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 degrees C, ionic strength 0.2 M (KCI). 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 pK(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 Bronsted type plots for amine basicity have slopes beta(N) ca, 0.2 for rate-determining amine attack (k(1)) and beta(N) ca. 0.8 for amine expulsion from T-+/- (k(-1)), in accord with the beta(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.