Synthesis of a,-unsaturated aldehydes and ketones

Reaction between two carbonyl compounds (aldehydes or ketones) in which at least one of these components has an -methyl or a-methylene group give aldols (/ -hydroxy carbonyl compounds) usually under very mild conditions, and these products lose water either spontaneously or at temperatures between 60° and 100° to give, / -unsaturated aldehydes or ketones. 

Suitable catalysts are alcoholic or aqueous alkali solutions, aluminum alkoxides, piperidine acetate, mineral acids, and ion-exchangers. 

A mixed aldol condensation between an aldehyde and a ketone can theoretically lead to four products. In practice, however, it proceeds in only one direction which is determined by the greater reactivity of the aldehyde than of the ketone as methylene component. For methyl ketones such as ethyl 

When acetone is used as methylene component care must be taken to have the acetone in excess, otherwise two molecules of aldehyde react with one of acetone.930 

Benzylideneacetone 931 Acetone (635 g, ca. 11 moles) is mixed with freshly distilled, acid-free benzaldehyde (420 g, 4 moles) and water (400 ml) in a round-bottomed flask (capacity 21) fitted with a stirrer. 10% Sodium hydroxide solution (100 ml) is then added, with stirring and water-cooling, at a rate such that the temperature in the flask remains between 25° and 31° this requires about 0.5-1 h. The mixture is stirred at room temperature for a further 2.25 h, then made acid to litmus by dilute hydrochloric acid. The heavier aqueous layer is separated and extracted with benzene (100 ml), and the benzene extract is united with the benzylideneacetone layer and shaken with water (100 ml). The organic layer therefrom is separated, the benzene is removed by distillation from a water-bath, and the residue is distilled in a vacuum, the water contaminant distilling almost completely in a forerun. The fraction boiling at 148-160°/25 mm is collected and on redistillation gives the product (375 to 450 g, 65-78%), b.p. 137-142°/16 mm. 

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