BY AMIT SHAH & SOHAM MULE, F.Y.B. PHARM, KMKCP. PTC (PHASE TRANSFER CATALYSIS) A SMALL TOPIC IN 2ND SEMESTER OF B.PHARM IN POC - 1 UNDER THE TOPIC SN REACTIONS. PTC FAVOURS SN2 REACTIONS.

1. Phase transfer catalysis
(PTC)
- BY AMIT SHAH & SOHAM MULE
F. Y. B. PHARM
KMKCP

2. What is catalyst ?
 It enhances the rate of reaction.
 The catalyst does not initiate a reaction.
 After the reaction there is no chemical change in the molecule (catalyst).
 It is regenerated at the end of the reaction.
 Definition–A substance that increases the rate of a chemical reaction without itself
undergoing any permanent chemical change.

3. Introduction :-
 Catalysis – catalysis is the process of increasing the rate of chemical reaction by adding a
substance known as a catalyst.
 The term phase transfer catalysis was introduced in the year 1965 for the first time.
 A phase transfer catalyst is used to carry a nucleophile from aqueous phase to organic
phase.
 The solvents used in this process should be immiscible with water and should be aprotic
in nature.
 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.

4.  Phase transfer catalysis is a special form of heterogeneous catalysis.
 Ionic reactants are often soluble in an 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.
 By using a PTC process, one can achieve faster reactions, obtain higher conversions or
yields, make fewer by products, eliminate the need for expensive or dangerous solvents
that will dissolve all the reactants in one phase, eliminate the need for expensive raw
materials and/or minimize waste problems.

5.  Phase-transfer catalysts are especially useful in green chemistry by allowing the use of
water, the need for organic solvents is reduced.
 Contrary to common perception, PTC is not limited to systems with hydrophilic and
hydrophobic reactants.
 PTC is sometimes employed in liquid/solid and liquid/gas reactions. As the name implies,
one or more of the reactants are transported into a second phase which contains both
reactants.

6. PRINCIPLE :-
 The principle of phase transfer catalyst is based on the ability of certain phase transfer
catalyst to facilitate the transfer of one reagent from one phase to another phase wherein
the other reagent exists.
 Reaction is made possible by bringing together the reagents which are originally in
different phases.
 It is also necessary that the transferred species is in an active state for effective phase
transfer catalytic action, and that it is generated during an organic reaction.
 In 1971, Starks introduced Phase Transfer Catalysis (PTC) to explain the role of
tetraalkylammonium or phosphonium salts in the reactions between two substances
located in different immiscible phases.

7.  e.g. the displacement reaction of 1-chlorooctane with aqueous sodium cyanide is
accelerated > 1000x by the addition of hexadecyltributylphosphonium bromide as a PTC.
 The key element of this tremendous reactivity enhancement is the generation of
quaternary phosphonium cyanide, which renders the cyanide anion organic soluble and
sufficiently nucleophilic.

8. THEORY BEHIND THE ROLE OF THE CATALYST :-
 The solutions are separated and a interface is formed due to their nature as aqueous and
organic phase.
 Since, the OH – ions are insoluble in organic phase and to move the process forward the
phase transfer catalysts are used to transfer the OH – ions in the organic phase.
 The phase transfer catalysts which are polar molecules, line up at the interface with
positively charged head in the aqueous phase and negatively charged tail in the organic
phase of the catalyst.
 So, the positively charged head of the catalyst attracts the negatively charged OH – ions
from the aqueous phase.
 Now on stirring at a greater speed, some of the molecules tilt upside down, the attraction
between the positively charged head of the catalyst and the negatively charged OH – ions
is so strong that it pull the OH – ions in the organic phase.

9.  The catalyst is then separated and regenerated at the end of the process.
 Hence, it is called phase transfer catalyst as it transfers OH – ions from one phase to
another, i.e from aqueous phase to organic phase .

10. Aqueous phase : OH–
ions
Organic phase : Non
polar ; R–X / CHCl3
Aqueous phase : OH–
ions
Interface
Organic phase : Non
polar ; R–X / CHCl3

11. COMMONLY USED phase transfer catalyst :-
 Quaternary ‘onium’ salts
 PEG
 Crown ethers
 Cryptands

12. Quaternary ‘onium’ salts :
 Quaternary ‘onium’ salts such as ammonium, phosphonium, antimonium and tertiary
sulphonium salts.
 Some of the PTC’s normally used are:-
NH4+ : tetra n – butyl ammonium halide
Aliquat 336 : N+CH3(C8H17)3 Cl
Methyl trioctyl ammonium chloride
Benzyl tri – methyl ammonium chloride or bromide (TMBA)
N+(CH3)3 CH2 C6H5 XBenzyl

13. Tri – ethyl ammonium chloride
N+(C2H5)3 CH2 C6H5 XCetyl
Tri – methyl ammonium chloride or bromide (CTMAB)
N+ (CH3)3 (CH2)15 CH3 X

14. PEG :
 PEG is linear polyether.
 High molecular weight polyethylene glycol (e.g. PEG 2000, PEG 8000).
 Polyethylene glycols (PEGs) are the cheapest and are stable in basic media and at
elevated temperatures.

15. Crown ether :
 When we add the KMnO4 to crown ethers. The kMnO4 is an oxidizing agent and the k+
gets trap in the ring and forms a bond with all the oxygen’s.

16. Cryptand :
 Cryptands are a family of synthetic bicyclic and polycyclic multidentate ligands for a
variety of cations.

17.  Crown ethers and cryptands: the most expensive.
 Crown ethers and cryptands, besides their high costs, are also toxic, and are to be avoided
whenever possible.

18. Mechanism of ptc :-
 A quaternary ammonium halide dissolved in the aqueous phase (Q+X-) undergoes anion
exchange with anion of the reactant dissolved in 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 organic phase , this is the phase transfer phase.
 In the organic phase , the anion of the ion-pair being nucleophilic undergoes a
nucleophilic substitution reaction with the organic reagent forming the desired product.
 The catalyst comes back subsequently to the aqueous phase and the cycle continues.

19. Overview of ptc :-
Interface Interface
Aqueous Phase
Organic Phase

20. ADVANTAGES OF PTC :-
 Do not require vigorous conditions and the reaction are fast.
 Do not require expensive aprotic solvents.
 Do not require high temperature ; the reaction usually occur at low temperature.
 There is no need for anhydrous conditions since water is used as one of the phases.
 With the help of PTC , the anion is available in organic solvent and so its nucleophilicity
increases.
 However, the main disadvantages of PTC, especially in commercial applications, is
the need to separate the catalyst from the product organic phase.
ADVANTAGES OF PTC :-