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Phase-transfer catalyst

In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of catalysis and can act through homogeneous catalysis or heterogeneous catalysis methods depending on the catalyst used. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase in the absence of the phase-transfer catalyst. The catalyst functions like a detergent for solubilizing the salts into the organic phase. Phase-transfer catalysis refers to the acceleration of the reaction upon the addition of the phase-transfer catalyst. PTC is widely exploited industrially. Polyesters for example are prepared from acyl chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTC-catalyzed alkylation of phosphothioates.

Types
is a typical industrial phase transfer catalyst. Phase-transfer catalysts for anionic reactants are often quaternary ammonium salts. Commercially important catalysts include benzyltriethylammonium chloride, methyltricaprylammonium chloride and methyltributylammonium chloride. Organic phosphonium salts are also used, e.g., hexadecyltributylphosphonium bromide. The phosphonium salts tolerate higher temperatures. An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. Crown ethers are used for this purpose on the laboratory scale. Polyethylene glycols and their amine derivatives are common in practical applications. One such catalyst is tris(2-(2-methoxyethoxy)ethyl)amine. These ligands encapsulate alkali metal cations (typically and ), affording lipophilic cations. Polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior. Chiral phase-transfer catalysts have also been demonstrated. Asymmetric alkylations are catalyzed by chiral quaternary ammonium salts derived from cinchona alkaloids. A variety of functionalized catalysts have been evaluated for PTC. One example is the Janus interphase catalyst, applicable to organic reactions on the interface of two phases via the formation of Pickering emulsion. Limitations Quaternary ammonium cations degrade by Hofmann degradation to amines, especially at higher temperatures preferred by process chemists. The resulting amines can be difficult to remove from the product. Phosphonium salt are unstable toward base, degrading to phosphine oxide. Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as tetra-n-butylammonium bromide and methyltrioctylammonium chloride in benzene/water systems. ==Phase-boundary catalysis==
Phase-boundary catalysis