M. Ali et Dpn. Satchell, KINETICS AND MECHANISM OF HYDROLYSIS OF OPEN-CHAIN THIOACETALS DERIVED FROM BENZOPHENONE AND THE REACTIVITY OF ALPHA-THIOPHENYL CARBOCATIONS, Perkin transactions. 2, (1), 1995, pp. 167-170
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.