Lab Exercise #5 Synthesis of Dibenzalacetone

University of the Southern CaribbeanOrganic Chemistry II CHEM 242 Lab Exercise # 5 Synthesis of dibenzalacetone (Macroscale) BACKGROUND: The base-catalyzed reaction of an aldehyde with a ketone is a mixed aldol condensation, known as the Claisen–Schmidt reaction. Dibenzalacetone is readily prepared by the condensation of acetone with two equivalents of benzaldehyde. The aldehyde carbonyl is more reactive than that of the ketone and, therefore, reacts rapidly with the anion of the ketone to give a ß hydroxyketone, which easily undergoes base-catalyzed dehydration. Depending on the relative quantities of the reactants, the reaction can give either mono- or dibenzalacetone. Dibenzalacetone is innocuous; its spectral properties (ultraviolet [UV] absorbance) indicate why it is used in sunscreens and sunblock preparations. In the present experiment, sufficient ethanol is present as a solvent to readily dissolve the starting material, benzaldehyde, and also the intermediate, benzalacetone. The benzalacetone, once formed, can then easily react with another mole of benzaldehyde to give the product, dibenzalacetone. IN THIS EXPERIMENT, an ethanolic solution of acetone and benzaldehyde is added to aqueous sodium hydroxide. The product, dibenzalacetone, crystallizes after a few minutes. The product is filtered from the mixture, washed, pressed dry, and recrystallized from an ethanol-water mixture. This very important reaction is easily carried out. BEFORE THE EXPERIMENT: Calculate the volume required to produce 0.0 1 25 mol of acetone. Calculate the volume of 2.2 “equivalents” of benzaldehyde. (In other words, 2.2 times as many moles of benzaldehyde as of acetone.) Note: the equation involves a simple 2:1 stoichiometry. Note: By using an actual 2.2:1 ratio, it ensures that the benzaldehyde is surplus and that the acetone is limiting. This is helpful for several reasons: Aldehyde oxidation. Aldehydes are often impure, because oxidation to carboxylic acid is fairly facile. By using 2.2 equivalents of benzaldehyde, then even if 10% of the benzaldehyde is corrupt, we ensure that we still have enough to f ully react with the acetone. Reaction Time. By having an excess of benzaldehyde, it makes it easier for the reaction to go to completion. Otherwise late in the reaction there isn’t much benzaldehyde left to react, so the reaction slows down a lot. By intentionally putting in some extra, it maintains at least a minimal concentration of electrophilic benzaldehyde till the very end, such that getting 100% conversion of isn’t so hard and doesn’t take so long. Ease of Product Purification: Disubstitution versus monosubstitution. Enabling complete conversion greatly simplifies purification. If complete conversion does not occur, either because benzaldehyde runs out or because insufficient time is used, the desired “disubstitution” product “dibenzalacetone”, in which two benzaldehydes have been incorporated, is contaminated by “benzalacetone”, the “monosubstitution” product in which only one benzaldehyde has been incorporated. Since the mono- and disubstituted products aren’t that different, it’s not that easy to remove the undesired side-product from the main desired product. But if you just make sure the reaction goes all the way to the desired product, then you don’t need to worry about it! PROCEDURE: Prepare a NaOH-Ethanol-Water solution mixture of 5 g of sodium hydroxide dissolved in 50 mL of water and 40 mL of ethanol at room temperature (25ºC) in a 125-mL Erlenmeyer flas k with a magnetic stirring bar and adjust the stirring dial to get a nice, even stirring action. To this add the calculated amount of benzaldehyde by syringe Add the calculated amount of acetone by syringe, last. (The acetone should go in last, after the benzaldehyde electrophile is already available. If the acetone goes in first, it could do aldol condensation on itself, in which enolate anions just attack neutral acetone carbonyls. Ketone carbonyls aren’t competitive with aldehyde carbonyls as electrophiles, but if there are no aldehydes available, ketones are better than nothing!) Stir the mixture for 30 minutes; then add 20 mL water and collect the product by suction filtration on a Büchner funnel. Break the suction and carefully pour 100 mL of water on the product. Reapply the vacuum. Repeat this process three times in order to remove all traces of sodium hydroxide. Finally, press the product as dry as possible on the filter using a cork, and then press it between sheets of filter paper to remove as much water as possible. Save a small sample for a melting-point determination; then recrystallize the product from ethanol , using a 125-mL Erlenmeyer inside your hot-water bath. A reasonable starting guess is to add ~4mL ethanol/gram product, and heat it up. Improvise if needed once hot, depending on what you see. (You can add more hot ethanol to increase solubility, or hot water to reduce solubility.) Note: The product has a low melting point, so it’s easy to think you’ve dissolved it when actually you’ve only melted it. Note: the water bath provides even heating and avoids overheating on the hot-plate surface. After cooling, rinse the crystals with an appropriate rinse solvent. (What might that be?) Dry thoroughly. Cleaning Up. Dilute the filtrate from the reaction mixture with water and neutralize it with dilute hydrochloric acid before flushing down the drain. The ethanol filtrate from the recrystallization should be placed in the organic solvents waste container. Note: If sodium hydroxide gets on the skin, wash until the skin no longer has a soapy feeling. Pre-Lab Questions Prepare a detailed mechanism for the synthesis of dibenzalacetone showing all intermediates and arrows to demonstrate electron movement . 10 mks Which side products do you expect in this reaction? How are they removed? 2 mks Post-Lab Question What evidence do you have that your product consists of a single geometric isomer or is a mixture of isomers? Does the melting point give such information? 4 mks