STEPWISE MECHANISMS OF THE AMINOLYSES OF 4-NITROPHENYL AND 2,4-DINITROPHENYL O-ETHYL DITHIOCARBONATES IN AQUEOUS-ETHANOL

The reactions of the title substrates with a series of secondary alicy clic amines are subjected to a kinetic study in 44 wt% aqueous ethanol , at 25.0 degrees C, ionic strength 0.2 M (maintained with KCl). Pseud o-first-order kinetics are found under amine excess. The order in amin e obtained is one in the reactions of all amines. The Bronsted-type pl ots for the overall second-order rate coefficients are biphasic with s lopes beta(1) = 0.3 (high pK(a)), beta(2) = 0.95 (low pK(a)) for the a minolysis of the 4-nitro derivative, and beta(1) = 0.3 and beta(2) = 0 .80 for the aminolysis of the dinitro compound; the pK(a) values at th e curvature center are pK(a)(0) = 9.8 and 9.5, respectively. From a co mparison of these pK(a)(0) values with those found in the same reactio ns in water and the shapes of the Bro;nsted-type plots, it is conclude d that these reactions are stepwise, with the formation of a zwitterio nic tetrahedral intermediate in the reaction pathway. (C) 1995 John Wi ley & Sons, Inc.