Ester hydrolysis in aqueous sulphuric acid. Effects of solvent ionizing power and nucleophilicity separated from the effects of protonation of substrate

Abstract

Rates of hydrolysis are reported for methyl, ethyl, and isopropyl toluene-p-sulphonates and for methyl, ethyl, isopropyl, cyclohexyl, and 2-endo-norbornyl methanesulphonates in 0–70% w/w aqueous sulphuric acid between 0 and 50 °C. N.m.r. studies of chemical shifts (¹H and ¹³C), showing the small extent of protonation of ethyl methanesulphonate, are also reported. The kinetic data are analysed using an equation developed from studies of solvolytic reactions in less acidic media: log (k/k₀)_ROTs=IN_OTs+mY_OTs, where k/k₀ refers to the rate of reaction in any solvent (k) relative to 80% ethanol/water (k₀), and I and m are empirical parameters for sensitivities of the substrate (ROTs) to solvent nucleophilicity (N_OTs) and ionizing power (Y_OTs) respectively. Values of N_OTs and Y_OTs are correlated with the Hammett acidity function H₀, and are used to calculate the relative proportions of substitution and of cyclization via triple bond participation in sulphonates of oct-6-yn-2-ol.