Synthesis of acetals using molecular sieves

Acetals can be prepared in high yield from the corresponding aldehydes or ketones and alcohols using molecular sieves to shift the equilibria by adsorption of the water formed. Examples include acetals of acetone, cyclohexanone, iso‐butyraldehyde, and benzaldehyde. Two batch techniques have been applied using the molecular sieve suspended in the reaction mixture at 0° or room temperature. The first method involves the use of dual‐function proton‐exchanged molecular sieves type Zeolon 500, AW 500, or mordenite. The outer surface of the sieve catalyzes the acetal formation whereas the inner surface selectively adsorbs the water formed. According to the second procedure molecular sieve and catalyst p‐toluene‐sulfonic acid are added to the reaction mixture. In this case proton exchange between catalyst and sieve occurs which appeared to be fast with the molecular sieves types KA and NaA, much less pronounced with the types CaA, Zeolon 500, and AW 500. All these sieves however can be used because of the high rate of acetal formation. With KA and CaA essentially quantitative conversions are obtained within a few minutes.