Kinetics of hydrolysis of 2-aryl-2-phenyl-1,3-dithianes in 10%(v/v) dioxane–water, containing perchloric acid. Acidity functions in this solvent and the reactivity of α-thio carbocations

Abstract

The acid-catalysed hydrolysis of 2-aryl-2-phenyl-1,3-dithianes in 10%(v/v) dioxane–water occurs at a convenient rate at 25 °C in the presence of 5–8 mol dm^–3 perchloric acid. The effect on the rate of changes in substituents, acidity and temperature, and of the use of a deuteriated solvent are described. Measurements of the H₀and X acidity functions for the solvent are reported; their values are very similar to those found for pure water over most of the acidity range. The mechanism of hydrolysis is believed to change from an AS_E2 scheme for the most reactive dithianes to an A2-like scheme for the least reactive. In very concentrated acid solutions the dithianes (and other suitable S,S-acetals) lead to stoichiometric amounts of the α-thio carbocations often postulated as low-concentration intermediates in S,S-acetal hydrolysis. The kinetics of the reaction of these ions with water to give the benzophenone (or corresponding carbonyl compound) are described, and compared with findings for similar α-oxo carbocations. Previous views on the mechanism of this reaction are criticised.