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 (0.02 ≤ [OH⁻]_T (mol dm⁻³) ≤ 0.5, 10% MeOH (v/v) + H₂O medium) without exhibiting [OH⁻] dependence. The reactivity trend follows I ~ II > > III ~ IV with low activation enthalpy {25.7±2.8 ≤ ΔH≠(kJ mol⁻¹) ≤ 64.8 ± 7.0}. The high negative and comparable values of activation entropy{−234 ± 8 ≤ ΔSΔ (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% D₂O), 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.