Kinetics and mechanism of the reactions of aryl chlorodithioformates with pyridines and secondary alicyclic amines

The reactions of pyridines and secondary alicyclic (SA) amines with phenyl and 4‐nitrophenyl chlorodithioformates (PClDTF and NPClDTF, respectively) are subjected to a kinetic study in aqueous ethanol (44 wt% ethanol) solution, at 25.0 °C, and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus amine] are linear and pH independent, with slope k_N. The Brønsted‐type plots (log k_N vs. pK_a of aminium ions) are linear for the reactions of PClDTF with SA amines (slope β of 0.3) and pyridines (β = 0.26) and those of NPClDTF with pyridines (β = 0.30). For the reaction of NPClDTF with SA amines the Brønsted‐type plot is biphasic, with slopes β₁ = 0.2 (at high pKa) and β₂ = 1.1 (at low pKa). The pK_a value at the center of curvature (pK) is 7.7. The magnitude of the slopes indicates that the mechanisms of these reactions are stepwise, with the formation of a zwitterionic tetrahedral intermediate as the rate‐determining step, except for the reaction of NPClDTF with SA amines where there is a change in the rate‐determining step, from formation to breakdown of the tetrahedral intermediate, as the amine basicity decreases. Copyright © 2009 John Wiley & Sons, Ltd.