11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

The fixed value of k app is called the apparent first-order reaction-rate constant. The overbardenotes the species in the organic phase. The reaction rate linearly increases withincreasing QY concentration. Equation (13) is established when the QY concentration isconstant. Most observed reaction rate would follow the pseudo-first-order kinetics for anexcess amount of aqueous reactant to that of organic reactant [37]. Wu [64] indicated thata pseudo-first-order hypothesis can be used to describe the PTC experiment data, eventhough the QY concentration is not kept constant. Wang and Wu [58] developed a comprehensivemodel in a sequential phosphazene reaction. Their experimental results wereconsistent with a first-order reaction rate; the pseudo-first-order reaction-rate constantwas not linearly related to the concentration of the catalyst, because the mass transferof catalyst between the two phases influenced the reaction. Wang and Yang [57,65] andWu [63] indicated that the QY concentration is constant over time when the molar ratio ofnucleophile to catalyst is larger than unity. Therefore, in the general case, the QY concentrationcannot vary with time only when the ion-exchange rate in the aqueous phase ismore rapid than that in the organic phase [66], no mass transfer resistance of catalystbetween the two phases occurs, the molar ratio of nucleophile to catalyst is larger thanunity, and the ionic strength in the aqueous phase is high [67].The complicated nature of the LLPTC reaction system is attributed to two masstransfer steps and two reaction steps in the organic and aqueous phases. The equilibriumpartition of the catalysts between the two phases also affects the reaction rate. On the basisof the above factors and the steady-state two-film theory [60,63,64,68], a phase-planemodel to describe the dynamics of a liquid–liquid PTC reaction has been derived. Thismodel offers physically meaningful parameters that demonstrate the complicated reactivecharacter of a liquid–liquid PT-catalyzed reaction. However, when the concentration ofaqueous solution is dilute or the reactivity of aqueous reactant is weak, the onium cationhas to exist in the aqueous phase. The mathematical model cannot describe this completely.When the onium cation exists in the aqueous phase, several important phenomenainvolved in the liquid–liquid reaction need to be analyzed and discussed.ð14ÞOn the basis of Eq. (12), and mechanism (14) [64,68], the species balance equationswere solved by eliminating the time variable (phase-plane model). The relevant rate equationsaredy od ¼ y 1oy ody 1ody o¼ P 1 QYy oym 1aQY 1y 1oð15Þð16ÞCopyright © 2003 by Taylor & Francis Group, LLC