Kinetics and mechanism of the pyridinolysis of aryl dithiobenzoates in acetonitrile

The kinetics and mechanism of the aminolysis of aryl dithiobenzoates (RC(S)SC 6 H 4 Z; R = Ph) with pyridines (XC 5 H 4 N) in acetonitrile at 60.0°C have been studied. A biphasic Brönsted plot is obtained with a change in slope from a large value (β X ≅ 0.7–0.8) to a small value (β X ≅ 0.2) at p K a ° = 5.2, which is interpreted to indicate a change of the rate‐determining step from breakdown to formation of the zwitterionic tetrahedral intermediate, T ± , at p K a = 5.2 as the pyridine basicity is increased. Rates are compared with those corresponding values for aryl dithioacetates (R = Me). The faster rates for R = Me than for R = Ph, for the rate‐limiting formation of T ± , can be attributed to the hyperconjugative charge transfer effect of the Me group, pseudo‐π Me → π* C=S . Clear‐cut change in the cross‐interaction constants, ρ XZ , from +1.47 to −0.20 supports the proposed mechanistic change. The breakpoint at p K a ° = 5.2 for R = Ph in the present work is in agreement with those for the pyridinolysis of R = Me and 2‐furyl, and attests to the insignificant effect of acyl group, R, on the breakpoint. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 434–440, 2004