Bulletin of the Korean Chemical Society vol. 32, no. 6, pp. 1907 - 1911
SCI; SCIE; SCOPUS; KCI
A kinetic study is reported for nucleophilic substitution reactions of 4-pyridyl X-substituted benzoates 7a-e with a series of alicyclic secondary amines in H 2O. The Brønsted-type plot for the reactions of 4-pyridyl benzoate 7c is linear with β nuc = 0.71. The corresponding reactions of 2-pyridyl benzoate 6, which is less reactive than 7c, resulted in also a linear Brønsted-type plot with β nuc = 0.77. The fact that the more reactive 7c results in a smaller β nuc value appears to be in accord with the reactivity- selectivity principle. The aminolysis of 7c has been suggested to proceed through a stepwise mechanism in which breakdown of the intermediate is the rate-determining step (RDS). The Hammett plot for the reactions of 7a-e with piperidine consists of two intersecting straight lines, i.e., ρ X = 1.47 for substrates possessing an electron-donating group (EDG) and ρ X = 0.91 for those possessing an electron-withdrawing group (EWG). In contrast, the corresponding Yukawa- Tsuno plot exhibits excellent linear correlation with ρ X = 0.79 and r = 0.56. Thus, it has been concluded that the nonlinear Hammett plot is not due to a change in the RDS but is caused by stabilization of the ground state of the substrates possessing an EDG through resonance interaction between the EDG and the C=O bond of the substrates.