11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

A summary of characteristic kinetic criteria to distinguish between the operation ofthe extraction and interfacial mechanisms has been suggested [28,49]. The extractionmechanism is characterized by: (1) increased rates with increased lipophilicity of catalyst,(2) reaction rates that are independent of stirring speed above a certain value, (3) firstorderor fractional dependence of reaction rate on catalyst concentration, and (4) pseudofirstor second-order kinetics if the reaction in the organic phase reaction is rate controllingor zero-order kinetics if diffusion across the interface is rate controlling.The interfacial mechanism is characterized by: (1) increased rates with increasedelectrostaticity of catalyst, (2) reaction rates are dependent on agitation rate, (3) fractionalkinetic order with respect to the catalyst concentration, and (4) the value of substrateacidity pK a is in the range 16–23.B. Kinetics of a Liquid–Liquid Phase Transfer Catalysis1. Starks Extraction MechanismA typical LLPTC cycle involves a nucleophilic substitution reaction, as shown in Eq. (8).A difficult problem in the kinetics of PT-catalyzed reactions is to sort out the rate effectsdue to equilibrium anion-transfer mechanism for transfer of anions from the aqueous tothe organic phase. The reactivity of the reaction by PTC is controlled by the rate ofreaction in the organic phase, the rate of reaction in the aqueous phase, and the masstransfer steps between the organic and aqueous phases [27–29]. In general, one assumesthat the resistances of mass transfer and of chemical reaction in the aqueous phase can beneglected for a slow reaction in the organic phase by LLPTC.Although a large number of papers have been published on the synthetic applicationsof PTC in the last three decades, little mathematical analysis of the phenomenon hasbeen done, and such an analysis is especially desirable in a large-scale application. Evansand Palmer [50] considered a process of interphase mass transfer and chemical reaction.Melville and Goddard [51] and Melville and Yortsos [52] presented an analysis of masstransfer in solid–liquid PTC. Chen et al. [53] derived algebraic expressions for the interphaseflux of QY and QX. The reaction parameters were estimated from experimental datausing a two-stage method of optimal parameters. Wang and Chang [54–56] studied thekinetics of the allylation of phenoxide with allyl chloride in the presence of PEG asLLPTC. A simple mathematical model describing the liquid–liquid PT-catalyzed reactionwith the two-film theory was analyzed [57–59]. The results of the model’s prediction areconsistent with experimental data. Such mathematical analysis appears desirable andneeded in view of the widespread interest in PTC in the chemical industry in whichtwo-phase transfer and triphase catalysis are the most common industrial processes.The reactivity in phase-transfer catalysis is controlled by: (1) the reaction rate in theorganic phase, (2) the mass transfer steps between the organic and aqueous phases, and (3)the distribution equilibrium of the quaternary salts between the two phases. The distributionof quaternary salts between two phases directly affects the entire system reactivity[60–62]. On the basis of the experimental data and earlier literature [27,28,63], a generalizedapproach describing a LLPTC reaction system uses a pseudo-first-order reaction. Therate expression is written asd½RXŠ¼ kdt int ½QYŠ½RXŠ¼ k app ½RXŠð12Þð13ÞCopyright © 2003 by Taylor & Francis Group, LLC