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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 12
BKCSDE 32(12)
December 20, 2011 
 
Title
 Kinetics and Mechanism of the Pyridinolysis of Ethylene Phosphorochloridate in Acetonitrile
Author
 Hasi Rani Barai, Hai Whang Lee *
Keywords
 Phosphoryl transfer reaction, Pyridinolysis, Ethylene phosphorochloridate, Biphasic concave upward free energy correlation, Isokinetic relationship
Abstract
 The nucleophilic substitution reactions of ethylene phosphorochloridate (2) with X-pyridines are investigated kinetically in acetonitrile at –20.0 oC. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ph. Unusual positive ρX (= +2.49) and negative βX (= –0.41) values are obtained with the weakly basic pyridines, and rationalized by the isokinetic relationship with isokinetic temperature at tISOKINETIC = 6.6 oC. The pyridinolysis rate of 2 with a cyclic five-membered ring is forty thousand times faster than its acyclic counterpart (3: diethyl chlorophosphate) because of great positive value of the entropy of activation of 2 (ΔS≠ = +49.2 eu) compared to negative value of 3 (ΔS≠ = –44.1 eu) over considerably unfavorable enthalpy of activation of 2 (ΔH≠ = 28.4 kcal mol–1) compared to 3 (ΔH≠ = 6.3 kcal mol–1). Great enthalpy and positive entropy of activation are ascribed to sterically congested transition state (TS) and solvent structure breaking in the TS. A concerted mechanism involving a change of nucleophilic attacking direction from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines is proposed.
Page
 4347 - 4351
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