Other effects on the phase-transfer catalytic reactions

6 Other effects on the phase-transfer catalytic reactions 

Simple mechanical separation such as filtration, centrifugation or phase separation can be used to separate the product and the phase-transfer catalyst by use of insoluble catalysts. However, the more frequently encountered technical problems in use of PTC for industrial applications is the need to separate the product and the phase-transfer catalyst by chemical equilibrium separation method in the liquid-liquid two-phase phase transfer catalytic reaction. The most commonly used methods for separation of products and PTC catalysts on an industrial scale are extraction and distillation. Other separation methods include sorption and reaction.  

The principle of extraction method used to separate PTC and product is based on solubility of quaternary ammonium salt in alkaline aqueous solution. For example, tetrabutylammonium bromide is soluble to the extent of 27% in dilute (1% NaOH) aqueous solutions, but when the solution is made more concentrated (15% NaOH), the solubility of Bu4N Br decreases to 0.07%. When the products are obtained in PTC system, they can be usually separated from PTC by distillation method. PTC catalyst in the distillation residue may sometimes be reusable. With quaternary ammonium salts as catalysts, temperatures above 100-120 C usually result in partial or total decomposition of the quaternary salts to trialkylamines and other products. Mieczynska et al. and Monflier et al. investigated the hydrogenation and hydroformylation under phase transfer catalytic conditions. They found that the yield of aldehydes obtained in hydroformylation of 1-hexene strongly depends on solvent 24% in toluene, 53-86% in toluene-water-ethanol mixture and 77-94% in water-ethanol solution. The mixture of water-ethanol as a solvent was also found to be the best for hydrogenation of 1-hexene (96% of hexane). Conversion of Ph2PCH(CH3)(COOH) phosphine into sodium salt Ph2PCH(CH3)(COONa) yields aldehyde in toluene, 92% in toluene-water and 94% in toluene-water-ethanol mixture. 

In principle, hydroxide anion is very difficult to transfer from aqueous to organic phases, yet it is one of the most valuable and most commonly used anions in the PTC systems. Addition of small amounts of alcohols to PTC systems requiring hydroxide transfer causes a dramatic increase in rates. Therefore, addition of alcohol enhances the PTC reaction as the cocatalytic effect. For example formation of alkoxide anions, RO, which are more readily transferred than the highly hydrated hydroxide anion, and which can serve as a strong base just as well as OH , and solvation of the hydroxide with alcohol rather than with water, making the hydroxide anion more organophilic and more easily transferred.  

Cyanide displacements catalyzed by quaternary ammonium salts usually do not proceed without the presence of water to facilitate exchange and transfer of anions. However, PTC displacement depends on alcohol structure. Benzyl alcohol is about 1.5-2 times as effective as either methanol or ethanol. 

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