Phase transfer catalysts for anion reactants are often quaternary ammonium salts. The corresponding catalysts for cations are often crown ethers. A PTC works by encapsulating the ion. The PTC-ion system has a hydrophilic interior containing the ion and a hydrophobic exterior.
For example, the nucleophilic aliphatic substitution reaction of an aqueous sodium cyanide solution with the alkyl halide 1-bromooctane does not ordinarily take place because 1-bromooctane does not readily dissolve in the aqueous solution. By the addition of small amounts of a phosphonium salt such as hexadecyltributylphosphonium bromide, cyanide ions can be ferried from the water phase into an organic phase (e.g., bromooctane + some hydrocarbon). With the phase transfer catalyst, nonanenitrile (1-cyanooctane) forms quantitatively in 90 minutes at reflux.
C8H17Br(org) + NaCN(aq) → C8H17CN(org) + NaBr(aq) (catalyzed by a R4P+Cl− PTC) Subsequent work by Herriott and Picker demonstrated that many such reactions can be performed rapidly at around room temperature by using catalysts such as tetra-n-butylammonium bromide or methyltrioctylammonium chloride in benzene/water systems.
By using a PTC process, one can achieve faster reactions, obtain higher conversions or yields, make fewer byproducts, eliminate the need for expensive or dangerous solvents which dissolve all the reactants in one phase, eliminate the need for expensive raw materials and/or minimize waste problems. Phase transfer catalysts are especially useful in green chemistry — by...