11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

With the saturation solubility CSeK 2in the organic solvent, the dissolution rate coefficientk S is dependent on the surface property and the degree of agitation.2. Formation of a transition complex [R–Br–Q–Br] (TC) in the organic phase withequilibrium constant K 1 .RBr ðorgÞþQBr ðorgÞ ÐTC ðorgÞð117ÞFormation of the active transition complex TC from the reaction of QBr and RBr isassumed to be fast and reversible.3. The substitution reaction of TC and dissolved SeK 2 in the organic film with thethird-order rate constant k.SeK 2 ðorgÞþ2TCðorgÞ !SeR 2 ðorgÞþ2 KBr ðorgÞþ2 QBr ðorgÞð118ÞThe adsorption of QBr on the solid surface of SeK 2 plays an insignificant role in thekinetic description. The solid–liquid equilibrium of KBr between its soluble parts andsolid parts is still existed. Wu [217] reported that the kinetic data for S-shape curveswere found in this system, as shown in Fig. 8 for different amounts of potassium sebacateused. This revealed that the catalytic transition complex [R–Br–Q–Br] in the organic phasewould lead to a long induction period for the reaction of SeK 2 with TC.The concentration of TC in the organic phase and the rate of change of componentsare derived as follows [216]:C TC;org ¼ K 1 C RBr;org C QBr;orgdC SeK2 ;orgdtdC SeR2 ;orgdt¼ k S C SeK 2¼ kC SeK2 ;orgC 2 TC;orgC SeK2 ;orgkC SeK2 ;orgC 2 TC;orgð119Þð120Þð121ÞEquation (120) is simplified by neglecting the term kC SeK2 ;orgC 2 TC;org to obtain the concentrationof SeK 2 in the organic film, C SeK2 ;org ¼ C SeK 2ð1 e k St Þ, which can be furthertransformed into the equation C SeK2 ;org ¼ C SeK 2ðk s tÞ n for ease of interpreting the kineticFIG. 8 Yields of product dibenzyl sebacate for different molar ratios (r) of dipotassium sebacate tobenzyl bromide: chlorobenzene 50 mL, benzyl bromide 0.01 mol, TBAB 0.0025 mol, agitation 350rpm, temperature 80 C; r values: (*) 0.125, (^) 0.25, (&) 0.5,()1.0, (*) 2.0.Copyright © 2003 by Taylor & Francis Group, LLC