Solvent-Independent Transition-State Structure for Acyl-Transfer Reactions. A Novel Strategy for Construction of a Bronsted Correlation

E. Buncel, I. H. Um, S. Hoz

Research output: Contribution to journalArticlepeer-review

122 Scopus citations

Abstract

The effect of changing solvent (DMSO-water) composition on the rate of the acyl-transfer reaction, for substituted phenoxides reacting with p-nitrophenyl acetate and p-chlorophenoxide reacting with substituted phenyl acetates, has been studied. A correlation of log k with a0in preference to σ, for leaving group substituent changes, supports a reaction pathway involving a tetrahedral intermediate, contrary to recent proposals of a concerted mechanism. The results are further analyzed by means of two kinds of Bronsted type correlations: the traditional method (A), where the pKaof the nucleophile is changed via substituent changes, and a novel method (B), where the change in pKais brought about by changes in the reaction medium, in this case by changing the DMSO-water composition. Method B was found to be applicable only over the range of 40–90 mol % DMSO in water where the physical properties of the media do not show any abrupt changes. It is shown that application of method A, which leads to the conclusion (via βnucvalues) that the transition-state structure changes with changing solvent composition (in accord with the RSP and Hammond's postulate), is inconsistent with the conclusion of a constant transition-state structure based on application of method B. The latter is shown to be the more reliable by independent criteria. It is proposed that this novel strategy for construction of Bronsted type plots could be applied to other reactions and different solvent systems.

Original languageEnglish
Pages (from-to)971-975
Number of pages5
JournalJournal of the American Chemical Society
Volume111
Issue number3
DOIs
StatePublished - 1 Feb 1989

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