| Abstract
| - Kinetic studies of the pyridinolysis (XC5H4N) of aryl dithioacetates (CH3C(S)SC6H4Z) are carriedout in acetonitrile at 60.0 °C. A biphasic Brönsted plot is obtained with a change in slope from alarge value (βX ≅ 0.9) to a small value (βX ≅ 0.4) at pKa° = 5.2, which is attributed to a change inthe 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 thecross-interaction constants ρXZ from a large positive value (ρXZ = +1.34) to a small negative value(ρXZ = −0.15) supports the mechanistic change proposed.
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