Kinetics and mechanism of the reactions of polyallylamine with aryl acetates and aryl methyl carbonates

The reactions of polyallylamine (PAA) with 4-nitrophenyl acetate (NPA), 2,4-dinitrophenyl acetate (DNPA), 2,4,6-trinitrophenyl acetate (TNPA), 4-nitrophenyl methyl carbonate (NPC), 2,4-dinitrophenyl methyl carbonate (DNPC) and 2,4,6-trinitrophenyl methyl carbonate (TNPC) at pH 7.0-11.5 were subjected to a kinetic investigation in aqueous solution at 25.0 degrees C and an ionic strength of 0.1 m (KCl). Potentiometric titration curves were obtained at different polymer concentrations under the same conditions as for the kinetic measurements. The degree of dissociation (alpha) and pK(app) values for PAA at each pH were found from the titration curves. The shape of these curves shows a conformational change of the polymer at alpha > 0.7. Similar behavior was observed through the dependence of logk(N) on either pH or alpha, where k(N) is the second-order rate constant for the title reactions. The k(N) value is influenced by the electrostatic interactions in the polymer chain and the conformational changes that PAA undergoes in solution. The Bronsted-type plots (logk(N) vs pK(app)) are linear with slopes (beta values) of 0.5, 0.4, 0.5, 0.7, 0.6 and 0.7 for the reactions of PAA with NPA, DNPA, TNPA, NPC, DNPC and TNPC, respectively. These data are consistent with concerted mechanisms. The kN values increase in the sequence TNPA > DNPA > NPA and TNPC > DNPC > NPC. These results are in accordance with those found for the reactions with monomeric amines, which are due to the increasing nucleofugality of the leaving groups, and also the increasing electrophilic character of the carbonyl carbon, as more nitro groups are added to the substrate. Acetates are more reactive than the corresponding methyl carbonates, which can be explained by the larger electron-releasing effect exerted by MeO relative to Me. PAA destabilizes the putative tetrahedral intermediate relative to the monomeric amines and the stability of tetrahedral intermediates would decrease in the sequence pyridines > anilines > secondary alicyclic amines > quinuclidines > PAA. Copyright (c) 2006 John Wiley & Sons, Ltd.