11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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ð10ÞThe dehydrohalogenation reactions of alkyl halides take place in the presence ofhydroxide ion and quaternary salts to form alkenes and alkynes [42–44]. The dehydrohalogenationis promoted by hydroxide ion. In general, two reaction conditions conducted inthis system were with highly lipophilic ammonium cation and 50% aqueous sodiumhydroxide. The reaction between 4-nitrobezenediazonium chloride and N-ethylcarbazolein aqueous media was accelerated by using a water–dichloromethane system containingsodium 4-dodecylbenzenesulfonate as a transfer catalyst for the diazonium ion [34].2. Mąkosza Interfacial MechanismThis reaction mechanism described by Mąkosza and Bialecka [45,46] is the acceptedcatalyst transport between the two phases. Reactions occurring in such systems involve:(1) transfer of ionic reactant from its normal phase and catalyst from the reaction phaseinto the interfacial region, (2) the ionic reactant reacting with catalyst in the interfacialregion to form intermediate catalytic reactant, (3) the intermediate catalytic reactanttransfer into the reaction phase to react with untransformed reactant to produce theproduct and catalyst. The reaction mechanism is expressed as follows:ð11ÞUsually, the aqueous salt could be too hydrophilic to allow the quaternary salt todissolve in the organic phase, and resided exclusively in the aqueous phase; anionexchange occured at or near the interface. The mechanism is applied to carbanion reactions,carbene reactions, condensation of polymerization, and C-alkylation of activemethylene compounds such as activated benzylic nitriles, activated hydrocarbons, andactivated ketones under PTC=OH . In most cases, the reaction involves the Q þ OHcomplex because QOH is highly hydrophilic and has extremely low solubility in theorganic phase.A mechanism can also be applied when the quaternary salt is too lipophilic todissolve in the aqueous phase, and resides exclusively in the organic phase, anion exchangeoccuring at or near the interface. This parallel mechanism is called the Bra¨ ndstro¨ m–Montanari mechanism. The ion-exchange reaction existing at the interface was verifiedby Landini et al. [47] and Bra¨ ndstro¨ m [48].Copyright © 2003 by Taylor & Francis Group, LLC
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
Page 1 and 2: 11Interfacial Mechanism and Kinetic
Page 3 and 4: Other factors in selecting asuitabl
Page 5 and 6: 1. Applications in BiologyOrsini et
Page 7 and 8: Tundo et al. [15] reported an effic
Page 9: transfer of catalyst from the react
Page 13 and 14: dy 1ady o¼ QYy ody 2ody o¼ QXy
Page 15 and 16: (a) Pseudo-Steady-State LLPTC model
Page 17 and 18: The order of magnitude of D Q for q
Page 19 and 20: E T Q þ ;H 2 O ¼ ½Qþ Š½X :jH
Page 21 and 22: Gustavii [101] and Bockries and Red
Page 23 and 24: The mass transfer resistances stron
Page 25 and 26: The extractive effectiveness factor
Page 27 and 28: Quaternary onium salts, crown ether
Page 29 and 30: 26 > Dowex 1 2 > A-27 IRA-410 > I
Page 31 and 32: C 6 H 6 > C 6 H 5 CH 3 and the tren
Page 33 and 34: Many experimental studies on three-
Page 35 and 36: FIG. 4 Yields of products and conve
Page 37 and 38: transfer of organic or aqueous reac
Page 39 and 40: [187-196]. Several reactions that c
Page 41 and 42: the solid salt directly reacts with
Page 43 and 44: solubility in the organic phase, th
Page 45 and 46: QCl þ AcONa ðsÞ ÐQCl:AcONa ðs
Page 47 and 48: curve of the product SeK 2 . The pa
Page 49 and 50: Taking the mass balance for the cat
Page 51 and 52: The ion-exchange reaction occurs at
Page 53 and 54: in both aqueous and organic phases
Page 55 and 56: Type I.The catalyst resides in the
Page 57 and 58: V orgk obs ¼V org þD A V thirdk o
Page 59 and 60: @C orgRX@t@C catQY@t@C catQX@t¼ D
Page 61 and 62:
tively. When the amount of the thir
Page 63 and 64:
V org volume of organic phase (L)Xa
Page 65 and 66:
44. Halpern, HA Zahalka, Y Sasson,
Page 67 and 68:
140. SL Regen. Angew Chem, Int Ed E
Page 69:
223. SD Naik, LK Doraiswamy. AIChE
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11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

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