Reaction Mechanism in Ionic Liquids: Kinetics and Mechanism of the Aminolysis of 4-Nitrophenyl Acetate

Article abstract of DOI:10.1002/kin.20994

In this work, we report a kinetic study of the reactions of the title compound with secondary alicyclic amines at different temperatures in acetonitrile and several ionic liquids (ILs). From this study, we have described that the reactions in MeCN and in

Full text of DOI:10.1002/kin.20994

Reaction Mechanism in
Ionic Liquids: Kinetics and
Mechanism of the
Aminolysis of 4-Nitrophenyl
Acetate
´
´
PAULINA PAVEZ, DANIELA MILLAN, MABEL ROJAS, JAVIERA I. MORALES, JOSE G. SANTOS
Facultad de Qu ´ı mica, Pontificia Universidad Cat o´ lica de Chile, Casilla 306, Santiago, 6094411, Chile
Received 8 October 2015; revised 29 December 2015; accepted 2 March 2016
DOI 10.1002/kin.20994
Published online 31 March 2016 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: In this work, we report a kinetic study of the reactions of the title compound with
secondary alicyclic amines at different temperatures in acetonitrile and several ionic liquids
(ILs). From this study, we have described that the reactions in MeCN and in the studied ILs occur
by a concerted mechanism and that the activation parameters are sensitive to the structural
properties of the ILs. The pKa values of all amines used in both MeCN and ILs were determined.
ꢀ
C
2016 Wiley Periodicals, Inc. Int J Chem Kinet 48: 337–343, 2016
INTRODUCTION
rameters have been used by D’anna and collaborates
to shed light on reaction mechanisms. They have ob-
served changes in activation parameter for the mononu-
clear rearrangement reactions of heterocycles when ILs
are used as a solvent in comparison with those found in
aqueous solution. These differences were attributed to
the solvation of the zwitterionic-like transition state by
dipole–dipole interactions with the ILs [5]. In addition,
Harper and collaborators have used activation parame-
ters in several organic reactions to understand the effect
of the ionic liquid on the reaction outcome [6]. They
found a “linear relationship between both the enthalpy
and entropy of activation and the Kamlet–Taft basic-
ity parameters” [6]e. On the other hand, Chiappe and
collaborates have used a Brønsted-type correlation to
understand the mechanism of the base-catalyzed eno-
lization reaction of 2-nitrocyclohexanone using sub-
stituted pyridines in several ILs. They found that the
Ionic liquids (ILs) show enormous potential as reac-
tion media in many organic reactions [1]. It has been
demonstrated that their remarkable properties as sol-
vents influence the rate and/or selectivity of these re-
actions [1,2]. Nevertheless, information about how re-
actions proceed in this ionic media at a molecular level
has received less attention [3].
Among the tools most used to understand the mech-
anisms of reactions are the activation parameters and
the Brønsted-type correlations [4]. In the past years,
in reactions occurring in ionic liquids activation pa-
Correspondence to: J. G. Santos; e-mail: jgsantos@uc.cl.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
ꢀC
2016 Wiley Periodicals, Inc.

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38
PAVEZ ET AL.
β values (β > 0.9) for this reaction agree with a mecha-
nism via solvent-stabilized enolate-like transition state
when ILs are the reaction media [7].
physicochemical properties of ILs on the outcome of
the reaction studied. Thus the anions were selected
taking into account their size and coordinating abil-
ity, whereas the cations to evaluate the hydrogen bond
donor ability of the imidazolium cation. Our goal is
to offer more tools to understand the mechanism of
the reactions in ILs. A kinetic study at different tem-
peratures was performed to obtain kinetic and ther-
modynamic parameters, which are used to clarify the
reaction mechanisms.
Bearing in mind that the IL cannot be considered
just as a “simple solvent,” structural aspects of these
solvents should also be considered to better under-
stand how the ionic media influences the result of a
particular reaction. Lately, it has been reported that
the properties of neat ILs (imidazolium-based ILs) can
be attributed to their structural organization, which in-
volves the formation of supramolecular aggregates [8].
Recently, Dupont et al. described that ILs are intimate
ion pair even in the presence of important quantities of
some organic cosolvents [9]. This is in accordance with
Harper et al., who demonstrated that in the ethanoly-
sis of diethyl chlorophosphate in [Bmim][(CF3SO2)2]-
ethanol-d6 mixtures the thermodynamic parameters do
not change significantly on varying the molar fraction
of ILs (XIL 0.31 and 0.72) [6]c.
EXPERIMENTAL
Material
SA amines, NPA, and all ILs were commercially
available from Merck-Germany with a nominal purity
>98%. ILs were dried to constant weight at 70°C for
24 h under reduced pressure and stored in a vacuum
Among the most studied reactions to investigate the
effects of solvents are the Diels–Alder [10] and nu-
cleophilic substitutions [11], since their mechanism in
different reaction media are well known. Particularly
in the case of nucleophilic reactions of aryl esters,
these can react by two possible mechanisms: (i) a con-
certed pathway, where the nucleophilic attack at the
electrophilic carbonyl carbon occurs simultaneously
with the leaving group departure within a single step;
and (ii) a stepwise mechanism, where the interaction
of the nucleophile with the electrophilic carbon leads
to the formation of a tetrahedral intermediate, from
which the leaving group detaches. In nucleophilic sub-
stitutions reactions, the Brønsted equation (log k vs.
pKa of amine) and particularly the charge developed
on the nucleophilic atom from reactants to the transi-
tion state (the slope of the Brønsted-type plot, β) has
been a very good tool to determine reaction mecha-
nisms in water and in organic solvents [12]. There is a
great deal of information in the literature about β values
for these reactions in conventional solvents; neverthe-
less, the information is scarce for the same reactions in
ILs [7,13].
vessel. SA amines were distilled prior to use. Water
content in ILs was less than 0.05% by Karl Fisher
titration (see Table S1 in the Supporting Information).
Kinetic Measurements
Kinetic measurements were performed by a UV–vis
spectrophotometer, following the appearance of p-
nitrophenoxide (400 nm) after at least four half-lives.
The kinetic experiments were carried out in IL so-
lutions at 25°C under pseudo–first-order conditions.
Each run was made in triplicate. In a typical spec-
trophotometric measurement, a quartz cuvette (light
path 0.2 cm) containing 500 µL of IL was thermostated
at 25°C during 10 min. Then a stock solution of NPA
−
4
(
(
50 µL, 2 × 10 M in MeCN) and stock amine
25 µL) were added. The spectra were recorded at
different reaction times, and pseudo–first-order rate
coefficients (kobs) were found for all reactions. The
activation parameters were determined in the same ex-
perimental conditions at the 20–40°C range by using
the Eyring equation.
Previously, we have studied the aminolysis reac-
tions of aryl esters in mixed aqueous solution showing
interesting mechanistic changes by varying the sol-
vent medium [14]. More recently, we also investigated
the aminolysis of 4-nitrophenyl acetate in [Bmim]BF4
Product Studies
The product, p-nitrophenoxide, was identified by com-
parison of the UV–vis spectra after completion of the
reactions with that of an authentic sample of the p-
nitrophenoxide ion under the same experimental con-
ditions (see Fig. S1 in the Supporting Information). To
confirm the presence of the products, extractions with
diethyl ether were made from the reaction media and
this extract analyzed by GC/MS. (Figs. S2 and S3 in
the Supporting Information).
[13].
In an attempt to shed more light on the ILs ef-
fect on the reaction mechanism, in this work we study
the reaction of 4-nitrophenyl acetate (NPA) with sec-
ondary alicyclic (SA) amines in different ILs, as shown
in Scheme 1. The ILs used have a variety of cations
and anions to allow investigation of the effects of the
International Journal of Chemical Kinetics DOI 10.1002/kin.20994

REACTION MECHANISM IN IONIC LIQUIDS
339
Scheme 1 General scheme reaction and ILs used.
Determination of the Relative Basicity
about the assignment of mechanism still remain. In
that study, we were prone to accept a stepwise mech-
anism for the title reaction in [Bmim]BF4 despite the
fact that the Brønsted plot slope indicated a borderline
between concerted and stepwise mechanisms. In this
work, we study the kinetic behavior of the reaction of
NPA with piperidine in anhydrous acetonitrile (MeCN)
and in five dried ILs (water content was less than 0.1%
by Karl Fisher titration), ([Hmim]NTf2; [Bmim]NTf2;
Constant (K ) of SA Amines in [Bmim]PF6
R
The KR values of SA amines in [Bmim]PF6 were
determined by using the method described by D’anna
et al. [15]. The method consist of determination of the
basic strength of amines in ILs by means of the forma-
tion of amine/p-nitrophenol ion pairs through UV–vis
measurements employing 4-nitrophenol as reference.
Following this procedure, the basicities of the four em-
ployed amines were evaluated in [Bmim]PF6 through
their KR value, where KR = [amonium cation] [nitro-
phenoxide]/[amine][nitrophenol]. The log KR values
are listed in Table III.
[
Bm2im]NTf2; [Bmim]OTf; [Bmim]PF6) as shown in
Scheme 1, at the 20–40°C range. The kobs values and
the experimental conditions are shown in Tables S2–
S7 in the Supporting Information. Also, the reactions
of NPA with 1-(2-hydroxyethyl)piperazine, morpho-
line, and 1-formylpiperazine are measured in MeCN,
[Bmim]NTf2, [Bm2im]NTf2, and [Bmim]PF6 at 25°C.
RESULTS AND DISCUSSION
Pseudo–first-order rate constants (kobs) were obtained
under amine excess; the kobs values and the experi-
mental conditions are shown in Tables S8–S11 in the
Supporting Information.
The kinetics of all the studied reactions obeys
Eq. (1), where kN is the rate coefficient for the
In our previous study, we have described that the reac-
tions of NPA with SA amines in [Bmim]BF4 proceed
by a stepwise mechanism [13]. Nevertheless, consid-
ering the arguments used in that study, some doubts
International Journal of Chemical Kinetics DOI 10.1002/kin.20994

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PAVEZ ET AL.
Table I Nucleophilic Rate Constants for the Reaction of NPA with Piperidine in Different Solvents and Temperatures
Solvents
kN (293.2 ± 0.1 K)
kN (298.2 ± 0.1 K)
kN (303.2 ± 0.1 K)
kN (307.6 ± 0.1 K)
kN (313.2 ± 0.1 K)
[
[
[
[
[
[
Bmim]BF4 [13]
Bmim]NTf2
Bmim]PF6
Bmim]OTf
Bm2im]NTf2
Hmim]NTf2
5.88
7.04
8.64
–
9.28
2.7 ± 0.2
4.1 ± 0.1
7.7 ± 0.5
–
1.81 ± 0.04
2.1 ± 0.2
4.8 ± 0.5
2.3 ± 0.1
1.69 ± 0.05
1.9 ± 0.2
2.04 ± 0.07
2.7 ± 0.1
5.4 ± 0.3
2.8 ± 0.2
1.91 ± 0.05
2.2 ± 0.2
3.5 ± 0.2
4.9 ± 0.3
–
4.0 ± 0.2
3.23 ± 0.09
3.3 ± 0.3
3.3 ± 0.1
6.2 ± 0.6
3.2 ± 0.1
2.16 ± 0.09
2.6 ± 0.1
2.56 ± 0.06
MeCN
2.9 ± 0.2
aminolysis of the substrate.
kobs = kN[amine]
(1)
The values of kN were obtained as a slope of linear
plots of kobs against amine concentration. Table I sum-
marizes the kN values of the reaction with piperidine
in different ILs at several temperatures.
There are two important results from Table I taking
into account the ILs sharing the anion NTf2: (i) The
similar values of kN for the reaction in [Bmim]NTf2 and
[
Hmim]NTf2 suggest that the alkyl change in the cation
does not affect the reaction rate, and (ii) the reaction in
Bmim]NTf2 presents kN values slightly smaller than
[
Figure 1 Eyring plots for the reaction of NPA with
piperidine in different solvents: acetonitrile ( ), [Bmim]PF6
in [Bm2im]NTf2, suggesting that the lack of Me in C-2
of the former IL probably stabilizes piperidine or NPA
more than the transition state and for that reason the rate
constant in [Bmim]NTf2 decreases. On the other hand,
we can also observe that the kN values in [Bmim]OTf
and [Bmim]BF4 are two or three times greater than
those in MeCN and in other ILs studied, suggesting
minor enthalpic and/or entropic cost.
With the kinetic values of Table I, the activation pa-
rameters in ILs and MeCN were determined. Figure 1
shows the Eyring plots for the above reaction in differ-
ent solvents.
(ꢀ), [Bmim]BF4 (ꢀ), [Bmim]OTf ( ), [Bmim]NTf2 ( ),
[Hmim]NTf2 ( ), and [Bm2im]NTf2 ( ).
From results of Table II, it is also possible to dis-
tinguish three types of behavior: (1) the reactions in
±
MeCN and [Bm2im]NTf2, with the lowest ꢀH val-
±
ues and the most negative ꢀS values; (2) reactions in
solvents with ꢀH values from 20 to 22 kJ/mol; and
ࣔ
(3) reactions in water and [Bmim]PF6, which present
From the slope and intercept of these straight lines
± ±
H and ꢀS , respectively, were determined. These
ꢀ
values are summarized in Table II; also the activation
parameters for the reaction in water are included [11]a.
It is worth mentioning that in the studied reactions
the media contained approximately 10% v/v MeCN
Table II Activation Parameters for the Reaction of
NPA with Piperidine in Water, Acetonitrile, and Six ILs
Solvent
ꢀHࣔ (kJ/mol)
ꢀSࣔ (J/Kmol)
(
see the Experimental part), which means that the mo-
Water [11]a
[Bmim]PF6
30.9
32 ± 2
20 ± 1
22 ± 1
22 ± 2
18 ± 2
22 ± 3
18.9 ± 0.4
–109
lar fraction of the ILs (XIL) is approximately 0.5. Con-
sidering that at high concentrations of IL (in binary
mixtures with less polar cosolvents) the IL effect as
solvent is due to the existence of ionic pairs [9], is ex-
pected that in the experimental conditions used in this
study the solvent effect may be the same as that of a
neat ionic liquid.
–130 ± 6
–162 ± 5
–155 ± 4
–166 ± 8
–176 ± 5
–163 ± 11
–174 ± 1
[
[
[
[
[
Bmim]BF4 [13]
Bmim]OTf
Hmim]NTf2
Bm2im]NTf2
Bmim]NTf2
MeCN
International Journal of Chemical Kinetics DOI 10.1002/kin.20994

REACTION MECHANISM IN IONIC LIQUIDS
341
Table III Nucleophilic Rate Constants for the Reactions of NPA with SA Amines in MeCN and in Three ILs, and pKa
or log KR) of the Amines in the Same Solventsa
(
−1
−1
M )
Amine
Solvent
kN (s
pKa (or log KR)
Piperidine
MeCN
Bmim]NTf2
Bm2im]NTf2
Bmim]PF6
MeCN
Bmim]NTf2
Bm2im]NTf2
Bmim]PF6
MeCN
Bmim]NTf2
Bm2im]NTf2
Bmim]PF6
MeCN
Bmim]NTf2
Bm2im]NTf2
2.2 ± 0.2
2.04 ± 0.07
2.8 ± 0.2
18.02 [17]
18.4 [16]
20.0 [16]
[
[
[
b
2.7 ± 0.1
3.09
1-(2-Hydroxyethyl)piperazine
0.42 ± 0.01
0.32 ± 0.02
0.33 ± 0.02
0.45 ± 0.07
17.6 [17]
15.8 [16]
18.8 [16]
[
[
[
b
1.45
Morpholine
0.069 ± 0.006
0.081 ± 0.003
0.060 ± 0.006
0.143 ± 0.008
0.093 ± 0.003
0.0087 ± 0.0004
0.049 ± 0.004
0.11 ± 0.01
16.6 [17]
14.5 [16]
16.1 [16]
0,759b
17.0 [18]
12.8 [16]
14.6 [16]
0.617b
[
[
[
1
-Formylpiperazine
[
[
[
Bmim]PF6
a
b
The k
N
and pK
a
were determined in the same reaction media. See the Experimental section.
ࣔ
ࣔ
the less negative ꢀS and more positive ꢀH values,
ously by us [16]. Therefore, we determined the relative
basicity constant (KR) of these amines in [Bmim]PF6
by using a method described by D’anna et al [15],
which has shown a good correlation between the pKa
and log KR for these amines in ILs [13].
the latter between 30 and 32 kJ/mol. The differences in
H values found between groups 1 and 2 could be at-
tributed to the H bond ability of solvents from group 2,
which would stabilize the reactants (mainly the amine)
by H bond formation, with the corresponding enthalpic
cost for the reaction.
Regarding the solvents from group 3, the gap of
almost 10 kJ/mol in ꢀH values and about 30 J/Kmol
in ꢀS values with respect to the other solvents studied
could be due to (i) a change of the mechanism from
concerted to stepwise or (ii) a structural response of
the ordered liquid state.
In our previous work, with only the activation pa-
rameters in water and in [Bmim]BF4, we estimated
±
ꢀ
By plotting the log kN vs. pKa (or pKR) values of
Table III, linear Brønsted plots were obtained (Fig. 2)
with slope values β = 0.68, 0.38, 0.41, and 0.56 for
the reactions in MeCN, [Bm2im]NTf2, [Bmim]NTf2,
and Bmim]PF6, respectively. Considering that the β
value describes the development of charge on the nu-
cleophilic atom (N atom) from reactants to the tran-
sition state, we can use the same described range of
β values in water and aqueous mixtures to assign the
mechanism in ILs. As the β values found in these re-
actions in ILs and in MeCN are in the range than those
described for concerted mechanism (0.3–0.8) [4]a, we
can safely conclude that the title reactions in ILs and
in MeCN are concerted.
ࣔ
±
ࣔ
that the ꢀH values in these solvents were similar. For
this reason, we were prone to accept the same step-
wise mechanism in both solvents. Nevertheless, with
the new results obtained in this work (Table II) we
notice that the activation parameters in [Bmim]BF4
are very similar to those in MeCN and, therefore, the
mechanism of the reaction in both solvents could be
the same.
To clarify this doubt, we carried out a kinetic study
of the aminolysis of NPA with four SA amines in
MeCN and in three ILs. Table III shows the kN values
for the aminolysis of NPA at 25°C in these solvents.
Table III also includes the pKa values of amines in both
MeCN and ILs. It is noteworthy that due to the high vis-
cosity of [Bmim]PF6, the pKa of these amines could not
be determined in this IL by the method described previ-
Since the reactions in ILs proceed by the same con-
certed mechanism, the different activation parameters
found in [Bmim]PF6, compared with those in the other
ILs, suggest a stronger stabilization of the reactants
(piperidine and NPA) relative to that of the transition
state. The stabilization of reactants causes an incre-
ࣔ
ment in ꢀH value (31.7 kJ/mol), whereas a relatively
more disordered transition state leads to a less negative
±
ꢀS (–130 J/K mol).
It is difficult to understand this behavior consider-
ing that the polarity of [Bmim]PF6 is not very dif-
ferent in comparison with the other ILs used [19].
International Journal of Chemical Kinetics DOI 10.1002/kin.20994

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PAVEZ ET AL.
Figure 2 Brønsted plots for the reactions in acetonitrile ( , β = 0.68): [Bmim]BF4 ( , β = 0.75) [13], [Bmim]NTf2
,β = 0.41), [Bm2im]NTf2 (ꢀ,β = 0.37), and [Bmim]PF6 ( ,β = 0.56).
(
Nevertheless, it has been described for [Bmim]PF6 that
the introduction of molecules (such as a solute) leads a
disruption of the H bond network, generating polar and
nonpolar nanoregions. This structural arrangement de-
pends on the geometry and the coordination ability of
In summary, we have described that the title reac-
tions with SA amines in MeCN and in the studied ILs
occur by a concerted mechanism and that the activation
parameters are sensitive to the structural properties of
the ILs.
−
the anion. In the case of the octahedral PF6 anion, the
equatorial F atoms participate preferentially in H bond-
ing network [8,20]. Therefore, the three-dimensional
structure of [Bmim]PF6 becomes more organized and
more difficult to disrupt. In these nanoregions, the re-
actants of the title reaction in this IL would be more
stabilized than the transition state (TS) because of the
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CONCLUSIONS
7
In this work, we reported a kinetic study of the title
substrate with secondary alicyclic amines at different
temperatures in acetonitrile and several ionic liquids.
The effect of a series of ionic liquids on the reaction
mechanism was studied through activation parameters
and Brønsted correlations.
8
9
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International Journal of Chemical Kinetics DOI 10.1002/kin.20994