Mechanism of Phase Transfer Catalysis, Examples of Phase Transfer Catalysts, Catalysis by Ionic Liquids, Examples of Ionic Liquids, Reactions involving Ionic Liquids.

1. Gopika M G
S6 Int MSc Chemistry
Amrita School of Arts and
Sciences,
Amritapuri Campus

2. Phase Transfer Catalysis
 In chemistry, a phase-transfer catalyst or PTC is
a catalyst that facilitates the migration of a reactant from
one phase into another phase where reaction occurs.
 Phase-transfer catalysis is a special form of heterogeneous
catalysis.
 Ionic reactants are often often soluble in aqueous phase
but insoluble in an organic phase in the absence of the
phase-transfer catalyst.
 The catalyst functions like a detergent for solubilizing the
salts into the organic phase.
 Phase-transfer catalysis refers to the acceleration of the
reaction upon the addition of the phase-transfer catalyst.

4. Principles of PTC
The principle of phase transfer catalysis (PTC) is brought
forth well by Reuben and Sjoberg (1981). The principle of
PTC is based on the ability of certain phase-transfer agents
(the PT catalysts) to facilitate the transport of one reagent
from one phase into another (immiscible) phase wherein the
other reagent exists. Thus, reaction is made possible by
bringing together the reagents which are originally in
different phases. However, it is also necessary that the
transferred species is in an active state for effective PT
catalytic action, and that it is regenerated during the organic
reaction.

5. Mechanism of PTC
The mechanism of PTC reaction was first proposed in
1971. According to Starksí original work, a quaternary
ammonium halide dissolved in the aqueous phase (Q
+X-) undergoes anion exchange with the anion of the
reactant dissolved in the aqueous solution. The ion-pair
formed (Q+X-) can cross the liquid-liquid interface due to its
lipophilic nature and diffuses from the interface into the
organic phase, this step being the ëphase-transferí. In the
organic phase, the anion of the ion-pair being quite
nucleophilic undergoes a nucleophilic substitution reaction
with the organic reagent forming the desired product (RY).
The catalyst subsequently returns to the aqueous phase and
the cycle continues.

7. Applications of PTC
 In nucleophilic substitution reactions and in reactions in
the presence of bases involving the deprotonation of
moderately and weakly acidic organic compounds.
 PTC has made possible the use of cheaper and easily
available alternative raw materials like potassium
carbonate and aqueous NaOH solution, thereby obviating
the need of severe anhydrous conditions, expensive
solvents, and dangerous bases such as metal hydrides
and organometallic reagents.

8.  When any kind of chemical reactions are carried out
in the presence of a PT catalyst in biphasic systems,
simple, cheap and mild bases like
NaOH and K2CO3 can be used instead of toxic alkali
metal alkoxides, amides, and hydrides
 In the field of Pharmaceuticals like Synthesis of various
drugs like dicyclonine, phenoperidine, oxaladine, ritaline,
etc.
 Polymeric bonded PTC for the determination of cyanide,
iodide, nitrite, sulphide and thiocyanate, led to easy layer
separation and PTC-free injection of the sample into the
chromatograph

9. IONIC LIQUIDS
 Ionic liquids have emerged as a promising alternative for
conventional solvents.
 Ionic liquid is defined as a salt with melting point below
the boiling point of water.
 Ionic liquids are known by several different names like
Neoteric Solvents, Designer Solvents, Ionic Fluids, and
Molten Salts.
 Most of the ionic liquids are composed of organic cation
and inorganic anions.

10.  In order to be liquid at room temperature, the cation
should preferably be unsymmetrical; that is, the alkyl
groups should be different.
 Polarity and hydrophilicity/hydrophobicity of ionic liquids
can be tuned by suitable combination of cation and anion.
 It is this property of ionic liquids which has earned them
the accolade “designer solvents.”
 As solvents, ionic liquids have found applications in a
number of reactions.

11. FACTS
 Salts that are liquid at ambient temperature.
 Have stable liquid range of over 300k.
 Have very low vapour pressure at room temperature.
 Selective solubility of water and organics.
 Potential to replace volatile organic solvents used in
processes.

12. 

13. PROPERTIES
 Easy separation.
 Very low vapour pressure.
 Non-flammable substance.
 High thermal stability.
 High mechanical stability.
 Electrochemically stable.
 Low toxicity.
 Non-volatile.

14. IONIC LIQUIDS AS CATALYST
 Because of the unique physical and chemical properties,
ionic liquids can be used for catalysing reactions.
 Depending upon the functional group attached to the
anion and/or cation, they may behave as an acidic, basic or
organocatalyst.

15. 1. As Acid Catalyst
 The application of acidic (Bronsted as well as Lewis) task
specific ionic liquids (TSILs) as a catalytic material is
growing rapidly in the field of catalysis.
 Combining the useful characteristics of solid acids and
mineral acids, TSILs have been synthesized to replace the
traditional mineral liquidacids, such as hydrochloric acid
and sulphuric acid, in the chemical reactions.
 In view of green chemistry, the substitution of harmful
liquid acids by reusable TSILs is one of the most promising
catalytic systems in chemistry.

16.  The acidic nature of Bronsted acidic ionic liquids as
catalysts has been exploited form any organic
transformations like:
Pechmann reaction
Koch carbonylation
Asymmetric Aldol condensation
Aza-Michael reaction
Beckmann rearrangemen
Synthesis of chalcones
Oxidation reactions and Prin’s reaction
Synthesis of furfural, etc...

17. EXAMPLES
 Esterification of alcohols by carboxylic acids has been carried out
in a halogen-free Bronsted acidic ionic liquid, Nmethyl-2-
pyrrolidinium methyl sulphonate under mild conditions without
additional solvents.
 The use of acidic ionic liquids for the saccharification of cellulose
and its subsequent conversion into important platform
molecules like hydroxy methyl furfural, furfural, and levulinic
acid has been well explored conditions, and without additional
solvent.

18.  Friedel-Crafts reaction of PCl3 and benzene in
[trEHAm]Cl-XAlCl3 ionic liquid for the clean synthesis of
dichloro phenyl phosphine (DCPP).
 Six different Bronsted acidic ionic liquids (BAILs) have
been synthesized and used as recyclable reaction media as
well as acid promoters for Pd-phosphine catalyzed
methoxy carbonylation of ethylene to produce methyl
propionate in excellent yields.

19. Advantages
 The advantage of this IL is that it could be reused upto 10 cycles
without any substantial loss of catalytic activity.
 The catalyst is versatile as it is also applicable to both aliphatic
and aromatic amines and in the synthesis of
bis(indolyl)methane.
 Compared with the classical methods this protocol allows the
simple product isolation and lesser reusable catalyst
consumption, which contributes to the greenness of the
procedure.

20.  The catalytic system has been found to be recyclable up to
fifteen cycles without any appreciable loss in activity.
 Ionic liquid catalyst being less oxophilicas compared to
mineral acids leads to greater selectivities.

21. 2. As Base Catalyst
 Basic functionalized ionic liquids have aroused
unprecedented interest because they showed more
advantages such as convenient recycling and higher
catalytic efficiency than the mixture of inorganic base and
ionic liquid for some base-catalyzed processes.

22.  Basic ionic liquids have been used to catalyze a number of
reactions like:
Aza-Michael addition reaction
Michael addition of active methylene compounds
Condensation reaction of aldehydes and ketones with
hydroxylamine
Synthesis of quinolines, pyrroles. Etc..

23. EXAMPLES
 A facile, mild, and quantitative procedure for the
preparation of tetrahydrobenzo[b]pyran derivatives in the
presence of an easily accessible basic ionic liquid[bmim]
OH as catalyst has been developed. The ionic liquid was
used for at least nine times with consistent activity.

24.  A green protocol for the Michael additionofN-
heterocyclesto𝛼,𝛽-unsaturated compound sat room
temperature using a basic ionic liquid[bmim] OH as a
catalyst and reaction medium.
 Basic ionic liquids choline hydroxide [ChOH], choline
methoxide(ChOMe), and choline imidazolium(ChIm) have
been synthesized and checked for their catalytic activity for
the production of biodiesel from soybean oil . Of all the
three ionic liquids, choline hydroxide was found to give the
best results in terms of yield,efficiency, and recyclability.

25.  Basic ionic liquid[bmim]OH has been successfully used as
an efficient catalyst for the synthesis of substituted ureas
starting from carbon dioxide and amines.
 Various[DABCO] based ionic liquids havebeen screened for
executing Knoevengeal condensation reaction. Of all these
ionic liquids, [C4dabco][BF4]was found to give the best
results.

26. Advantages
 The main advantages of this methodology are solvent-free
reaction conditions, no need of dehydrating agents to
remove the water formed as a by-product, recyclability of
catalyst, and operational simplicity.
 No product purification was required and the catalyst was
found to be recyclable up to seven cycles without any
decrease in activity.
 There action is highly stereoselective giving alkenes with
E-geometry only.

27. 3. As Organocatalyst
 Ionic liquids have the potential to have a huge impact in
this area of catalysing organic compounds.
 One of the promising approaches to organocatalysis is
through hydrogen bonding interactions, and the reactions
to which this has been most often applied are Diels-
Aldercycloadditions and their derivatives.
 Though ionic liquids are green solvents, they are
synthesized from the materials which use fossil fuels as
their resource.

28. EXAMPLES
 A functionalized chiral ionic liquid, an efficient reusable
organocatalyst is used for asymmetric Michael addition of
ketones/aldehydes with nitroalkenes.
 Starting from (S)-proline, several chiral ionic liquids have
been synthesized by Vasiloiu and coworkers . These ionic
liquids were successfully used as organocatalysts to execute
asymmetric aldol condensation giving good yields
andselectivity.

29.  A sulphur functionalized chiral ionic liquid which has been
used as an organocatalyst for epoxidation reaction of
various aromatic aldehydes with benzyl bromide in water
giving trans-epoxides with high diastereo selectivity and
enantioselectivity.
 Imidazolium based ionic liquids can be used as precatalysts
for N-heterocyclic carbene catalyzed reactions whereby the
catalyst can be obtained by deprotonation.

30. CONCLUSION
 Synthesizing ionic liquids from renewable raw materials
will add to the green attributes of ionic liquids.
 Sugars are suitable, abundantly available raw material for
the synthesis of ionic liquids.
To conclude it can be said that the field of ionic liquid
catalysis holds enormous possibilities to be explored.