Kinetics study of a Diels-Alder reaction in mixtures of an ionic liquid with molecular solvents

Article abstract of DOI:10.1002/poc.1377

The second-order rate constants for cycloaddition reaction of cyclopentadiene with naphthoquinone were determined spectrophotometrically in various compositions of 1-(1-butyl)-3-methylimidazolium terafluoroborate ([bmim]BF₄) with water and methanol at 25°C. Rate constants of the reaction in pure solvents are in the order of water > [bmim]BF ₄ > methanol. Rate constants of the reaction decrease sharply with mole fraction of the ionic liquid in aqueous solutions and increase slightly to a maximum in alcoholic mixtures. Multi-parameter correlation of logk ₂ versus solute-solvent interaction parameters demonstrated that solvophobicity parameter (Sp), hydrogen-bond donor acidity (α) and hydrogen-bond acceptor basicity (β) of media are the main factors influencing the reaction rate constant. The proposed three-parameter model shows that the reaction rate constant increases with Sp, α and β parameters. Copyright

Full text of DOI:10.1002/poc.1377

Research Article
Received: 21 November 2007,
Revised: 9 March 2008,
Accepted: 20 March 2008,
Published online in Wiley InterScience: 29 May 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1377
Kinetics study of a Diels-Alder reaction in
mixtures of an ionic liquid with molecular
solvents
Ali Reza Harifi-Mooda, Aziz Habibi-Yangjehb
a
*
and Mohammad Reza Gholami
The second-order rate constants for cycloaddition reaction of cyclopentadiene with naphthoquinone were deter-
mined spectrophotometrically in various compositions of 1-(1-butyl)-3-methylimidazolium terafluoroborate
([bmim]BF4) with water and methanol at 25 -C. Rate constants of the reaction in pure solvents are in the order of
water > [bmim]BF4 > methanol. Rate constants of the reaction decrease sharply with mole fraction of the ionic liquid in
aqueous solutions and increase slightly to a maximum in alcoholic mixtures. Multi-parameter correlation of logk2
versus solute–solvent interaction parameters demonstrated that solvophobicity parameter (Sp), hydrogen-bond
donor acidity (a) and hydrogen-bond acceptor basicity (b) of media are the main factors influencing the reaction rate
constant. The proposed three-parameter model shows that the reaction rate constant increases with Sp, a and b
parameters. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: ionic liquid; Diels-Alder; solvent effect
understood.[24] However, a few investigations deal with the ILs
media and the solvent effects on chemical kinetics of DA
reactions.[24,26–27]
INTRODUCTION
Reactions involving isopolar-activated complexes, such as
Diels-Alder (DA) reactions, normally exhibit small solvent effects
and, consequently, studies on this topic have traditionally been
scarce.[1] However, interest in the role of the solvent in DA
reactions has increased over the last decade because of the
noticeable improvement in these reactions achieved by the use
of water or aqueous solutions.[2–7] The fact that rate constants for
DA reactions in water are dramatically larger than those in
organic solvents contradicts with common notion that DA
reactions are rather insensitive to solvent effects.[7] In addition to
obvious economical and environmental advantages, water has
surprisingly beneficial effects on organic reactions, which has
popularized water as a reaction medium.[8–11] Chemical reactions
can be affected by the solvent through several kinds of
interactions. Studies on solvent effects are generally carried
out by means of relationships between reactivity properties, that
is reaction rate or several types of selectivity, and empirical
parameters representing different kinds of solute–solvent
interactions.[7,12–15]
Ionic liquids (ILs) have recently been regarded as an
eco-friendly alternative to replace volatile organic solvents
in current chemical processing, due to their unique physical
and chemical properties.[16–19] Furthermore, there are wide
ranges of cations and anions that have been used in the
preparation of ILs, giving a great potential for synthetic
variation.[20,21] ILs seem to be the suitable media for DA reactions
and have been used as both solvents and catalysts for such
reactions.[1,22–24] Tiwari and Kumar[25] have recently shown that
ILs are not as effective as water in accelerating DA reaction. Many
DA reactions have been studied in this media and the solvent
influences on the endo/exo selectivity of the reaction are well
In continuing our studies on ILs,[28,29] in the present work, we
report a kinetics study of the cycloaddition reaction between
cyclopentadiene (1) and naphthoquinone (2) (Scheme 1).
Second-order rate constant of the reaction was determined in
various compositions of 1-(1-butyl)-3-methyl imidazolium tetra-
fluoroborate ([bmim]BF4), as a well-known IL, with water and
methanol at 25 8C and the results were interpreted by various
solute–solvent interaction parameters.
EXPERIMENTAL SECTION
Material
Naphthoquinone (m. p. 126 8C) was obtained from Merck and
purified by recrystalization from methanol-light petroleum.
Cyclopentadiene, purchased from Merck, was used freshly
by distillation of its dimer. Methanol (>99.5%) was supplied
by Merck and was used without further purification.
Doubly distilled water was used in all solvent samples.
*
Correspondence to: M. R. Gholami, Department of Chemistry, Sharif University
of Technology, P.O. Box 11365-9516, Tehran, Iran.
E-mail: gholami@sharif.edu
a
A. R. Harifi-Mood, M. R. Gholami
Department of Chemistry, Sharif University of Technology, Tehran, Iran
b
A. Habibi-Yangjeh
Department of Chemistry, Faculty of Science, University of Mohaghegh
Ardabili, Ardabil, Iran
J. Phys. Org. Chem. 2008, 21 783–788
Copyright ß 2008 John Wiley & Sons, Ltd.

A. R. HARIFI-MOOD, A. HABIBI-YANGJEH AND M. R. GHOLAMI
Table 2. Second-order rate constants of the reaction
between (1) and (2) in methanol–[bmim]BF4 mixtures at 25 8C
102 ꢀ k2/Mꢁ1 sꢁ1
ETN
a
b
*
p
xIL
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.95 ꢂ 0.04
2.33 ꢂ 0.03
2.34 ꢂ 0.03
2.32 ꢂ 0.04
2.25 ꢂ 0.04
2.19 ꢂ 0.03
2.16 ꢂ 0.05
2.11 ꢂ 0.07
2.07 ꢂ 0.07
2.06 ꢂ 0.08
2.05 ꢂ 0.13
0.76
0.83
0.83
0.82
0.81
0.8
0.79
0.77
0.76
0.75
0.67
1.16
1.15
1.1
1.06
1.03
0.99
0.96
0.92
0.9
0.8
0.58
0.79
0.85
0.88
0.91
0.93
0.93
0.93
0.93
0.94
0.89
0.68
0.63
0.6
0.57
0.55
0.55
0.54
0.54
0.53
0.59
Scheme 1.
1-(1-Butyl)-3-methylimidazolium tetrafluoroborate (>98%), stored
under nitrogen, was purchased from Solvent-Innovation GmbH
and was used as received. Karl Fischer titrations showed no
detectable water was present in freshly purchased [bmim]BF4.
0.87
0.75
Kinetic measurements
In order to check stability, the reactants and product of the
reaction were stored in [bmim]BF4-solvent media for 24 h and did
not find any competitive reaction. The kinetics of the reaction was
studied spectrophotometrically by running the reaction in the
thermostated cells of spectrophotometer at 25 8C. A GBC UV-vis
cintra 40 spectrophotometer coupled with a thermocell was used
with 1.00 cm silica cells. The absorbance variation with time was
recorded at l ¼ 330–340 nm for naphthoquinone of the reaction
regarding to composition of media. The reaction rate is slow;
therefore, the kinetics of reactions was studied under pseudo-
first-order conditions and initial rate method. However, in
water-rich regions, the reaction rate is fast enough to use the
rate-integrated method.
basicity (b), normalized polarity parameter (ETN) and solvopho-
bicity parameter (Sp) have been proposed in order to explain the
variations in rate of the reactions by modifying the reaction
media. The solvatochromic parameters for solutions of [bmim]BF4
with water and methanol at various compositions have been
determined in our laboratory (Tables 1 and 2).[29]
The rate constants in mixtures of methanol and water with
[bmim]BF4 follow different patterns. A nonlinear dependence of
k2 on the mole fraction of IL is observed. This behaviour is
attributed to preferential solvation effects. In alcoholic solutions,
rate constants have a maximum in alcohol-rich region while the
rate constant is moderately invariant in IL-rich area. Since, the
dienophile is capable of accepting hydrogen bonds, the variation
of rate constant with IL as cosolvent may cause change in
hydrogen bond abilities and polarity of media, we have proved
that hydrogen bond acceptor/donor abilities and dipolarity/
polarizability have been demonstrated synergism.[29] This
behaviour is attributed to a hydrogen bond interaction between
IL and alcohol to give hydrogen-bonded complex in the media,
which is more polar and a better hydrogen bond donor (HBD)
than the two constituents of the mixture.[30] Of course, variation
RESULTS AND DISCUSSION
The second-order rate constants of the cycloaddition reaction
between cyclopentadiene (1) and naphthoquinone (2) in various
compositions of [bmim]BF4 with water and methanol were
determined at 25 8C (Tables 1 and 2).
Several main types of parameters representing solute–solvent
*
interactions, such as dipolarity/polarizability parameter (p ),
hydrogen-bond donor acidity (a), hydrogen-bond acceptor
of the reaction is moderate and this makes
a reliable
interpretation of these small effects extremely difficult; however,
it can be a good example of different solvent effects compare
with the aqueous solutions.
Table 1. Second-order rate constants of the reaction
between (1) and (2) in water–[bmim]BF4 mixtures at 25 8C
Many papers referred that imidazolium-based ILs show polarity
like short chain alcohols.[29,31–36] Thus, the rate constant value of
the reaction in pure IL, which is near to its value in pure methanol,
reflects the influence of the polarity on the reaction rate as only
one of the effective solvent parameters in this reaction.
In aqueous solution, the rate constant reduces sharply with IL
content. Solvent polarity, hydrogen bonding and enforced
hydrophobic interactions are important factors that may affect
the rate constant. The absence of hydrophobic interactions and
weaker hydrogen bonding in ILs may be the important reasons
for the observed difference in the rates between water and ILs.
For example, the HBD ability in water is approximately twice that
in [bmim]BF4.[29] The rate constant of DA reaction carried out in
different ILs have been correlated with the ET(30) parameter.[25]
Since ET(30) parameter has correlation with H-bonding ability, the
rate constant of these reactions decreases with the decrease in
HBD ability of ILs.
*
p
xIL
102 ꢀ k2/Mꢁ1 sꢁ1
ETN
a
b
0
415 ꢂ 8
1.00
0.91
0.84
0.81
0.78
0.76
0.75
0.75
0.73
0.73
0.75
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
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
85.7 ꢂ 0.9
34.3 ꢂ 0.8
15.9 ꢂ 0.6
9.79 ꢂ 0.14
7.34 ꢂ 0.11
4.90 ꢂ 0.12
3.67 ꢂ 0.08
3.40 ꢂ 0.09
2.86 ꢂ 0.09
2.37 ꢂ 0.11
2.05 ꢂ 0.13
1.06
1.03
0.99
0.99
0.96
0.96
0.96
0.96
0.95
0.94
0.89
0.95
0.91
0.89
0.86
0.85
0.81
0.82
0.87
0.75
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 783–788

DIELS-ALDER REACTION IN MIXTURES OF AN ILS
Table 3. Sp values in water–[bmim]BF4 and metha-
nol–[bmim]BF4 mixtures
Sp in water–
[bmim]BF4
system
Sp in methanol–
[bmim]BF4
system
xIL
0
1
0.1998
—
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.7981
0.6935
0.5874
0.5264
0.5026
0.4717
0.4686
0.4579
0.4473
0.4366
0.4259
0.2755
0.3198
0.3488
0.3691
0.3846
0.3961
0.4058
0.4125
0.4192
0.4259
Figure 1. Variation of the solvophobicity of water–[bmim]BF4 (^) and
methanol–[bmim]BF4 (~) mixtures with mole fraction of [bmim]BF4
shows a good correlation of the calculated logk2 by Eqn (2) with
the experimental values. Changes in solute–solvent interactions
during the activation process will influence the rate constant.
Reactions, which are accompanied by an increase in the charge
separation in their activation process, are accelerated in solvents
of increasing polarity. Thus, increasing the reaction rate constant
is attributed to a major interaction of the polar media with
the activated complex relative to the reactants. Similar results
have been reported in molecular solvents.[2–4,37] In addition, the
solvent would destabilize the reactants through its solvophobi-
city parameter because in the reaction of two apolar reactants,
the unfavourable water contacts with reactants are reduced and
then the apolar reactants have a tendency to stick together in
aqueous solutions. Therefore, solvophobicity parameter is one of
the factors that affect the reaction rate constant. It can be seen
that the rate constants have a fair correlation with solvophobicity
of the media (Eqn 3).
Correlation with the composition of the media
In order to observe the solvent effects on the reaction, single and
multi-parameter correlation between logk2 and parameters of
solvent was investigated. We have attempted to rationalize the
observed trends in rate constants of the reaction in terms of
solvophobicity parameter, normalized polarity parameter or
*
Abraham-Kamlet-Taft (AKT) parameters including p , a and b. For
all mixtures, values of ET , a, b and p are available.[29] Values of Sp
N
*
for the pure IL were calculated based on the Abraham method[37]
and solubility data for rare gases and alkanes in [bmim]BF4 are
[38]
also demonstrated (Table 3).
Values of Sp for many binary
mixtures give a fair linear correlation with the volume fraction of
constituents.[15,37] Thus, we have estimated the Sp values for
water–[bmim]BF4 and methanol–[bmim]BF4 mixtures by the
following equation:
log k2 ¼ ꢁ3:239 ðꢂ0:103Þ þ 3:954 ðꢂ0:174Þ Sp
(3)
n ¼ 12; r ¼ 0:990; s ¼ 0:102; F1;10 ¼ 517:87
Sp ¼ f1 Sp1 þ ð1 ꢁ f1Þ Sp2
(1)
Obviously the reaction rate constant increases with the
increasing solvophobicity parameter. This equation shows that
hydrophobic effects only play a role in aqueous solutions and
demonstrates the better correlation with logk2 than the
normalized polarity (Fig. 3). Therefore, the normalized polarity
parameter and solvophobicity parameter of the media are not
individually the main factor in determining solvent effects on the
reaction rate. Merging these effects in one equation produces a
good correlation as dual-parameter regression of logk2 versus ETN
and Sp (Eqn (4)).
where f1 is the volume fraction of [bmim]BF4 and Sp1 and Sp2 are
solvophibicity parameter in pure [bmim]BF4 and water (or
methanol), respectively. The Sp values of the corresponding mole
fractions and values between those listed for mixtures of
[bmim]BF4 and water were linearly interpolated and have been
summarized in Table 3 and plotted in Fig. 1.
In order to suggest a complete and reasonable model for
solvent effects on this reaction, mixtures of solvents were
separately investigated.
log k2 ¼ ꢁ 5:021 ðꢂ0:793Þ þ 3:572 ðꢂ1:580Þ ETN
Water–[bmim]BF4 system
Single-parameter correlation of logk2 values versus ETN in
water–[bmim]BF4 mixtures gives an acceptable result (Eqn (2)).
(4)
þ 2:139 ðꢂ0:816Þ Sp
n ¼12; r ¼ 0:994; s ¼ 0:086; F2;9 ¼ 367:85
where standardized coefficients for ETN and Sp are 0.536 and
0.462, respectively. As can be seen, ETN and Sp have approximately
equal effects on increasing the reaction rate in aqueous solution
of [bmim]BF4. In order to show efficiency of the suggested
dual-parameter equation, the calculated values of the reaction
rate constant in mixtures of water with [bmim]BF4 using Eqn (4)
have been plotted versus the experimental values. Figure 4
log k2 ¼ ꢁ 7:015 ðꢂ0:283Þ þ 7:646 ðꢂ0:357Þ ETN
(2)
n ¼12; r ¼ 0:989; s ¼ 0:108; F1;10 ¼ 459:38
where n, r, s and F are the number of data, regression coefficient,
standard error and statistical Fisher number, respectively. As can
be seen, the second-order rate constant of the reaction increases
with the increasing normalized polarity parameter. Figure 2
J. Phys. Org. Chem. 2008, 21 783–788
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. R. HARIFI-MOOD, A. HABIBI-YANGJEH AND M. R. GHOLAMI
Figure 2. Plot of the calculated values, by Eqn (2), versus the exper-
imental values of logk2 in aqueous solutions of [bmim]BF4
Figure 4. Plot of the calculated values, by Eqn (4), versus the exper-
imental values of logk2 in aqueous solutions of [bmim]BF4
Figure 5. Plot of the calculated values, by Eqn (5), versus the exper-
imental values of logk2 in aqueous solutions of [bmim]BF4
Figure 3. Plot of the calculated values, by Eqn (3), versus the exper-
imental values of logk2 in aqueous solutions of [bmim]BF4
demonstrates a good agreement between the experimental and
the calculated values of logk2.
the reaction rate. This reaction is an example of a typical DA
reaction, in which the dienophile is capable of accepting
hydrogen bonds, that promotes the rate of the reaction.
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 reaction. Thus, the
reaction rate constant increases with hydrogen-bond donor and
acceptor parameters. Figure 5 shows the ability of predicting the
rate constants by recent equation.
The normalized polarity parameter is a blend of dipolarity/
polarizability and hydrogen-bond donor acidity of the media. The
main advantage of single-parameter correlations versus ETN is its
simplicity and a major drawback is that correlations are still
difficult to interpret. The multi-parameter approach in terms of
AKT models produces acceptable models in which the influence
of every parameter on the reaction rate has been illustrated. Thus,
*
multi-parameter correlations of logk2 versus a, p , b and Sp were
considered and the following equation was obtained:
Methanol–[bmim]BF4 system
In spite of similar trend between logk2 and ETN, single-parameter
correlation of logk2 versus ETN do not give reasonable result in
methanol–[bmim]BF4 solutions (r ¼ 0.784). Multi-parameter cor-
relation were considered for methanol–[bmim]BF4 system. Unlike
water–[bmim]BF4 system, dual-parameter regression of logk2
values versus ETN and Sp don’t indicate any fair correlation.
The consideration of the hydrogen-bond acceptor basicity of
the media can produce a better fit compared with the
log k2 ¼ ꢁ 5:548 ðꢂ0:298Þ þ 2:569 ðꢂ0:511Þ a
þ 1:915 ðꢂ0:304Þ b þ 1:950 ðꢂ0:448Þ Sp
n ¼12; r ¼ 0:999; s ¼ 0:038; F3;8 ¼ 1249:90
(5)
It is clear that the reaction rate constant increases with a, b and
Sp values. The standardized coefficients are 0.563, 0.117 and
0.488 for a, b, and Sp, respectively. These coefficients demons-
trate that a and Sp parameters are more effective than the b on
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 783–788

DIELS-ALDER REACTION IN MIXTURES OF AN ILS
Figure 7. Plot of the calculated values, by Eqn (9), versus the exper-
imental values of logk2 in all mixtures of the [bmim]BF4
Figure 6. Plot of the calculated values, by Eqn (6), versus the exper-
imental values of logk2 in mixtures of methanol with [bmim]BF4
dual-parameter regression (Eqn (6)).
where standardized coefficients are 0.273 and 0.774 for ETN and
Sp, respectively. It is clear that solvophobicity of the media has
main effect on controlling the reaction rate in the mixtures.
*
Substitution of the normalized polarity parameter with p and a
parameters and modification of the model confirms a fair
correlation of logk2 with a, b and Sp as shown in Eqn (9):
log k2 ¼ ꢁ 4:462 ðꢂ0:462Þ þ 1:377 ðꢂ0:180Þ EN
þ 1:613 ðꢂ0:313Þ b þ 2:103 ðꢂ0:390ÞTSp
n ¼11; r ¼ 0:966; s ¼ 0:008; F3;7 ¼ 32:713
(6)
Also, this model can be modified by removing the normalized
polarity parameter and investigating the dipolarity/polarizability
and hydrogen-bond donor acidity of media. There is a reasonable
correlation between logk2 and the three empirical parameters
log k2 ¼ ꢁ 4:919 ðꢂ0:133Þ þ 0:681 ðꢂ0:092Þ a
þ 2:143 ðꢂ0:223Þ b þ 3:664 ðꢂ0:085Þ Sp
n ¼22; r ¼ 0:997; s ¼ 0:049; F3;17 ¼ 1118:12
(9)
*
including a, b and p (Eqn (7)).
log k2 ¼ ꢁ 3:453 ðꢂ0:111Þ þ 0:147 ðꢂ0:013Þ a
þ1:189 ðꢂ0:095Þ b þ 1:072 ðꢂ0:068Þ pꢃ
n ¼11; r ¼ 0:994; s ¼ 0:004; F3;7 ¼ 181:05
The standardized coefficients are 0.154, 0.232 and 1.041 for a, b
and Sp, respectively. Figure 7 shows the ability of the model for
prediction of the reaction rate constant using Eqn (9). This
equation also confirms the importance of the solvophobicity
parameter in the reaction rate. As can be seen, the experimental
and calculated values of logk2 are in good agreement for all
solutions.
(7)
The standardized coefficients are 0.702, 3.637 and 4.300 for a, b
*
and p , respectively. These coefficients demonstrate that the
influence of hydrogen-bond donor acidity of media on the
*
reaction rate is negligible. On the other hand, b and p of media
Because of large changes in the rate constants in water–
[bmim]BF4 relative to methanol–[bmim]BF4 media, aqueous
solutions play a major role in the modelling of the system (Eqns
(8) and (9) are similar to Eqns (4) and (5)). As a result, we suggest
that the increase in the reaction rate in the solutions can be
attributed to two factors: stabilization of the activated complex of
the reaction relative to the reactants owing to the hydro-
gen-bonding interactions (donor and acceptor) of the media, and
a substantial decrease in the hydrophobic surface area of the
reactants during the activation process or enforced hydrophobic
interactions that destabilize the reactants relative to the activated
complex. The obtained results in the mixture of [bmim]BF4 with
water and methanol as aqueous and alcoholic media are similar
to the previous studies in molecular solvent,[2–7] and it proves
that the rate of DA reactions is mainly resulted in a combination
of hydrophobic and hydrogen-bonding interactions. Thus, unlike
what has been reported in a few papers,[39,40] results indicate that
the solvent effects in mixtures of the IL with conventional
molecular solvents can be described by means of the
solute–solvent interactions. As a result, in continuing our
studies,[28,29] it is obvious that [bmim]BF4 behaves like methanol
or water as a molecular solvent and not as a salt in the
reactions.[41]
have approximately equal effects on the reaction rate. The a
parameter demonstrates a trend with a maximum, however in
the b trend, a minimum is observed. Summation of these effects
shows a synergetic effect on the reaction rate with a maximum
around mole fraction of 0.1 for [bmim]BF4. Figure 6 shows a good
agreement between the experimental and the calculated values
of logk2 in the mixtures of methanol with [bmim]BF4.
Because of small changes in the rate constants in these media,
a simple model has not been obtained. Since, it can be a good
example to compare the solvent effects in alcoholic solutions
with aqueous solutions.
All mixtures
Single-parameter regression of the reaction rate constants with
the normalized polarity parameter does not demonstrate good
correlation in all solutions (r ¼ 0.777). Similar to water–[bmim]BF4
system, introducing solvophobicity parameter into the model
shows a better correlation (Eqn (8)).
log k2 ¼ ꢁ 4:497 ðꢂ0:416Þ þ 2:429 ðꢂ0:607Þ ETN
þ 2:724 ðꢂ0:240Þ Sp
(8)
n ¼22; r ¼ 0:974; s ¼ 0:146; F2;19 ¼ 176:76
J. Phys. Org. Chem. 2008, 21 783–788
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. R. HARIFI-MOOD, A. HABIBI-YANGJEH AND M. R. GHOLAMI
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