Kinetics and mechanism of the benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates.
The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 degrees C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Brønsted-type plots for the nucleophilic rate constants are linear with slopes beta = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Brønsted plot was found for the reactions of DNPC and DNPTC at pK(a) ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's "hard and soft acids and bases" principle.
Castro
Enrique A
EA
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile. ecastro@puc.cl
Pavez
Paulina
P
Santos
José G
JG