Phase transfer catalyst
Encyclopedia
In chemistry
, a phase transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase transfer catalysis is a special form of heterogeneous catalysis
. 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.
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 that will 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.
Contrary to common perception, PTC is not limited to systems with hydrophilic and hydrophobic 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 both reactants.
s. Typical catalysts include benzyltrimethylammonium chloride and hexadecyltributylphosphonium bromide.
For example, the nucleophilic aliphatic substitution reaction of an aqueous sodium cyanide
solution with an ether
eal solution of 1-bromooctane does not readily occur. The 1-bromooctane is poorly soluble in the aqueous cyanide solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile:
Via the quaternary phosphonium cation, cyanide
ions are "ferried" from the aqueous phase into the organic phase.
Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as tetra-n-butylammonium bromide
or methyltrioctylammonium chloride in benzene/water systems.
An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. In the research lab, crown ether
s are used for this purpose. Polyethylene glycol
s are more commonly used in practical applications. These ligands encapsulate alkali metal cations (typically Na+ and K+), affording large lipophilic cations. These polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior.
of phosphothioates. One of the more complex applications of PTC involves asymmetric alkylations, which are catalyzed by chiral
quaternary ammonium salts derived from cinchona alkaloids.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a phase transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase transfer catalysis is a special form of heterogeneous catalysis
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...
. 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
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
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.
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 that will 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
Green chemistry
Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...
— by allowing the use of water, the need for organic solvents is reduced.
Contrary to common perception, PTC is not limited to systems with hydrophilic and hydrophobic 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 both reactants.
Types of phase transfer catalysts
Phase transfer catalysts for anionic reactants are often quaternary ammonium and phosphonium saltPhosphonium salt
A phosphonium salt is a salt containing the phosphonium ion such as phosphonium iodide . More commonly, phosphonium refers to a quaternary organic derivative such as tetraphenylphosphonium chloride, 4P+ Cl- and tetramethylphosphonium iodide, [P4]+I−.Alkyltriphenylphosphonium salts are widely...
s. Typical catalysts include benzyltrimethylammonium chloride and hexadecyltributylphosphonium bromide.
For example, the nucleophilic aliphatic substitution reaction of an aqueous sodium cyanide
Sodium cyanide
Sodium cyanide is an inorganic compound with the formula NaCN. This highly toxic colorless salt is used mainly in gold mining but has other niche applications...
solution with an ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
eal solution of 1-bromooctane does not readily occur. The 1-bromooctane is poorly soluble in the aqueous cyanide solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile:
- C8H17Br(org) + NaCNSodium cyanideSodium cyanide is an inorganic compound with the formula NaCN. This highly toxic colorless salt is used mainly in gold mining but has other niche applications...
(aq) → C8H17CN(org) + NaBrSodium bromideSodium bromide is an inorganic compound with the formula NaBr. It is a high-melting white, crystalline solid that resembles sodium chloride. It is a widely used source of the bromide ion and has many applications.-Synthesis, structure, reactions:...
(aq) (catalyzed by a R4P+Cl− PTC)
Via the quaternary phosphonium cation, cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
ions are "ferried" from the aqueous phase into the organic phase.
Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as tetra-n-butylammonium bromide
Tetra-n-butylammonium bromide
Tetra-n-butylammonium bromide is a quaternary ammonium salt with a bromine counterion commonly used as a phase transfer catalyst.-See also:*Tetrabutylammonium tribromide, with an additional Br2 unit*Tetrabutylammonium fluoride...
or methyltrioctylammonium chloride in benzene/water systems.
An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. In the research lab, crown ether
Crown ether
Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-. Important members of this series are the tetramer , the pentamer , and the hexamer...
s are used for this purpose. Polyethylene glycol
Polyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
s are more commonly used in practical applications. These ligands encapsulate alkali metal cations (typically Na+ and K+), affording large lipophilic cations. These polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior.
Applications
PTC is widely exploited industrially. Polyester polymers for example are prepared from acid chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTC-catalyzed alkylationAlkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene . Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in...
of phosphothioates. One of the more complex applications of PTC involves asymmetric alkylations, which are catalyzed by chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
quaternary ammonium salts derived from cinchona alkaloids.