11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

The reaction steps involve [197]:1. Dissolution of solid potassium nitrobenzoate:CE org þ KNB solid Ð CE KNB org ð82Þ2. Intrinsic reaction in the organic phase:CE KNB org þ PhCH 2 Br org ! PhCH 2 Br org þ CE KBr org ð83Þ3. Release of crown ether:CE KBr org Ð CE org þ KBr solid ð84ÞWith further additions of water, the overall reaction rate does not inevitably increase, butreaches a maximum with an optimal amount of water added.2. Solubilization of Solid Salt by Quaternary Ammonium SaltsSLPTC can also be conducted by using quaternary ammonium salt as the catalyst. Thisphenomenon is somewhat different from using crown ether. Vander Zwain and Hartner[198] concluded that, for the reaction of acetate and adeninyl anions in the solid–liquidPT-catalyzed reaction using tricaprylmethylammonium chloride, showed better efficiencythan crown ether. Yadav and Sharma [199] investigated the kinetics of the reaction forbenzyl chloride and sodium acetate/benzoate by SLPTC. They found that cetyldimethylbenzylammoniumchloride was the most efficient catalyst among those studied in thetemperature range 90–139 C, and the rate of reaction in the presence of water was lessthan that in the absence of water. The solubilities of NaOAc and NaCl in toluene assolvent at 101 C are 3:85 10 5 and 3:24 10 5 gmol/mL, respectively, while being 2:1910 5 gmol=mL for the former in the presence of dimethylhexadecylbenzyl chloride. Theconcentrations of chlorides and acetates are 6:25 10 5 and 5:6 10 5 gmol=mL.Obviously, the solubilities of these two salts are affected by the reaction with the PTcatalyst. Yee et al. [200] showed that the slower reactions catalyzed by quaternary saltsin a well-mixed batch reactor were caused by the limited effectiveness of quaternary saltsin solubilizing the solid reactant.Yang and Wu [201] investigated the esterification of dipotassium phthalate withbenzyl bromide in a solid–liquid system. We found that the catalytic intermediate, formedby the solid reactant with tetrabutylammonium bromide, was the key-reacting componentin SLPTC. Yang and Wu [202] explored the kinetics of the O-allylation of sodium phenoxidewith allyl bromide in the presence of quaternary ammonium salt catalyst in a solid–liquid system. The behaviors of the catalytic intermediate tetrabutylammonium phenoxide,formed from the reaction of solid sodium phenoxide and tetrabutylammonium bromidein the solid–liquid phases, are important in conducting the etherification, andpseudo-first-order kinetics are observed.The past efforts in SLPTC show that not only can the reactions be catalyzed byquaternary ammonium salt, but the interfacial reaction of the solid reactant with thequaternary ammonium salt is also important in this type of reaction. Moreover, thebehaviors of the active intermediate are also influenced by the addition of water in conductingthe quaternary salts catalyzed reactions. A conceptual scheme describing thereaction mechanism for SLPTC was proposed by Melville and Goddard [203,204], i.e.,heterogeneous solubilization and homogeneous solubilization, by considering the solubilityof solid salts in the organic phase. For the heterogeneous solubilization mechanism,Copyright © 2003 by Taylor & Francis Group, LLC