Intermolecular general-acid catalysis in acetal hydrolysis

Abstract

The structural requirements for the hydrolysis of an acetal to be general-acid catalysed are discussed. It is concluded that the hydrolysis of mixed aryl alkyl acetals of benzaldehyde should show general-acid catalysis. This is confirmed by showing that the hydrolysis of benzaldehyde methyl phenyl acetal is general-acid catalysed in acetate, formate, chloroacetate, and phosphate buffers. The α-value of the Brønsted equation is calculated from the results of the first three of these buffers to be 0·60 at 20°. The solvent isotope effect of the acetic acid-catalysed reaction is k(HOAc)/k(DOAc)= 2·11 and of the hydronium ion-catalysed reaction k(H₃O⁺)/k(D₃O⁺)= 1·02 at 20°. The ρ-value for the acetic acid-catalysed hydrolyses of a series of benzaldehyde methyl aryl acetals is +0·98. A concerted A–S_E2 mechanism is preferred to one involving a slow proton transfer followed by a rapid breakdown of the resulting conjugate acid.