- Phase transfer catalyst
A phase transfer catalyst or PTC in
chemistryis a catalystwhich facilitates the migration of a reactant in a heterogeneoussystem from one phase into another phase where reaction can take place. Ionic reactants are often solublein an aqueousphase but insoluble in an organic phase unless the phase transfer catalyst is present. Phase transfer catalysis (also PTC) refers to the acceleration of the reaction by the phase transfer catalyst.
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 hydrophilicinterior containing the ion and a hydrophobicexterior.
For example, the
nucleophilic aliphatic substitutionreaction of an aqueous sodium cyanidesolution with the alkyl halide 1-bromooctanedoes not ordinarily take place because 1-bromooctane does not readily dissolve in the aqueous solution. By the addition of small amounts of a phosphonium saltsuch as hexadecyltributylphosphonium bromide, cyanideions 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.cite journal
author = Starks, C.M.
year = 1971
title = Phase-transfer catalysis. I. Heterogeneous reactions involving anion transfer by quaternary ammonium and phosphonium salts
J. Am. Chem. Soc.
volume = 93
issue = 1
pages = 195–199
doi = 10.1021/ja00730a033]
Subsequent work by Herriott and Pickercite journal | author = Herriott, A.W.
coauthors = Picker, D. | year = 1975 | title = Phase transfer catalysis. Evaluation of catalysis | journal =
J. Am. Chem. Soc.| volume = 97 | issue = 9 | pages = 2345–2349 | doi = 10.1021/ja00842a006] demonstrated that many such reactions can be performed rapidly at around room temperature by using catalysts such as tetra-n-butylammonium bromideor methyltrioctylammonium chloridein 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 allowing the use of water, the need for organic solvents is reduced. [cite journal | author = J. O. Metzger | title = Solvent-Free Organic Syntheses | year = 1998 | journal = Angewandte Chemie International Edition| volume = 37 | issue = 21 | pages = 2975–2978 | doi = 10.1002/(SICI)1521-3773(19981116)37:21<2975::AID-ANIE2975>3.0.CO;2-A] [cite journal | journal = Pure Appl. Chem.| volume = 72 | issue = 7 | pages = 1399–1403 | year = 2000 | title = Phase-transfer catalysis. A general green methodology in organic synthesis | author = Mieczyslaw Makosza | doi = 10.1351/pac200072071399]
Contrary to common perception, PTC is not limited to systems with aqueous and organic soluble reactants. PTC is sometimes employed in liquid/solid and liquid/gas reactions. As the name implies, one or more of the reactants are transported into a second phase, which contains the other reactants.
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