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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 30, Number 10
BKCSDE 30(10)
October 20, 2009 

 
Title
Kinetics and Mechanism of Nucleophilic Displacement Reactions of Y-Substituted Phenyl Benzoates with Z-Substituted Phenoxides
Author
Se Won Min, Jin A Seo, Ik Hwan Um*
Keywords
Acyl-group transfer, Aryl benzoates, Brønsted-type plot, Concerted mechanism, Yukawa-Tsuno plot
Abstract
Second-order rate constants have been measured for two series of nucleophilic displacement reactions, i.e., reactions of 4-nitrophenyl benzoate with Z-substituted phenoxides and those of Y-substituted phenyl benzoates (1a-h) with 4-chlorophenoxide (4-ClPhO-) in 80 mol% H2O/20 mol% DMSO at 25.0 ± 0.1℃. The Brønsted-type plot for reactions of 4-nitrophenyl benzoate with Z-substituted phenoxides exhibits an excellent linear correlation with βnuc = 0.72. Reactions of 1a-h with 4-chlorophenoxide result in also a linear Brønsted-type plot with βlg = -0.62, a typical βlg value for a concerted mechanism. The Hammett plots correlated with σo and σ- constants show many scattered points for reactions of 1a-h with 4-chlorophenoxide. In contrast, the corresponding Yukawa-Tsuno plot exhibits an excellent linear correlation with ρY = 2.26 and r = 0.53, indicating that expulsion of the leaving group occurs at the rate-determining step (RDS) either in a concerted mechanism or in a stepwise pathway. However, a stepwise mechanism with leaving group departure being the RDS is excluded since the leaving Y-substituted phenoxide is less basic and a better nucleofuge than the incoming 4-ClPhO-. Thus, the reactions have been concluded to proceed through a concerted mechanism, in which bond formation between the nucleophile and electrophilic center is more advanced than expulsion of the leaving group in the transition state on the basis of the magnitude of βnuc and βlg values.
Page
2403 - 2407
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