Chapter 1

General Introduction

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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

3

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

8

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

9

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,

13

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.