Effect of Amine Nature on Rates and Mechanism: Pyridinolyses of 4-Nitrophenyl Benzoate

Abstract

Pseudo-first-order rate constants (kobs) have been measured spectrophotometrically for the reactions of 4-nitrophenyl benzoate with a series of pyridines in H2O containing 20 mol % DMSO. The plots of kobs vs pyridine concentration are linear up to ca. 0.1 M pyridines, indicating that the effect of self-association of pyridines with their conjugate acids are insignificant in this concentration range. The Brønsted-type plot has been obtained to be linear with a β nuc value 1.11, suggesting that the pyridinolyses proceed through a rate-determining breakdown of the zwiterionic addition intermediate. The pyridines studied have shown higher reactivity than isobasic primary and secondary amines toward the substrate. 1-Benzoyl-4-dimethylamino-pyridinium ion, a possible intermediate, has not been detected since the rate of its hydrolysis in the reaction condition is comparable with or even faster than its formation.