High viscosity of ionic liquids causes rate retardation of Diels-Alder reactions

Article abstract of DOI:10.1007/s11426-012-4684-9

Second order rate constants, k₂ have been determined for three bi-molecular Diels-Alder reactions to demonstrate that the high viscosity of ionic liquids can be a detrimental property in carrying out Diels-Alder reactions, if ionic liquids are employed as solvent media. It is possible to enhance the reaction rates of the reaction if a co-solvent is mixed in pure ionic liquid used as a solvent. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-012-4684-9

SCIENCE CHINA
Chemistry
August 2012 Vol.55 No.8: 1633–1637
doi: 10.1007/s11426-012-4684-9
• ARTICLES •
· SPECIAL ISSUE · Ionic Liquid and Green Chemistry
High viscosity of ionic liquids causes rate retardation of
Diels-Alder reactions
KUMAR Anil* & PAWAR Sanjay S.
Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India
Received March 10, 2012; accepted May 10, 2012; published online July 6, 2012
Second order rate constants, k2 have been determined for three bi-molecular Diels-Alder reactions to demonstrate that the high
viscosity of ionic liquids can be a detrimental property in carrying out Diels-Alder reactions, if ionic liquids are employed as
solvent media. It is possible to enhance the reaction rates of the reaction if a co-solvent is mixed in pure ionic liquid used as a
solvent.
ionic liquids, viscosity, kinetics, Diels-Alder reactions
termolecular Diels-Alder reaction between cyclopentadiene
1 Introduction
and acrylates is seriously hampered due to high viscosities
of ionic liquids employed as solvent media [8]. In fact, these
simple Diels-Alder reactions were faster in water than in
any ionic liquids. It is important to note here that the role of
viscosity in organic reactions has been a controversial issue.
An initial increase in rates with increasing viscosity, con-
trary to the general expectation of deceleration due to vis-
cosity, was reported several years ago [9]. These observa-
tions were explained on the basis of vibrational activation
theory. On the other hand, it was asserted that the rates of
organic reaction were independent of viscosity [10]. In an
independent study, we observed a strong correlation be-
tween the retarding effects on the kinetics of the Diels-
Alder reaction in the lithium perchlorate-diethyl ether
(LPDE) medium and the high viscosity of the medium [11].
The LPDE solution above 5 M concentration showed a
sudden decline, which was primarily due to the 800% in-
crease in viscosity beyond that composition. In another
study from our laboratory, the reaction rates were noted to
increase with the rise in viscosity, leveling off at 1.2–1.3 cP
before dropping with increasing viscosity beyond 1.3 cP.
This investigation was acknowledged for establishing the
characteristic effects of frictional forces on reaction kinetics
The past few years have witnessed an increasing number of
publications related to ionic liquids. Ionic liquid or room
temperature ionic liquids have been considered as alter-
nate solvents to the volatile organic compounds (VOCs)
responsible for environment pollution. The interest in ionic
liquids was initiated because of their advantageous physi-
cochemical properties such as negligible vapor pressure,
high thermal and electrochemical stability, high solvating
power, etc. In the past decades, ionic liquids have been in-
creasingly used for diverse applications such as organic
synthesis, catalysis, electrochemical devices and solvent
extraction of a variety of compounds in addition to many
other areas [1–7].
While the advantages of ionic liquids are desirable, there
exists a serious problem in handling these new materials.
Ionic liquids possess high viscosity. This poses operational
problems while making physical measurements and also
carrying out reactions. High viscosity of these ionic liquids
leads to the solvent friction that seems to determine the re-
activity. It has been shown from our laboratory that the in-
*Corresponding author (email: a.kumar@ncl.res.in)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

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Kumar A, et al. Sci China Chem August (2012) Vol.55 No.8
in conventional solvents [12].
lium hexaflourophosphate [BMIM][PF6], 1-butyl-3-methyl
imidazolium iodide [BMIM]I, 1-octyl-3-imidazolium hex-
aflourophosphate [OMIM][PF6], 1-octyl, 3-methyl pyri-
dinium tetraflouroborate [O3MPy][BF4], 1-butyl pyridinium
tetraflouroborate [BP][BF4] and 4-methyl,1-butylpyridinium
tetraflouroborate [4MBP][BF4] were synthesized by the
reported procedure [20–23]. The ionic liquids were thor-
oughly dried by heating at 70 °C under high vacuum for
several hours before each kinetic run. The water content in
all the ionic liquids as measured by Karl-Fisher coulometer
was less than 50 ppm. The purity of ionic liquids was
Based on the limited data from the study of Diels-Alder
reaction of cyclopentadiene with methyl acrylate, Welton
had concluded that an increase in viscosity accelerated the
Diels-Alder reactions [13]. Neta and coworkers acknowl-
edged the role of the high viscosity of ionic liquids for dif-
fusion-controlled reactions like elementary radical reactions
in ionic liquids [14]. A study of the activation parameters of
the bromination of alkynes in ionic liquids demonstrated
that viscosity affected the rate of reaction by increasing the
activation energy for bond breaking in the transition state
[5].
1
checked using H NMR spectroscopy. All manipulations
As a part of our ongoing research program on ionic liq-
uids [15–18], we now present three Diels-Alder reactions
(Schemes 1) in ionic liquids of varying viscosities with a
view to understanding the effect of viscosity of ionic liquids
on the kinetic profiles of reactions.
were carried out under an atmosphere of dry nitrogen to
exclude moisture.
2.2 Kinetic analysis
For a standard kinetic run, the dienophile was added to the
ionic liquid (1 mmol in 1 mL of ionic liquid) and was al-
lowed to equilibrate at the desired temperature. The temper-
ature was controlled using a Julabo constant temperature
bath at 298.150.01 K. The reaction was initiated by addi-
tion of 1 (1 mmol in 1 mL). The reaction progress was mon-
itored at appropriate time intervals by extraction of aliquots
with ether followed by appropriate dilution and GC analysis.
The rate constants thus determined were reproducible to
within 4%. A representative plot for the kinetic analysis is
shown in Figure 1. The original references for understand-
ing the reaction conditions for obtaining the kinetics of the-
se reactions should be consulted. The rate constant k2 was
determined from the slope of the graph.
2 Experimental
2.1 Materials
2,3-Dimethylbutadiene 1, 1,4-naphthoquinone 2, cyclopen-
tadiene 4, () menthyl acrylate 5, methyl (E)--cyanocin-
namate 7 were either procured commercially or synthesized
as reported [19]. Cyclopentadiene 1 was freshly cracked
from dicyclopentadiene prior to its use. The purities of the
materials were checked using 1H NMR spectroscopy.
Ionic liquids used in the current investigations were syn-
thesized by standard procedures. 1-ethyl-3-methyl tetra-
flouroborate [EMIM][BF4], 1-butyl-3-methyl imidazolium
tetraflouroborate [BMIM][BF4], 1-butyl-3-methyl imidazo-
2.3 Viscosity measurement
The viscosity was measured using a cone and plate viscom-
eter. The temperature was maintained at 298.15±0.01 °C
with the help of a Julabo constant temperature bath. The
viscosities thus measured were reproducible within 3%.
The viscosity measurements were carried out with the same
Scheme 1 The reactions of 2,3-dimethylbutadiene 1 with 1,4-naphtho-
quinone 2, cyclopentadiene 4 with () menthyl acrylate 5 and 4 with me-
thyl (E)--cyanocinnamate 7.
Figure 1 Plot of [x/(a(ax))] against t for the reaction of 1 with 2 in
[BMIM][BF4] at 298.15 K.

Kumar A, et al. Sci China Chem August (2012) Vol.55 No.8
1635
sample of ionic liquids as used for the kinetic runs and at
the same temperatures, in order to eliminate undesirable
discrepancies.
3 Results and discussion
In order to investigate the effect of viscosity, several ionic
liquids with graded viscosities were selected in which three
Diels-Alder reactions were carried out. The investigated
reactions were of 2,3-dimethylbutadiene 1 with 1,4-naph-
thoquinone 2, cyclopentadiene 4 with () menthyl acrylate 5
and 4 with methyl (E)--cyanocinnamate 7 (Scheme 1).
These reactions were selected on the basis of their kinetic
data obtained in organic solvents as reported in the literature
[24]. The second order rate constants, k2 in organic solvents
differed largely from each other thus offering us the test of
the role of viscosity in a wide range of reaction rates. Simi-
larly, the lowest viscosity of the ionic liquid selected for the
study was as low as 20 cP for [EMIM][BF4], and as high as
1000 cP for [BMIM]I providing us the range in which the
kinetics could be examined. In Figure 2 are plotted the k2
values for all the three reactions as a function of viscosity, 
of eight ionic liquids. The viscosity values measured by our
technique were [EMIM][BF4] = 20.2, [BMIM][BF4] = 92.1,
[BMIM][PF6] = 173, [BMIM]I = 1000, [OMIM][BF4] =
600, [O3MPy][BF4] = 668, [BP][BF4] = 145 and [4MBP]
[BF4] = 167 cP at 298 K.
The reaction of 4 with 7 is the fastest among the reac-
tions studied, while the reaction of 1 with 2 is the slowest
one. It is very interesting to note that the k2 values in each
case fall with an increase in the viscosity of ionic liquids.
First, the reaction of 4 with 7 decreases very sharply in the
ionic liquids possessing viscosities about 20 to 170 cP.
Secondly, the k2 value for this reaction in [BMIM]I is de-
creased by 50% when compared to in [EMIM][BF4], while
the viscosity increases from 20 to 1000 cP. This decrease in
k2 values for the reaction of 4 with 5 is about 3-times under
identical conditions of viscosity. This observation is seen in
all three reactions possessing the order of reactions from
105 to 103 dm3 mol1 s1 carried out in the ionic liquids
with viscosity ranging from about 20 to 2000 cP. This effect
can be attributed to the presence of the encounter-controlled
region in the high viscosity zones [12]. In this zone, the
reactants cannot see each other due to diffusion problems
resulting into the decline in the reaction rates.
Figure 2 Second order rate constant k2 as a function of viscosity  of the
ionic liquids. (∆) reaction of 1 + 2 with multiplication factor of 105 in k2;
(■) 4 + 5 with multiplication factor of 104 in k2; (O) 4 + 7 with multiplica-
tion factor of 103 in k2.
are expected to influence kinetic parameters of reactions.
The polarity of many ionic liquids has been studied in
terms of ET(30) (electronic transition energy in kcal/mol)
and the Kamlet-Taft polarity parameters, α the hydrogen
bond donor ability HBD: acidity, β the hydrogen bond ac-
ceptor ability HBA: basicity and π* polarizability. Though
the kinetic parameters of these reactions cannot be correlat-
ed with accuracy using a multiple regression approach, the
ET(30) and π* did not provide a correlation of rate constants
with these polarity parameters. Similarly, surface tension
data of ionic liquids do not correlate well with the rate con-
stant values. Considering our previous reports and the fact
that the rates were observed to be viscosity dependent, this
lack of correlation was not very surprising. When a process
is dominated by one of the solvent parameters governing the
reaction, the effect of other variables, though present, tends
to get “masked”. In order to overcome this difficulty, the
rate constant, k for the reaction in ionic liquids was correct-
ed for its viscosity dependence to give a modified rate con-
stant, k′ as follows:
ln k′ = ln B – a ln 
With the above corrections, it is possible to fit the ln k’ val-
ues with the help of the following correlation:
For the reaction 1 + 2:
Successful and effective application of ionic liquids as
solvent media in organic transformations, extraction, elec-
trochemical and microbiological processes require a precise
knowledge of polarity of these compounds. Recently, our
laboratory has contributed significantly to the measurement
and understanding of polarity parameters of several ionic
liquids [15, 16, 23, 25]. Since solvation is an important
process during the reaction involving organic moieties, the
polarity parameters of any solvent, including ionic liquids
ln k′ = 11.3584 + 0.99 (  0.13 )  + 5.55
( 1.11)  + 15.28 (  3.46 ) *
(r2 = 0.794)
For the reaction 4 + 5:
ln k′ = 8.5282 + 0.82 (  0.09 )  + 4.87 ( 0.98) 
+ 11.38 (  2.22 ) *
(r2 = 0.914)

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Kumar A, et al. Sci China Chem August (2012) Vol.55 No.8
For the reaction 4 + 7:
viscosity of ionic liquids before they are used as solvent
media. The viscosity of ionic liquids can be lowered by
adding a co-solvent in a very small quantity. This solvent
medium when used for carrying out organic reactions can
lead to high reaction rates. It is hoped that the attenuation of
viscosity of viscous ionic liquids can be carried out with
ease for carrying out the accelerated reactions.
ln k′ = 4.5554 + 0.78 (  0.09 )  + 3.58 ( 0.89) 
+ 11.33 (  2.88 ) *
(r2 = 0.811)
The above correlations can be treated useful to correlate
the rate constants of these three reactions. In Figure 3 are
shown the correlated log k′cor values against those obtained
experimentally, log k′exp confirming the utility of solvent
parameters once the viscosity effects are considered.
The author thanks Department of Science and Technology, New Delhi for
awarding him a J. C. Bose National Fellowship for enabling him to carry
out this work.
Several years ago, we had demonstrated that high viscos-
ity of molten organic salt, called tetra-n-butyl ammonium
picrate can be significantly lowered upon addition of a sol-
vent [26]. Recently, we carried out the similar experiment
of adding a solvent in pure ionic liquid and observed a dras-
tic reduction in the viscosity of pure ionic liquid [27]. This
addition of a solvent has been called as a co-solvent, a sol-
vent system that has been used in speeding up of the reac-
tions. Using this experimental observation, we used such a
solution of ionic liquid (mole fraction of [BMIM]I = 99.5%
by weight with methanol as a co-solvent) for carrying out
all the three reactions. The viscosity measured of [BMIM]I
at this composition was 53 cP as compared to the pure one
being 1000 cP. Surprisingly, the value of k2 for the reactions
of 1 with 2, 4 with 5 and 4 with 7 were noted to be 12.3 ×
105, 45.3 × 104 and 179.2 × 103 dm3 mol1 s1, respec-
tively. It is clear that addition of methanol as a co-solvent in
a pure ionic liquid enhances the rate constants by approxi-
mately 5, 4.5 and 4 times for the reactions of 1 with 2, 4
with 5 and 4 with 7, respectively. These results demonstrate
that addition of a co-solvent in pure ionic liquid, if em-
ployed as a solvent media for a Diels-Alder reaction.
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