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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 34, Number 1
BKCSDE 34(1)
January 20, 2013 

Nucleophilic Substitution Reactions of Phenyl Y-Substituted-Phenyl Carbonates with Butane-2,3-dione Monoximate and 4-Chlorophenoxide: Origin of the α-Effect
Min-Young Kim, Se-Won Min, Ik-Hwan Um*
The α-Effect, Transition-state stabilization, Solvent effect, Concerted mechanism, Rate-determining step
Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Ysubstituted- phenyl carbonates 7a-g with butane-2,3-dione monoximate (Ox–) in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. The α-nucleophile Ox– is 53-95 times more reactive than the corresponding normalnucleophile 4-ClPhO– toward 7a-g, indicating that the α-effect is operative. The magnitude of the α-effect (e.g., the kOx−/k4-ClPhO− ratio) is independent of the electronic nature of the substituent Y. The cause of the α- effect for the reactions of 7a-g has been suggested to be ground-state (GS) effect rather than transition-state (TS) stabilization through a six-membered cyclic TS, in which Ox– behaves a general acid/base catalyst. This idea is further supported by the result that OH– exhibits negative deviation from the linear Brønsted-type plot composed of a series of aryloxides, while Ox– deviates positively from the linearity. Differential solvation of the GS of Ox– and 4-ClPhO– has been suggested to be responsible for the α-effect exerted by Ox–.
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