Kinetics and Mechanism of the Aminolysis of Phenyl and 4-Nitrophenyl Ethyl Thionocarbonates

The reactions of the title substrates (PTOC and NPTOC, respectively) with secondary alicyclic amines are subjected to a kinetic study in aqueous solution at 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients ( k obsd ) are found throughout. The order in amine is one for the reactions of piperidine but is of intermediate order between 1 and 2 for the reactions of the other amines. The kinetic results can be accommodated by a reaction scheme with two hypothetical tetrahedral intermediates: a zwitterionic (T ± ) and an anionic (T - ) one, whereby amine catalysis (deprotonation of T ± to give T - ) is kinetically important. Both the p K a of T ± and the rate coefficient for proton transfer ( k 3 ca. 10 10 s - 1 M - 1 ) are estimated. The values of the other rate microcoefficients of the scheme are found by a nonlinear least-squares fitting, and these values are compared with those exhibited in the aminolysis of phenyl thionoacetate (PTOA), and S -phenyl and S -(4-nitrophenyl) O -ethyl dithiocarbonates (PDTC and NPDTC, respectively). The Brönsted type plots for amine basicity have slopes β N ca. 0.2 for rate-determining amine attack ( k 1 ) and β N ca. 0.8 for amine expulsion from T ± ( k - 1 ), in accord with the β N values found in similar aminolyses. The general base catalysis by amine found in the aminolysis of NPTOC, in contrast with the lack of such catalysis in the aminolysis of 4-nitrophenyl methyl carbonate, is explained by a smaller rate coefficient for expulsion of 4-nitrophenoxide ( k 2 ) from T ± (which competes with amine deprotonation of T ± ) relative to the same expulsion from the analogous oxy intermediate.