Kinetics and mechanism of hydrolysis of open-chain thioacetals derived from benzophenone and the reactivity of α-thiophenyl carbocations

Abstract

In a 40%(v/v) dioxane–water solvent, in the presence of 0.3–4.0 mol dm^–3 perchloric acid, the rates of hydrolysis of diethyl and diphenyl thioacetals derived from substituted benzophenones exhibit substituent effects, acidity dependencies, activation parameters and solvent isotope effects which all suggest that the hydrolyses follow the A1 mechanism. The diethyl acetals are ca. 10⁴-fold less reactive than their O,O-analogues and ca. 10⁴-fold more reactive than the corresponding dithanes, for both of which classes of acetal the AS_E2 mechanism of hydrolysis has been suggested. In concentrated aqueous perchloric acid the diaryl thioacetals are, like the diethyl compounds, rapidly and quantitatively converted into the corresponding α-thin carbocations, which then undergo slow hydrolysis to the benzophenone. Kinetic measurements show that the α-thiophenyl carbocation Ph₂C⁺–SPh is ca. 20-fold more reactive towards hydrolysis than is Ph₂C⁺–SEt, but that substituents in the thiophenyl group have little effect on reactivity (ρ≃ 0.6). The detailed kinetic results are compatible with our previous suggestions about the mechanism of hydrolysis of α-thin carbocations.