Abstract

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 ?1 = 0.2 (high pKa) and 32 = 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± the amine pKa decreases. For the aminolysis of MPDNPC, a slightly curved Brönsted-type plot for kN is obtained, with ?1 = 0.1 (low pKa) ?2 = 0.55 (high pKa). This is consistent with a concerted mechanism.