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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 11
BKCSDE 32(11)
November 20, 2011 

 
Title
Kinetics and Mechanism of the Pyridinolysis of Aryl Ethyl Chlorothiophosphates in Acetonitrile
Author
Keshab Kumar Adhikary, Hai Whang Lee
Keywords
Biphasic free energy correlation, Phosphoryl transfer reaction, Pyridinolysis, Aryl ethyl chlorothiophosphates
Abstract
The nucleophilic substitution reactions of Y-aryl ethyl chlorothiophosphates with X-pyridines are studied kinetically in acetonitrile at 55.0 ºC. The Hammett and Brönsted plots for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Me. The substituents of X = 4-CN and 4-Ac show great positive deviations from both the Hammett and Brönsted plots. The Hammett plots for substituent Y variations in the substrates exhibit biphasic concave upwards with a minimum point at Y = H. The obtained values of the cross-interaction constants (ρXY) are all null in spite of the biphasic free energy correlations for both substituent X and Y variations, since the ρX values with both the strongly and weakly basic pyridines are almost constant. A stepwise mechanism with a rate-limiting leaving group departure from the intermediate is proposed where the distance between X and Y does not vary from the intermediate to the second transition state. A frontside attack is proposed with the strongly basic pyridines based on the considerably great magnitudes of ρX and βX values and a backside attack is proposed with the weakly basic pyridines based on the relatively small magnitudes of ρX and βX. The positive deviations of the two strong π-acceptor parasubstituents, X = 4-Ac and 4-CN, from both the Hammett and Brönsted plots are rationalized by the great extents of bond formation and breaking.
Page
3947 - 3951
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