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

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, kH2O/kD2O+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

Effect of solvent on the reactions of coordination complexes. Part 2. Kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water and ethylene glycol–water media

The kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated in aqueous methanol (MeOH) and aqueous ethylene glycol (EG) media (0-80% by weight of MeOH or EG) at 45-64.7 °C. The logarithm of the pseudo-first-order rate constants for MeOH-water media exhibits linear dependence with the reciprocal of the bulk dielectric constant (D-1s), the mole fraction of MeOH (XMeOH) and the solvent ionizing power Y(Y1-AdCl) as determined by the solvolysis rates of 1-adamantyl chloride. Similar plots (log ksobsvs.xEG or D-1s) for EG-water media are non-linear. It is evident that the solvation phenomenon plays dominant role and the rate of solvolysis is mediated by the dual solvent vectors, the overall acidity and basicity of the solvent mixtures. The relative transfer free-energy calculations indicate that the mixed solvent media exert more destabilizing effect on the transition state as compared to the initial state. The activation enthalpy and entropy vs.Xorg(where Xorg is the mole fraction of the organic solvent component) plots display maxima and minima indicating that the solvent structural changes play significant role in the activation process. The activation free energy at a given temperature, however, increases only marginally and linearly with increasing Xorg. The mutual compensatory effect of activation enthalpy and entropy on the activation free energy is in keeping with the fact that the perturbations of the reaction zone and the solvent network remain approximately proportional to each other with increasing Xorg so that the isodelphic and the lyodelphic components of ΔH± and ΔS± correlate well with each other

Computational Mechanism Design: A Call to Arms

Game theory has developed powerful tools for analyzing decision making in systems with multiple autonomous actors. These tools, when tailored to computational settings, provide a foundation for building multiagent software systems. This tailoring gives rise to the field of computational mechanism design, which applies economic principles to computer systems design

Kinetics & Mechanism of Base Hydrolysis of Malonato & Succinato Complexes of Pentaamminecobalt(III) Ion

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Base-catalysed hydrolysis of cis-(imidazolato)-, cis-(benzimidazolato)- and cis-(N-methylimidazole)-bis-(ethylenediamine)halogenocobalt(III) cations. A comparison of the reactivities of the deprotonated imidazole and benzimidazole complexes

The base hydrolysis of the complexes cis-[Co(en)2B(X)]n+ [en = ethylenediamine; B = imidazole (Him), N-methylimidazole (mim), or benzimidazole (Hbzim); X = Cl or Br] has been investigated at 20-35°C and [OH-]τ=(4.7-99.7)×10-3 mol dm-3(I= 0.10 mol dm-3) under which conditions the co-ordinated imidazole and benzimidazole undergo complete NH deprotonation. The activation parameters (85≤ΔH‡/kJ mol-1 ≤ 97, +84 ≤ ΔS‡/J K-1 mol-1 ≤ +97), and the rate dependence on the leaving groups and the non-labile amine ligands are consistent with a SN1 CB mechanism. The electron-displacement properties of the N-co-ordinated imidazolate (im) and benzimidazolate (bzim) ions appear to enhance the pKNH of the co-ordinated ethylenediamine, the effect being relatively more significant for the former anion. Analysis of the activation entropy data in terms of the dissociative activation model for the conjugate bases, cis-[Co(en)(en - H)B(X)]n+[B = im or bzim (n= 0); mim (n= 1)] indicates that the configurational rearrangement at the cobalt(III) centre most likely occurs in the transition state of the actual acta of the substitution for the less reactive chloro complexes. The presumed trigonal-bipyramidal intermediate is efficiently scavenged by azide

Decarboxylation of Hydrogencarbonatopentamminecobalt(III) in Aquo-organic solvent media

The decarboxylation of hydrogencarbonatopentaamminecobalt(III) has been investigated in aqueous, 99% D2O and aquo-organic solvent media (0-70 wt.% of cosolvent) at 15 ≤ t/°C ≤ 40 (I = 0.02 mol dm-3), using methanol, propan-2-01, tert-butyl alcohol, ethylene glycol, acetone, acetonitrile, DMSO and ethylene carbonate as cosolvents. The solvent isotope effects on rate (kH2O/kD2O=O 1.0 at 1535°C) and activation parameters (ΔH≠ = 77.7 ± 1.0, 77.8 ± 0.9 kJ mol-1 and ΔS≠ = 16 ± 3, 16 ± 3 J K-1 mol-1 for aqueous and 99% D,O media, respectively) were negligible. The decarboxylation rate constant increased with increasing mole fraction (Xorg) of the cosolvent and the effect was pronounced at relatively high values of Xorg for the dipolar aprotic cosolvents. This was attributed to a greater degree of destabilisation of the initial state as compared to the transition state with increasing mole fraction of the cosolvent. The In ks vs. 1/εs plots (ks is the rate constant and εs, the bulk relative permittivity) showed marked dependence on the nature of the cosolvents; the gradients of such plots generally increased with increasing dipole moment of the cosolvent molecules, indicating thereby that the solvation of the initial state and the transition state of the substrate is governed by the ion-dipole interactions between the water and cosolvent molecules. The relative transfer free energy of activation, [ΔΔGt≠](s←w), decreased linearly with Xorg for all mixed-solvent media, indicating that the preferential solvation effect is not significant. The activation enthalpy and entropy vs. Xorg plots displayed extrema suggesting that these thermodynamic parameters are sensitive to the structural changes in the bulk solvent phase. The solvent effects on ΔH≠ and ΔS≠ are mutually compensatory