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

Article abstract of DOI:10.1002/poc.1787

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 k₁ (amine attack to form T^±), k_-1 (its back step), k₂ (nucleofuge expulsion from T^±), and k ₃ (proton transfer from T^± to the amine) are determined for some reactions. The Bronsted plots for k₁ are linear with slopes β₁ = 0.2-0.4 in accordance with the slope values found when T^± formation is the rate-determining step. The sensitivity of log k₁ and log k_-1 to the pK _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 k₂ and k₃ steps are rate determining. Copyright