11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

the solid salt directly reacts with the quaternary catalyst at the solid–liquid interface toproduce the intermediate, which then transfers into the solvent and reacts with the organicsubstrate to form the product. For the homogeneous solubilization mechanism, the solidreactant can be dissolved in an organic solvent of generally higher polarity, and then reactswith the catalyst to form the intermediate. Melville and Yortsos [205] performed a theoreticalstudy regarding rapid homogeneous reactions based on a simple stagnant filmmodel in the system of SLPTC.Naik and Doraiswamy [206] reported that the homogeneous solubilization could befurther subdivided into four types, models A to D, for the following reactions:QX org þ MY s=aq Ð QY org þ MX s=aqQY org þ RX org ! QX org þ RY orgð85Þð86ÞModel A assumes that the solid dissolution and mass transfer steps are very fast comparedwith the organic reaction and that the solid particles MY and MX are present at theirequilibrium solubility levels in the organic phase. The concentrations of QY and QX in theorganic phase are both constant, i.e.,C QYo ¼ Kq 0K þ ; C QXo ¼ q 0K þ ð87ÞModel B assumes that both the ion-exchange reaction in Eq. (85) and the organic reactionin Eq. (86) are under kinetic control with the solid dissolution and mass transfer steps stillfast, and a differential equation describing the variation of QY with reaction time in theorganic phase is required. Model C assumes that MY is no longer at saturation concentrationin the organic phase, but is at some finite value. The rate of dissolution is governedby the interfacial area per unit volume of the organic phase, the dissolution rate constant,and the driving force between the saturation and the instant concentrations. Both the ionexchangeand the organic reactions take place in the bulk organic phase, and the transportof species from the solid surface to the bulk liquid is assumed to be fast; in addition, thevariation of the interfacial area according to the progress of the reaction should also beaccounted for. Model D accounts for the effect of transport of QY from the thin filmoutside the solid surface to the bulk liquid, and incorporates the rate of the organicreaction. The ion-exchange reaction is assumed to be fast and completed within the film.In order to describe the solubilization of solid reactant in the organic phase, Yangand Wu [200] performed the ion-exchange reaction of sodium phenoxide with tetrabutylammoniumbromide in a solid–liquid system. The interfacial reaction and mass transfersteps are shown as follows. The independent ion-exchange reaction isPhONa ðsÞþQBr ðorgÞ !PhOQ ðorgÞþNaBr ðsÞð88ÞThis reaction involves several steps:(a) Dissolution of PhONa. Traces of water are present in the solid reactantPhONa:3H 2 O, and the omega phase around the solid particle is formed to enhance thesolubilization of PhONa in the solution. The rate of dissolution of PhONa is very fast,leading to the solid part of PhONa readily in equilibrium with its soluble part. The concentrationof PhONa at the interface is thus kept at its saturation state.(b) Reaction of PhONa with QBr. PT catalyst QBr reacts with the soluble part ofPhONa to form PhOQ at the solid–liquid interface. The film reaction is assumed to bereversible with the equilibrium constant K 1 :Copyright © 2003 by Taylor & Francis Group, LLC