Aldol condensation: A simple teaching model for organic laboratory
To synthesize trans-dibenzalacetone from acetone and benzaldehyde using NaOH as the catalytic base in a 1:1 water/ethanol solvent
Prepare a 0.5M NaOH (20 mmol) solution in 40 mL of distilled water and prepare a 1M solution of benzaldehyde (40 mmol) in 40 mL of ethanol. Mix the two solutions throughly. Add 0.8 g (1.1 mL, 15 mmol) of acetone to the reaction mixture and stir for 30 minutes. Recover the product by suction filtration using a Buchner funnel.
Amount: 3.044 g
Appearance: Bright yellow crystals
Melting Point: 103-104°C (lit mp 113°C)
Purity: Very pure sample of trans-dibenzalacetone
This procedure produced pure trans-dibenzalacetone without having to recrystallize. The two characteristic doublets of trans-dibenzalacetone had coupling constants of 16.25Hz (located at 7.7ppm) and 15.5Hz (located at 7.1ppm). These coupling constants of near 15Hz indicates that the trans product was recovered.
The limiting reagent was the acetone (0.8 g, 15 mmol) and the precent yield of this reaction was 87%. An excess of benzaldehyde (1.33 equivalent) was used and the benzaldehyde was dissolved into the ethanol prior to being introduced into the reaction mixture.
3.044 g of pure trans-dibenzalacetone was produced in a yield of 87%
Mech-Dibenzalacetone is readily prepared by the crossed Aldol condensation between benzaldehyde and acetone under alkaline conditions. Acetone is a carbonyl compound that contains alpha hydrogens. Therefore, it can participate in the various condensation reactions that involve removal of an alpha hydrogen. Benzaldehyde, on the other hand, does NOT contain any alpha hydrogens. The following sequence of reactions indicates the pathway for the formation of dibenzalacetone.
The carbanion produced by this step reacts with the more active benzaldehyde rather than with a second molecule of acetone. Thus, the next step becomes:
Because a 2:1 molar ratio of benzaldehyde is used, the reaction continues.
The overall balanced equation is: