Phase transfer catalysts

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Phase transfer catalysts (PTCs) are a group of chemicals that do not directly catalyze a chemical reaction but facilitate or allow the transfer of individual reagents from one thermodynamic phase to another. This is critical for the reaction when it occurs in an environment where more than one thermodynamic phase exists, and one or more reactants are soluble in one phase and insoluble in the other. Phase transfer catalysts for anionic reagents are typically quaternary ammonium salts of RN + X- (R-alkyl or aryl, X-halogen), and for cationic reagents - crown ethers. Examples

An example of a phase transfer catalyst reaction is the addition of dichlorocarbene to double cyclohexene bonding. Cyclohexene dissolved in chloroform is added aqueous sodium hydroxide (NaOH). Since the aqueous and organic phases do not mix, the reaction does not occur. If, however, a small amount of benzyltriethylammonium chloride is added, the reaction takes place immediately. Benzyltriethylammonium benide, although positively charged, is dissolved in chloroform by the presence of four hydrocarbon substituents at the nitrogen atom. When the positively charged benzyltriethylammonium ion goes to the organic layer, the negatively charged counterion must follow it to maintain the balance of the electrical charge. As much more hydroxyl ions (OH) are present in the water phase than chloride ions, the OH- ions are mainly transferred to the organic phase, where chloroform reacts immediately.

The use of crown ether as phase transfer catalyst is illustrated by the following example: Potassium permanganate is insoluble in benzene. However, by shaking the solution of potassium permanganate with benzene slightly dibenzo-18-crown-6, a purple solution is obtained by the complexation of potassium ions. The corona-potassium ether complex can penetrate into the benzene solution, "pulling" the negative permanganate and causing a purple coloration of the benzene. The so-called "purple benzene" is a valuable reagent in the oxidation of alkenes.

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