Nucleophilic Substitution Reactions of Aryl Thiophene-2-carbodithioates with Pyridines in Acetonitrile

The kinetics of reactions between Z-aryl thiophene-2-carbodithioates and X-pyridines in acetonitrile at 60.0 $^{\circ}C$ have been investigated. The Bronsted plots obtained for the pyridinolysis of aryl thiophene-2-carbodithioates are curved, with the center of curvature at $pK_a$ ~ 5.2 ($pK_a^{\circ}$). The Bronsted plots for these nucleophilic reactions show a change in slope from a large ( ${\beta}_X{\cong}$0.78-0.87) to a small ( ${\beta}_X{\cong}$0.33-0.35) value, which can be attributed to a change in the rate-determining step from breakdown to formation of a zwitterionic tetrahedral intermediate in the reaction path as the basicity of the pyridine nucleophile increases. A clear-cut change in the crossinteraction constants, ${\rho}_{XZ}$, from +0.92 to -0.23 supports the proposed mechanistic change. The breakpoint at $pK_a$ = 5.2 for R = thiophene ring in the present work is in agreement with those for the pyridinolysis of R = Me and 2-furyl, and attests to the insignificant effects of acyl group, R, on the breakpoint.