Famous examples are quaternary ammonium salts possessing long alkyl groups and crown ethers. These catalysts can move across the two layers and are called amphiphilic. Phase-transfer catalysts (PTC) are used to promote ionic reactions like S N2 substitution in biphasic systems consisting of water and nonpolar organic solvents. These solvents have high boiling points and are relatively expensive, thus are not ideal for large scale applications. These reactions often necessitate the use of polar nonprotic solvents such as DMF and DMSO in order to increase the reactivity and the solubility. Reactions such as S N2 substitution involve deprotonation by strong bases and combining the resulting anions with electrophiles.
