Ultrasonically enhanced phase transfer catalysis_Hielscher0 pages

Ultrasonically induced and enhanced Phase Transfer Catalysis
Kathrin Hielscher, Hielscher Ultrasonics GmbH
kathrin@hielscher.com; www.hielscher.com

Phase Transfer Catalysis
Phase Transfer Catalysis (PTC) is a special form of heterogeneous catalysis and known as a
practical methodology for organic synthesis. By using a phase transfer catalyst, it becomes
possible to solubilize ionic reactants, which are often soluble in an aqueous phase but
insoluble in an organic phase. General advantages of phase transfer catalysis are the little
efforts for preparation, simple experimental procedures, mild reaction conditions and the
use of inexpensive and environmentally benign reagents, such as quaternary ammonium
salts, and solvents, and the possibility of conducting large scale preparations (Ooi et al.
2007).
A variety of liquid–liquid and liquid–solid reactions have been intensified and made selective
by using simple phase-transfer (PT) catalysts such as quats, polyethylene glycol-400, etc.,
which allow ionic species to be ferried from aqueous phase to organic phase. Thus, the
problems associated with extremely low solubility of the organic reactants in the aqueous
phase can be overcome. In the pesticide and pharmaceutical industries, PTC is used
extensively and has changed the fundamentals of business. (Sharma 2002)
Power Ultrasound
The application of power ultrasound is a well-known tool to create extremely fine emulsions.
In chemistry such extremely fine-size emulsions are used to enhance chemical reactions. This
means that the interfacial contact area between two or more immiscible liquids becomes
dramatically enlarged and provides thereby a better, more complete and/or faster course of
the reaction.
For phase transfer catalysis - the same as for other chemical reactions – enough kinetic
energy is needed to start the reaction.
This has various positive effects regarding the chemical reaction:
1. A chemical reaction that will normally not occur because of its low kinetic energy can
get started by ultrasonication.
2. Chemical reactions can be accelerated by ultrasonically-assisted PTC.
3. Complete avoidance of phase transfer catalyst.
4. Raw materials can be used more efficient.
5. By-products can be reduced.
6. Replacement of cost-intensive hazardous strong base with inexpensive inorganic
base.
By these effects, PTC is an invaluable chemical methodology for organic synthesis from two
and more immiscible reactants: Phase transfer catalysis (PTC) enables to use raw material of