Abstract

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 ß of 0.3) and pyridines (ß - 0.26) and those of NPClDTF with pyridines (ß - 0.30). For the reaction of NPClDTF with SA amines the Brønsted-type plot is biphasic, with slopes ß 1 - 0.2 (at high pKa) and ß 2 - 1.1 (at low pKa). The pK a value at the center of curvature (pK 0 a) 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. © 2009 John Wiley & Sons, Ltd.