Mechanism of the Brønsted acid catalysed hydrolysis of 2-aryl-2-methyl-1,3-dithianes in aqueous perchloric acid

Abstract

The acid catalysed hydrolysis of five para-substituted 2-aryl-2-methyl-1,3-dithianes in aqueous perchloric acid (3–9 mol dm^–3) has been studied kinetically. The effect of changes in substituents (ρ=–3.1), temperature (ΔS^‡=–38–60 J K^–1 mol^–1) and acidity (m^‡= 0.54–0.74), and the solvent isotope effects (k_obs^D₂O/k_obs^H₂O < 1), are all compatible with an essentially AS_E2 mechanism for the MeO, Me, H and Cl derivatives. For the p-NO₂ compound the acidity dependence, and the values of ΔH^‡ and ΔS^‡(ca.–200 J K^–1 mol^–1) suggest a change to an A2-like mechanism. Our results are discussed in the light of previous work on acetal hydrolysis.