Effect of Solvation on β-Values of Formyl Acetyl, and Pivaloyl Transfer between Sulfur and Oxygen Nucleophiles

Article abstract of DOI:10.1021/ja00528a040

Second-order rate constants were measured in aqueous solution for the reaction of a series of oxy anions and thiol anions with the formate and pivalate esters of p-nitrophenol and p-nitrothiophenol.These data, along with literature data provide a picture of the effect of alkyl substitution on the rates of uncatalyzed acyl transfer between sulphur and oxygen nucleophiles.Evidence is provided in support of the proposal that β values for oxy anions are altered by solvation, whereas those for thiol anions are not.The change of rate-determining step from formation to breakdown of tetrahedral intermediate occurs for formate and pivalate esters in the same manner that this occurs for acetate esters.The greater kinetic reactivity of formate esters compared to acetate esters is not a function of the entering or leaving nucleophile but is an inherent difference in the esters reflecting the relative ease of formation of the tetrahedral intermediates.