Kinetics of crossed aldol condensations in high-temperature water

We examined two crossed aldol condensations in pure liquid water at temperatures of 250, 300, and 350°C. We synthesized benzalacetone from benzaldehyde and acetone, and chalcone from benzaldehyde and acetophenone. We provide evidence that these reactions are acid and base catalyzed in high-temperature water. Benzalacetone was always the major reaction product in its experiments and chalcone was almost always the major product in its synthesis experiments. At 350°C and long reaction times (5 and 8 hours) the chalcone yields were surpassed by those of 3-phenylpropiophenone. The maximum molar yield from the benzalacetone synthesis was 24% at 250°C and 5 hours, and the maximum yield of chalcone was 21% at 250°C and 15 hours. At higher temperatures, the yield of the unsaturated ketone products was lower due to formation of degradation products such as benzoic acid, benzyl alcohol, benzylacetone, acetophenone, E-stilbene, propiophenone and diphenylethane. A reaction network with reversible formation of the unsaturated ketone, its degradation, and a path for benzaldehyde disproportion provided the basis for a quantitative reaction model. These results provide another illustration of the ability of HTW to facilitate acid- and base-catalyzed reactions without any added acid or base.