11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

V orgk obs ¼V org þD A V thirdk org x org þk inter x aq þk third D A x thirdð171Þwhere x A denotes the mole fraction of the catalyst existing in the different phases, and K Arepresents the distribution of A(benzyl chloride) between an organic phase and athirdphase in equilibrium and is defined asD A C A;thirdC A;org¼ m A;thirdV orgm A;org V thirdð172ÞTaking the logarithm for Eq. (170), one can determine the observed rate constant k obsfrom the experimental data by plotting ½ lnðy A;org ÞŠversus time t:V orgmlnðy A;org Þ ln¼k catV org þD A V obs tð173Þthird V org(c) Reactant and Alkali Salt in the Aqueous Phase. The overall reaction rate inTLPTC usually increases with the increase in amount of strong base reactant in theaqueous phase. In contrast with abase-catalyzed elimination reaction, the third liquidphase already formed will be precipitated under the excess base to dehydrate the catalystphase. In the presence of 49% of NaOH, two liquids and one solid are observedinstead of three liquids at asomewhat lower base concentration.Ido et al. [227] found that increasing the aqueous reactantKBrincreases the reactionrateinTLPTC.Theionicstrengthintheaqueousphasealsoaffectstheeaseofformingthethird liquid phase, since adding extra salts tends to salt out ion pairs produced from theaqueous reactant with the quaternary salt. In the system of n-butyl bromide reacted withsodium phenolate [225], the water molecules form hydrogen bonds with NaOPh as well aswith QOPh, leading to the amount of tetrabutylammonium salts in the organic phase andin the third liquid phase increasing with the amount of NaOPh added, which in turnenhances the overall reaction rate.(d) Organic Solvent and the Reaction Temperature. In general, the more polar theorganic solvent the faster is the overall reaction rate in LLPTC due to the increasingsolubility of the catalytic intermediate in the organic phase, and leading to much easiertransport of ion pairs into the solvent to react with the organic substrate. In contrast,in TLPTC, the solubility of the catalytic ion pairs in the organic solvent should be lowenough to push the catalyst to form aseparate phase. Thus, asolvent with low polarityor anonpolar one is favorable. Under the same conditions of using KBr and acatalyst,the reaction rate in dodecane was observed to be much faster than in toluene [227].Increasing the reaction temperature accelerates the reaction rate [221–226,230–233].However, the catalyst existing in the third liquid phase as well as in the organic phaseshould still be maintained. Under strong base conditions in TLPTC, the catalyst and theactive intermediate have the tendency to decompose at ahigh temperature, hence, alimitingreaction temperature should be kept in maintaining the third liquid phase.C. Kinetic Modeling for TLPTCYang[234]hasdevelopedatheoreticalmodeltoinvestigatetheeffectsofmasstransferanddistribution of the catalyst within the third liquid phase and organic or aqueous phase ontheoverallreactionrate.Themodelingconsidersthedispersedorganicdropletsurroundedbyaninterfacialcatalystlayerunderagitationconditions,asshowninFig.10.Thistypeofdroplet is similar to some oil/water emulsions in the presence of surfactants. The reactantCopyright © 2003 by Taylor & Francis Group, LLC