Kinetics and mechanism of the aminolysis of phenyl dithioacetate in aqueous solution

Abstract

The reactions of the title substrate with a series of secondary alicyclic amines have been subjected to a kinetic study in water at 25°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 (pK_–1) and PhS^–(K₂) from 1 are both smaller than those for expulsion from 3. The Brønsted plots for K₁(rate coefficient for formation of 1), K₁(=K₁/K_–1) and K_–1 have slopes β= 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^o= 10.3 ± 0.5, which is smaller than that derived from the aminolysis of S-phenyl thioacetate (pK_a^o= 12.2).