11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

E T Q ¼ ½Qþ Š½X Š2 þ½Q þ Š½X Š2EQX T ½QXŠ¼½Q þ Š½X Š2E T Q 2 X 2¼ ½Q 2X 2 Š½Q þ Š½X Š 2 ð45Þð46Þð47ÞBy using Eqs (42)–(47), the values of E T Q þ, ET QX, EQ T 2 X 2, and the distribution constantm are evaluated. Corrections for the mean activity coefficient in the organic phase weremade using the Marshall and Grunwald expression, and the values of m, K da , K do , and were calculated by a numerical iteration method. Beronius and Bra¨ ndstro¨ m [91] evenclarified the identical value of K do at ½QXŠ ¼0 within the limits of experimental errorand the conductance measurement. In view of past reports [87–92], most K da valueswere located in the range between 1 and 10; K do values were located in the range between10 1 and 10 5 . The dissociation ability of quaternary salt in the aqueous phase is greaterthan that in the organic phase.The quaternary salts QX can be completely dissociated to free ions (Q þ and X )inthe aqueous phase (, ½Q þ Š=½QXŠ > 100Þ and partially dissociated in the organic phasewhen the concentration of the quaternary salt is 0.0125 kmol/m 3 . The quaternary salts QXcan be partially dissociated to free ions in the aqueous and the organic phases when theconcentration of quaternary salt is 0:1 kmol=m 3 . The incremental rules of the dissociationdegree of the quaternary salts were obtained as follows: (1) increasing the charge-tovolumeratio of the central cation or counteranion (e.g., P þ > N þ or I > Br > Cl ),(2) increasing the electron-releasing groups on the quaternary cation (e.g., Aliquat336 > TBAC), and (3) increasing the electron-withdrawing groups on the quaternaryanion (e.g., TBA-TBPO > TBA-BPO > TBAC). Electron-releasing (or electron-withdrawing)groups apparently make the transition state more stable on the quaternarycation (or anion) while the ion-pair type of quaternary salts transferring through theinterface between two phases is a transition state. Bockries and Reddy [93] reportedthat the association constant decreased when the effective ionic radius of the ion pairwas increased.Quaternary salts in an organic phase must be determined experimentally to knowwhether the salts are dissociated or associated, and, if so, to what degree. The hydration ofthe anion plays an important role in dissociating the catalyst. Furthermore, the solvationof the anions increases the size of the ions, decreases their mobility and diffusion rate, andreduces the reactivity of the reactant. How many molecules of the coextracted water doeseach quaternary salt carry? Hence, the equation for the distribution of a tetralkylammoniumhalide into an organic phase can be written as [94,95]Q þ þ X þjH 2 O Ð Q þ þ X :jH 2 OQ þ þ X þjH 2 O Ð Q þ X :jH 2 Oð48Þð49ÞDepending on whether the species in the organic phase is dissociated as free ions [Eq. (48)]or associated as ion pairs [Eq. (49)], the corresponding equilibrium constants can bewritten asCopyright © 2003 by Taylor & Francis Group, LLC