Hydrotalcites as base catalysts. Kinetics of Claisen-Schmidt condensation, intramolecular condensation of acetonylacetone and synthesis of chalcone Original Research Article

The activity of hydrotalcites has been investigated by means of several reactions able to evidence their basic properties and the mechanism involved, and also to show their interest in the synthesis of fine chemicals of great practical interest for pharmaceutical applications. With this aim, the Claisen-Schmidt condensation of benzaldehyde with excess acetone has been performed in solid-liquid conditions. The kinetic study suggests a Langmuir-Hinshelwood mechanism, in which the rate-determining step would be the condensation of benzaldehyde and acetone, in agreement with former results of Hammett correlations. The reaction rate goes through a maximum as a function of calcination temperature of the catalyst, or the water content of the solvent, suggesting that the active sites are basic hydroxyls. These results are obtained by the intramolecular condensation of acetonylacetone as test reaction giving selectively 3-methyl-2-cyclopentanone, and showing, indeed, the presence of mainly basic sites on the hydrotalcite. Taking these results into account, the synthesis of chalcone has been performed over a suitably activated hydrotalcite.