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 ( k obs ) are obtained. Plots of k obs against [amine] are linear, with k N as slopes. A biphasic Brönsted‐type plot for k N is obtained for the aminolysis of MPNPC, with slopes β 1 = 0.2 (high p K a ) and β 2 = 0.9 (low p K a ). 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 p K a decreases. For the aminolysis of MPDNPC, a slightly curved Brönsted‐type plot for k N is obtained, with β 1 = 0.1 (low p K a ) and β 2 = 0.55 (high p K a ). This is consistent with a concerted mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 309–315, 2002