aldol

Organic Chemistry II | Lecture | Laboratory

Organic Chemistry Laboratory II
Dibenzalacetone by the Aldol Condensation
Experimental Procedure

Introduction
In this experiment, students will work in pairs to prepare dibenzalacetone from benzaldehyde and acetone via a mixed aldol condensation. The reaction scheme for this synthesis is given in Figure 1 below.

Figure 1: Reaction Scheme for Preparation of Dibenzalacetone

The first step of the reaction involves generation of the enolate of acetone, using sodium hydroxide as base. The enolate then reacts with the carbonyl carbon of the benzaldehyde in a nucleophilic acyl addition. A second enolate of acetone is generated which then reacts with another molecule of benzaldehyde. The resulting dibenzalacetone contains a ketone and two alkene functional groups, that are conjugated.

The dibenzalacetone product will be characterized by melting point and TLC analysis, and the percent yield will be determinied. The overall purpose of this experiment is to illustrate the mixed aldol condensation as a way to prepare a,b-unsaturated carbonyl compounds.

Procedure

Figure 1: Reaction Set up

Clamp a 250ml Erlenmeyer flask to a ring stand and set the flask on a hot plate/stirrer. Add a magnetic stir bar to the flask. Weigh out 5g of NaOH (wear gloves!) and add it to the flask, along with 40 ml of ethanol and 50 ml of DI water. Stir the mixture until the sodium hydroxide is completely dissolved. Add 5ml of benzaldehyde. After the NaOH in the first flask is completely dissolved, add 1ml of acetone to the flask. Continue stirring for 5-10 minutes, then add another 1ml of acetone. A solid will form in the flask and it may become difficult to stir. Allow the reaction mixture to stir (or swirl the flask periodically if necessary) for 20 minutes.

After 20 minutes, clamp a 125ml vacuum flask to a ring stand. Insert an adapter and Buchner funnel, fitted with a piece of filter paper, into the top of the flask. Connect the side arm of the vacuum flask to the aspirator vacuum. Turn on the vacuum and wet the filter paper with ~10ml of ethanol. Pour the reaction mixture through the funnel to isolate the solid product (dibenzalacetone) in the Buchner funnel. Break the vacuum and discard the filtrate, leaving the solid in the funnel. Reassemble the vacuum filtration set-up and wash the solid three times with ~50ml of water. Leave the vacuum on for an addition 5-10 minutes to assist in drying out the product. Use a second piece of clean filter paper to press the solid down, squeezing out excess water.

Transfer the solid to a 125ml Erlenmeyer flask. Add 10ml of ethanol to the flask. Clamp the flask to a ring stand and set it in a water bath. Warm the mixture gently in the hot water bath until the solid product completely dissolves. Remove the flask from the water bath and cool it, undisturbed, in an ice bath until a precipitate forms. Set up a clean, dry 125ml vacuum flask, fitted with an adapter and Buchner funnel. Clamp the flask to a ring stand. Pour the ethanol/product mixture through the funnel to isolate the solid. Keep the vacuum on for an additional 5-10 minutes to dry the product. Transfer the product to a watch glass and allow it to dry for an additional 15 minutes.

Figure 2: Vacuum filtration

TLC Analysis
Before the product is completely dry, the TLC analysis can be run. In a small vial or beaker, dissolve ~30mg of the product in acetone. Prepare a TLC plate (silica gel) by marking a line along the bottom of the plate, with two perpendicular tick marks. Label the tick marks "1" (product) and "2" (benzaldehyde). Spot the tick mark #1 with the solution of the product and tick mark #2 with the solution of benzaldehyde using a capillary pipette. Develop the plate using a developing solvent that will provide an Rf of ~0.5. Start with 30:70 ethyl acetate:hexane and make appropriate adjustments in the developing solvent ratio, if necessary. View the plate under UV light and in the iodine chamber. Record your results.

Weight and Melting Point Determination of Dibenzalacetone
Inspect your solid product and if it appears to be dry, weigh it and calculate the percent yield. Calibrate the thermometer of the melting point apparatus with benzoic acid (lit mp = 122^oC). Record the melting point of the product.

IR Spectroscopic Analysis
Run an IR spectrum of benzaldehyde, acetone and the dry, purified product. (One spectrum of acetone and benzaldehyde can be run per bench, but each student pair must run a spectrum of their product). Compare the spectra of the experimental product to the spectrum of the known dibenzalacetone and to the starting materials, benzaldehyde and acetone . Record all the major peaks in the spectrum in your notebook.

Chemical Tests
Perform the following chemical tests on benzaldehyde, acetone and your product.

Jones Test	Set up three small (75 X 12mm) test tubes in a test tube rack in the hood. Label the test tubes #1-3. Add ~5 mg of the solid product or 0.25ml of a liquid reagents to each test tube. Add ~1ml of acetone to each tube. Use test tube #1 for the product, tubes # 2 for benzaldehyde and tune #3 for acetone. Add 2 drops of the Jones reagent to tubes #1-3. Observe each tube for an immediate (2-5 sec) color change and record results.
Brady's Test	Set up three small (75mm X 12mm) test tubes in a test tube rack in the hood. Be sure that the test tubes are clean and dry. Label the test tubes #1-3. In test tube 1, place 2 drops of benzaldehyde, in test tube 2, place two drops of acetone, in test tube 3 place ~10mg of the product. Add ~2 ml of ethanol to each tube and vortex to dissolve the compounds. Add ~2 ml of the Brady’s reagent to each test tube. Vortex each mixture and observe each for the formation of a orange-yellow precipitate or color change. If no preciptate forms, allow the tubes to sit for ~15 minutes and observe again. Record the results.
Iodoform Test	Set up a test tube rack containing three, small (75mm X 12mm) test tubes. Label the test tubes 1-3. In test tube #1, dissolve ~5mg of product ~3 ml of dioxane. In test tube #2, add 2-4 drops benzaldehyde dissolved in ~1 ml dioxane and in test tube #3, add 2-4 drops acetone dissolved in ~1 ml dioxane . Vortex the tubes for ~30 seconds. Add 1 ml of 3M NaOH to each tube and vortex again for 30-40 seconds. Add ~2ml of the iodoform reagent to each tube and heat the tubes (clamp them!) in a hot water bath for 2 minutes. If the dark color is dissipated after heating for 2 minutes, add additional iodoform reagent, 5-7 drops at a time, until the dark color remains after heating for 2 minutes. Add 3 ml of distilled water to each tube and allow them to stand for ~15 minutes. Observe each tube for the formation of a precipitate. Record results from each tube.

Waste Disposal
Dispose of NaOH solutions in the basic aqueous waste. Dispose of ethanol and acetone in the non-halogenated organic waste. Filter paper should be disposed of in the solid waste containers.