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

• Published in: Journal of physical organic chemistry 2009-11, Vol.22 (11), p.1030-1037
• Main Authors: Castro, Enrique A., Gazitúa, Marcela, Santos, José G.
• Format: Article
• Language: English
• Subjects: aminolysis; aryl chlorodithioformates; Brønsted plot; kinetics; mechanism; tetrahedral intermediates
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1555

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 (kobs) are found. Plots of kobs versus [amine] are linear and pH independent, with slope kN. The Brønsted‐type plots (log kN vs. pKa 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 pKa value at the center of curvature (pK a0) 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. For the reactions of PClDTF with secondary alicyclic (SA) amines and pyridines and those of NPClDTF with pyridines, the formation of a tetrahedral intermediate (T±) is rate determining. For the reaction of NPClDTF with SA amines, there is a change in the rate‐determining step with amine basicity.