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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 4
BKCSDE 32(4)
April 20, 2011 

Kinetics and Mechanism of the Pyridinolysis of Aryl Phenyl Chlorothiophosphates in Acetonitrile
Md. Ehtesham Ul Hoque, Shuchismita Dey, Chan Kyung Kim, *Hai Whang Lee *
Biphasic concave upward/downward free energy correlation, Phosphoryl transfer reaction, Pyridinolysis, Y-aryl phenyl chlorothiophosphates
Kinetic studies for the reactions of Y-aryl phenyl chlorothiophosphates with X-pyridines have been carried out in acetonitrile at 35.0 ºC. The Hammett and Brönsted plots for substituent X variations in the nucleophiles are biphasic concave upwards with a break point at X = 3-Ph, while the Hammett plots for substituent Y variations in the substrates are biphasic concave downwards (and partially upwards) with a break point at Y = H. The signs and magnitudes of the cross-interaction constant (ρ XY) are strongly dependent upon the nature of substituents, X and Y. The proposed mechanism is a stepwise process with a rate-limiting step change from bond breaking with the weaker electrophiles to bond formation with the stronger eletrophiles. The nonlinear free energy correlations of biphasic concave upward plots for substituent X variations in the nucleophiles are rationalized by a change in the attacking direction of the nucleophile from a backside with less basic pyridines to a frontside attack with more basic pyridines.
1138 - 1142
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