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 ASE2 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.