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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 31, Number 9
BKCSDE 31(9)
September 20, 2010 

Kinetic and Theoretical Studies on Pyridinolysis of 2,4-Dinitrophenyl X-Substituted Benzoates: Effect of Substituent X on Reactivity and Mechanism
Ik Hwan Um*, Eun Hee Kim, Li Ra Im, Masaaki Mishima
Pyridinolysis, Brønsted-type plot, Intermediate, Rate-determining step, Reaction mechani
Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 2,4-dinitrophenyl X-substituted benzoates (X = 4-MeO, H and 4-NO2) with a series of Z-substituted pyridines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The Brønsted-type plots exhibit downward curvature (e.g., β2 = 0.89 ~ 0.96 when pKa < 9.5 while β1 = 0.38 ~ 0.46 when pKa > 9.5), indicating that the reaction proceeds through a stepwise mechanism with a change in rate-determining step (RDS). The pKao, defined as the pKa at the center of Brønsted curvature, has been analyzed to be 9.5 regardless of the electronic nature of the substituent X in the benzoyl moiety. Dissection of kN into the microscopic rate constants k1 and k2/k-1 ratio has revealed that k1 is governed by the electronic nature of the substituent X but the k2/k-1 ratio is not. Theoretical calculations also support the argument that the electronic nature of the substituent X in the benzoyl moiety does not influence the k2/k-1 ratio.
2593 - 2597
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