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

The reactions of the title substrate with a series of secondary alicyclic amines have been subjected to a kinetic study in water at 25-degrees-C, ionic strength 0.2 mol dm-3. Under amine excess pseudo-first-order rate coefficients (k(obs)) are found throughout. The order in amine varies from 1-2 according to the amine basicity and the experimental conditions. A general reaction scheme with a zwitterionic and an anionic tetrahedral intermediate (1 and 2, respectively) accounts for the rate equations found. The pK(a) of 1 is estimated along with some microconstants in the scheme. From the values of the microconstants, either obtained experimentally or estimated, it is deduced that 1 is more stable than its analogue (3) formed in the aminolysis of S-phenyl thioacetate, and therefore, 1 has time for proton transfer to a base to yield 2. The rate coefficients for expulsion of an amine (k-1) and PhS-(k2) from 1 are both smaller than those for expulsion from 3. The Bronsted plots for k1 (rate coefficient for formation of 1), K1 (=k1/k-1) and k-1 have slopes beta = 0.2, 1.2 and -1.0, respectively. The pK(a) value of an (hypothetical)amine for which k-1 = k2 is deduced: pK(a)-degrees = 10.3 +/- 0.5, which is smaller than that derived from the aminolysis of S-phenyl thioacetate (pK(a)-degrees = 12.2).