Phase transfer catalyst

A phase transfer catalyst or PTC in chemistry

Chemistry

Chemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...

 is a catalyst which facilitates the migration of a reactant in a heterogeneous

Heterogeneous

Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations. It is the opposite of homogeneous, which means that an object or system consists of multiple identical items...

 system from one phase into another phase where reaction can take place. Ionic reactants are often soluble in an aqueous phase 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.

A phase transfer catalyst or PTC in chemistry

Chemistry

Chemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...

 is a catalyst which facilitates the migration of a reactant in a heterogeneous

Heterogeneous

Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations. It is the opposite of homogeneous, which means that an object or system consists of multiple identical items...

 system from one phase into another phase where reaction can take place. Ionic reactants are often soluble in an aqueous phase 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 ether

Crown ether

Crown ethers are heterocyclic 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 ,...

s. 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

Sodium cyanide

Sodium cyanide is an inorganic compound with the formula NaCN. This highly toxic colourless salt is used mainly in gold mining but has other niche applications...

 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

Phosphonium salt

A phosphonium salt is a salt containing the phosphonium ion such as phosphonium iodide...

 such as hexadecyltributylphosphonium bromide, cyanide

Cyanide

A cyanide is any chemical compound that contains the cyano group , which consists of a carbon atom triple-bonded to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN^-. Organic compounds that have a -C≡N functional group bonded to...

 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

Sodium cyanide

Sodium cyanide is an inorganic compound with the formula NaCN. This highly toxic colourless salt is used mainly in gold mining but has other niche applications...

(aq) → C₈H₁₇CN(org) + NaBr

Sodium bromide

Sodium bromide, also known as sedoneural is a salt with the formula NaBr, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries. Its action is due to the bromide ion . It is a high-melting white, crystalline solid that resembles sodium chloride...

(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

Tetra-n-butylammonium bromide

Tetra-n-butylammonium bromide is a quaternary ammonium salt with a bromine counterion commonly used as a phase transfer catalyst.A closely related compound is tetra-n-butylammonium tribromide with an additional Br₂ unit....

 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

Green chemistry

Green chemistry, also called sustainable chemistry, is a chemical philosophy encouraging the design of products and processes that reduce or eliminate the use and generation of hazardous substances. Whereas environmental chemistry is the chemistry of the natural environment, and of pollutant...

 — by allowing the use of water, the need for organic solvents is reduced.

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.