Phase transfer catalyst

Phase transfer catalyst is one kind of catalyst that facilitates the transfer of one reacting species from one phase (aqueous phase) to another phase (organic phase) in order to carry out a reaction.

what is phase transfer catalysis?

A substance that has the ability to transfer one of the reactants from one phase (say aqueous) to another phase ( say organic) having another reactant, to carry out a reaction is called phase transfer catalyst and phenomenon is called phase transfer catalysis.

There are mainly two types of phase transfer catalysts. 1. Quaternary ammonium or phosphonium Salts 2. Crown ethers and Cryptands

These catalysts have somewhat slightly different actions but the effects are the same. Both types transfer the anions from one phase to another phase and allow them to be free to react with the substrate.

Some examples

Phase transfer catalyst mechanism

Organic phase

Aqueous phase

Where Q+ = Quaternary ammonium salt or phosphonium salt

If Q+ ions have such a low solubility in water, the exchange of ions takes place across the interface of the aqueous and organic phases

Interface

Advantages of phase transfer catalyst

1. It helps to reduce the consumption of organic solvents and raw materials. 2. Using it, the reaction can be carried out in mild reaction conditions. 3. It helps to control both reaction conditions, reaction rate, and yields

Limitations of phase transfer catalyst

1. Ordinary tetraalkyl quarternary salts decompose at high temperatures i.e greater than 120-149 degrees Celcius. 2. Even at low temperature under highly alkaline conditions, these catalyst decomposes. 3. These salts undergo decomposition in the presence of highly nucleophilic anions such as phenoxide

4. Crown ethers and cryptands are stable under these conditions but are much more expensive than quaternary salts. 5. For the transfer of divalent or trivalent anions, no superior catalysts have been introduced. 6. One of the disadvantages of crown ether catalysts is that it is limited by hole size relationship. For example; 15-crown-5 has a hole size similar to sodium cation (Na+).

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