Neighboring Carboxyl Group Participation in the Hydrolysis of Acetals. Hydrolysis of o-Carboxybenzaldehyde cis- and trans-1,2-Cyclohexanediyl Acetals

Abstract

The plot of log kobsd vs pH for the hydrolysis of o-carboxybenzaldehyde trans-1,2-cyclohexanediyl acetal at 50 °C in H2O has four unit changes of slope in the pH range 2−9. The plot is here described by proceeding from low pH to high pH. The observed hydronium ion- and water-catalyzed reactions at pH < 6 have rate constants that are similar, but not identical, to those for hydrolysis of the acylal 3-[(trans-2-hydroxycyclohexyl)oxy]phthalide, which was isolated from the reaction at pH 3, and synthesized independently. The pH−log rate constant profile for hydrolysis of the acetal bends downward near pH 6 to give a slope of −1.0. Oxocarbonium ion hydrolysis is then a water reaction. At pH 7 the mechanism of the reaction changes to attack of OH- on the oxocarbonium ion intermediate. A change in rate-determining step takes place at pH 8 to hydronium ion-catalyzed ring opening of the anionic species of the acetal, or the kinetically equivalent intramolecular general acid catalysis in ring opening of the neutral...