Active Site for Alumina-catalyzed Aldol Condensation

Abstract

Ethyl methyl ketone flowing through a layer of neutral alumina undergoes self-condensation on the methyl group to 5-methyl-4-hepten-3-ope at room temperature. By using the same method, acetophenone forms dypnone, and diethyl ketone forms 4-methyl-5-ethyl-4-hepten-3one, but self-condensation of propiophenone does not proceed (Table 1). In the same manner, ethyl methyl ketone, acetophenone and diethyl ketone condense with benzaldehyde to form benzalmethy1 ethyl ketone, chalcone and 2-methyl-1-phenyl-1-penten-3-one, respectively. Propiophenone does riot condense with benzaldehyde (Table 2). As mentioned above, aluminacatalyzed aldol condensation occurs by attack of the carbonyl group on the less hindered α carbon atom. According to Abbott's and Harries's reports, ethyl methyl ketone undergoes aldol condensation on the ethyl group with acid catalysts. On the other hand, the same ketone undergoes aldol condensation on the methyl group with basic catalysts. From these results described above, it is concluded that the, active site of alumina for the aldol condensation is the base site.