STUDIES IN SOLUTION KINETICS

Abstract

Part one of this project concerns a study on the kinetics of the general acid- and base-catalysed mutarotation of tetra-methylglucose at 25°C in mixed water-p-dioxane solvents for the catalysts water, pyridine, acetate ion, benzoate ion and benzoic acid, The concentrations of water used in the mixed solvents range from 11M to 52M, For each catalyst used, at least five runs of different compositions of water-p-dioxane solvents are performed, the concentrations of catalysts being around O.1M except for benzoic acid (O.2M) and water. The apparent orders of reaction with respect to water, obtained by the least squares method, are two, three, three and zero for the catalysts water, pyridine, acetate ion and benzoate ion respectively, The concentration scale used in computing the order of reaction is based on molarity rather than mole fraction. Cyclic transition states which can perhaps be formed via the step-wise mechanism for the mutarotation of tetramethylglucose with the intervention of the solvent water molecules are proposed and illustrated in Chap. IV page 84, 86-89. These cyclic transition states are strictly applicable in water-p-dioxane solvents but it seems highly probable that they would also apply in water or even in water-deuterium oxide mixtures are reinterpreted. Knowing the number of water molecules in the transition states, the kinetic data previously obtained for water-deuterium oxide mixtures are reinterpreted. The theoretical kn/kH ratios at differernt n values are calculated for the catalysts water, pyridine and acetate ion and are compared with the experimental values obtained by Huang and his coworkers, The results in Chap, IV page 96-97 show the extent to which the earlier interpretation needs to be modified. Part two of this project deals with the investigation of the kinetics of the thermal decarboxylation of cyanoacetic acid in the solvent water. The rate measurements are carried out at four different temperatures, namely, 156°C, 161.5°C, 169.4°C and 176°C and the results are shown in Chap, VI page 122. The activation energy of the reaction is found to be 40-43 Kcal per mole and is compared with that of nitroacetic acid. A mechanism is proposed which is similar to those of the decarboxylations of other substituted acetic acids.