Concerted Acetyl Group Transfer between Substituted Phenolate Ion Nucleophiles: Variation of Transition-State Structure as a Function of Substituent

Article abstract of DOI:10.1021/ja00189a045

Second-order rate constants (k_ArO) have been measured for the concerted displacement of aryl oxide from aryl acetates in aqueous solution by substituted phenoxide ions.Values of k_ArO obey linear Bronsted correlations when either the leaving group or the attacking phenolate ion structures are varied.The Bronsted coefficients obey the equations β_nuc = 0.20pK_1g - 0.68 and β_1g = 0.15pK_nuc - 1.73 to a good degree of precision, and the variation indicates that the structure of the transition-state changes within the range of phenolate ions studied; this alsoprovides confirmation that a concerted mechanism operates.The equations for β_nuc and β_1g predict the equation (log k_ii = 0.17pK_a² - 2.41pK_a + C) for k_ii, the rate constant for the reaction of aryl oxide ion with acetates bearing identical aryl oxide leaving groups.The identity rate constants may be interpolated from the observed rate constants (k_ArO) and exhibit excellent fit to the above equation with the single disposable parameter, C, set at 6.5.This is the first report of curvature in a Bronsted plot of identity rate constants.Effective charge development and loss on leaving and attacking oxyanions is fully balanced in the transition state when entering and leaving nucleophiles have a pK_a of 7.1.Tetrahedral or acylium ion-like transition-state structures are predicted for hypothetical phenols with pK_a's of 11.7 and 2.0, respectively.