Phase Transfer Catalysis
Definition of a Phase Transfer Catalyst:

A phase transfer catalyst is a catalyst which facilitates the migration of a reactant in a
heterogeneous system from one phase into another phase where reactions can take
place. Ionic reactants are often soluble in an aqueous phase but are insoluble in an
organic phase unless the phase transfer catalyst is present.

Phase transfer catalysis or PTC refers to the acceleration of the reactions by the phase
transfer catalyst.

PTC for anions reactant are often quaternary ammonium salts. PTC for cations are
often crown ethers.
Types of Phase-transfer Catalysts:
Phase-transfer catalysts for anionic reactants are often quaternary ammonium and
phosphonium salts. Typical catalysts include benzyltrimethylammonium chloride and
hexadecyltributylphosphonium bromide.
E.g. C8H17Cl + NACN (in presence of H2O) give No Reaction.
The 1-chlorooctane and sodium cyanide solution form two separate layers. Heating of this
mixture under reflux and vigorous stirring for 1-2 days gives no reaction.
C8H17Cl + NACN (in presence of R4N+) gives C8H17CN + NaCl
When an appropriate quaternary ammonium salt is added, tetrahexylammonium chloride, the
displacement occurs rapidly in near 100% in 2-3hr.
In this process the ammonium salt catalyst:
1. Transfers the cyanide into the organic phase.
2. Activates the transferred cyanide for the reaction with the alkyl halide.
3. Transfers the displaced chloride anions back to the aqueous phase to start a
new catalytic cycle.
An alternative to the use of “quat salts” is to convert alkali metals cations into hydrophobic
cations . In the research lab , crown ethers are used for this purpose. Polyethyleneglycols are
more commonly used in practical applications. These ligands encapsulate alkali metal cations
(typically Na+ and K+ ) affording large lipophilic cations. These polyethers have a
hydrophilic “interiors” containing the ion and a hydrophobic exterior.
Principle of Phase-Transfer Catalysis :
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 the reaction 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 .
Mechanism:
According to Starks’ 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
lopophilic nature and diffuses from the interface into the organic phase, this step being the
‘phase-transfer’. 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.
Aqueous phase:
Q+Y- + X- → Y- + Q+X-
Organic phase:
QY + RX → RY + QX
A prerequisite for a substance to function as a PT-Catalyst is to form ion-pairs soluble in the
organic phase and to be transferred in a highly active state.
Advantages of PTC :

Elimination of organic solvents.

Use of simple and inexpensive reactants (NaOH, KOH, K2CO3 etc. instead of NaH,
KHMDS t-BuOK etc).

High yields and purity of products.

Simplicity of the procedure.

Highly scalable.

Low energy consumption and love investment cost.

Minimization of Industrial waste.
Applications:

PTC is particularly useful for reaction of organic anions with nonpolar organic
reactants.

PTC is also applicable for numerous reactions in which anions are intermediates for
generation other active species such as carbenes, nitrenes and organometallic
reagents.

Reactions have been done in the following areas :
Alkylations. Substitutions. Aldol and related condensations
Carbenes reactions
Oxidations and reductions. Organometallic transformations.
Conclusion: In the recent years, a lot of research has gone into developing new techniques
for reaction rate enhancement. Usually, these techniques are more chemistry-intensive
than what traditionally chemical engineers have been used to. The present paper has thus
concentrated on the methods of modelling PTC reactions, using both soluble and
immobilized forms of the catalyst. PTC is “green chemistry”, for this reason industrial
applications are expanding.
References:

en.wikipedia.org ( Wikipedia )

Mac Millan Lab, Group Meeting , Anthony Mustachio , April 10 2008

International Journal of Scientific And Technology Research Volume 1 , Issue 3 , April
2012

W.P. Weber and G.W.GOkel, Phase Transfer Catalysis in Organic Synthesis, SpringerVerlag Berlin Heidelberg, 1977.