Lab Name: Dibenzalacetone by the Aldol Condensation - SYNTHESIS OF DIBENZALACETO

Question

Lab Name: Dibenzalacetone by the Aldol Condensation - SYNTHESIS OF DIBENZALACETONE
Procedure:
Into a 10 100-mm reaction tube, place 4 mL of 3 M sodium hydroxide solution. To this solution, add 3.2 mL of 95% ethanol and 0.424 g of benzaldehyde.2 Then add 0.15 g of acetone to the reaction mixture. Cap the tube immediately and shake the mixture vigorously. The benzaldehyde, initially insoluble, goes into solution, resulting in a clear, pale-yellow solution. After a minute or so, it suddenly becomes cloudy and a yellow precipitate of the product forms. Continue to shake the tube from time to time for the next 30 minutes. If the product fails to crystallize, open the tube and scratch the inside of the tube with a glass rod. Remove the liquid from the tube using a Pasteur pipette by squeezing the bulb of the pipette, lowering the pipette into the liquid and pressing the tip against the bottom of the tube, and then aspirating the liquid into the pipette, leaving the crystals in the tube (Fig. 37.1). Add 3 mL of water, cap, and shake the tube vigorously. Remove the wash liquid as before, and wash the crystals twice more with 3-mL portions of water. After the final washing, add 3 mL of water to the tube and collect the crystals on a Hirsch funnel using vacuum filtration. Use the filtrate to complete the transfer of the crystals. Squeeze the product between sheets of filter paper to dry it, and then recrystallize the crude dibenzalacetone from a 70:30 ethanol-water mixture. After cooling the tube for several minutes in ice, collect the product by removing the solvent with a pipette (Fig. 37.1) and washing once with about 0.5 mL of ice-cold 70% ethanol while the tube is in ice. Alternatively, collect the crystals on a Hirsch funnel (Fig. 37.2) or Wilfilter and wash once with ice-cold 70% ethanol. Dry the product under vacuum by attaching the tube to an aspirator (Fig. 37.3). Determine the weight of the dibenzalacetone and its melting point, and calculate the percent yield. In a typical experiment, the yield will be 0.10 g (mp 110.5°C–112°C).
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QUESTIONS:
1. What are all the possible errors that could have happened during the lab?
2. How does this lab relate to a organic chemistry II class lecture and the real world of chemistry?

Explanation / Answer

Preparation of dibenzalacetone

1. Possible error in the reaction could be, not all the product crystalls have formed from solution and this would lead to lower percent yield. Acetone may undergo mono aldol condensation with benzaldehyde. Acetone may also undergo a self aldol reaction. All these factors would lower the product yield.

2. In organic chemistry, aldol reaction plays a significant role in synhtesis of large organic structures starting with smaller substrates. The selecticity to a product may be increased by increasing the concentration of the other starting material and/or by using a more electrophilic substrate in combination with a more nucleophilic enolate. In real life also we can incorporate these factors to obatain product of interest using this strategy.

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