General acid catalysed hydrolysis of benzaldehyde aryl methyl acetals

Abstract

The kinetics of hydrolysis of substituted-benzaldehyde methyl phenyl acetals, XC₆H₄·CH(OMe)(OPh)(X =m-NO₂, m-MeO, H, p-Me, or p-Meo), and of benzaldehyde methyl substituted-phenyl acetals, Ph·CH(OMe)-(OC₆H₄Y)(Y =m-NO₂, m-Br, m-F, m-MeO, p-Me, or p-MeO), catalysed by pivalic, acetic, 3-chloropropionic, formic, and chloroacetic acid, and by hydronium ions have been studied. The reactions of the benzaldehyde methyl substituted-phenyl acetals have non-linear Hammett plots which were thought to arise from a shift in the structure of the transition state from one bearing a negative charge on the phenolic oxygen to one bearing a positive charge as the substituent changes from being electron withdrawing to electron releasing. The α values for these reactions become larger the more strongly electron releasing the substituent. The α values for the reactions of the substituted-benzaldehyde methyl phenyl acetals increase as the electron withdrawing power of the substituent increases. The mechanisms of these reactions are discussed with reference to the relevant More O'Ferrall–Jencks diagram.