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Chapter 1
General Introduction
1
The scientific discipline known as Ionic liquids (ILs from here) consists of an
emerging class of materials with a diverse and extraordinary set of properties. The
explosion of interest in ILs continues apace, both because they led to fascinating
chemical physics problems and because of the multiplicity of their uses. The
realization that task-specific ILs can be created via simple and systematic chemical
modifications of the constituent ions takes these systems beyond the promise of
designer solvents to highly useful systems for diverse applications including drug
delivery and as active pharmaceutical ingredients, solvents for green processing of
otherwise insoluble bio-molecules such as cellulose, highly energetic materials, and
novel electrolytes for energy applications such as batteries, fuel cells, and solar photo-
electrochemical cells. Understanding the origins of these properties and how they can
be controlled by design to serve valuable practical applications presents a wide array
of challenges and opportunities to the chemists and physicist.
1.1. Brief History of Ionic Liquids
As is well known, the whole field of ILs started with Humphrey Davy‘s
pioneering work on the electrolytic decomposition of simple molten salts under the
influence of an applied dc electric field, to yield the elements that initially had been
chemically combined in the salt under study. Davy [1] worked primarily with the
high-melting simple salts. This situation was radically altered by the work that has
been led by Paul Walden [2] in 1914, and reports the first IL ethyl-ammonium nitrate.
It has probably been studied in more detail, and for more purposes, than any other IL.
After that, the work pioneered by Osteryoung, Hussey, and Wilkes, is admirably
described in Bockris and Reddy‘s book ―Modern Electrochemistry: Ionics‖ published
in 1998, which manifests 1-butyl-pyridinium chloride was the first truly room
temperature pure electrolyte (i.e., a system consisting of ions without a solvent)
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reported by Osteryoung et al. in 1975 [3]. Thereafter, Wilkes, in particular, has
developed the use of l-ethyl-3-methyl-imidazolonium chloride (Me-Et-ImCl). In this
respect a comprehensive review article was published by Wilkes [4] in 2002, which
describes the journey of ILs from molten salt, classified ILs on the basis of their
constituents and thermal response, as well as differentiates ILs from molten salts. This
article first time coined the term ―neoteric solvents‖ for ILs.
With the great development in the range and scope of IL field there has of
course, been the need, from time to time, to have conferences, books and review
articles describing much of the new knowledge and its applications. Though a book
by Dyson and Geldbach ―Metal Catalyzed Reactions in Ionic Liquids,‖ appeared in
2005 [5], it has had a record of the very rapid modern applications of ILs. The volume
―Ionic Liquids‖ [6] edited by Barbara Kirchner, in 2009. is a comprehensive account
of the known developments in the field of ILs, particularly in so far as physical
properties, structural elucidation, relation with spectroscopy, synthesis and after
synthesis purification was concerned. This book also contains many articles
describing researches which it turns out were to be most important in defining how
the subject was developed during the ensuing decade. It is easy to understand that, as
the range of applications of ILs becomes wider and wider, more specialized books
will tend to appear. A book edited by P. Wasserscheid and T. Welton, entitled ―Ionic
Liquids in Synthesis‖ [7], gives an excellent account of the scope and applications of
ILs. Thus, Koel‘s outstanding recent book, ―Ionic Liquids in Chemical Analysis‖ [8],
reflects this trend. This book gives an inimitable account of the applications of ILs in
analytical science. It is particularly noteworthy for its account of the important recent
work on the use of ILs in chemical analysis. Another recent book which covers
molecular array of ILs is Schroder‘s. ―Molecular ordering of ILs at a sapphire hard
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wall a high energy x-ray reflectivity study.‖[9] The development of the field of ILs
opened the door to much more IL research. Scientists realized the uses of ILs are so
diverse that no one IL is suitable in all instances. Consequently, one finds in the
literature numerous description of tuning ability of ILs. Hence, the Designing ability
of ILs fascinates not only the alteration of anion, but also the alteration in alkyl chain
lengths on the cation. The ability to control or tune or design ions either cation or
anion in ILs gave way to the ability to vary properties of the IL as desired, which of
course opens new horizons for the synthesis and applications of thousands of new ILs.
Since then there has been an amazingly rapid increase in the numbers and types of
ILs, and the range of applications has broadened so greatly, some of the more
commonly studied and applied cations and anions are shown below.
common cations:
N r'r N
rr' im
N r'r N
Me
2-Me-rr' im
N r
rpy
R4NR4P
N
r'r
rr' pyr
N
O
'r r
N r
N
N r
N rN
rvim
rr' mor
rpz
4-vinyl-rpy
common anions:
.
Now, popularity of ILs as a research topic is reflected in the numerous reviews
[10, 11], monographs [12], conference proceedings [13-15], and special issues of the
reputed journals of the esteemed societies [16-18] that have been published recently.
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As has been indicated above, the impact of ILs has been so great and its influence so
widespread in every branch of recent day science that any attempt even to mention all
the material in the published literature would surely produce a work of tremendous
dimensions. Perhaps, that is something which should be done: the present work is not
however offered in any way as an encyclopedia of ILs.
1.2. Classes of Ionic Liquids
Molten salts were first discovered over 200 years ago. A molten salt is a liquid
that is entirely composed of ions and typically has a very high melting point. Today,
the term IL is applied to any molten salt which is liquid below . A great
contribution of, C. A. Angell and coworkers [19-29] in the field of ILs astonishingly
boosted the application window of ILs from glass forming material to reaction media
and so many, they classified an ILs into four categories, 1) Aprotic ionic liquids, 2)
Protic ionic liquids, 3) Inorganic ionic liquids and 4) Solvate ionic liquids, and
documented such genuine work of the classification of ILs on the basis of their
thermal response, constituents and utility so nicely in the form of review article [1]. A
brief discussion on the classification of these neoteric materials is given in following
pages,
1.2.1. Aprotic ionic liquids
The majority of ILs, and certainly those responsible for the meteoric rise in the
number of publications in this area since the mid- 90‘s, are liquids in which, the
cations are organic molecular-ions. Such cations are usually charge-compensated by
anions of oxidic character like nitrate, perchlorate or more frequently fluorinated-
oxidic character like triflate. Among the most common of the latter are the
triflate (trifluoro-methane sulfonate,
), and bis-trifluoro-methane-sulfonyl-
imide, or ) ions. The fluorinated anions are prominent because of
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the viscosity-lowering reduction of the Vander-Waals interactions (thanks to the
tightly bound, hence unpolarizable, character of the fluorine electrons). Such ILs tend
to be chaotropic salts, the ―ionicity‖ of aprotic ILs is the property that is responsible
for their characteristic low vapor pressures, in other senses they have depressed
melting points as a result of low symmetry ions which contain charge delocalization,
and weak directional intermolecular interactions.
1.2.2. Protic ionic liquids
Subsets of ILs are protic ionic liquids (PILs), the key properties that
distinguish PILs from other ILs is the proton transfer from the acid to the base,
leading to the presence of proton-donor and proton-acceptor sites, which can be used
to build up a hydrogen-bonded network [30], and are easily produced through the
combination of a Bronsted acid and Bronsted base (These are formed by the simple
transfer of a proton from pure Bronsted acid to pure Bronsted base equation 1 [31]),
In all these PILs have a proton available for hydrogen bonding and usually have non-
negligible vapor pressure and some are distillable media, where their boiling point
occurs at a lower temperature than decomposition. In some cases PILs are ―poor‖ ILs,
by inspection of two of their properties (conductivity and fluidity) in the so called
Walden plot [31]. Though it is not possible to differentiate whether this is due to
incomplete proton transfer, aggregation, or the formation of ion complexes [32], very
surprisingly, the concentration of neutral species present in PILs should still lead to
the mixture being defined as an IL. Few accounts [33] have been published on the
presence of neutral species in PILs, where the properties of the mixture are clearly of
the IL rather than of the neutral species. Indeed, by taking advantages of the features
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accounted of the PILs, the PILs have found very important applications in recent
years [34].
1.2.3. Inorganic ionic liquids
Systematic exploration of the role of fused salts in organic chemistry is of
recent date to the late development of the fundamentals of fused-salt chemistry itself;
the bulk of our knowledge of inorganic melts has evolved since about 1950. Though
organic applications are limited by the high freezing point and low solvent power for
organic non-electrolytes displayed by liquid inorganic salts, but a huge application
window of such materials in electrochemistry, and in analytical sciences for their role
as ―engineering fluids‖ in batteries and fuel cells is well recognized. The progress of
research on these materials favoring the development for applications in new, and
sometimes, surprising areas of chemistry and technology; as biosensors, in
lubrification, as rocket propulsions, in textile industries etc. These may be obtained, in
both aprotic and protic forms, by taking advantage of the same packing problems that
lead to low-melting ILs of the organic cation type, and the examples like lithium
chlorate (melting point ), and its glassforming eutectic with lithium perchlorate,
and protic examples like hydrazinium nitrate are well known [35].
1.2.4. Solvate (chelate) ionic liquids
These form a largely unstudied class of ILs that needs to be recognized
because the class includes cases of multivalent cation salts that would not ordinarily
be able to satisfy the criterion of Tm < . The first recognized members of this
class were molten salt hydrates, like , whose mixtures with alkali
metal salts were found to be almost ideal mixtures, and most with liquidus
temperatures well below ambient. These were hailed as a ‗‗new class of molten salt
mixtures‘‘, but there has been some question about the life time of the water
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molecules in the cation coordination shell. This should be long with respect to the
diffusion time scale for the ‗‗ILs‘‘ classification to be unambiguous. Recently the
Watanabe laboratory has described new cases where long lifetime is guaranteed
because the ligating groups all belong to the same molecule.
1.3. Solvent properties of Ionic Liquids
It is well known that all the solutions have an ability to perform the role of
solvent. As is well known, the whole field of solvent and solvent effects developed
from the time of alchemists. The alchemist‘s search for a universal solvent, the so-
called ‗‗Alkahest‘‘ or ‗‗Menstruum universale‘‘, as it was called by Paracelsus (1493–
1541), indicates the importance of the solvents and the process of dissolution.
C. Reichardt [36] in their comprehensive book entitled ―Solvents and Solvents effects
in Organic Chemistry‖ published in 2003, admirably described the history,
importance, and applications of the solvent and solvent systems in traditional as well
as modern organic synthesis.
In a classical chemical process, solvents are used extensively for dissolving
reactants, extracting and washing products, separating mixtures, cleaning reaction
apparatus, and dispersing products for practical applications. While the invention of
various exotic organic solvents has resulted in some remarkable advances in
chemistry, the legacy of such solvents has led to various environmental and health
concerns. Consequently, as part of green chemistry efforts, a variety of cleaner
solvents have been evaluated as replacements [37, 38]. However, an ideal and
universal green solvent for all situations does not exist. Among the most widely
explored greener solvents are ILs, supercritical , and water. These classical
solvents and solvent systems complementing each other both in properties and
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applications. Importantly, the study of green solvents goes far beyond just solvent
replacement. The use of green solvents has led science to uncharted territories.
An excellent report on the solvent properties of ILs was made by Chiappe et al
[39] in 2005, which covers an exceptional survey about the synthesis, structure
induced properties, and an ability of ILs to replace volatile organic compounds. An
article by Castnerand Wishart [40] highlights on solvation and solvation dynamics in
ILs. The cationic and anionic components of ILs offer a wide range of chemical and
physical properties that can be independently tuned to provide a wide variety of
molecular environments. The cationic charge can be localized as in ammonium or
phosphonium cations, or delocalized as is the case for imidazolium and pyridinium
cations. In addition to the contribution of the charges to the total electrostatic energy,
the effective dipole moments play a substantial role. Typical ions that form ILs are
often highly polarizable. Another alternative is to add alkyl or perfluoroalkyl groups
to IL cations or anions to thereby minimize charge- and higher-order electrostatic
interactions in favour of Vander- Waals interactions.
1.4. Ionic Liquids and Green Chemistry
The role of chemistry is essential in ensuring that our next generation of
chemicals, materials, and energy is more sustainable than the current generation.
Worldwide demand for environmentally friendly chemical processes and products
require the development of novel and cost-effective approaches to pollution
prevention. One of the most attractive concepts in chemistry for sustainability is
Green Chemistry, which is the utilization of a set of principles that reduces or
eliminates the use or generation of hazardous substances in the design, manufacture,
and applications of chemical products. Although some of the principles seem to be
common sense, their combined use as a designer framework frequently requires the
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redesign of chemical products or processes. It should be noted that the rapid
development of Green Chemistry is due to the recognition that environmentally
friendly products and processes will be economical on a long term [41].
The key notion of green chemistry is ‗‗efficiency‘‘, including material
efficiency, energy efficiency, man-power efficiency, and property efficiency (e.g.,
desired function vs. toxicity). Any ‗‗wastes‘‘ aside from these efficiencies are to be
addressed through innovative green chemistry means. ‗‗Atom-economy‘‘ and
minimization of auxiliary chemicals, such as protecting groups and solvents, form the
pillar of material efficiency in chemical productions. By far, the largest amount of
―auxiliary wastes‘‘ in most chemical productions is associated with solvent usage.
The principal requirements of solution phase processes in the utilization and
transformation of biomass opens a wide field of enormous potential impact for green
solvents in the supply chain of fuels and chemicals.
Are ILs really green? A weakly argued letter from Albrecht Salzer has raised
this nevertheless valid question. Robin Rogers gave a tactful, and lucid, response, and
Welton quote directly from this: ―Salzer has not fully realized the magnitude of the
number of potential IL solvents. I am sure, for example, that we can design a very
toxic IL solvent. However, by letting the principles of green chemistry drive this
research field, we can ensure that the ILs and IL processes developed are in fact green
[7].
1.5. Applications of Ionic Liquids
ILs comprises an extremely broad class of molten salts those are attractive for
many practical applications because of their useful combinations of properties.
Scientific American published an article by Michael Lemonick [42] in May 2011,
made a courtesy on ILs; a novel material composed entirely of ions. Its applicability
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to absorb carbon dioxide is extremely good as compare to the other, chemically
similar absorber. The phase change (i.e. from solid to liquid) ability of ILs is quite
interesting which releases of heat and the disposal of carbon recycles ILs for another
applications. The potential of ILs in various areas of chemistry is discussed in several
papers, including Lanthanides and Actinides in ILs [43], catalysis in ILs [44], solar
Cells [45], electrochemistry and spectroscopy [46], in polymer science [47], in
separation science [48], ILs from Nanoscience to Supramolecules [49, 50], in
environmental and life sciences [51, 52]. To the date variety of organic
transformations are performed in ILs such as Diels-Alder reaction [53], Friedel-Craft
reaction [54], Cross-Coupling reactions [55, 56], nucleophilic substitution [57]
reactions were also studied in ILs.
One of the key areas of Green Chemistry is the elimination of solvents in
chemical processes or the replacement of hazardous solvents with environmentally
benign solvents. The development of solvent-free alternative processes is, of course,
the best solution. The application of alternative solvents such as water, fluorous and
ILs, supercritical media, and their various combinations is rapidly increasing. From
this perspective, an ILs have been on the forefront of the use of alternative and
greener solvents in the chemical industry.
There are myriad synthetic organic reactions that are widely applied in small-
scale synthesis. Much of the chemical literature over the last century has been devoted
to the invention and application of new synthetic methodology. Nonetheless, there are
so very few reactions that stand up to the most stringent of tests: scale-up and
manufacture. Process chemistry is the practical application of organic synthesis. For a
chemical process to be functional on large scales it not only needs to be robust and
predictable; it should also be operationally simple, safe, and straight forward. Ideally,
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reactions should use inexpensive, environmentally benign starting materials, reagents,
and solvents and produce the target compound not only in high yield but also in very
high quality as well, with a minimum of impurities that are easily removed, preferably
by crystallization. If the process is catalytic, turnover numbers and turnover
frequencies must be high and the product must be free of trace contaminants such as
heavy metal salts or complexes. Few synthetic transformations meet these rigorous
criteria. During development, the process chemist seeks to understand completely the
chemistry involved and conducts reactions aimed at finding the limits of acceptability
of critical variables. In the current view, the process chemist develops not only
commercial routes but also enabling and supply routes as a drug candidate,
agrochemical, or fine chemical moves from the laboratory to full-scale production.
Good process chemistry is necessarily green chemistry; it always has been, long
before this phrase became fashionable [58].
1.6. Statement of the work
The launch of Green Chemistry in 1999 coincided with the explosion of
interest in ILs that was associated with the arrival of good quality, accessibly priced,
commercial ILs. This led to large numbers of publications on the potential of ILs to
act as solvents for chemicals synthesis, often justified by the ILs being a green
alternative to conventional molecular solvents. Also, a number of papers reporting the
synthesis and properties of ILs were published in the journals.
Green chemistry and sustainability essentially go hand in hand. Sustainable
development is meeting the needs of the present generation without compromising the
ability of future generations to meet their own needs. We need greener chemistry-
chemistry that efficiently utilises (preferably renewable) raw materials, eliminates
waste and avoids the use of toxic and or hazardous solvents and reagents in both
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products and processes in order to achieve this noble and lofty goal. Green chemistry
embodies two main components. First, it addresses the problem of efficient utilisation
of raw materials and the concomitant elimination of waste. Second, it deals with the
health, safety and environmental issues associated with the manufacture, use and
disposal or reuse of chemicals. Our laboratory has developed experimental methods
and theoretical avenues to understand the catalytic action and reuse of the various
catalysts and catalyst systems including ILs in organic synthesis. In addition to that
hydrophobic hydration/interaction and water structural effects for few model
compounds like drug molecules, cyclodextrins, and ammonium salts in aqueous and
non-aqueous, non-electrolyte/electrolyte solutions have been successfully studied as
well [59-62].
Solvents are perhaps the most active area of Green Chemistry research. They
represent an important challenge for Green Chemistry because they often account for
the vast majority of mass wasted in syntheses and processes. Moreover, many
conventional solvents are toxic, flammable, and/or corrosive. Their volatility and
solubility have contributed to air, water and land pollution, have increased the risk of
workers‘ exposure, and have led to serious accidents. There is one more neglected
issue, it is the time scale of the reaction. Time is money and hence the processes
which are fast or catalyzed so that products are obtained in short span are produced
for industrial application. Seen in this light, the companies are making high profits for
selling tuneable ILs. Therefore for researchers in this field, the starting materials are
very expensive. Hence, there is a need for process development to synthesize ILs.
Recovery and reuse, when possible, is often associated with energy-intensive
distillation and sometimes cross contamination. In an effort to address all those
shortcomings, chemists started a search for safer solutions. Solvent less systems,
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water, supercritical fluids (SCF) and more recently ILs are some examples of those
new ‗‗green‘‘ answers. In those earlier days the claim that ILs were green solvents
largely rested on the (then believed) non-volatility of ILs and its associated properties
(low flammability, ease of containment etc.). This claim has since been challenged on
several occasions, particularly with the toxicity and environmental persistence of the
most widely used ILs being noted as important negative green factors. In response to
these challenges ILs has been being designed for low toxicity and biodegradability.
We have developed a methodology to obtain ILs using molten tetra-butyl-ammonium
bromide as a catalyst which is greener and inexpensive. Using the method, we have
carried out the synthesis of 1-ethyl-3-methyl-imidazolium bromide, 1-propyl-3-
methyl-imidazolium bromide, 1-butyl-3-methy-limidazolium bromide, N-butyl-
pyridinium bromide, N-octyl-pyridinium bromide, 4-vinyl-N-butyl-pyridinium
bromide, N-butyl-benzimidazolium bromide, and heterocycles like 2,4,5-
triarylimidazole, and few bis(indolyl)methane derivatives satisfactorily in molten
tetra-butyl-ammonium bromide medium. Further the strategy involving molten N-
butyl-pyridinium bromide medium has also been implemented in the synthesis of 3-
3‘-bis-(indolyl)-phenylmethane, 3-3‘-bis-(indolyl)-4-chlorophenylmethane, 3-3‘-bis-
(indolyl)-4-methoxyphenylmethane, 3-3‘-bis-(indolyl)-3,4-(dimethoxy)-
phenylmethane, 3-3‘-bis-(indolyl)-cinnamyl-phenylmethane.
The scope of gas-phase ion/ion chemistry accessible to mass spectrometry is
largely defined by the available tools. Due to the development of novel
instrumentation, a wide range of reaction phenomenologies has been noted, many of
which have been studied extensively and exploited for analytical applications. It was
felt that because of unavailability mass spectrometry facilities, the neat and correct
spectroscopic analysis of fragmentation pattern of ILs have not been studied by
14
synthetic chemists. The tandem positive electrospray mass spectrometry
fragmentation data of ILs (or molten salts) incorporating the tetra-butyl-ammonium
bromide, 1-methyl-imidazolium and 1-butyl-pyridinium ring have been obtained and
analyzed. The influence of chain length of alkyl group in imidazolium based ILs is
studied. The presence of the cationic ring system produces intense, even electron
molecular cations in electrospray that undergo multiple stages of quadrupole system
to yield fragments which often are radical cations. Unusual losses of methyl and
hydrogen radicals are frequently noted. The mechanism by means of which
fragmentation occurs has been advanced and described as a part of this thesis.
There is a problem for hydrophobic and hydrophilic ILs, regarding solubility
of water. Hydrophilic ILs generally contains water, it has been estimated and it also
affects the key properties like diffusion coefficient, glass forming temperature (Tg),
viscosity etc. It is desirable to know the correct molecular weight or number of water
molecules associated with the main frame work atom assembly of IL. In literature,
one can find examples like Fe(III)NO39H2O, α-cyclodextrin 6H2O, CuSO4 5H2O etc.
which are solid and treated as hydrates. By mass spectrometry or other spectroscopy,
it is difficult to assess water molecules quantitatively. Therefore one has to use KF
titration methodology or osmotic pressure measurements to ascertain the exact
molecular formulae or structure of IL. We faced the above mentioned difficulty in
case of N-butyl-pyridinium bromide. Therefore we employed the techniques KF
titrimetry as well the vapor pressure osmometry. As an example, the osmometrys
application to determine the structural formula for N-butyl-pyridinium bromide is
described in this dissertation. For this densities and osmotic coefficient measurements
for aqueous solutions of N-butyl-pyridinium bromide have been reported at 298.15 K
using vibrating tube digital densitometer and vapor pressure osmometer respectively.
15
The partial molar volume of salt at infinite dilution, osmotic coefficient and osmotic
of aqueous solutions of N-butyl-pyridinium bromide have been estimated using the
density and osmolality data. The results are explained in terms salt hydration, ion-ion
interactions and molecular weight determination as an example for N-butyl-
pyridinium bromide.
The thesis with the description of experimental methodology, mass data
analysis, applications of molten salt as efficient, non-volatile, non-explosive, easy to
handle, an effective medium for synthesis of ILs and applications to synthesize
variety of heterocycles, stresses the importance of the field called as ILs. The brief
account of the conclusions drawn and suggestions for further work are also included
in the summary.