Nucleophilic substitution reactions of aryl dithioacetates with pyridines in acetonitrile

Kinetic studies of the pyridinolysis (XC5H4N) of aryl dithioacetates (CH3C(=S)SC(6)H(4)Z) are carried out in acetonitrile at 60.0degreesC. A biphasic Bronsted plot is obtained with a change in slope from a large value (beta(X) congruent to 0.9) to a small value (beta(X) congruent to 0.4) at betaK(a)degrees = 5.2, which is attributed to a change in the rate-limiting step from breakdown to formation of a zwitterionic tetrahedral intermediate, T+/-, in the reaction path as the basicity of the pyridine nucleophile increases. A clear-cut change in the cross-interaction constants rho(XZ) from a large positive value (rho(XZ) = +1.34) to a small negative value (rho(XZ) = -0.15) supports the mechanistic change proposed.