Bulletin of the Korean Chemical Society

Korean Chemical Society (대한화학회)
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The Effect of Polarizability on Rate and Reaction Mechanism: Reactions of S-Aryl Substituted Thiobenzoates with $HO-$ and Aryloxide Ions

  • Ik-Hwan Um (Department of Chemistry, Ewha Womans University) ;
  • Sang-Eun Chun (Department of Chemistry, Ewha Womans University) ;
  • Dong-Sook Kwon (Department of Chemistry, Ewha Womans University)
  • Published : 1991.10.20
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Abstract

Second-order rate constants have been determined spectrophotometrically for reactions of S-p-nitrophenyl substituted thiobenzoates with various phenoxide ions and S-aryl substituted thiobenzoates with $HO^-$ ion. Thiol esters have been found to be more reactive than the corresponding oxygen esters toward phenoxide ions. The high reactivity of thiol esters relative to oxygen esters becomes insignificant as the basicity of the nucleophile increases. Furthermore, the highly basic $HO^-$ ion is less reactive toward thiol esters than oxygen esters. The significant dependence of the reactivity of thiol esters on the basicity of nucleophiles has been attributed to the nature of the HSAB principle. The present kinetic study has also revealed that the reactivity of thiol esters compared to oxygen esters is not so pronounced as expected based on the enhanced nucleofugicity of thiol esters. However, the effects of substituents in the nucleophile and in the acyl moiety of the substrate on rate appear to be significant. These kinetic results have led to a conclusion that the present reactions proceed via a rate-determining formation of a tetrahedral intermediate followed by a fast breakdown of it. The magnitude of the ${\beta}$ values shows no tendency either to increase or to decrease with the intrinsic reactivity of the reagents. The constancy of ${\beta}$ values in the present system is suggestive that the RSP should have limited applicability.

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