11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

[187–196]. Several reactions that cannot be performed in liquid–liquid phases can becarried out efficiently in solid–liquid systems.Starks et al. [183] have addressed several questions regarding the mechanistic detailsof SLPTC, and those include what are the mechanisms of transport of anions from thesolid phase to the organic phase, the mechanisms of formation of reactive ion pairs, themechanisms of exchange of product anions located in the organic phase with reactantanions located in the solid phase, the effects of particle size on the rates of reaction, themechanistic differences between quaternary cation and crown ethers as PT catalysts, andthe mechanistic role of small quantities of water in SLPTC. Obviously, the behavior of theactive ion pairs or catalytic intermediates is important in realizing the mechanism ofSLPTC.A. Interfacial Phenomena1. The Omega PhaseFor solid–liquid PT-catalyzed reactions using crown ethers as the catalyst, the correspondingcation of the solid reactant has some limitations, e.g., a potassium salt system can onlyuse 18-crown-6 as the catalyst, while 15-crown-5 can only catalyze the reaction of asodium salt. This is because metal salts carried by crown ethers depend on their molecularstructures with the cation size just fitting into the cage of the crown ether; the activecomplex is then transported into the organic phase. Moreover, the solubility of this activecomplex is related to its lipophilicity in the organic solvent [184,185].In many solid–liquid systems using crown ethers as the catalyst, adding smallamounts of water enhances the reaction rate greatly. A trace amount of water inSLPTC obviously plays an important role. When small quantities of water are added,the solid particles are surrounded by water molecule to form a thin layer. This interfaciallayer between the solid and the organic phases is termed the omega phase, whereby thesolubility of solid reactant in the solution is enhanced to produce easily the active intermediate.Liotta et al. [186] indicated that, using 18-crown-6 as the catalyst for the solid–liquid reaction of benzyl halide and potassium cyanide, 92% of the 18-crown-6 (as asolution in toluene) and inorganic salts KCN and KCl resided in the toluene phase;however, about 1–2% of the crown ether was transferred on to the surface of the saltand coated the surface of the salt particles to form a third phase when adding smallamounts of water.When the omega phase is formed, the overall reaction rate can be described bypseudo-first-order kinetics with respect to the organic reactant. While the reaction followspseudo-zero-order kinetics as the substitution reaction is conducted in the presence ofcrown ether and in the absence of water, it is independent of the benzyl halide concentration.Crown ether directly dissociates the cation of the reacting salt. A reaction mechanismwas proposed for the esterification reaction of solid potassium 4-nitrobenzoate and benzylbromide by using crown ether [197]. The overall reaction isO 2 N C 6 H 4 COO K þ CHCl 3 ;25 Cþ C 6 H 5 CH 2 Br !O 2 N C 6 H 4 COOCH 2 C 6 H 5 þ KBr ð81ÞCopyright © 2003 by Taylor & Francis Group, LLC