Aldol Condensation: Synthesis of Dibenzalacetone using theClaisen-Schmidt Reaction Abstract: In this experiment, dibenzalacetone was prepared via an aldol condensation reaction between acetone and benzaldehyde. Acetone and benzaldehyde were mixed with sodium hydroxide and ethanol, and mixed vigorously for 30 minutes. The crystals were rinsed with water then centrifuged. This was repeated a total of three times. The dibenzalacetone was then characterized using melting point analysis. The observed melting point was 93.5°C- 109. 2°C compared to a cited value of 110°C. The sample was not completely dry and could have contributed to the lower and broader observed melting point. The percent yield was 117 %. This percentage occurred because of the excess moisture in the sample. Fig 1. Equation of Acetone and Benzaldehyde to form Dibenzalacetone Introduction: Dibenzalacetone, or DBA, is often used as an ingredient in sunscreen lotions and sprays because it has spectral properties that make it capable of absorbing UV light. It is also known to possess anti-inflammatory properties and can therefore be used to treat pain, swelling and inflammation. 1 Dibenzalacetone can also be used as a ligand in organometallic chemistry, and its derivatives are often utilized as polymer crosslinkers.2 This experiment demonstrated the Claisen-Schmidt condensation of mixed aldols. Enolate ions were formed when the molecules with hydrogens alpha to carbonyl groups were reaction with a base (NaOH.) Fig. 2 Acetone reaction and Sodium Hydroxide Reaction When an enolate forms from an aldehyde, it normally reacts with unreacted aldehydes to form an aldol addition/condensation. Ketones are less reactive toward nucleophilic additions and therefore, the enolate formed from a ketone can be used to react with an aldehyde. This is known as the Claisen-Schmidt reaction. When the product formed still has a reactive alpha hydrogen and a hydroxide adjacent to an aromatic ring, the reaction will quickly undergo dehydration giving a condensation product. Fig. 3 Claisen-Schmidt Reaction Because of the symmetry of acetone, the reaction can now be repeated on the other side of the carbonyl which then form the final product, dibenzalacetone. Procedure: 0.2206g of benzaldehyde was transferred to a centrifuge tube. 1.6 ml of ethanol and 2.0 ml of 3M sodium hydroxide were measured then transferred to the centrifuge tube via clean, dry syringes. Using a micropipette syringe, 0.073 ml of acetone were measured then injected into the centrifuge tube. The tube was then capped, and shaken vigorously by hand for approximately 30 minutes. The solution was then centrifuged for one minute on level one. The excess liquid was removed with a pipette, and then rinsed with 3-4 ml of de-ionized water. The centrifuge tube was capped and shaken, then placed back in the centrifuge again for one minute on level one. This rinse procedure was completed twice. After the final rinse and removal of excess liquid, the crystals were removed from the centrifuge tube, placed on a filter in a sealed Hirsch filter, then vacuumed. After vacuuming, the crystals were removed from the Hirsch filter and placed between 6 paper filters to absorb the excess moisture. The crystals were then weighed. After weighing the crystals, the melting point was determined using a Vernier melt station and Vernier LabQuest 2. Results and Calculations: Mass of product: 0.1646 g Theoretical Yield: 0.1407 g Percent Yield: 116.99% Finding Limiting Reagent Acetone 0.073mL | 1 mol A | 1 mole prdct | 224g prdct | 58.08 g | 1 mol A | = 0.2815 g dibenzalacetone Benzaldehyde 0.226 g B | 1 mol B | 1 mole prdct | 224g prdct | 58.08 g B | 2 mol B | = 0.4358 g dibenzalacetone The limiting reagent for this reaction is acetone as it yields the least amount of dibenzalacetone. Aldol Condensation: Synthesis of Dibenzalacetone Theoretical Yield Acetone 0.073mL | 1 mol A | 1 mole prdct | 224g prdct | 58.08 g | 1 mol A | = 0.2815 g dibenzalacetone Percent Yield (mass of DBA crystals) (theoretical yield) = 0.3294 g DBA 0.2815 g DBA x 100 = = 117.02% yield Discussion and Conclusions: In this experiment, an aldol condensation was carried out under basic conditions. In these types of reactions, carbonyl compounds act as either the enolate ion or the electrophile, or sometimes both. The carbonyl compound which becomes the enolate ion must possess at least 1 α-carbon. In this experiment, the enolate ion originates from acetone because benzaldehyde lacks an α-carbon. Benzaldehyde, therefore, acts as the electrophile in this reaction. The enolate ion derived from acetone then reacts with benzaldehyde to form a βhydroxyketone. This β-hydroxyketone will then undergo dehydration to form an enome, a conjugated system of an alkene and a ketone. This product is also called an α,β-unsaturated ketone which is stabilized by conjugation with the phenyl substituent. This α,β-unsaturated ketone then undergoes a second aldol condensation to give the final product, dibenzalacetone. References 1 Smerbeck, R. V. & Pittz, E. P. (1986). U.S. Patent No. 4,587,260. Washington, DC: U.S. Patent and Trademark Office. 2 Heidt, P. C. & Elliott M. L. (2001). U.S. Patent No. 6,194,615. Washington, DC: U.S. Patent and Trademark Office. 3 http://1chemistry.blogspot.com/2011/11/objective-1.html 4 http://www2.volstate.edu/chem/2020/Labs/Aldol_Condensation.html
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