KINETICS AND MECHANISM OF THE AMINOLYSIS OF 4-NITROPHENYL DITHIOACETATE

The reaction of the title substrate with a series of secondary alicylic amines has been the subject of a kinetic study in aqueous solution, 25-degrees-C, ionic strength 0.2 M (KCl). With the amine in excess, pseudo-first-order rate constants (k(obsd)) are observed. The order in amine varies from 1 to 2 depending on the basicity of the amine and the reaction conditions. A reaction scheme is deduced on the basis of the existence of zwitterionic (T+/-) and anionic (T-) tetrahedral intermediates. Proton transfer from T+/- to an amine or base (to yield T-) seems to compete with 4-nitrothiophenoxide (NPS-) expulsion from T+/-. The pK(a) of T+/- and all the rate microconstants of the scheme are estimated. The rates of expulsion of NPS- and amine from T+/- are smaller than those from analogous T+/- formed in the aminolyses of O-ethyl S-(4-nitrophenyl) dithiocarbonate and 4-nitrophenyl thiolacetate. It is claimed that substitution of Me by RO (R = alkyl) or S- by O- in T+/- destablizes this intermediate.