Abstract

"The title reactions are subjected to a kinetic analysis in 44 wt% ethanol-water, at 25.0°C, ionic strength 02 (KCI). With a large excess of amine over the substrate, pseudo-firstorder rate coefficients (kobs) are obtained, which are linearly dependent on the amine concentration. The nucleophilic rate constants (kN) are determined from plots of kobs vs. amine concentration. The Brönsted-type plot obtained with the kN values is linear, with slope ?= 0.63. The magnitude of this slope suggests that the mechanism is concerted, as opposed to a stepwise process with rate-determining breakdown of a zwitterionic tetrahedral intermediate (T?), in which the value of ? is usually 0.8- 1.0. The pyridinolysis of the same substrate in the same solvent is stepwise with the breakdown of T? as the rate-determining step. The change to a concerted mechanism for the title reactions is attributed to the superior nucleofugality of the alicyclic amines, compared to the isobasic pyridines, which destabilizes kinetically the ""intermediate"" T? in such a way that it does not exist, and the mechanism becomes enforced concerted. © 1998 John Wiley & Sons, Inc."