Principle of Phase Transfer Catalysis
Many inorganic salts are easily soluble in water, insoluble or difficult to dissolve in organic solvents. On the contrary, most organic substances are soluble in organic solvents but difficult to dissolve in water. Therefore, if you want to make inorganic salts and organic substances react homogeneously, you have to use a solvent that can dissolve both reactants, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF) or hexamethylphosphoric acid triamide (HMPT).
If there is a catalyst that can cross the interface between the two phases and transfer the reaction entity from the aqueous phase to the organic phase, so that it reacts quickly with the substrate and brings another negative ion in the reaction into the aqueous phase, and the catalyst is not lost when transferring the reaction entity, but repeatedly plays the role of "transferring" negative ions. This catalyst is called a phase transfer catalyst. The term that describes this phenomenon and process is phase transfer catalysis.
The most typical example of phase transfer catalysis is the reaction of solid salts or their aqueous solutions with organic substances dissolved in non-polar solvents. The reaction process is as follows:
In this reaction, the phase transfer catalyst QX, which is soluble in both the organic phase and the aqueous phase, exchanges negative ions with sodium cyanide NaCN in water, and then the catalyst that has exchanged negative ions is transferred to the organic phase in the form of ion pairs, that is, the oil-soluble catalyst positive ion Q+ brings the negative ion CN- into the organic phase. The degree of solvation of this negative ion in the organic phase is greatly reduced, so the reaction activity is very high and it can react quickly with the substrate. Subsequently, the catalyst positive ion Q+ returns to the aqueous phase with the negative ion X-, and the negative ions are continuously transferred back and forth across the interface.
However, some researchers believe that the catalysts generally used are very lipophilic, and the concentration of its positive ions in the aqueous phase is very low. Most of them stay in the organic phase and only exchange negative ions at the interface. However, the research only shows that the exchange sites of negative ions are different, while the reaction mechanism of the actual substitution reaction is consistent, and the above reaction mechanism has been confirmed.
Kinetic measurements of some specific examples show that in non-polar or low-polar solvents such as dichloromethane, chloroform, and benzene, the above-mentioned substitution reactions are mainly carried out through ion pairs. Only in solvents with higher dielectric constants, some ion pairs dissociate into negative ions to participate in the reaction. The reaction rate of phase transfer catalysis is related to the ability of the catalyst cation to bring the required negative ions into the organic phase, but it is not proportional. This is because factors such as the solvation of negative ions and the interaction between ion pairs and sub-negative ions in the organic phase also affect the reaction rate.
Types of Phase Transfer Catalysts
Most phase transfer catalyst reactions require the catalyst to transfer negative ions to the organic phase. In addition, some catalysts transfer positive ions or neutral molecules from one phase to another. Commonly used phase transfer catalysts are as follows:
(1) Onium salts: This is a type of catalyst with a wide range of uses and low prices. The most commonly used is quaternary ammonium salts. Other salts of the same type include phosphonium salts, sulfonium salts, and arsenic salts, but the latter are less commonly used.
(2) Crown ethers: One of their important uses is to complex metal ions. This property is also used in phase transfer reactions. Crown ethers can complex with alkali metal ions to form pseudo-organic cations, which are very similar to the cations of quaternary ammonium salts. Therefore, they can also make organic and inorganic alkali metal salts soluble in non-polar organic solvents. However, since they are more expensive than other catalysts such as quaternary ammonium salts and have greater toxicity, they have not been widely used.
(3) Three-phase catalysts: Three-phase catalysts are catalysts used in the recently developed triphase catalysis. This is an insoluble solid catalyst used to accelerate the reaction of a water-organic two-phase system. It is solid itself, so it forms a three-phase system, called a three-phase catalyst. The advantages of using this catalyst are: simple operation, easy separation after reaction, and the catalyst can be quantitatively recovered. Although this method has only been around for a short time, it has aroused great interest in the chemical industry because it requires low energy and capital and is suitable for automated continuous production. Therefore, this method has great development potential.
Examples of Phase Transfer Catalysis in Organic Synthesis
(1) Alkylation
(2) Nucleophilic substitution reaction
(3) Elimination reaction
