11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

Gustavii [101] and Bockries and Reddy [93] indicated that the dissociation constantof quaternary salts increased when the dielectric constant of the solvents was increased.Nagata [108] reported that the logarithmic value of the association constant of a quaternarysalt was proportional to the reciprocal of the dielectric constant of the mixed solvent.According to Eq. (53), the dissociation constant decreased slightly with increasing valuesof the reciprocal of the dielectric constant. Parker et al. [109] demonstrated that the freeenergies of transfer are very useful in correlation with the solvent effects on S N 2 effects inPTC. The free energies of transfer for the quaternary salts of dissociated ions from waterto the solvent can be written asG t Q þ þX ¼ RT ln a !Qþ a X¼ RT ln E T a Qþ aQXK do ð54ÞXThe free energies of transfer for free ions from water to the solvent can be written asG t i ¼ RT ln a i; i ¼ Q þ or X ð55Þa iAbraham [88], Czapkiewicz et al. [110], and Taft et al. [111] have reported the freeenergies of transfer of (C n H 2nþ1 Þ 4 NX (n ¼ 1–3) for ion pairs and dissociated ions. The freeenergies of transfer for quaternary salts of ion pairs and dissociated ions from water tofour kinds of organic solvents were determined in these studies. The free energies oftransfer for ion pairs were less than those for dissociated ions, i.e., the transfer abilityof ion pairs was greater than that of dissociated ions. The result of the stronger cation–anion attraction in ion pairs is to reduce significantly the magnitudes of the endoergicsolvent cavity terms, as well as the exoergic anion–solvent attractive terms. The stability ofquaternary salts for ion pairs was greater than that for dissociated ions from water to theorganic phase. The result corresponds to that of Taft et al. [111]. The sequences of freeenergy of transfer for quaternary salts are of three sorts: (1) P þ > N þ , (2)TBPO < I < BPO < Br < Cl , and (3) the long chain of an alkyl group is of lowvalue (Aliquat 336 < TBAC). The stability of ion pairs in dichloromethane (or dissociatedions in chloroform) was the highest among the four kinds of solvents. These results revealthat the incremental charge localization in the anion and decrement in the cation increasesthe stability of quaternary salt in the organic phase.D. Mass Transport in LLPTCUsually, it is recognized that the rate-determining step is controlled by the chemicalreaction in the organic phase under LLPTC conditions. For a fast mass transfer rate ofcatalyst between the two phases, the influence of mass transfer on the reaction can beneglected. In the past, the reaction rate was assumed to be independent of agitation andthe surface area of the interface beyond a minimum stirring rate ( 300 rpm). However,the reaction rates can increase with increased agitation in cases where the transfer rate ofanion between both phases is slower than the organic reaction. The phenomenon of masstransfer of quaternary salt between the two phases has received little attention. The reactivityof the reaction by PTC is controlled by the rates of the organic and aqueous reactions,the partition equilibrium, and the mass transfer steps of the quaternary saltsbetween the organic and aqueous phases [27,28]. The partition equilibrium of quaternaryammonium salts was obtained in our previous work [85,86,92].Copyright © 2003 by Taylor & Francis Group, LLC