KINETICS AND MECHANISM OF THE AMINOLYSIS OF 2,4-DINITROPHENYL O-ETHYL DITHIOCARBONATES AND 2,4,6-TRINITROPHENYL O-ETHYL DITHIOCARBONATES

The reactions of a series of secondary alicyclic amines with the title substrates (DNPDTC and TNPDTC) are subject to a kinetic study in water at 25.0-degrees-C, ionic strength 0.2 M (KCl). The reactions are first order in both the amine and the substrate. The Bronsted-type plots obtained are nonlinear and are explained by a stepwise mechanism through a zwitterionic tetrahedral intermediate (T+/-), although a concerted process is not rigorously excluded for the reactions of the less basic amines with TNPDTC. An equation is reported which describes the dependence of the basicities of the amine and leaving group on the nucleofugality ratio of these groups from T+/- (k-1/k2). This equation satisfactorily predicts the pK(a) values at the Bronsted breaks (pK(a)degrees). The experimental pK(a)degrees values found in the present reactions correlate well with those in the aminolysis of p-nitro- and unsubstituted phenyl O-ethyl dithiocarbonates. The values of both k-1 and k2 are larger in the reactions of phenyl O-ethyl dithiocarbonate than in the aminolysis of phenyl dithioacetate due to the additional push exerted by EtO in T+/-. The T+/- formed in the aminolysis of DNPDTC is less unstable than that in the aminolysis of S-(2,4-dinitrophenyl) O-ethyl thiocarbonate (whose mechanism is concerted), and this is attributed to a smaller ability of S- in T+/- to form a double bond and expel a nucleofuge.