Kinetics of hydrolysis of 1-acetoxy-, 1-acetoxy-8-hydroxy-, and 1,8-diacetoxy-napthalenes; intramolecular participation by a hydroxy group

Abstract

Consecutive hydrolysis of the acetoxy groups in 1,8-diacetoxynaphthalene in aqueous alkaline solution can be studied kinetically because hydrolysis occurs more rapidly for the diacetate than for 1-acetoxy-8-hydroxynaphthalene which is mostly present in the ionised form. The dependence of the rate coefficient (k₂) for hydrolysis of 1-acetoxy-8-hydroxynaphthalene on the concentration of hydroxide ion is complex because of the equilibrium between the ionised and un-ionised forms and their different reactivities. The equilibrium between the ionised and un-ionised forms has been measured separately and the data are combined with the kinetic results to show that the linear dependence of k₂ on [OH^–] at high concentrations is due to reaction of the ionised form with OH^–. At low concentrations, the curved dependence of k₂ on [OH^–] can be explained by reaction of ionised 1-acetoxy-8-hydroxy-naphthalene with solvent. The value of the rate coefficient (k₄) for this reaction, in comparison with the value for the reaction of 1-acetoxynaphthalene with water, shows that the hydroxy group assists the attack of water by intramolecular general base catalysis. A kinetic solvent isotope effect k₄(H₂O)/k₄(D₂O) 2.2 ± 0.5 is observed.