Solvent effects on kinetics of the reaction between 2-chloro-3,5- dinitropyridine and aniline in aqueous and alcoholic solutions of [bmim]BF 4

Article abstract of DOI:10.1002/kin.20282

Rate constants, k_A. for the aromatic nucleophilic substitution reaction of 2-chloro-3,5-dinitropyridine with aniline were determined in different compositions of 1-(1-butyl)-3-methylimidazolium terafluoroborate ([bmim]BF₄) mixed with

Full text of DOI:10.1002/kin.20282

Solvent Effects on Kinetics
of the Reaction between
2-Chloro-3,5-dinitropyridine
and Aniline in Aqueous
and Alcoholic Solutions
of [bmim]BF4
ALI REZA HARIFI-MOOD,1 AZIZ HABIBI-YANGJEH,2 MOHAMMAD REZA GHOLAMI1
1Department of Chemistry, Sharif University of Technology, P. O. Box 11365-9516, Tehran, Iran
2Department of Chemistry, Faculty of Science, University of Mohaghegh Ardebili, P. O. Box 179, Ardebil, Iran
Received 20 April 2007; revised 2 July 2007; accepted 23 July 2007
DOI 10.1002/kin.20282
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Rate constants, kA, for the aromatic nucleophilic substitution reaction of 2-chloro-
3,5-dinitropyridine with aniline were determined in different compositions of 1-(1-butyl)-3-
methylimidazolium terafluoroborate ([bmim]BF4) mixed with water, methanol, and ethanol
at 25◦C. The obtained rate constants of the reaction in pure solvents are in the following
order: water > methanol > ethanol > [bmim]BF4. In these solutions, rate constants of the
reaction decrease with the mole fraction of the ionic liquid. Single-parameter correlations
of log kA versus normalized polarity parameter (ETN), hydrogen bond acceptor basicity (β),
hydrogen bond donor acidity (α), and dipolarity/polarizability (π∗) do not give acceptable
results in all solutions. Dual-parameter correlations of log kA versus ETN and β also α and β
gave reasonable results (e.g., in solutions of water with [bmim]BF4, the correlation coefficients
are 0.994 and 0.996, respectively). The proposed dual-parameter models demonstrate that the
reaction rate constant increases with ETN, β, and α. The increase in the rate constant is attributed
to hydrogen-bonding interactions (donor and acceptꢀor) of the media with an activated complex
C
of the reaction that has the zwitterionic character. 2007 Wiley Periodicals, Inc. Int J Chem
Kinet 39: 681–687, 2007
INTRODUCTION
The energetic level of molecules may be modified
by interactions with surrounding species, and it may
be difficult to relate chemical properties to molecular
structures [1]. The solvent effects play a key role in
many chemical and physical processes in solutions.
Correspondence to: Mohamad Reza Gholami; e-mail: gholami
@sharif.edu.
c
ꢀ
2007 Wiley Periodicals, Inc.

682
HARIFI-MOOD, HABIBI-YANGJEH, AND GHOLAMI
Scheme 1
The strong influence of solvents on chemical and
physical processes (reaction rates, selectivity, chemi-
cal equilibria, position and intensity of spectral absorp-
tion bands and liquid chromatographic separations) has
been well established [2]. Solvent effects are closely
related to the nature and extent of solute–solvent inter-
actions, locally developed in the immediate vicinity of
the solutes. The study of solute–solvent interactions in
binary mixed solvent systems is more complex than in
pure solvents. The solute can be preferentially solvated
by any of the solvents present in the mixture. On the
other hand, solvent–solvent interactions can strongly
affect solute–solvent interactions [1,3]. The problem
in studying solvent effects on the reaction rates is to
identify and assess the relative importance of various
factors on the solvent effects.
Room temperature ionic liquids (RTILs) have re-
cently attracted greater attention as environmentally
benign alternatives to conventional organic solvents in
a variety of synthetic [4,5], catalytic [6], and electro-
chemical applications [7], as a result of their unique
physical and chemical properties and the relative ease
with which these properties can be fine-tuned by al-
tering the cationic or anionic moieties comprising the
RTILs [5]. They continue to excite interest for a num-
ber of reasons. First, ionic liquids are environmentally
friendly solvents, particularly because they have very
low-vapor pressures under ambient conditions [8]. Sec-
ond, they might provide improved reactivities in a num-
ber of chemical processes [9,10]. Finally, the fact that a
wide range of cations and anions can be employed gives
chemists the potential to design the solvents with spe-
cific properties. In such case, ionic liquids are described
as “designer solvents” [8]. Among various ionic liquids
used as reaction media, those based on dialkylimida-
zolium salts have attracted particular attention, as they
possess a wide range of liquids and are easy to prepare
and handle [10,11].
In continuing our studies of solvent effects on or-
ganic reactions [15,16] and similar studies [17], we
were interested in effects of mixtures on ionic liquids
with molecular solvents. In this work, the solvent ef-
fects on the kinetics of nucleophilic heteroatomic sub-
stitution reaction of 2-chloro-3,5-dinitropyridine with
aniline was studied in aqueous and alcoholic solutions
of 1-(1-butyl)-3-methyl imidazolium tetrafluoroborate
([bmim]BF4) at 25◦C. In our previous studies, we re-
portedtheeffects of various solvatochromicparameters
on different organic reaction rates [16].
The suggested reaction mechanism of 2-chloro-3,5-
dinitropyridine with aniline or similar primary or sec-
ondary amines is shown in Scheme 1 [16e,18].
Application of the steady-state approximation gives
Eq. (1) in which kA is the observed second-order rate
constant and B is a second molecule of the amine or an
added base:
ꢀ
ꢁ
k1 k2 + k3[B]
kA =
(1)
k
−1 + k2 + k3[B]
Either the formation of an intermediate or its decom-
position into products, in these reactions, may be con-
sidered as a rate-determining step. Therefore, different
cases may be established, which have been reported
elsewhere [16e]. If the formation of the intermedi-
ate is the rate-determining step, that is, (a) k
ꢁ
−1
(k2 + k3 [B]), then Eq. (1) reduces to kA = k1, and
the amine does not catalyze the reaction. But, if the
decomposition of the intermediate to products is the
rate-determining step, that is, (b) k ꢂ (k2 + k3 [B]),
−1
Eq. (1) is converted to Eq. (2) and the reaction is cat-
alyzed by amines.
ꢀ
ꢁ
kA = k1k2/k−1 + k1k3/k [B]
(2)
−1
The mechanism of both the catalyzed and the un-
catalyzed reactions has been discussed in the presence
of the primary and secondary amines [16e]. The re-
action rate of this compound is much faster than that
of 1-halo-2,4-dinitrobenzene, because of (a) electron-
withdrawing of an aza group in addition to nitro groups
and (b) the possibility of intramolecular hydrogen
bonding between ammonium hydrogen and the aza
group in the intermediate (Scheme 2). The latter is
Several organic reactions were performed in ionic
liquids successfully, but quantitative aspects of reac-
tions in ionic liquids have been much less investigated
[12,13]. The complexity of even the simple reactions
in RTILs is significantly greater than those of reactions
in molecular solvents. Therefore, there is a continu-
ing challenge to understand the role of RTILs in the
reactions [13,14].
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE REACTION BETWEEN 2-CHLORO-3,5-DINITROPYRIDINE AND ANILINE
683
Table I Second-Order Rate Constants at Different
Concentrations of Aniline in Several Solvent
Compositions at 25◦C
102 × k (M−1 s−1
)
A
[Aniline] (M)
x1a
0.0184
0.0369
0.0553
0.0922
Scheme 2
Mixtures of [bmim]BF4 and water
0.0
0.3
0.6
1.0
122.1
9.76
3.88
1.60
120.3 121.5 121.0
9.72 9.63 9.65
confirmed by high and negative activated entropy val-
ues of the reaction [18b].
3.76
1.72
3.70
1.70
3.80
1.63
Mixtures of [bmim]BF4 and methanol
33.4 32.5 33.6
6.56 6.63 6.62
2.33 2.30 2.28
Mixtures of [bmim]BF4 and ethanol
53.0 52.5 53.2
5.70 5.77 5.75
3.22 3.20 3.15
0.0
0.3
0.7
34.2
6.59
2.39
EXPERIMENTAL SECTION
Materials
2-Chloro-3,5-dinitropyridine (m.p. 64◦C) was ob-
tained from Merck (Darmstadt, Germany) and purified
by recrystallization from methanol-light petroleum as
yellow needles. Anilinewas purchasedfromMerckand
purified by vacuum distillation. Methanol (>99.5%)
and ethanol (>99.8%) were supplied by Merck and
were used without further purification. Doubly distilled
water was used in all solvent samples. 1-(1-Butyl)-3-
methylimidazolium tetrafluoroborate (>98%), stored
under argon, was purchased from Solvent-Innovation
GmbH (Cologne, Germany) and was used as received.
Karl Fischer titrations showed no detectable presence
of water in freshly purchased [bmim]BF4.
0.0
0.4
0.6
53.9
5.70
3.18
a x1 is mole fraction of [bmim]BF4.
influence of aniline concentration as a base on the re-
action rate was studied at different compositions of
solvents. The second-order rate constants of the reac-
tion at various concentrations of aniline are shown in
Table I.
These data show that no significant acceleration in
the rate constant occurred with the increase in the
aniline concentration. Similar results for this reac-
tion in aqueous solutions of aliphatic alcohols have
been observed [16e]. Similar results have also been
reported for an aromatic nucleophilic substitution re-
action of 1-fluro-2,4-dinitrobenzene and 1-chloro-2,4-
dinitrobenzene with piperidine in aqueous solutions
of various alcohols [16a,b,d]. Hence, the reaction is
not catalyzed by a base. It can be concluded that the
formation of the zwitterionic intermediate is the rate-
determining step of the reaction, and the first case of
Eq. (1) satisfies this situation.
Kinetic Measurements
The kinetics of the reaction was studied spectrophoto-
metrically, by running the reactions in the thermostated
cells of spectrophotometer at 25◦C. A GBC UV–vis
cintra 40 spectrophotometer coupled with a thermocell
was used with 1.00-cm silica cells, and the absorbance
variation with time was recorded at λ = 350–360 nm
(in different solvent compositions) for the product of
reaction. The kinetics of reactions was studied under
pseudo-first-order conditions with respect to 2-chloro-
3,5-dinitropyridine. In all the cases, the infinity ab-
The second-order rate constants of the reaction, kA,
in mixtures of [bmim]BF4 with water, methanol, and
ethanol were obtained at various mole fractions of
[bmim]BF4 at 25◦C, and the results are summarized
in Tables II–IV.
sorbance, A , was determined experimentally for each
∞
kinetic run and used to calculate the reaction rate con-
stant from Eq. (3):
The solvatochromic parameters for solutions of
[bmim]BF4 with water, methanol, and ethanol at vari-
ous compositions have been determined in our labora-
tory (Tables II–IV) [19]. Figure 1 demonstrates a plot
of the reaction rate constant versus mole fraction of
[bmim]BF4. As can be seen, the rate constant of the re-
action decreases sharply with the ionic liquid content.
The second-order rate constant of the reaction follows
ln(A∞ − At ) = − kobs t + ln(A∞ − A0)
(3)
RESULTS AND DISCUSSION
To determine the rate-determining step of the reaction
between 2-chloro-3,5-dinitropyridine and aniline, the
International Journal of Chemical Kinetics DOI 10.1002/kin

684
HARIFI-MOOD, HABIBI-YANGJEH, AND GHOLAMI
Table II Second-Order Rate Constants in Mixtures of
Water with [bmim]BF4 at 25◦C
Table IV Second-Order Reaction Rate Constants in
Mixtures of Ethanol with [bmim]BF4 at 25◦C
102 × k
102 × k
x
1
(M−1 s−A1
)
ETN
α
β
π*
x
1
(M−1 s−A1
)
ETN
α
β
π∗
0.0
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
121.0
38.0
22.3
14.1
1.00
0.91
0.84
0.81
0.78
0.76
0.75
0.75
0.73
0.73
0.72
0.67
1.3
1.12
1.0
0.46
0.55
0.6
0.62
0.6
0.59
0.57
0.56
0.54
0.54
0.53
0.59
1.1
0.0
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
53.2
27.0
17.4
10.5
0.65
0.71
0.76
0.77
0.77
0.76
0.76
0.75
0.74
0.73
0.72
0.67
0.97
1.00
1.02
0.99
0.96
0.93
0.92
0.88
0.86
0.84
0.8
0.9
0.51
0.65
0.76
0.82
0.86
0.89
0.9
0.93
0.93
0.94
0.94
0.89
1.06
1.03
0.99
0.99
0.96
0.96
0.96
0.96
0.95
0.94
0.89
0.81
0.74
0.68
0.63
0.6
0.59
0.56
0.55
0.54
0.53
0.59
0.95
0.91
0.89
0.86
0.85
0.81
0.82
0.81
0.75
9.65
7.70
6.83
5.42
3.80
2.82
2.17
1.84
1.63
5.75
4.56
3.15
2.60
1.95
1.74
1.63
0.75
the following sequence: water > methanol > ethanol
> [bmim]BF4.
number, respectively. Therefore, the normalized po-
larity parameter and hydrogen bond acceptor basicity
of the media are not individually the main factor in
determining solvent effects on the reaction rate. So,
dual-parameter correlations of log kA versus ETN and β
were considered and are summarized in Eqs. (5)–(7).
These parameters give good results in all the solutions:
In mixtures of water with [bmim]BF4,
Although the normalized polarity parameter (ETN)
of water is higher than that of ethanol and methanol, a
single-parameter correlation of log kA versus ETN does
not give a reasonable result in all the solutions. For
example in solutions of [bmim]BF4 with methanol, the
regression coefficient of log kA versus ETN is 0.510.
Also, there is no acceptable correlation between log kA
and hydrogen bond acceptor basicity (β) in any of the
solutions, for example, it was observed in the solution
of methanol with [bmim]BF4 as in Eq. (4):
log kA = −8.123 ( 0.445) + 6.776 ( 0.261) ETN
+ 2.963 ( 0.553)β
(n = 12, r = 0.994, SE = 0.070, F2,10 = 364.12) (5)
log kA = −4.202( 0.386) + 4.791( 0.626)β
(n = 12, r = 0.924, SE = 0.181, F1,11 = 58.53) (4)
standardized coefficient of ETN is 1.075, and standard-
ized coefficient of β is 0.221.
where n, r, SE, and F are number of data, regres-
sion coefficient, standard error, and statistical Fisher
Table III Second-Order Rate Constants in Mixtures of
Methanol with [bmim]BF4 at 25◦C
102 × k
x
1
(M−1 s−A1
)
ETN
α
β
π∗
0.0
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
33.6
21.4
15.4
0.76
0.80
0.83
0.83
0.82
0.81
0.8
0.79
0.77
0.76
0.75
0.67
1.16
1.15
1.15
1.1
1.06
1.03
0.99
0.96
0.92
0.9
0.8
0.58
0.69
0.79
0.85
0.88
0.91
0.93
0.93
0.93
0.93
0.94
0.89
0.74
0.68
0.63
0.6
0.57
0.55
0.55
0.54
0.54
0.53
0.59
9.54
6.62
5.10
3.80
2.82
2.28
1.84
1.68
1.63
Figure 1 Second-order rate constants of reaction versus
0.87
0.75
mole fraction of [bmim]BF4 in its mixtures with methanol
◦
ꢀ
( ), ethanol ( ), and water (ꢂ) at 25 C.
ꢁ
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS OF THE REACTION BETWEEN 2-CHLORO-3,5-DINITROPYRIDINE AND ANILINE
685
In mixtures of methanol with [bmim]BF4,
strate that the reaction rate increases with the increase
in α. However, a good single parameter correlation be-
tween log kA and α parameter was not observed in all
the solutions. For example in solutions of methanol
with [bmim]BF4, the following result was obtained:
log kA = −6.990 ( 0.228) + 3.796 ( 0.286) ETN
+ 4.492 ( 0.147)β
(n = 12, r = 0.996, SE = 0.042, F2,10 = 631.24) (6)
log kA = −4.580 ( 0.394) + 3.290 ( 0.390)α
(n = 12, r = 0.936, SE = 0.167, F1,11 = 71.13)(8)
standardized coefficient of ETN is 0.377, and standard-
ized coefficient of β is 0.866.
In mixtures of ethanol with [bmim]BF4,
Hence, multiparameter correlations were considered.
Multiparameter correlations of log kA versus α and β
demonstrate acceptable results in all the solutions (see
Eqs. (9)–(11)):
log kA = −6.808 ( 0.458) + 3.640 ( 0.541) ETN
+ 4.544 ( 0.180) β
(n = 12, r = 0.993, SE = 0.064, F2,10 = 326.17) (7)
In mixtures of water with [bmim]BF4,
standardized coefficient of ETN is 0.288, and standard-
ized coefficient of β is 1.079.
log kA = −6.686 ( 0.344) + 4.043 ( 0.133)α
+ 3.263 ( 0.478)β
(n = 12, r = 0.996, SE = 0.060, F2,10 = 499.87) (9)
The standardized coefficient or beta coefficient is
the estimate of an analysis performed on variables that
have been standardized, so that they have variance of 1.
This is usually done to find out which of the indepen-
dent variables have a greater effect on the dependent
variable in the multiple regression analysis, when the
variables are measured in different units of measure-
ment. Before fitting the multiple regression equation,
all variables (independent and dependent) can be stan-
dardized by subtracting the mean and dividing by the
standard deviation. The standardized regression coef-
ficients, then, represent the change in a dependent vari-
able that result from a change in one standard deviation
in an independent variable.
standardized coefficient of α is 1.087, and standardized
coefficient of β is 0.244.
In mixtures of methanol with [bmim]BF4,
log kA = −4.862 ( 0.090) + 1.960 ( 0.129)α
+ 2.649 ( 0.190)β
(n = 12, r = 0.997, SE = 0.037, F2,10 = 816.90) (10)
standardized coefficient of α is 0.558, and standardized
coefficient of β is 0.511.
As can be seen, the second-order rate constant of
the reaction increases with the increase in the normal-
ized polarity parameter and hydrogen bond acceptor
basicity. The activated complex leading to the zwitte-
rionic intermediate (Scheme 2) is expected to have the
zwitterionic character and to be favored by the increase
in the normalized polarity parameter, because zwitte-
rionic molecules were more stabilized in high-polar
media than in the media with lower polarity. Also,
hydrogen-bonding interactions of the media (solvent
as acceptor with β parameter) with positive charge on
the activated complex of the reaction stabilize the ac-
tivated complex higher than the reactants; therefore,
the increase in the β parameter accelerates the reaction
rate. It is clear that in solutions of methanol and ethanol
with [bmim]BF4, the effects of β on the increase in the
reaction rate is higher than that of ETN, because the
standardized coefficient of β is higher than that of ETN.
The normalized polarity parameter is a blend of π∗
(dipolarity/polarizability) and α (hydrogen bond donor
acidity) of media. Thus, correlations of log kA versus α
in all the solutions were considered. The results demon-
In mixtures of ethanol with [bmim]BF4,
log kA = −5.108 ( 0.171) + 2.227 ( 0.255)α
+ 2.896 ( 0.185)β
(n = 12, r = 0.996, SE = 0.051, F2,10 = 514.41)
(11)
standardized coefficient of α is 0.383, and standardized
coefficient of β is 0.688.
To show the efficiency of the suggested dual-
parameter correlations, for example, the calculated
values of the reaction rate constants in mixtures of
methanol with [bmim]BF4 using Eq. (10) have been
plotted vs. the experimental values. Figure 2 shows
a good agreement between the experimental and the
calculated values of log kA. Thus, unlike what has
been reported in a few papers [13], results indicate
that the media effects on this reaction can be described
by means of the solvatochromic parameters in the pres-
ence of the ionic liquid in conventional solvents.
International Journal of Chemical Kinetics DOI 10.1002/kin

686
HARIFI-MOOD, HABIBI-YANGJEH, AND GHOLAMI
CONCLUSIONS
Changes in the solvent composition showed a dramatic
effect on the rate of the aromatic nucleophilic sub-
stitution reaction of 2-chloro-3,5-dinitropyridine with
aniline. The second-order rate constants of the reaction
represent a falloff with the mole fraction of [bmim]BF4.
Unlike many reactions, the rate of the reaction de-
creases with the addition of [bmim]BF4. In all these
cases, formation of the zwitterionic intermediate is the
rate-determining step of the reaction. Normalized po-
larity, hydrogen bond donor acidity, and hydrogen bond
acceptor basicity of media have parallel and positive ef-
fects on the rate of the reaction. Thus, solvatochromic
parameters of media can describe solvent effects on
the reaction rate and represent a theoretical model for
similar cases in the ionic liquid mixed with molecu-
lar solvents. The results of dual-parameter correlations
of log kA versus ETN and β (also α and β) in all the
solutions represent improvements with regard to the
single-parameter models.
Figure 2 Plot of the calculated values of −log k by
A
Eq. (10) versus the experimental values of it in mixtures
of methanol with [bmim]BF4 at 25◦.
It is clear that the reaction rate constant increases
with α and β in all the solutions. The negative charge
of the activated complex of the reaction is on a
pyridine ring. Hydrogen-bonding interactions of the
media with negative charge on the activated complex
of the reaction stabilize the activated complex more
than the reactants; therefore, the reaction rate increases
with the α parameter. The activated complex leading
to the intermediate of the reaction has the zwitterionic
character with positive charge on nitrogen of aniline
and negative charge on the pyridine ring. Hydrogen-
bonding interactions of media (donor and acceptor)
with the charges on the activated complex stabilize the
activated complex more than the reactant of the reac-
tion. Thus, the reaction rate constant increases with
hydrogen bond donor and acceptor parameters. As can
be seen, in solutions of water with [bmim]BF4 and
methanol with [bmim]BF4, the effect of α on the in-
crease in the reaction rate is higher than β, because
in these solutions the standardized coefficient of α is
higher than that of β.
The α parameter of RTILs is largely affected by the
nature of the cation, but there is also a smaller anion
effect [20]. It has been known that in [bmim]BF4 all
three imidazolium ring hydrogen atoms are acidic. The
α value for [bmim]BF4 is moderately high but is lower
than that of water, methanol, and ethanol [19]. The β
parameter of RTILs is mainly dominated by the na-
ture of the anion. The anion of [bmim]BF4 is known
to have a compact structure, possessing much weaker
basicity in comparison to alcohols. Then, [bmim]BF4
has lower hydrogen bond donor acidity and hydro-
gen bond acceptor basicity relative to water (except
for β), methanol, and ethanol. Therefore, the second-
order rate constant of the reaction is in the following
sequence: water > methanol > ethanol > [bmim]BF4.
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