Mechanisms of Acylal Hydrolysis in Sulfuric Acid Solutions

Abstract

The acid-catalyzed hydrolysis (in aqueous H2SO4) of acylals ArOCH2OAc is reported, and compared with that of normal acetate esters. On the basis of r plot behavior and the effects of the aromatic substituents, an AAc-2 ester hydrolysis mechanism is shown to occur in dilute acids, with a changeover to a mechanism analogous to AA1-1 ester hydrolysis (intermediate cation ArOCH2+) in more concentrated acids, the position of the changeover being dependent on the substituent. Methylene diacetate is also investigated and shown to display the same behavior, with an A-1 mechanism involving an intermediate CH3COOCH2+ becoming important at about 75% H2SO4. The relative AA1-1 reactivities for acetates ROAc, R = CH3OCH2, tert-butyl, C6H5OCH2, C6H5CH2, 4-NO2C6H4OCH2, CH3COOCH2, isopropyl, are 6 × 103, 5 × 102, 1, 7 × 10−2, 4 × l0−2, 1.1 × 10−2, 1.3 × 10−4 respectively.