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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 31, Number 7
BKCSDE 31(7)
July 20, 2010 

Pyridinolyses of 2,4-Dinitrophenyl Phenyl Carbonate and 2,4-Dinitrophenyl Benzoate: Effect of Nonleaving Group on Reactivity and Mechanism†
Ik Hwan Um*, Min Ji Son, Song I Kim, Kalsoom Akhtar
Brønsted-type plot, 2,4-Dinitrophenyl phenyl carbonate, Pyridinolysis, Rate-determining step, Stepwise mechanism
Second-order rate constants (kN) have been measured for reactions of 2,4-dinitrophenyl phenyl carbonate (2) with a series of pyridines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC, and compared with the kN values reported for the corresponding reactions of 2,4-dinitrophenyl benzoate (1) to investigate the effect of nonleaving group on reactivity and mechanism. The reactions of 2 result in larger kN values than those of 1. The Brønsted-type plot for the reactions of 2 exhibits a downward curvature (i.e., β2 = 0.84 and β1 = 0.16), which is typical for reactions reported to proceed through a stepwise mechanism with a change in rate-determining step. The pKa at the center of the Brønsted curvature, defined as pKao, has been found to be 8.5 and 9.5 for the reactions of 2 and 1, respectively. Dissection of kN into the microscopic rate constants (e.g., k1 and k2/k-1 ratio) has revealed that the reactions of 2 result in larger k1 values than those of 1, indicating that PhO behaves as a stronger electron-withdrawing group than Ph. However, the k2/k-1 ratio has been found to be independent of the electronic nature of Ph and PhO.
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