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Kinetics and mechanism of the reactions of S ‐4‐nitrophenyl 4‐methylthiobenzoate with secondary alicyclic amines and pyridines
Author(s) -
Castro Enrique A.,
Aguayo Raul,
Bessolo Jorge,
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.1055
Subject(s) - alicyclic compound , chemistry , amine gas treating , medicinal chemistry , reaction rate constant , kinetics , stereochemistry , organic chemistry , physics , quantum mechanics
Abstract The reactions of the title substrate with a series of six secondary alicyclic amines and a series of eight pyridines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, and an ionic strength of 0.2 mol · dm −3 . Under amine excess pseudo first‐order rate coefficients ( k obs ) are obtained. Plots of k obs against free amine concentration at constant pH are linear, with the slope ( k N ) independent of pH. The Brønsted‐type plots (log k N against p K a of the conjugate acids of the amines) are non‐linear, with the curvature center located at p K a (p K a 0 ) 9.7 and 9.4, for the reactions of secondary alicyclic amines and pyridines, respectively. The plots are consistent with a zwitterionic tetrahedral intermediate (T ± ) on the reaction path and a change in rate‐determining step. The greater p K a 0value for secondary alicyclic amines than pyridines is explained by the greater nucleofugality from T ± of secondary alicyclic amines compared to isobasic pyridines. These p K a 0values are lower than those found for the reactions of S ‐4‐nitrophenyl 4‐Y‐substituted thiolbenzoates (Y = H, Cl, NO 2 ) with the corresponding amine series. These results indicate that electron donation from the non‐leaving group in T ± favors leaving group expulsion from T ± relative to amine expulsion. Copyright © 2006 John Wiley & Sons, Ltd.