11. Interfacial Mechanism and Kinetics of Phase-Transfer Catalysis

The order of magnitude of D Q for quaternary salts is Aliquat 336 > TBA-TBPO >TBAI > TBPB > TBAB > TBAC. The sequence of D Q for solvents is CHCl 3 >CH 2 Cl 2 > 1;2-C 2 H 2 Cl 2 > C 6 H 5 Cl. The order of influence on the extraction capabilityof quaternary salts is Br 3 C 6 H 2 O > I > Br < Cl and P þ > N þ for the anion andcentral cation, respectively. Reasons for these behaviors have been discussed in previouswork [48,76,81,85,86]. The D Q value increased on increasing the temperature.The true extraction constants of quaternary salts QX corresponding to their infinitelydilute solutions in a two-phase system were calculated using the following equation:a QXEQX T ¼a Qþ; a X½QXŠ¼½Q þ Š½X Š2ð36Þwhere a and 2 are the activity and the mean ionic activity coefficient of the quaternarysalts, respectively.The distribution constant of quaternary salt at equilibrium between two phases ism ¼ ½Qþ X Š½Q þ X Šð37ÞThe dissolved Q þ Xin the aqueous and organic phase may dissociate toQ þ X Ð Q þ þ X ð38ÞQ þ X Ð Q þ þ X ð39ÞThus, the dissociation constants K da and K da of QX in the aqueous and organic phases arewritten asK da ¼ ½Qþ Š½X Š 2 ½Q þ X ŠK do ¼ ½Qþ Š½X Š 2 ½Q þ X Šð40Þð41ÞThe dissociation constant in aprotic organic solvents can be derived from fundamentalprinciples based on Bjerrum’s theory for ion pairs. In most organic media, thedissociation constant of ion pairs is very low (of the order of around 10 5 ) [48].Bra¨ ndstro¨ m [87] reported that the ionic aggregation states of quaternary salts existingin the organic phase were of various types, i.e., dissociated ions (Q þ þ X ), ion pairs(Q þ X ), quadruples ½ðQ þ X Þ 2 Š, etc. Hence, the total concentration of quaternary salt inan organic phase can be written asC Q ¼½Q þ Šþ½QXŠþ2½Q 2 X 2 Šþð42ÞSince the organic system is in electrical neutrality,½Q þ Š¼½X ŠEquation (42) can be transformed intoð43ÞC Q ¼ E T1=2Q þ ½Q þ Š½X Š 1=2þETQX 2 ½Q þ Š½X Š þ 2E T Q 2 X 22 4 ½Q þ Š½X Š 2þ ð44Þwhere EQ T þ, ET QX, and EQ T 2 X 2are the concentration quotients represented asCopyright © 2003 by Taylor & Francis Group, LLC