Solvent Effect on Amide Hydrolysis I-Formamide Hydrolysis in Dioxane-Water Mixtures

Chemistry Department, Faculty of Science, Mansoura University, Egypt

Manuscript received 27 May 1976, revised 15 June 1977, accepted 26 July 1977

The kinetics of formamide hydrolysis was studied in acidic and basic dioxane-water mixtures at temperatures within the range 25-60^°. The variation of rate constant with temperature, dielectric constant, acidity function and solvent composition was discussed. The refined distances of closest approach for the activated complexes and the thermody­namic parameters of activation were calculated and discussed as further evidences for the solvation effects. The results were explained by the proposed mechanisms of the acid and base hydrolysis. Water ionisation constants in several dioxane-water mixtures up to 80% by weight of dioxane were also determined at eight temperatures ranging from 25 to 60° at 5° intervals. An excellent linear dependence of pK_w on the mole fraction of water in the solvent was recorded. This can be represented by an equation which facilitate the calculation of any unknown pK_w value. Plots of pK_w versus log C_w, at any temperature, give two excellent linear portions separated by a sharp boundary at a water concentration corresponding to 50 wt%.of dioxane. As a surprising result, this solvent composition was coincident with the minimum in the reaction rate. The reaction rate was thus correlated with the ionisation constant of the water which is present as a com­ponent of the solvent system.