Hydration of alkynes using Br?nsted acidic ionic liquids in the absence of Nobel metal catalyst/H2SO4

Article abstract of DOI:10.1016/j.molcata.2012.04.010

In this study, a variety of imidazole based sulfonic acid group functionalized Br?nsted acidic ionic liquids (BAILs) were synthesized. BAILs have been successfully developed as task specific ionic liquids for hydration of alkynes under mild conditions to give high yields of ketones as a selective product. Acidity of BAILs was determined using volumetric titration and UV-visible spectroscopic methods. The Hammett acidity order and acid value of BAILs correlate well with the activity order observed for most of the BAILs in the hydration reaction of phenylacetylene. Theoretical studies demonstrate that hydrogen bonding plays a key role in tuning the acidity of BAILs. Density function theory calculations are also able to explain the difference in activity observed in these BAILs. The activity of ionic liquids was theoretically studied by computing the activation energy for the hydration reaction. Recycling experiments suggest that these novel BAILs can be reused without significant loss in activity. BAILs are simple and easy to prepare and exhibit excellent activity toward the hydration of a variety of alkynes to ketones. Moreover, the reaction involving BAILs does not involve heavy metal catalysts or H ₂SO₄. Novel BAILs offer several attractive features such as sustainable synthetic route, low cost, moisture stability, high yields, and recyclability.

Full text of DOI:10.1016/j.molcata.2012.04.010

Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Hydration of alkynes using Brönsted acidic ionic liquids in the absence of Nobel
metal catalyst/H2SO4
Rajkumar Kore, T.J. Dhilip Kumar, Rajendra Srivastava∗
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a variety of imidazole based sulfonic acid group functionalized Brönsted acidic ionic liquids
(BAILs) were synthesized. BAILs have been successfully developed as task specific ionic liquids for hydra-
tion of alkynes under mild conditions to give high yields of ketones as a selective product. Acidity of BAILs
was determined using volumetric titration and UV–visible spectroscopic methods. The Hammett acidity
order and acid value of BAILs correlate well with the activity order observed for most of the BAILs in the
hydration reaction of phenylacetylene. Theoretical studies demonstrate that hydrogen bonding plays a
key role in tuning the acidity of BAILs. Density function theory calculations are also able to explain the
difference in activity observed in these BAILs. The activity of ionic liquids was theoretically studied by
computing the activation energy for the hydration reaction. Recycling experiments suggest that these
novel BAILs can be reused without significant loss in activity. BAILs are simple and easy to prepare and
exhibit excellent activity toward the hydration of a variety of alkynes to ketones. Moreover, the reaction
involving BAILs does not involve heavy metal catalysts or H2SO4. Novel BAILs offer several attractive
features such as sustainable synthetic route, low cost, moisture stability, high yields, and recyclability.
© 2012 Elsevier B.V. All rights reserved.
Received 1 February 2012
Received in revised form 18 April 2012
Accepted 20 April 2012
Available online 26 April 2012
Keywords:
Brönsted acidic ionic liquids
Hydration reaction
Acid catalyst
Hammett acidity
Acid value
DFT calculations
1. Introduction
for the appointed reaction. In recent years, Brönsted acidic ionic
liquids, as environmental-friendly catalysts, have received great
Low volatility, chemical and thermal stability, reusability and
eco-friendliness are the key properties to develop green and sus-
tainable chemical processes. Ionic liquids (ILs) have been developed
as green reaction media and structure directing agents due to their
negligible volatility and excellent thermal stability [1,2]. The low
vapor pressure of ILs makes them potential substituent for highly
volatile organic solvents thus reducing the amount of pollution
caused by solvent evaporation. Recently, ILs have been success-
fully employed as dual nature (solvents as well as catalysts) for
a variety of reactions, but their use as catalyst under solvent-
free condition needs to be given more attention [3]. Most of the
common ILs are based on the nitrogen-containing heterocyclic
cations such as imidazolium, benzimidazolium and pyridinium
[1–6]. Functionalization ability of these heterocyclic cation forms
new families and generations of ILs with more specific and tar-
geted properties. Recently, the synthesis of “task-specific” ILs with
special functions according to the requirement of a specific reac-
tion has become an attractive field [7–10]. All these studies allow
us to investigate the possibility of suitable ionic liquid catalysts
attention of researchers, and many organic reactions, such as ester-
ification, polymerization, alkylation, carbonylation, and Beckmann
rearrangement were reported with excellent selectivity [11–16].
The addition of water to alkynes is a synthetic method for the
preparation of carbonyl compounds [17]. The transformation is
classified as a dihydro, oxo-biaddition, but usually is divided into a
hydro-hydroxylation of a triple bond, followed by tautomerization
of the intermediary alkenol [18]. The hydration of terminal alkynes
gives either a methyl ketone (Markovnikov addition) or an alde-
hyde (anti-Markovnikov addition). Unlike many other syntheses
of carbonyl compounds, the hydration reaction of alkynes involves
simple addition of a water molecule with 100% atom efficiency. It is
regarded as a convenient and efficient method for the production
of ketones and aldehydes, and has been revealed as a useful tool
in total synthesis [19]. Traditionally Hg(II) catalysts are used for
the hydration reaction of terminal alkynes to form methyl ketones
[20,21]. Although this process offers very good yields, but it is not
an environmentally benign and sustainable approach. The pollu-
tion problems associated with the handling and disposal of toxic
mercury compounds discourage its industrial application.
Much research has been devoted to find catalysts based on
less toxic metals, the most promising being gold(I), gold(III), plat-
inum(II), and palladium(II) [22–25]. Catalytic anti-Markovnikov
∗
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.04.010

62
R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
hydration of terminal alkynes to aldehydes has been realized in
an efficient manner using ruthenium(II) complex catalysts [26].
However, the use of expensive metal-transition catalysts limits the
process and open up a thrust area to find an economical solution.
Therefore, a metal-free hydration of alkynes has been developed
under acidic conditions including concentrated sulfuric acid, formic
acid, p-toluenesulfonic acid (PTSA), trifluoromethanesulfonic acid
or trifluoromethanesulfonimide [27,28]. While these catalysts are
suitable in the case of robust substrates, many of them require
harsh conditions (large excess of strong acid, high temperature
(>373 K), organic solvent, long reaction times (18–72 h) [27–29].
Very recently, H2SO4 (0.5–8 mol equivalent to substrate) catalyzed
hydration reaction of alkynes to ketones in excess amount of ILs
has been developed, which is again a non eco-friendly route [30].
The objective of this study is to obtain an economical and
eco-friendly solution for the hydration reaction of alkynes. In
this manuscript, we are reporting for the first time that SO3H-
functionalized imidazole based Brönsted acidic ionic liquids (BAILs)
can be used for the hydration reaction. It may be noted that
this method is eco-friendly and economical, because it does not
involve corrosive H2SO4 or any costly noble metal catalysts. The
SO3H functionalized task specific BAILs catalysts offer prod-
uct under mild reaction conditions, which is rather difficult to
obtain with other acid catalysts reported in the literature. By
systematic variation in the anion {HSO4, Cl, CF3COO and p-
toluene sulfonate (TSO)} and cation {1-methyl, 3-H imidazolium
(Hmim), 1-methyl, 3-sulfonyl imidazolium (SO3Hmim), 1-methyl,
3-(3-sulfo-propyl)-imidazolium (C3SO3Hmim), 1-methyl 3-(4-
sulfo-benzyl)-imidazolium (Benz-SO3Hmim)}, it was possible to
make a library of task specific BAILs for the hydration reac-
tion of alkynes. Structure–activity relationship was established
using acidity measurements and density function theory (DFT)
calculations.
ı (ppm) 8.55 (s, 1H), 7.62–7.60 (m, 3H), 7.35 (d, 1H), 7.34–7.28
(m, 2H), 3.83 (s, 3H), 2.32 (s, 3H); 13C NMR (D2O): ı (ppm) 140.33,
135.51, 130.02, 125.95, 123.62, 120.16, 118.10, 115.21, 36.12, 21.13.
MS (ESI) for [SO3Hmim]+ m/z 163.03 and for [TSO]− m/z 171.00.
[SO3Hmim][CF3COO]: IR (KBr, ꢀ, cm−1) = 3151, 3075, 2982,
2871, 2810, 1772, 1588, 1454, 1148, 1024, 860, 828, 690. 1H
NMR (400 MHz, D2O): ı (ppm) 8.77 (s, 1H), 7.28–7.20 (m, 2H),
4.10 (s, 3H); 13C NMR (D2O): ı (ppm) 161.74, 139.26, 128.85,
124.98, 114.19, 35.15. MS (ESI) for [SO3Hmim]+ m/z 163.03 and
for [CF3COO]− m/z 162.95.
[SO3Hmim][HSO4]: IR (KBr, ꢀ, cm−1) = 3320, 3160, 2997, 2881,
2813, 1730, 1588, 1366, 1144, 1033, 889, 765, 624. 1H NMR
(400 MHz, D2O): ı (ppm) 8.37 (s, 1H), 7.16–7.05 (m, 2H), 3.65 (s,
3H); 13C NMR (D2O): ı (ppm) 135.56, 125.31, 122.40, 34.90. MS
(ESI) for [SO3Hmim]+ m/z 163.03 and for [HSO4]− m/z 96.93.
[Benz-SO3Hmim][TSO]: IR (KBr, ꢀ, cm−1) = 3094, 2965, 2817,
1708, 1584, 1352, 1152, 894, 686. 1H NMR (400 MHz, D2O): ı (ppm)
8.28 (s, 1H), 7.29 (d, 2H), 7.06 (m, 3H), 6.98 (m, 3H), 6.86 (d, 2H), 4.90
(s, 2H), 3.43 (s, 3H), 1.93 (s, 3H); 13C NMR (D2O): ı (ppm) 141.87,
139.32, 135.59, 133.18, 129.11, 128.99, 128.76, 128.34, 124.91,
123.51, 121.75, 52.49, 35.46, 20.16. MS (ESI) for [Benz-SO3Hmim]+
m/z 253.08 and for [TSO]− m/z 171.00.
[Benz-SO3Hmim][CF3COO]: IR (KBr, ꢀ, cm−1) = 3146, 3089,
3010, 2808, 1771, 1583, 1449, 1153, 1037, 881, 722. 1H NMR
(400 MHz, D2O): ı (ppm) 8.48 (s, 1H), 7.22–7.18 (m, 6H), 5.11 (s, 2H),
3.63 (s, 3H); 13C NMR (D2O): ı (ppm) 162.71, 142.12, 135.66, 133.38,
129.14, 128.57, 128.27, 120.16, 115.21, 52.59, 35.58. MS (ESI) for
[Benz-SO3Hmim]+ m/z 253.08 and for [CF3COO]− m/z 162.95.
2.2. Catalytic reaction
In a typical procedure, phenylacetylene (1.0 mmol) was mixed
with water (3.0 mmol) and BAILs (required amount) and the reac-
tion mixture was heated at 333 K. After 10 h, mixture was diluted
with H2O and extracted with chloroform. The combined organic
layer was washed with water and dried over anhydrous Na2SO4.
The reaction mixture was analyzed using gas chromatography
(Yonglin 6100; BP-5; 30 m × 0.25 mm × 0.25 ␮m). The products
were identified by GC–MS (Shimadzu QP-5000; 30 m long, 0.25 mm
i.d., with a 0.25-␮m thick capillary column, DB-1) and authentic
samples obtained from Aldrich.
Aqueous portion of the reaction mixture was evaporated to
remove the water. Residue was washed three to four times with
diethyl ether to remove any organic impurity. Finally, ionic liquid
portion was dried under vacuum at 353 K for 4 h. The recovered
ionic liquid was used in the recycling experiments.
2. Experimental
2.1. Synthesis of BAILs
Syntheses of some of the ILs used in this work are
already reported (Scheme 1) [15,16,31–33]. However,
syntheses of several BAILs such as [SO3Hmim][HSO4],
[SO3Hmim][CF3COO], [SO3Hmim][TSO], [Benz-SO3Hmim][TSO],
and [Benz-SO3Hmim][CF3COO] are reported here for the first time.
For
the
synthesis
of
[SO3Hmim][HSO4]/
solution of
[SO3Hmim][CF3COO]/[SO3Hmim][TSO],
a
H2SO4/PTSA/CF3COOH (10 mmol) in CH2Cl2 (10 mL) was added
drop wise to a solution of [SO3Hmim][Cl] (10 mmol) in 10 mL
CH2Cl2, and the reaction mixture was stirred at 273 K for
10 min. The resulting reaction mixture was refluxed for 24 h.
After completion of the reaction, solvent was removed by
evaporation under reduced pressure. Further, residue was
then washed several times (5 times) with ethyl acetate to
obtain [SO3Hmim][HSO4] (Yield = 91%), [SO3Hmim][CF3COO]
(Yield = 89%) and [SO3Hmim][TSO] (Yield = 92%).
3. Results and discussion
3.1. Synthesis and characterizations of BAILs
BAILs were synthesized by one step or multi step synthesis route
(Scheme 1). Samples were characterized by using FT-IR, NMR, and
ESI-MS. The influence of anions and cations of BAILs in the catalytic
activity was investigated. For−this study, BAILs having CF3COO−,
HSO4−, Cl−, and CH3C6H4SO3 (TSO) anions were prepared. Four
sets of BAILs having [Hmim], [SO3Hmim], [C3SO3Hmim], and [Benz-
SO3Hmim] cations were prepared.
Acidity of BAILs was measured using UV–visible spectropho-
tometer with a basic indicator by following the method reported
in the literature [34–36]. Acidity of BAILs was investigated in water
medium using 4-nitroanline as indicator. With the increase of acid-
ity of the BAILs, the absorbance of the unprotonated form of the
basic indicator decreased, whereas the protonated form of the indi-
cator could not be observed because of its small molar absorptivity
For
the
synthesis
first
of
[Benz-SO3Hmim][TSO]/[Benz-
1-benzyl-3-methylimidazolium
SO3Hmim][CF3COO],
chloride was prepared by following the reported proce-
dure [16]. It was then reacted with stoichiometric amount of
PTSA/CF3COOH in dichloromethane to obtain [Benz-mim][TSO]
(Yield = 94%)/[Benz-mim][CF3COO] (Yield = 90%) (Scheme 1). It was
then sulfonated using stoichiometric amount of chlorosulfonic acid
in CH2Cl2 to obtain [Benz-SO3Hmim][TSO] (Yield = 86%)/[Benz-
SO3Hmim][CF3COO] (Yield = 85%).
[SO3Hmim][TSO]: IR (KBr, ꢀ, cm−1) = 3150, 2992, 2876, 2813,
1721, 1592, 1449, 1139, 1019, 876, 686. 1H NMR (400 MHz, D2O):

R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
63
Scheme 1. Schematic representation for the synthesis of BAILs investigated in this study.
and its location. Therefore, the [I]/[IH+] (I represents indicator) ratio
can be determined from the differences of measured absorbance
after the addition of BAILs and Hammett function, H0, can be
calculated using Eq. (1). This value can be regarded as the relative
acidity of the BAILs.
(1). It has been observed that BAILs having HSO4 anion have
more Hammett acidity compared to BAILs having other anions.
UV–visible study shows that among the four sets of cations
investigated in this study, [SO3Hmim] exhibited higher Hammett
acidity (low H0 value) when compared with other cations. Acid
values of BAILs obtained using volumetric titration method are
consistent with the Hammett acidity (obtained using UV–visible
spectroscopic) trend for the same series of BAILs (having same
cation). BAILs were investigated for the hydration of alkynes for
the synthesis of ketones. Activity data can be explained based
on the acidity obtained using UV–visible spectroscopy/titration
method for several BAILs investigated in this study. Struc-
ture activity relationship was also established by performing
DFT calculations, which is discussed in the later part of this
manuscript.
ꢀ
ꢁ
(I)
(IH+)
H0 = pK(I)aq + log
(1)
Under the same concentration of 4-nitroanline (3 mg/L,
pK(I)aq = pKa = 0.99) and BAILs (50 mmol/L) in H2O, H0 values
of all BAILs were determined. The maximal absorbance of the
unprotonated form of the indicator was observed at 380 nm in
water. When the BAIL was added, the absorbance of the unproto-
nated form of the basic indicator decreased (Fig. 1 and Table 1).
Hammett acidity (H0) of these BAILs was calculated using Eq.

64
R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.4
Referance
Referance
(a)
(b)
[SO Hmim][CF COO]
[Benz-SO Hmim][TSO]
3
3
[SO Hmim][TS3O]
[Benz-SO Hmim][HSO ]
3
4
3
1.2
1.0
0.8
0.6
0.4
0.2
0.0
[SO Hmim][HSO ]
[Benz-SO Hmim][CF3COO]
3
3
4
[SO Hmim][Cl]
[C SO Hmim][Cl]
3
3
3
[(SO H)2im][Cl]
[C SO Hmim][CF COO]
3
3
[C SO Hmim][TS3O]
[Hm3im][HSO ]
3
3
4
[Hmim][TSO]
[C SO Hmim][HSO ]
3
3
4
[Hmim][Cl]
[Hmim][CF COO]
3
300
350
400
450
500
550
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorbance spectra of 4-nitroaniline after addition of (a) Hmim and SO3Hmim sets of BAILs and (b) C3SO3Hmim and Benz-SO3Hmim sets of BAILs in water.
3.2. BAILs assisted hydration of alkynes
(DMF), N-methylpyrrolidone (NMP) were investigated (Table 2).
Among the solvent investigated, DCM and DCE afforded the highest
yield (Table 2). However, it may be noted that without any sol-
vent, the yield was even higher than that of DCM/DCE (Table 2).
Hence, this process follows the green chemical principle in which
the reaction was performed in solvent-less medium to afford the
high yield of the product. Effect of amount of BAILs in the reac-
tion demonstrates that by increasing the BAILs concentration, yield
of the product increased (Table 2). A reasonably good yield of the
product was obtained when equivalent amount of BAILs was used.
Reaction can be done even at ambient conditions, however the yield
was very low (Table 2). With increase in the temperature, product
yield increases. A reasonably good yield of the product was obtained
at 333 K (Table 2). After evaluating several reaction parameters,
Activities of BAILs were assessed with the help of hydration
of alkynes. No product was obtained in the absence of BAILs.
Previous work in this direction led us to conclude that [Benz-
SO3Hmim][HSO4] is the best catalyst among the BAILs investigated
[16]. Hence, in this study, preliminary reactions were performed
using [Benz-SO3Hmim][HSO4] to find the optimized reaction con-
dition. In the initial investigations, phenylacetylene was chosen
as a model substrate for alkyne hydration. Influence of temper-
ature, amount of [Benz-SO3Hmim][HSO4], solvents and reaction
time were investigated. A variety of polar and non-polar sol-
vents such as toluene, dichloromethane (DCM), 1,2-dichloroethane
(DCE), methanol (MeOH), acetonitrile (ACN), dimethylformamide
Table 1
H0 and acidity values of BAILs in water at 298 K and comparison of the reactivity of BAILs for the hydration reaction of phenylacetylene.
Entry no.
BAILs
Amax
[I] (%)
100
[IH+] (%)
H0
Yield (%)
Acid value (mg KOH/mol)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
No BAIL
0.95
0.64
0.74
0.77
0.80
0.64
0.69
0.71
0.74
0.61
0.65
0.68
0.71
0.95
0.86
0.92
0.94
0.77
0.77
0.77
0.77
–
33
22
19
16
33
27
25
22
36
32
28
25
–
9
3
1
19
19
19
19
–
0
35
75
95.5
39
30
79
86
65
74
68
79
82
6
8
11
14
–
[Benz-SO3Hmim][HSO4]
[Benz-SO3Hmim][CF3COO]
[Benz-SO3Hmim][TSO]
[C3SO3Hmim][Cl]
[C3SO3Hmim][HSO4]
[C3SO3Hmim][CF3COO]
[C3SO3Hmim][TSO]
[SO3Hmim][Cl]
[SO3Hmim][HSO4]
[SO3Hmim][CF3COO]
[SO3Hmim][TSO]
[(SO3H)2im][Cl]
[HMim][Cl]
67
78
81
84
67
73
75
78
64
68
72
75
100
91
97
99
81
81
81
81
1.30
1.54
1.62
1.71
1.30
1.42
1.46
1.54
1.24
1.32
1.40
1.46
–
1.99
2.50
2.98
1.62
1.62
1.62
1.62
120.89
101.3
96.1
18.9
81.3
39.7
37.2
89.3
178.5
130.2
122.5
106.1
48.7
85.6
66.4
51.8
96.1
96.1
96.1
96.1
[HMim][HSO4]
[HMim][CF3COO]
[HMim][TSO]
[Benz-SO3Hmim][TSO]a
[Benz-SO3Hmim][TSO]b
[Benz-SO3Hmim][TSO]c
[Benz-SO3Hmim][TSO]d
60.5
21.0
68.9
86.3
Indicator: 4-nitroanline.
Reaction condition: phenylacetylene (1.0 mmol), deionized water (3.0 mmol), BAIL (1.0 mmol), temperature (333 K), reaction time (10 h).
a
BAIL (0.5 mmol).
BAIL (0.2 mmol).
BAIL (0.2 mmol), reaction time (24 h).
Catalytic activity data after 5th recycling.
b
c
d

R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
65
Table 2
100
Influence of reaction parameters such as solvent, amount of [Benz-
SO3Hmim][HSO4], and temperature on the hydration of phenylacetylene.
Entry no.
Solvent
Amount of
catalyst
Temperature
(K)
Yield (%)
95
90
85
80
(mmol)
1
2
3
4
5
6
7
8
9
10
11
12
13
Acetonitrile
Methanol
Toluene
Dimethylformamide
N-methylpyrrolidone
Dichloromethane
1,2-Dichloroethane
None
None
None
None
None
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
2.0
1.0
1.0
1.0
333
333
333
333
333
333
333
333
333
333
298
313
353
3
7
1
2
4
32
28
35
19
64
2
10
76
None
Reaction condition: phenylacetylene (1.0 mmol), deionized water (3.0 mmol), reac-
tion time (10 h).
0
1
2
3
4
5
Number of cycles
an optimum condition (reaction temp. = 333 K, BAILs = 1 equiv. with
respect to alkyne, and solvent = none, reaction time = 10 h) was cho-
sen to investigate the influence of a variety of BAILs for this reaction.
The activities of various BAILs were compared in neat condi-
tion at 333 K for 10 h (Table 1). Mechanism for the BAILs assisted
hydration reaction is illustrated in Scheme 2. The reaction mecha-
nism of acid-catalyzed alkyne hydration proceeds via rate-limiting
protonation of the carbon-carbon triple bond by forming a cyclo-
propenylium ion intermediate (I) (with H+ of BAILs). Addition of
water to the intermediate (I) leads to the formation of an enol (II),
which converts to keto form via keto–enol tautomerism (III). H+
liberated during the addition of the water, reacts with the interme-
diate (III) to form ketone as a selective product.
Fig. 3. Recycling study of the [Benz-SO3Hmim][TSO] for the hydration of pheny-
lacetylene.
H2O
R
H
H
R
H
BAIL
(I)
R
O
BAIL
H
BAIL
H
R
O
H
(II)
It is evident from the hydration reaction that the activity of the
BAILs are governed by the functionality ( SO3H) of BAILs. Activity
of BAILs having SO3H functionalization in cation was found to be
more than that of BAILs without SO3H functionalization. Results
R
O
BAIL
H
H
H
H
H
(III)
Scheme 2. Mechanism for the acid (BAILs) catalyzed hydration of alkynes.
0.30
1.8
1.0 equivalent
0.25
0.20
0.15
0.10
0.05
0.00
show that Benz-SO3Hmim cation and TSO anion were found to be
the best among the cations and anions investigated in this study. In
the case of SO3H functionalized BAILs, in addition to the conven-
tional acid catalyzed hydration reaction (Scheme 2), SO3H group
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.5 equivalent
R
H
1.0 equivalent
H2O
0
50
100
150
O
O
S
R
H
O
R'
H
O
R'
O
S
S
R
O
O
R'
O
0.5 equivalent
O
HO
H
R
O
R'
S
O
R
O
O
0
100 200 300 400 500 600
O
H
H
Time (min)
H
Fig. 2. Kinetics of hydration reaction using 0.5/1.0 equiv. with respect to pheny-
lacetylene over [Benz-SO3Hmim][TSO].
Scheme 3. Mechanism for the SO3 containing BAILs in the hydration of alkynes.

66
R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
Table 3
[Benz-SO3Hmim][TSO] assisted hydration of alkynes.
Entry no.
Alkyne
Product
Yield (%)
O
1
68.9 (21)a
O
CH3
4
4
H3C
CH3
2
3
40.5 (15.2)a
34.7 (13.5)a
O
O
4
21.6 (5)a
OH
O
OH
5
80.5 (25)a
OH
O
OH
MeO
MeO
6
(100)a
Reaction condition: phenylacetylene (1.0 mmol), deionized water (3.0 mmol), BAIL (0.2 mmol), temperature (333 K), reaction time (24 h).
a
Parenthesis represent the yield obtained after 10 h of reaction.
synergistically facilitate the hydration reaction by following the
plausible mechanism shown in Scheme 3. First step of the hydra-
tion reaction using SO3H functionalized BAILs proceeds through
more stable five membered transition state (Scheme 3) compared
with three membered transition state in conventional (H+) acid cat-
alyzed reaction (Scheme 2). Based on the result obtained, one can
conclude that the activity of BAILs in hydration reaction is highly
dependent on the SO3/SO3H functionalization.
Kinetics of the hydration reaction was monitored using two dif-
ferent amounts of [Benz-SO3Hmim][TOS] (0.5 and 1.0 equiv. with
respect to phenylacetylene) under the optimized reaction condi-
tion. From the plot of −ln(C/C0) vs time (Fig. 2), it is very clear that
the reaction is first order when the kinetics is monitored in the
time domain of 0–150 min of the reaction. Fig. 2 (inset) shows that
the plot of −ln(C/C0) vs time is straight line with slope 9 × 10−4
and 1.94 × 10−3 for the reaction conducted with 0.5 and 1.0 equiv.
of [Benz-SO3Hmim][TSO] with respect to phenylacetylene, respec-
tively.
investigated (Table 1). It is clear that the catalytic amount of [Benz-
SO3Hmim][TSO] can also be utilized to carry out this reaction but
the yield was low under the optimized condition (Table 1). How-
ever, the yield of the reaction can be increased by continuing the
reaction for longer time (Table 1). To demonstrate the robust-
ness and durability of BAILs, [Benz-SO3Hmim][TSO] was recycled 5
times. At the end of each cycle, the reaction product was extracted
for the determination of yield by gas chromatography. Throughout
all the recycling, comparable yields (on average 90%) were observed
(Fig. 3). The results strongly suggest that the BAILs are a robust and
recyclable system that can constantly promote the transformation
of phenylacetylene to acetophenone without losing its reactivity.
Hydration reaction proceeded well, delivered good product yields,
and accommodated a wide range of alkynes (Table 3). Both aromatic
and aliphatic alkynes could be converted to ketones under the given
conditions. Compared to aromatic terminal alkynes, aliphatic ter-
minal alkynes are less reactive. Internal alkynes are less reactive
than terminal alkynes. Alkynes having propargyl function pro-
vides higher yield, especially when aromatic ring has methoxy
substitution at para position. This is due to the fact that alkynes
having a propargylic function could provide a four membered cyclic
Having found the highest activity of [Benz-SO3Hmim][TSO]
in the hydration reaction under the optimized condition, the
influence of the amount of BAIL required for the reaction was

R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
67
Fig. 4.
Activation energy for the protonation step in the hydration reaction using Benz-SO3Hmim set of BAILs. A1 is the adduct
of phenylacetylene and [Benz-SO3Hmim][HSO4], A2 is the adduct of phenylacetylene and [Benz-SO3Hmim][CF3COO] and A3 is the adduct of phenylacetylene and [Benz-
SO3Hmim][TSO]. TS(A1), TS(A2) and TS(A3) are the corresponding TS of A1, A2 and A3, respectively.
oxonium intermediate which could result from an intra-molecular
oxygen atom-addition (Scheme 4). Further nucleophilic ring open-
ing by water gives the carbonyl compound.
of BAILs were determined by performing DFT geometry opti-
mizations at the B3LYP/6-31G using the Gaussian09 program
[37]. Geometrical parameters of the optimized structures of BAILs
(Figs. S1–S5 of supporting information) are provided in Tables 4–7.
In addition to this, at−omic charge analysis on the protons of SO3H,
(N2)C H and HSO4 groups of BAILs has also been performed
(Table T1–T2, supporting information), which is consistent with the
experimental evidences (Table 1). Theoretical study confirmed that
hydrogen bonding exists in the BAILs. Molecular geometry study
3.3. The molecular geometries of BAILs
Since the hydration reaction is performed in water, there-
fore, theoretical studies have been performed by considering H2O
molecules in the calculation. The minimum-energy geometries

68
R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
Table 4
The geometry parameters of Hmim set of BAILs calculated at B3LYP/6-31G.
Observed parameters
[Hmim][Cl]
[Hmim][HSO4]
[Hmim][TSO]
[Hmim][CF3COO]
˚
(N· · ·H) bond distance (A)
H13· · ·N8 = 1.047
C3 H7 = 1.075
–
H5· · ·Cl14 = 2.057
H5· · ·N4 = 1.043
C3 H8 = 1.077
H8· · ·O17 = 2.302
H5· · ·O16 = 1.695
H31· · ·N14 = 1.056
C1 H4 = 1.078
H20· · ·N4 = 1.061
C3 H7 = 1.077
H7· · ·O13 = 2.350
H20· · ·O14 = 1.610
(N)2C
(N)2C H· · ·O
Other hydrogen bonds in BAILs (A)
H
H4· · ·O12 = 2.217
˚
H31· · ·O11 = 1.626
Table 5
The geometry parameters of C3SO3Hmim set of BAILs calculated at B3LYP/6-31G.
Observed parameters
[C3SO3Hmim] [Cl]
[C3SO3Hmim]
[HSO4]
[C3SO3Hmim]
[TSO]
[C3SO3Hmim]
[CF3COO]
˚
H
O bond distance of sulfonic acid (A)
H23 O22 = 0.989
H22 O21 = 1.040
H32 O31 = 0.989
C3 H7 = 1.081
H7· · ·O28 = 2.038
H22· · ·O29 = 1.549
H16· · ·O28 = 2.591
H10· · ·O28 = 2.418
H22 O21 = 1.083
H22 O21 = 1.049
(N)2C
(N)2C H· · ·O
Other hydrogen bonds in BAILs (A)
H
C3 H7 = 1.086
C3 H7 = 1.084
C3 H7 = 1.088
H7· · ·Cl12 = 2.5198
H7· · ·O27 = 2.027
H22· · ·O28 = 1.411
H16· · ·O27 = 2.270
H33· · ·O43 = 2.593
H7· · ·O27 = 1.980
H22· · ·O28 = 1.494
H14· · ·O28 = 2.517
H16· · ·O28 = 2.518
˚
H15· · ·Cl12 = 2.402
Table 6
The geometry parameters of SO3Hmim set of BAILs calculated at B3LYP/6-31G.
Observed parameters
[SO3Hmim] [Cl]
[SO3Hmim] [HSO4]
[SO3Hmim] [TSO]
H17 O16 = 1.60
[SO3Hmim]
[CF3COO]
[(SO3Him)2] [Cl]
H19 O18 = 1.049
˚
H
O bond distance of sulfonic acid (A)
H17 O16 = 1.134
C3 H7 = 1.076
H23 O19 = 1.477
H22 16 = 0.991
H17 O16 = 1.536
O
H13
O12 = 1.050
(N)2C
H
(N)2C H· · ·O
C3 H7 = 1.082
H7· · ·O13 = 2.053
H23· · ·O21 = 1.257
C1 H4 = 1.086
H4· · ·O12 = 1.949
H17· · ·O30 = 1.228
C3 H7 = 1.079
H7· · ·O18 = 2.267
H17· · ·O18 = 1.230
C3 H7 = 1.077
˚
Other hydrogen bonds in BAILs (A)
H17· · ·Cl18 = 1.697
H13· · ·Cl14 = 1.939
H19· · ·Cl14 = 1.951
Table 7
The geometry parameters of Benz-SO3Hmim set of BAILs calculated at B3LYP/6-31G.
Observed parameters
[Benz-SO3Hmim]
[HSO4]
[Benz-SO3Hmim]
[TSO]
[Benz-SO3Hmim]
[CF3COO]
˚
H
O bond distance of sulfonic acid (A)
H32 O31 = 0.99
H28 O27 = 0.99
H28 O27 = 0.99
H16
O15 = 0.988
(N)2C
(N)2C H· · ·O
Other hydrogen bonds in BAILs (A)
H
C1 H4 = 1.087
H4· · ·O14 = 1.969
H7· · ·O13 = 2.415
H24· · ·O14 = 2.367
C1 H4 = 1.088
H4· · ·O11 = 1.933
H20· · ·O43 = 2.356
H7· · ·O12 = 2.406
H37· · ·O43 = 2.66
H39· · ·O11 = 2.66
C1 H4 = 1.095
H4· · ·O12 = 1.887
H29· · ·O11 = 2.248
H20· · ·O11 = 2.429
˚
reveals that anions interact with the SO3H and imidazole ring
through C H· · ·O or O H· · ·O type hydrogen bonding.
Geometrical parameters and hydrogen bonding data of
[C3SO3Hmim] set of BAILs correlate well with the activity order
obtained from catalytic investigation (Tables 1 and 5). It may be
noted that in this case, the activity of [C3SO3Hmim][HSO4] is sig-
nificantly low compared to [C3SO3Hmim][CF3COO]. A very low
activity of [C3SO3Hmim][HSO4] can be explained based on the
bond distance of O H and (N)2C H. The large difference in the
Geometrical parameters and hydrogen bonding data of [Hmim]
set of BAILs clearly state that [Hmim][Cl] is the least active BAIL in
this set (Tables 1 and 4). DFT calculation and H-bonding suggest
that [Hmim][CF3COO] is more active than [Hmim][HSO4] because
the N H is more labile in case of [Hmim][CF3COO] (Table 4).
Table 8
Activation energy for the protonation step in the hydration of phenylacetylene over BAILs.
Entry no.
BAILs
Adduct of
(BAIL + phenylacetylene)
(Hartrees)
TS (Hartrees)
Activation
energy
(kcal mol−1
)
1
2
3
4
5
6
7
8
9
[Benz-SO3Hmim][HSO4]
[Benz-SO3Hmim][TSO]
[Benz-SO3Hmim][CF3COO]
[C3SO3Hmim][TSO]
[C3SO3Hmim][HSO4]
[C3SO3Hmim][CF3COO]
[C3SO3Hmim][Cl]
[SO3Hmim][HSO4]
[SO3Hmim][CF3COO]
[SO3Hmim][TSO]
−2167.603
−2362.724
−1994.30
−2210.346
−2015.227
−1841.921
−1776.054
−1897.32
−1723.98
−2092.375
−1273.762
−2167.560
−2362.69
−1994.262
−2210.31
−2015.18
−1841.882
−1776.01
−1897.281
−1723.939
−2096.34
−1273.712
26.97
21.94
23.86
22.59
29.49
24.47
27.61
24.47
25.73
21.96
31.38
10
11
[Hmim][HSO4]

R. Kore et al. / Journal of Molecular Catalysis A: Chemical 360 (2012) 61–70
69
of imaginary frequencies. The activation energies for the pro-
tonation step of phenylacetylene with [Benz-SO3Hmim] set of
BAILs were obtained (Table 8, Fig. 4). The activation energy of
the [Benz-SO3Hmim][TSO] is found to be lowest with a value of
21.94 kcal mol−1 of all the BAILs studied. Experimentally, the activ-
ity of [Benz-SO3Hmim][TSO] is found to be the maximum (ketone
yield = 95%). The activation energy of the [Benz-SO3Hmim][HSO4]
is found to be the highest with a value of 26.97 kcal mol−1 and
the yield of the product was only 35%. The activation energy
for [Benz-SO3Hmim][CF3COO] is calculated to be 23.86 kcal mol−1
with the experimental ketone yield of 75%. The activation ener-
gies for the protonation rate-limiting step in the hydration reaction
using several other BAILs are provided in Table 8 (Figs. S6–S13, sup-
porting information). Based on these results, one can conclude that
acidity measurement along with DFT calculation is needed to estab-
lish the structure activity relationship for the hydration reaction
using BAILs.
Scheme 4. Proposed mechanism for the BAILs assisted hydration of alkynes having
propargyl function.
activity is mainly due to the (N)2C H bond distance in these
two cases (Table 5). The strong hydrogen bonding (C3 H7 = 1.081)
in [C3SO3Hmim][HSO4] makes proton less labile compared to
other BAILs of this series. Optimized structure and related
parameters clearly show that a significant interaction exists
4. Conclusions
−
between HSO4
and
SO3H, whereas such interaction is
absent in Cl− and SO3H (Table 5). H23 of SO3H group in
[C3SO3Hmim][Cl] has no interaction, whereas H22 of SO3H
group in [C3SO3Hmim][HSO4] significantly interact with O29
In summary, an effective and recyclable protocol using a variety
of Brönsted acidic ionic liquids without using costly Nobel metal
catalysts or H2SO4 for the transformation of alkynes to ketones
under mild conditions has been successfully developed. BAILs are
easy to prepare and exhibit excellent reactivity and stability over
multiple cycles. BAILs having more number of SO3H functional-
ization exhibited comparatively high activity. The activity of ionic
liquids in the hydration reaction is well supported by the acidity
measurements, geometrically optimized parameters, and the acti-
vation energy of the rate determining protonation step using DFT
calculations. Based on the results, one can conclude that to under-
stand the activity difference of these ionic liquids catalysts, along
with UV–visible/volumetric titration investigations, DFT calcula-
tion is very essential.
−
of HSO4 (H22· · ·O29 = 1.549) and inhibits the accessibility of
H22 to the reactant molecules for the hydration reaction and
hence [C3SO3Hmim][HSO4] activity was found to be less com-
pared to the activity of [C3SO3Hmim][Cl]. The highest activity of
(C3SO3Hmim][TSO] in this set of BAILs is due to the presence of
an additional SO3 group in anion, which enhances the activity by
following the mechanism shown in Scheme 3.
Geometrical parameters and hydrogen bonding data of
[SO3Hmim] set of BAILs correlate well with the activity order
obtained from catalytic investigation (Tables 1 and 6). Less activ-
ity of [SO3mim][Cl] can be correlated well with the shorter
bond distance of H17· · ·O16 (Table 6). It may be noted that
in this case, opposite trend in the activity was observed
([SO3Hmim][CF3COO] < [SO3Hmim][HSO4]). Again this activity
order can be very well explained using hydrogen bonding. H23 and
H7 are more labile in [SO3Hmim][HSO4] due to the weak hydro-
gen bonding (Table 6). The highest activity of [SO3Hmim][TSO] in
this set of BAILs is due to the presence of an additional SO3 group
in anion, which enhances the activity by following the mechanism
shown in Scheme 3.
Geometrical parameters and hydrogen bonding data of
[Benz-SO3Hmim] set of BAILs correlates well with the activ-
ity order obtained from catalytic investigation (Tables 1 and 7).
It may be noted that in this case also the activity of
[Benz-SO3Hmim][CF3COO] is significantly higher than [Benz-
SO3Hmim][HSO4]. Low activity of [Benz-SO3Hmim][HSO4] can
Acknowledgements
Authors thank Department of Science and Technology, New
Delhi for financial assistance (DST grant SR/S1/PC/31/2009).
Authors are grateful to Prof. M. K. Surappa, Director IIT Ropar for
his constant encouragement.
Appendix A. Supplementary data
Supplementary
can be
data
found,
associated
in the
with
online
this
version,
arti-
at
cle
http://dx.doi.org/10.1016/j.molcata.2012.04.010.
References
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H bond distance
(C3 H7 = 1.083) compared to other BAILs of this series. The highest
activity of [Benz-SO3Hmim][TSO] in mainly due to (N)2C H bond
distance (C3 H7 = 1.087) and the presence of an additional SO3
group in the anion, which enhance the activity by following the
mechanism shown in Scheme 3.
The reaction mechanism of acid-catalyzed alkyne hydration
proceeds via the rate-limiting protonation of the carbon-carbon
triple bond, followed by rapid addition of water to generate an enol,
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of BAILs were obtained. The vibrational analysis was performed
for the minimum and transition state optimized geometries. The
vibrational analysis data confirmed the structures from the number
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