KINETICS AND MECHANISM OF HYDROLYSIS OF OPEN-CHAIN THIOACETALS DERIVED FROM BENZOPHENONE AND THE REACTIVITY OF ALPHA-THIOPHENYL CARBOCATIONS

In a 40% (v/v) dioxane-water solvent, in the presence of 0.3-4.0 mol d m(-3) perchloric acid, the rates of hydrolysis of diethyl and diphenyl thioacetals derived from substituted benzophenones exhibit substituen t effects, acidity dependencies, activation parameters and solvent iso tope effects which all suggest that the hydrolyses follow the A1 mecha nism. The diethyl acetals are ca. 10(4)-fold less reactive than their O,O-analogues and ca, 10(4)-fold more reactive than the corresponding dithanes, for both of which classes of acetal the AS(E)2 mechanism of hydrolysis has been suggested. In concentrated aqueous perchloric acid the diaryl thioacetals are, like the diethyl compounds, rapidly and q uantitatively converted into the corresponding alpha-thio carbocations , which then undergo slow hydrolysis to the benzophenone. Kinetic meas urements show that the alpha-thiophenyl carbocation Ph(2)C(+)-SPh is c a. 20-fold more reactive towards hydrolysis than is Ph(2)C(+)-SEt, but that substituents in the thiophenyl group have little effect on react ivity (p similar or equal to 0.6). The detailed kinetic results are co mpatible with our previous suggestions about the mechanism of hydrolys is of alpha-thio carbocations.