Kinetics and mechanism of the aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl 4‐methylphenyl carbonates in aqueous ethanol

The reactions of 4‐methylphenyl 4‐nitrophenyl carbonate (MPNPC) and 4‐methylphenyl 2,4‐dinitrophenyl carbonate (MPDNPC) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo‐first‐order rate coefficients (kobs) are obtained. Plots of kobs against [amine] are linear, with kN as slopes. A biphasic Brönsted‐type plot for kN is obtained for the aminolysis of MPNPC, with slopes β1 = 0.2 (high pKa) and β2 = 0.9 (low pKa). This is in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T±), and a change in the rate‐determining step, from formation to breakdown of T± as the amine pKa decreases. For the aminolysis of MPDNPC, a slightly curved Brönsted‐type plot for kN is obtained, with β1 = 0.1 (low pKa) and β2 = 0.55 (high pKa). This is consistent with a concerted mechanism.