Phase transfer catalyst

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A phase transfer catalyst or PTC in chemistry is a type of chemical compound, often a quaternary ammonium salt, which facilitates the migration of a particular chemical component in one phase into another phase in a heterogeneous system. The chemical component is soluble in one phase but insoluble in the other unless the phase transfer catalyst is present. For example the nucleophilic aliphatic substitution reaction of an aqueous sodium cyanide solution with the alkyl halide 1-bromobutane does not ordinarily take place because both solutions will not mix. By the addition of 1% of the quaternary ammonium salt tetrahexylammonium chloride however cyanide ions are ferried into the organic phase from the water phase and 1-cyanobutane forms quantitatively in a matter of minutes. A PTC works by encapsulating the ion. The PTC-ion system has a hydrophilic interior containing the ion and a hydrophobic exterior. Crown ethers are also efficient phase transfer catalysts.