• Sarat C Dash

      Articles written in Journal of Chemical Sciences

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

      Sarat C Dash Anadi C Dash

      More Details Abstract Fulltext PDF

      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 III > > IIIIV with low activation enthalpy {$25.7\pm 2.8 ≤ \Delta H^{\neq}$(kJ mol-1) $≤ 64.8\pm 7.0$}). The high negative and comparable values of activation entropy {$-234 \pm 8 ≤ \Delta S^{\neq}$(J K-1 mol-1) $≤ −127 \pm 20$} 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, $k^{H2O}/k^{D2O+H2O} \sim 1$ (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.

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