11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

2. A quantitative parameter for characterizing accessibility was suggested [28]based on the strong dependence of electrostatic interaction on the distance of closestapproach between the cation and anion (which is determined by steric factors). Thisparameter, termed q, is simply the sum of the reciprocals of the length of the linearalkyl chains attached to the central nitrogen of the quaternary cation;q ¼ 1=C# 1 þ 1=C# 2 þ 1=C# 3 þ 1=C# 4 , where C# is the number of carbon atoms in eachof the four alkyl chains in the quaternary cation.3. Fukunaga et al. [102] had presented a correlation function based on hydrophile–lipophile balance (HLB) ideas to assess the efficiency of quaternary salts in the benzene–watersystem in terms of Hildebrand parameters ½Dð QX Þ¼ð QX Þ 2 =ð QX Þ 2 Š where QX , and are, respectively, the solubility parameters of the catalyst, water, and organic solvent.4. Sirovski [103] proposed that the structure–activity relationship for quaternarysalts can be described quantitatively using Hansch -hydrophobicity constants. Theseconstants are defined analogously to Hammett and Taft constants [104]: x ¼ log P x log P H , where P H is the distribution coefficient for the standard compound,and P x is the same from its derivative with the X substituent in the standard 1-octanol–water system, which has low ion selectivity in relation to halide and hydroxide ions.The former two relationships (paragraphs (1) and (2) above) were focused on to accessthe distribution of quaternary cations. The equilibrium property cannot reveal when thetotal carbon number for various quaternary salts is the same. In paragraph 3, theHildebrand parameter cannot be easily obtained for all quaternary salts. Hence, we tookthe results of paragraphs 1–3 and the concept of HLB for the surfactant to show that thedispersal efficiency of surfactant or emulsifier molecules is a function of the relative interactionsof their polar, hydrophilic ‘‘heads’’ with the aqueous phase and of their nonpolar,lipophilic ‘‘tails’’ with the hydrocarbon phase [105,106]. We developed a new model as0:475HLB ¼ qðM T M H Þ þ 9:4 M TBABð52ÞM NXin which 0.475 and 9.4 are hydrophilic group numbers of CH 2 and N, respectively, whichwere defined by Davies [107]. The equation of the HLB was developed in respect of theextraction of quaternary salts between two phases based on molecular weights of hydrophilicand lipophilic groups. A linear relationship between extraction constant and HLBwas observed for ammonium cations. An average decrease in log E QX is about 10:5 2unts per HLB value for various counteranions. The free energies of transfer for ion pairsand dissociated ions were determined and were shown to correspond to the experimentaldata in the literature.It is of interest to determine the crude free energies of phase transfer between organicand aqueous phases for the quaternaries. This is combined with the free energies oftransfer for halide ions to give the free energies for the tetrabutylammonium and tetrabutylphosphoniumions, which are not well established. Do different salts give the samevalues? Tseng [92] reports the free energy of transfer of some anions from water to variouskinds of solvents based on the distribution data for quaternary salts, and evaluates theextraction behavior of quaternary onium salts in order to understand their performance ina PT-catalyzed reaction system.An extensive and self-consistent set of data on free energies of transfer of someinorganic salts has been reported [89]. The free energy of the extraction constant, distributionconstant, and dissociation constant are expressed asG i ¼ RT lnðiÞ; i ¼ E QX ; E T QX; m; K da ; or K do ð53ÞCopyright © 2003 by Taylor & Francis Group, LLC