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

✍ Scribed by Enrique A. Castro; Monica Andujar; Paola Campodonico; José G. Santos

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 134 KB
• Volume: 34
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.10046

✦ Synopsis

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 (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