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 degreesC, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (k(obs)) are obtained. Plots of kobs against [amine] are linear, with k(N) as slopes, A biphasic Bronsted-type plot for kN is obtained for the aminolysis of MPNPC, with slopes beta(1) = 0.2 (high pK(a)) and beta(2) = 0.9 (low pK(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 pK(a) decreases. For the aminolysis of MPDNPC, a slightly curved Bronsted-type plot for kN is obtained, with beta(1) = 0.1 (low pK(a)) and beta(2) = 0.55 (high pK(a)). This is consistent with a concerted mechanism. (C) 2002 Wiley Periodicals, Inc.