The Role of Water in Phase Transfer Catalysis

The role of water in phase transfer catalysis is, like the weather, much discussed and little understood. Several groups have examined the organic phase in two-phase aqueous-organic systems and found that water is present to the extent of a few molecules per ion [10, 19, 57, 58]. The amount of water present depends on the catalyst, the cation, the anion, and the quaternary onium salt and concentration. 

Phase transfer processes rely on the catalytic effect of quaternary onium or crown type compounds to solubilize in organic solutions otherwise insoluble anionic nucleophiles and bases. The solubility of the ion pairs depends on lipophilic solvation of the ammonium or phosphonium cations or crown ether complexes and the associated anions (except for small amounts of water) are relatively less solvated. Because the anions are remote from the cationic charge and are relatively solvation free they are quite reactive. Their increased reactivity and solubility in nonpolar media allows numerous reactions to be conducted in organic solvents at or near room temperature. Both liquid-liquid and solid-liquid phase transfer processes are known the former ordinarily utilize quaternary ion catalysts whereas the latter have ordinarily utilized crowns or cryptates. Crowns and cryptates can be used in liquid-liquid processes, but fewer successful examples of quaternary ion catalysis of solid-liquid processes are available. In most of the cases where amines are reported to catalyze phase transfer reactions, in situ quat formation has either been demonstrated or can be presumed. 

Brandstrom, A. Preparative Ion Pair Extraction. Lakemedel Apotekarsocieteten, 

Ugelstadt, J., EUingsen, T., Berge, A. Acta Chem. Scand. 20, 1593 (1966). 

Pearson, R. G. Hard and Soft Acids and Bases. Stroudsburg, Pa. Dowden, Hutchinson and Ross, Inc. 1973. 

---