Solvent effects on the rates of some acid-catalysed reactions

Abstract

Rate constants for some acid-catalysed reactions are reported together with H0 acidity functions in dilute solutions of perchloric acid in dioxan–water mixtures at various temperatures. Rate constants in dioxan–water mixtures containing 0·1M-hydrochloric acid are also reported. In all cases graphs of log (observed rate constant) against H0 are linear for reactions in perchloric acid solutions of fixed solvent composition. The slopes of these graphs are relatively insensitive to solvent composition except for the A-2 reactions studied, for which the slopes show an appreciable solvent-dependence. The effects of changing solvent composition on the reaction rates in solvents containing a constant concentration of mineral acid catalyst are compared by plotting graphs of H0+ log k against log (activity of water). With 0·1M-perchloric acid catalyst approximately linear correlations are obtained which are discussed in terms of the reaction mechanism. Comparison of results obtained with perchloric and hydrochloric acid catalyst also throws light on the reaction mechanisms.