Proximity effect on the general base catalysed hydrolysis of amide linkage: The role of cationic surfactant, CTABr

Abstract

The bis phenoxide forms of (1,2)bis(2-hydroxybenzamido)ethane(I), (1,5)bis(2-hydroxybenzamido)3-azapentane(II), (1,3)bis(2-hydroxybenzamido)propane(III), and (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane(IV) undergo facile hydrolysis of one of the amide groups (0.02The reactivity trend follows I ∼ II >> III ∼ IV with low activation enthalpy {25.7 ± 2.8 ≤ H = (kJ mol −1 ) ≤ 64.8 ± 7.0}. The high negative and comparable values of activation entropy{−234 ±8 ≤ S = (J K −1 mol −1 ) ≤ −127 ± 20} are consistent with closely similar, and ordered transition states which can be assembled by favourably oriented phenoxide groups. The solvent kinetic isotope effect for I, k H2O /k D2O+H2O ∼ 1 (20 and 50 volume% D 2 O), indicates that proton transfer is not involved as a part of the rate controlling process. The observed slowing down of the rate of this reaction for I in the micellar pseudo phase of CTABr also supports the proposed mechanism. Under premicellar conditions, however, rate acceleration is observed, a consequence believed to be associated with the capping effect of the hydrophobic tail of the surfactant cation forming the reactive ion-pair, CTA + , (I-2H) 2− exclusively in the aqueous pseudo phase.