Medium Effects on Activation Parameters of Alkaline Hydrolysis of Monomethylsuccinate Ion

Department of Chemistry, Jadavpur University, Jadavpur, Calcutta-700 032

The effect of the solvent on the activation parameters for the reaction between mono­methylsuccinate and hydroxide ions has been studied by using water, 20% ethanol-water and 20% ethylene glycol-water. Two sets of isodielectric ethanol-water mixtures of dielectric constants 68.66 and 50.38 and one set of isodielectric ethylene glycol-water mixtures of dielectric constant 68.66 have been used. The activation parameters have been dissected into the electrostatic and nonelectrostatic components. An analysis of the activation energy, entropy of activation and nonelectrostatic potential reveals that solva­tion of the transition state is larger in ethanol-water than in ethylene glycol-water of the same dielectric constant. The free energy of activation for the isodielectric (68.66) etha­nol-water has a much lower positive value than for the corresponding isodielectric glycol-water medium, implying poor compensation between ∆H‡ and T∆S‡, as well as struc­rural differences between the two solvent systems.