11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

E T Q þ ;H 2 O ¼ ½Qþ Š½X :jH 2 OŠ 2 ½Q þ Š½X Š½H 2 OŠ j 2 EQX;H T ½QX:jH2 O ¼2 OŠ½Q þ Š½X Š½H 2 OŠ j 2ð50Þð51ÞThe j value can be calculated by dividing ðH 2 OÞ by the amount of quaternary salts in theorganic phase. The water content difference in the organic phase ððH 2 OÞÞ equals thedifference between the measured water content in the solvent and that in the solution atthe same temperature.The order of magnitude of H 2 O in the organic phase for quaternary salts is Aliquat336 > TBA-TBPO > TBAI > TBPB > TBAB > TBAC. The sequence of ½H 2 OŠ for solventsis 1,2-C 2 H 4 Cl 2 > CH 2 Cl 2 > CHCl 3 > C 6 H 5 Cl. This tendency of the sequence of thecoextracted water is identical to that of the solubility of water in the organic phase of 1,2-C 2 H 4 Cl 2 ð1:3Þ > CH 2 Cl 2 ð0:81Þ > CHCl 3 ð0:08Þ > C 6 H 5 Cl ð0:05Þ at 20 C. The orders ofinfluencing extraction capability of H 2 O are Cl > Br 3 C 6 H 2 O > Br > I and N þ > P þfor the anion and central cation, respectively. The trend for water content in the organicphase varied with increasing temperature. Landini et al. [96] indicated that the solvatingcapability between quaternary salt and water could reduce the quaternary salt’s reactivityin the organic phase in a PT-catalyzed reaction. This result was confirmed by previous work[61,76]. Hence, it is significant to study the liquid–liquid PT-catalyzed reaction and toevaluate how many molecules of the coextracted water are carried by each quaternarysalt. The water content in the organic phase increased with increasing temperature. The ½H 2 OŠ value increased when the charge-to-volume ratio of the anion increased and when thepolarity of the solvent increased, but decreased as the lipophilicity of the quaternary saltincreased. These tendencies correspond to those reported by Landini and coworkers[97,98]. Kenjo and Diamond [95] reported that the average water contents in a nitrobenzene/watersystem at 23 C were 3.3, 1.8, and 1 (mol/mol quaternary salt) for Cl ,Br , andI , respectively. Starks and Owens [99] reported that the hydration numbers ofC 16 H 33 Bu 3 P þ X were 0.4, 4, and 5 for NO 3 ,Cl , and CN , respectively. The averagewater content in the organic phase ð½H 2 OŠÞ was about 1–3 mol/mol quaternary salt, exceptfor TBAC. Because the hydration numbers for different anions were different when thequaternary salt was TBA þ [(n-C 4 H 9 Þ 4 N þ Š, the results demonstrate that the water of hydrationis primarily associated with the anion, rather than with the quaternary cation.Quaternary ammonium ions are used as PT catalysts because they are least likely tointerfere in chemical reactions. According to the experimental results of Bra¨ ndstro¨ m [48],Herriott and Picker [100], and Landini et al. [97], the organophilic quaternary cationsserved as more effective PT catalysts than quaternary cations with small alkyl chains.Thus, the incremental number of C atoms surrounding the central atom (e.g., N) of aquaternary salt will increase its lipophilicity, thus raising the extraction constant.However, these researchers did not give the relationship between the extraction constantand the structure of quaternary salts. According to the literature, four relationships forquaternary cations have been reported.1. Gustavii [101] observed a linear relationship between log E QX and n, the numberof C atoms in an ammonium ion. He extracted picrates into methylene chloride usingprimary amines as well as symmetrical secondary and tertiary amines and symmetricalquaternary ammonium salts. The relationships for quaternary ammonium salts islog E Q picrate ¼ 2:0 þ 0:54n.Copyright © 2003 by Taylor & Francis Group, LLC