Abstract
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