37156-55-1Relevant articles and documents
Hydrolysis of 1-(X-substituted-benzoyl)-4-aminopyridinium ions: Effect of substituent X on reactivity and reaction mechanism
Um, Ik-Hwan,Kim, Eun-Hee,Kang, Ji-Sun
experimental part, p. 8062 - 8067 (2012/01/04)
A kinetic study is reported for hydrolysis of 1-(X-substituted-benzoyl)-4- aminopyridinium ions 2a-i, which were generated in situ from the nucleophilic substitution reaction of 2,4-dinitrophenyl X-substituted-benzoates 1a-i with 4-aminopyridine in 80 mol% H2O/20 mol% DMSO at 25.0 ± 0.1 °C. The plots of pseudo-first-order rate constants kobsdvs. pyridine concentration are linear with a large positive intercept, indicating that the hydrolysis of 2a-i proceeds through pyridine-catalyzed and uncatalyzed pathways with the rate constant kcat and ko, respectively. The Hammett plots for kcat and ko consist of two intersecting straight lines, which might be taken as evidence for a change in the rate-determining step (RDS). However, it has been proposed that the nonlinear Hammett plots are not due to a change in the RDS but are caused by stabilization of 2a-i in the ground state through a resonance interaction between the π-electron-donor substituent X and the carbonyl functionality. This is because the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with ρX = 1.45 and r = 0.76 for kcat while ρX = 1.39 and r = 0.72 for ko. A possibility that the hydrolysis of 2a-i proceeds through a concerted mechanism has been ruled out on the basis of the large ρX values. Thus, the reaction has been concluded to proceed through a stepwise mechanism in which the leaving group departs after the RDS since OH- is more basic and a poorer nucleofuge than 4-aminopyridine. The Royal Society of Chemistry 2011.
