TABLE OF CONTENTS Phase transfer catalysis (PTC) refers to the transfer of ions or organic molecules between two liquid phases (usually water/organic) or a liquid and a solid phase using a catalyst as a transport shuttle. The most common system encountered is water/organic, hence the catalyst must have an appropriate hydrophilic/lipophilic balance to enable it to have compatibility with both phases. The most useful catalysts for these systems are quaternary ammonium salts. Commonly used catalysts for solid-liquid systems are crown ethers and polyglycol ethers. Starks (Figure 4.5) developed the mode of action of PTC in the 1970s. In its most simple form it involves transport of the reactive anion X - from the aqueous phase into the organic phase, reaction with the substrate and return of the catalyst to the aqueous phase often associated with a leaving anion.
There are many possible 'green' advantages in using PTC. These include:
Higher productivity - PTC catalysed reactions are often very rapid, one reason being that anions in the organic phase have few associated water molecules, making them highly reactive through reduction in activation energy. This of course will normally translate into reduced energy usage and greater reactor throughput.
Higher selectivity - Because of reduced activation energy these reactions can often be run at lower temperatures, which may reduce by-product formation.
Ease of product separation - Since the organic layer, substantially free from water-soluble contaminants, can simply be decanted off, product separation is often simple, resulting...
