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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 13, Number 6
BKCSDE 13(6)
June 20, 1992 

The Effect of Solvent on the α-Effect(3) : Nucleophilic Substitution Reactions of Aryl Acetates in MeCN-H2O Mixtures of Varying Compositions
Ik-Hwan Um*, Gee-Jung Hahn, Gwang-Ju Lee, Dong-Song Kwon
Second-order rate constants have been measured spectrophotometrically for the reactions of substituted phenyl acetates with butane-2,3-dione monoximate and p-chlorophenoxide anions in MeCN-H2O mixtures of varying compositions. The reaction rate, unexpectedly, decreased remarkably upon initial additions of MeCN to H2O up to 30-40 mole % MeCN, and followed by a gradual increase upon further additions of MeCN. The change in solvent composition also influenced the magnitude of the α-effect, i.e., the α-effect increased as the mole % MeCN increased. The solvent dependent α-effect for the present system appears to indicate that the differential solvation between the α-effect nucleophile and the corresponding normal nucleophile is not solely responsible but the difference in the transition-state stabilization is also responsible for the α -effect in organic solvent-rich region.
642 - 647
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