Mechanism of acyl transfer between 1-acetoxy-8-hydroxynaphthalene and phenolate ions; investigation of a leaving group with potential hydrogen bond stabilisation

Abstract

The kinetics of the acyl transfer between 1-acetoxy-8-hydroxynaphthalene and substituted phenol buffers have been studied and the rates are compared with those for the reaction of 1-acetoxynaphthalene. The variation of the first-order rate coefficient with buffer concentration at different buffer ratios is compatible with reaction between undissociated 1-acetoxy-8-hydroxynaphthalene and phenolate ion or with reaction of the anion of 1-acetoxy-8-hydroxynaphthalene with undissociated phenol. Analysis of the data according to the latter reaction gives a rate coefficient for reaction of the anion of 1-hydroxy-8-acetoxy-naphthalene with phenol which is larger than that for the reaction of 1-acetoxynaphthalene with phenolate ion and is, therefore, rejected. The rate coefficients calculated for the preferred reaction of undissociated 1-acetoxy-8-hydroxynaphthalene with substituted phenolate ions give a linear Brønsted plot of slope β 0.48. The rate coefficients for reaction with phenolate and 3,4-dichlorophenolate ions are respectively 5.5 and 13 times higher than the values for the reaction of 1-acetoxynaphthalene with the same ions. The enhanced rate may be the result of weak assistance of the departure of the leaving group by partial formation of an intramolecular hydrogen bond involving the 8-hydroxy group.