KINETICS AND MECHANISM OF THE AMINOLYSIS OF PHENYL THIONOACETATE IN AQUEOUS-SOLUTION

The reactions of a series of secondary alicyclic amines with the title substrate have been subjected to a kinetic study in water at 25-degrees-C, ionic strength 0.2 M. Pseudo-first-order rate coefficients (k(obsd)) are found throughout, under amine excess. The order in amine varies from 1 to 2 according to its basicity and the reaction conditions. A reaction mechanism consisting of a zwiterionic (T+/-) and an anionic (T-) tetrahedral intermediate is proposed to account for the results. Base catalysis by OH-and the amine is observed for conversion of T+/- to T-. The microscopic rate coefficients involved in the reaction scheme are either estimated or determined. The Bronsted slopes for formation of T+/- and for its back step are beta(N) = 0.16 and -0.82, respectively. The rate coefficients for formation of T+/- (k1) and that for expulsion of PhO- from T+/- (k2) are smaller than those in the phenyl acetate (PA) and phenyl dithioacetate (PDTA) reactions. The rate coefficient for amine expulsion from T+/-(k-1) is smaller than that in the PA reactions, but it is similar to that in the aminolysis of PDTA and 4-nitrophenyl dithioacetate. It is claimed that there is little sensitivity of k-1 to the leaving group (ArO or ArS) basicity when the C-S- bond is involved in T+/-, and reasons for this are given.