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Kinetics and mechanism of the reactions of polyallylamine with aryl acetates and aryl methyl carbonates
Author(s) -
Castro Enrique A.,
Echevarría Gerardo R.,
Opazo Alejandra,
Robert Paz S.,
Santos José G.
Publication year - 2006
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1007
Subject(s) - chemistry , potentiometric titration , ionic strength , aqueous solution , aryl , dissociation (chemistry) , titration , reaction rate constant , kinetics , medicinal chemistry , stereochemistry , inorganic chemistry , organic chemistry , alkyl , ion , physics , quantum mechanics
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 °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 (α) and p K 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 α > 0.7. Similar behavior was observed through the dependence of log k N on either pH or α, 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 Brønsted‐type plots (log k N vs p K app ) are linear with slopes (β 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 k N 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 © 2006 John Wiley & Sons, Ltd.