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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 24, Number 8
BKCSDE 24(8)
August 20, 2003 

Kinetics and Mechanism of the Pyridinolysis of Aryl Phenyl Isothiocyanophosphate in Acetonitrile
Keshab Kumar Adhikary, Hai Whang Lee, Ikchoon Lee
Pyridinolysis, Aryl phenyl isothiocyanophosphates, Cross-interaction constant
The kinetics and mechanism of the pyridinolysis (XC5H4N) of Y-aryl phenyl isothiocyanophosphates (1;(YC6H4O)(C6H5O)P(=O)NCS) are investigated in acetonitrile at 55.0 oC. The Hammett plots for substituent (Y) variations in the substrate (log k2 vs sY) exhibit a convex upward biphasic type with breaks at Y = H. For electron-donating Y groups the Hammett coefficients, rY, are positive and cross-interaction constant rXY is negative, while those for electron-withdrawing Y groups rY values are negative with a positive rXY. These results are interpreted to indicate mechanistic change at the breakpoint ( sY = 0) from a concerted to a stepwise mechanism with rate-limiting expulsion of the -NCS group from a trigonal bipyramidal pentacoordinated (TBP-5C) intermediate. Biphasic plots of log k2 vs sX or pKa(X) with steeper slopes for the more basic nucleophiles are obtained suggesting an equatorial nucleophilic attack in contrast to an apical attack for the less basic nucleophiles with smaller magnitude of rX or bX.
1135 - 1140
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