), intercept (Y), and the parameters s, a, and b (and their standard
errors) and the number of data points (n) corresponding to logk ¼Y þ sp* þ aa þ bb for the reaction between 2,4-DNFB and BU in
A
binary mixtures (including the data in pure molecular solvents)
2
r (r )
SD
Y (s
Y
)
s (s
s
)
a (s
a
)
b (s
b
)
n
a
0
0
0
.947 (0.896)
.904 (0.817)
.946 0.895
0.0432
0.065
0.073
ꢂ3.249 (1.359)
ꢂ0.385 (0.259)
ꢂ0.587 (0.274)
2.661 (0.915)
1.281 (0.427)
1.668 (0.418)
ꢂ0.0059 (0.453)
ꢂ0.103 (0.261)
ꢂ0.463 (0.195)
6.076 (2.111)
0.821 (0.252)
0.988 (0.233)
10
15
b
c
20
a
b
c
AN; AN þ [bmim][BF
4
] mixtures (XAN 0.1/XIL 0.9).
AN; AN þ [bmim][BF
4
] mixtures (XAN 0.1/XIL 0.9), AN þ [bmim][PF
6
] mixtures (XAN 0.1/XIL 0.5).
AN, DMF, AN þ [bmim][BF
4
] mixtures (XAN 0.1/XIL 0.9), AN þ [bmim][PF
6
] mixtures (XAN 0.1/XIL 0.5), DMF þ [bmim][PF
6
] mixtures (XAN
0.1/X 0.4).
IL
J. Phys. Org. Chem. 2008, 21 87–95
Copyright ß 2007 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

P. M. MANCINI ET AL.
mixtures with AN þ [bmim][PF6]. Considering that the
reactions are noncatalyzed, these results can be related to a
more efficient solvation of the transition state when the anion
is [BF ]. Moreover, the higher kA values detected for the
4
reaction performed in DMF þ [bmim][PF
6
] mixtures with
respect to the mixtures with AN can also be related with
the high HBA ability of the media.
The observed special kinetic synergetic effect in AN þ
[
4
bmim][BF ] mixtures can be principally attributed to the
solvent basicity being an evidence that the mixed solvents
behave as a ‘new solvents’ with particular solvating properties.
Moreover it can be noted that this particular kinetic effect was
not observed previously in noncatalyzed S
developed in binary organic solvent systems.
N
Ar reactions
EXPERIMENTAL
Reagents and solvents
[
30,31]
2
,4-DNFB and BU were purified as previously reported.
[bmim][BF ] and [bmim][PF ] were prepared according to literature
The crude ILs were purified through alumina and
dried under vacuum. Solvatochromic indicators [2,6-diphenyl-4-(2,
,6-triphenyl-1-piridinio)-1-phenolate; N,N-diethyl-4-nitroaniline;
4-nitroanisole; 4-nitroaniline; 4-nitrophenol], and molecular sol-
Figure 3. Plot of the experimental logk
,4-DNFB and 1-BU versus the predicted values calculated by the data
of Table 4 for n ¼ 20
A
values for the reaction of
4
6
[32,33]
2
procedure.
4
The correspondence between the experimental values of
logk and the corresponding values calculated by the data
[30,31]
A
vents were prepared and/or purified as previously reported.
reported in Table 4 for n ¼ 20 was investigated. The results
show that the linear correlation is good: log kAðexp:Þ¼0:039þ
The binary mixtures were prepared prior to use and stored under
anhydrous conditions.
0
:970ꢄ log kAðcalc:Þ; r ¼ 0.945; SD ¼ 0.067. The corresponding
plot is presented in Fig. 3. In connection with this and in order to
illustrate the applicability of these results, the second-order rate
Solvatochromic parameter measurements and calculations
constants at X[bmim][PF6] ¼ 0.9 were estimated: kA(ANþ[bmim][PF6])
¼
The spectroscopic data were obtained with a Perkin-Elmer
Lambda 40 UV/Vis spectrophotometer, equipped with a
ꢂ1 ꢂ1
ꢂ1 ꢂ1
s ).
7.43 (l mol
s
); kA(DMFþ[bmim][PF6]) ¼ 8.71 (l mol
thermostatic cell holder. The indicator solutions were prepared
N
just prior to use. The E (30) and ET parameters were calculated
T
[1]
CONCLUSIONS
according to the procedure reported elsewhere. The micro-
scopic properties p*, b, and a were calculated according to
[22]
ꢆ The comparison of the molecular–microscopic solvent
parameters corresponding to the explored binary mixtures
with both ILs considered at similar mixed-solvent composition
reveals that the difference between the explored mixtures is
centered on the basic character.
For the proposed ‘new solvents’ 1, 2, 5, and 6, while it can be
observed that the dipolarity/polarizability and basicity are
similar to those of AN and DMF, the acidity is quite different
being highly increased, particularly in the case of AN.
The most important difference between solvents 3 and 4
resides fundamentally in their basic character.
general expressions collected by Marcus.
Kinetic procedures
The kinetics of the reactions was studied by UV–Vis spectropho-
[30,31]
tometry, essentially by the procedure previously described.
DNFB was dissolved in the pure solvents and the corresponding
binary mixtures were prepared at the proper concentrations in
order to yield the required IL mole fractions in the cell.
The individual selected solvent mixtures with MeOH
solvents 7 and 8) clearly differ in the b property.
The particular properties of these selected ‘new solvents’
converted them in an appropriate media for chemical
processes.
Acknowledgements
(
We are indebted to the Universidad Nacional del Litoral (UNL),
Rep u´ blica Argentina. This work received financial support from
the Science and Technology Secretariat, UNL, CAI R D Program
(
Projects 2005-7-45, 2006-33- 183, and 2006-33-182).
ꢆ The multiparameter correlation treatment of the kinetic data
with the molecular–microscopic solvent properties allowed us
to determine the incidence of each type of solvent property on
the kinetics of the reaction. For the explored reactive systems
the solvation effects are dominated by both the dipolarity/
polarizability and the basicity of the media, contributing these
solvent properties to accelerating the chemical process.
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Copyright ß 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 87–95

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