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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 6
BKCSDE 32(6)
June 20, 2011 

Kinetic Study on Aminolysis of 4-Pyridyl X-Substituted Benzoates: Effect of Substituent X on Reactivity and Reaction Mechanism
Jong Pal Lee*, Ae Ri Bae, Ik-Hwan Um*,
Aminolysis, 4-Pyridyl benzoate, Rate-determining step, Brønsted-type plot, Yukawa-Tsuno plot
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 H2O. The Brønsted-type plot for the reactions of 4-pyridyl benzoate 7c is linear with βnuc = 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 βnuc = 0.77. The fact that the more reactive 7c results in a smaller βnuc 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., ρX = 1.47 for substrates possessing an electron-donating group (EDG) and ρX = 0.91 for those possessing an electron-withdrawing group (EWG). In contrast, the corresponding Yukawa- Tsuno plot exhibits excellent linear correlation with ρX = 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.
1907 - 1911
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