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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 1
BKCSDE 32(1)
January 20, 2011 

 
Title
Alkaline Hydrolysis of Y-Substituted Phenyl Phenyl Thionocarbonates: Effect of Changing Electrophilic Center from C=O to C=S on Reactivity and Mechanism
Author
Song I Kim, Hey Ran Park, Ik Hwan Um*
Keywords
Alkaline hydrolysis, Polarizability, Concerted mechanism, Stepwise mechanism, Hammett plot
Abstract
Second-order rate constants (kOH‒) have been measured spectrophotometrically for reactions of Y-substituted phenyl phenyl thionocarbonates (4a-i) with OH in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The kOH‒ values for the reactions of 4a-i have been compared with those reported previously for the corresponding reactions of Y-substituted phenyl phenyl carbonates (3a-i) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and mechanism. Thionocarbonates 4a-i are less reactive than the corresponding carbonates 3a-i although 4a-i are expected to be more reactive than 3a-i. The Brønsted-type plot for reactions of 4a-i is linear with βlg = ‒ 0.33, a typical βlg value for reactions reported to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step (RDS). Furthermore, the Hammett plot correlated with σo constants results in much better linearity than that correlated with σ constants, indicating that expulsion of the leaving group is not advanced in the RDS. Thus, alkaline hydrolysis of 4a-i has been concluded to proceed through a stepwise mechanism with formation of an intermediate being RDS, which is in contrast to the forced concerted mechanism reported for the corresponding reactions of 3a-i. Enhanced stability of the intermediate upon modification of the electrophilic center from C=O to C=S has been concluded to be responsible for the contrasting mechanisms.
Page
179 - 182
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