11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

2. Mąkosza Interfacial MechanismThe interfacial mechanism is the most widely accepted mechanism for PTC reactions in thepresence of a base. However, although there are numerous industrially important applications,very few kinetic studies or mathematical models for this mechanism are reported. Ingeneral, the mechanism is also described by a pseudo-first-order hypothesis.Juang and Liu [74,75] proposed and discussed a possible mechanism based on amixed Mąkosza and modified interfacial mechanism. The reaction rate for the etherificationof a substituted phenylacetic acid by PTC was measured using a constant interfacialarea cell, and expressed asR f ¼ k½R 0 XŠ 1=3 ½RHŠ½QXŠ½OH Š 5=21 þ k a ½QXŠ 1=2 ½OH Šþk b ½RHŠ 1=2 ½OH Šð34ÞC. Thermodynamic Equilibrium in LLPTCQuaternary salts are generally used as normal liquid–liquid PT catalysts. In general, thefunctional groups of the quaternary cation will affect the dissolution of the catalyst in theorganic phase. Further, the phase transfer of the anion will also affect the reaction rate inthe two-phase reaction. Therefore, a proper choice of PT catalyst is very important inpromoting the reaction rate. Unfortunately, a universal guideline is unavailable for selectingthe proper PT catalyst to enhance the reaction. The reactivity in PTC is controlled by:(1) the reaction rate in the organic phase, (2) the mass transfer steps between the organicand aqueous phases, and (3) the distribution equilibrium of the quaternary salts betweenthe two phases. The distribution of quaternary salts between two phases directly affects theentire system reactivity [60–62].In general, anion transfer and anion activation are the important steps involved intransferring anions from the aqueous phase to the organic phase where the reaction takesplace. Factors affecting the extraction ability of the anion from the aqueous to organicphase include cation–anion interaction energies, the ionic strength in the aqueous phase,ion-pair hydration, the lipophilicity of the catalyst, and the polarity of the organic phase.The extraction behavior and distribution coefficients of quaternary salts in various mediahave also been investigated [76–86].Bra¨ ndstro¨ m [48] indicated that the distribution of quaternary salt between two(liquid–liquid) phases exists as complicated multiequilibrium constants, which dependon the structure of the anion, cation, and solvent, as well as on pH, ionic strength, andconcentrations in the aqueous solution. Such equilibrium properties have not yet beenevaluated completely. The relationship between quaternary salt and extraction constant isan important consideration for PTC work.The distribution coefficient of quaternary cation D Q was obtained by measuring theconcentrations of quaternary cation (Q) in the organic and aqueous phases, respectively.The distribution coefficient is highly dependent on the nature and concentration of thequaternary salts:D Q ¼ ½QŠ obs½QŠ obsð35ÞThe distribution coefficient of quaternary cations between both the phases not onlyprovides information on the phases to facilitate the modeling of the two-phase transfercatalysis system, but it can also give a criterion for evaluating the suitability of the catalyst.Copyright © 2003 by Taylor & Francis Group, LLC