Kinetics and mechanism of the aminolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl thiolacetates

Abstract

The reactions of the title substrates with alicyclic secondary amines and pyridines are subject to a kinetic study in water [25 °C, ionic strength 0.2 mol dm^–3(KCl)]. The Brønsted-type plots obtained for all reaction series are non-linear, indicating the existence of a tetrahedral intermediate (T^±) in the reaction pathway and a change in the rate-limiting step involving formation and breakdown of T^±. Calculated curves based on the above hypothesis fit the experimental data well. From estimations of both the pK_a of the T^± formed in the reactions with secondary amines and the rate of arylthiolate ion expulsion from T^±(k₂) it is deduced that the rate coefficient for proton transfer from T^± to a base is lower than k₂. The centre of the Brønsted-type curvature for the reactions of the dinitro derivative lies at pK_a= pK_a°= 8.9 (alicyclic amines) and 6.6 (pyridines). For 2,4,6-trinitrophenyl thiolacetate, pK_a°= 7.8 (alicyclic amines) and 4.9 (pyridines). These figures show that pyridines leave T^± at a rate (k_–1) slower than isobasic secondary amines (the k₂ value is independent of the amine nature). From estimations of k_–1 and k₂ based on the aminolysis of other aryl thiolacetates the pK_a^o values of both reaction series are predicted. These show reasonable agreement with the experimental values. Comparison of these reactions with the aminolyses of aryl acetates indicates that the ‘push’ provided by ArO (to expel a given amine) from an analogous oxy-T^± is slightly stronger than that exerted by an isobasic ArS group in T^±.