Reactions of O ‐aryl S ‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

The reactions of O ‐(4‐methylphenyl) S ‐(4‐nitrophenyl), O ‐(4‐chlorophenyl) (4‐nitrophenyl), O ‐(4‐chlorophenyl) S ‐phenyl, and O ‐(4‐methylphenyl) S ‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients ( k obs ) are found. For some of the reactions, plots of k obs vs . free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T ± ) and the other anionic (T − ). In some cases, there is a kinetically significant proton transfer from T ± to an amine to yield T − . Values of the rate micro constants k 1 (amine attack to form T ± ), k −1 (its back step), k 2 (nucleofuge expulsion from T ± ), and k 3 (proton transfer from T ± to the amine) are determined for some reactions. The Brønsted plots for k 1 are linear with slopes β 1  = 0.2–0.4 in accordance with the slope values found when T ± formation is the rate‐determining step. The sensitivity of log k 1 and log k −1 to the p K a of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k 2 and k 3 steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd.