Synthesis of Dibenzalacetone by Aldol Condensation The aldol condensation is a reaction between two aldehydes aldehydes or ketones, catalyzed by a base or acid, generating a molecule having both alcohol and aldehyde functional groups. The aldol product is either a β-hydroxyald β-hydroxyaldehyde ehyde or a β-hydroxyketone. This reaction reaction is an important important synthetic mechanism that produces large molecules through the formation of carbon-carbon bonds. The product results from the addition addition of one molecule molecule of an aldehyde aldehyde or ketone to a second molecule in such a way that the α-carbon of the first molecule becomes attached to the carbonyl carbon of the second molecule. Conjugation of the newly formed double bond with the carbonyl group stabilizes the unsaturated product and provides the thermodynamic thermodynamic drivin driving g force for the dehydration dehydration process. The overall two-step sequence of reactions involves aldol formation and dehydration. The steps involve an acid-base reaction between a strong base such as the hydroxide ion and a hydrogen located α to a carbonyl group of the aldehyde or ketone. The acidity acidity of the organic species is is large relative relative to most hydrogens that are bonded to carbon due to the resonance stabilizati stabilization on of the enolate that is formed. The enolate attacks the carbonyl group and the aldol undergoes an acid-base reaction with the remaining acidic α-hydrogen, followed by the loss of OH – as a leaving group to give an enol. The net loss of H+ and OH – represents the loss of water, therefore the term aldol condensation. In this experiment, you will run an aldol condensation between benzaldehyde and acetone. The reaction is shown below. The product precipitates out of solution and can be collected by filtration. Melting temperature analysis will be used to characterize the product. O
O O H
2
+
R
N a O H (a q ) R'
E tO H
+ R
2 H2O
R'
OBJECTIVES In this experiment, you will • • •
Perform an aldol condensation between benzaldehyde and acetone. Purify the product by recrystallization. Characterize the product by melting temperature analysis.
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MATERIALS Part I Synthesis of Dibenzalacetone
50 mL round-bottom flask stir bar three 250 mL beakers 10 mL graduated cylinder magnetic stirrer/hot plate ring stand with utility clamp Büchner funnel filtering flask filter paper disposable Pasteur pipets and bulb Temperature Probe or thermometer
acetone benzaldehyde 2.5 M sodium hydroxide solution ethanol, denatured ice distilled water balance weighing paper watch glass spatula
Part II Recrystallization
18×150 mm test tube two 250 mL beakers hot plate ring stand with utility clamp Büchner funnel filtering flask filter paper disposable Pasteur pipets and bulb spatula
glass stirring rod watch glass ethanol, denatured ice distilled water balance weighing paper compressed air
Part III Melting Temperature
LabQuest or computer interface LabQuest App or Logger Pro Vernier Melt Station glass capillary tubes, one end closed
tissues (preferably lint-free) sample from part II mortar and pestle
PROCEDURE Part I Synthesis of Dibenzalacetone
1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood. 2. Prepare an ice water bath in a 250 mL beaker. 3. Prepare the reaction mixture. a. Place the 50 mL round bottom flask containing a stir bar in a clean 250 mL beaker and tare the weight on a balance. b. Slowly add acetone dropwise to the flask until the mass is about 0.25 g. Record the actual mass to the nearest 0.01 g. c. Tare the weight again and add benzaldehyde dropwise until the mass is 1.0 g. Record the actual mass to the nearest 0.01 g. d. Clamp the round bottom flask to a ring stand and lower the flask onto the stir plate. e. Add 5 mL of ethanol to the flask. f. Add 1.5 mL of 2.5 M NaOH to the flask.
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Synthesis of Dibenzalacetone by Aldol Condensation
4. Stir the reaction for 15 minutes with the flask open to air. 5. The product from the reaction will be a yellow precipitate. If very little precipitate forms, warm the reaction mixture in a 50°C hot water bath for 10 minutes. 6. After the precipitate forms, place the flask in an ice water bath for 10 minutes. Clamp the flask to prevent it from tipping. 7. While waiting, set up a vacuum filtration using a Büchner funnel. Weigh and record the mass of the filter paper to the nearest 0.01 g. 8. Collect the precipitate. a. b. c. d.
Filter the precipitate by vacuum filtration. Wash with cold distilled water. Rinse the solid with 5–8 drops of ethanol. Keep the vacuum filtration on for an additional 10 minutes to help air dry the solid.
9. Weigh the dried solid on the filter paper and record the mass to 0.01 g. Part II Recrystallization
10. Prepare a 60–70°C hot-water bath in the 250 mL beaker. Monitor the temperature with a Temperature Probe or thermometer. 11. Transfer your crude product to a test tube and add about 5 mL of ethanol. Use the minimum amount of solvent needed to dissolve your solid. Crystals will not form if too much ethanol is used. 12. Stir the solution with the glass stirring rod for 5 minutes. If the solid does not dissolve, add another 0.5 mL of ethanol. Continue stirring until the solid has completely dissolved. 13. Once the solid has completely dissolved, cool the solution in an ice water bath to promote crystallization. 14. Collect the product. a. Set up a vacuum filtration using a Büchner funnel. Be sure to weigh and record the mass of the filter paper to the nearest 0.01 g before filtering the solid. b. Allow to completely air dry, or direct a gentle stream of air above the Büchner funnel for 10–15 minutes to completely dry the solid. 15. Weigh the dried recrystallized product on the filter paper and record the mass to 0.01 g. Part III Test the Melting Temperature of Dibenzalacetone.
16. Obtain a small amount of the solid organic compound. The solid should be in a powdered form. If it is not, use a mortar and pestle to carefully grind the solid to a powder.
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17. Prepare a sample for melting. a. Pack a capillary tube 3–4 mm (~1/8 inch) deep with your sample by inserting the open end into a small pile of the solid. A small amount of the solid will be pushed up into the tube. b. Tap the closed end of the capillary tube on the table top to compress the sample into the closed end. c. Check the control knob on the Melt Station to confirm that it is in the Off position. d. Carefully insert the capillary tube of solid into one of the sample holders of the Melt Station. 18. Connect the Melt Station power supply to a powered electrical outlet. 19. Connect the Melt Station sensor cable to LabQuest or to a computer interface. 20. Start the data-collection program, then choose New from the File menu. You are now set up to take melting temperature data for up to 20 minutes. 21. In the first trial, you will want to observe the melting process and make a rough estimate of the melting temperature of your sample. Don’t worry if the heating rate is a bit too rapid, and the sample melts too quickly. To do this: a. Start data collection. b. On the Melt Station, turn the control knob to a setting of 180ºC. The red light will turn on indicating active heating. c. Carefully observe your sample. If the solid begins to melt, click Mark to mark the temperature on your graph (or press the D key on the computer or the OK button on LabQuest). When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance. d. If the solid does not melt by the time the temperature gets to 150ºC, turn the control knob to the 220ºC setting. Continue observing your sample, and if the sample begins to melt, mark the temperatures on the graph as previously described. e. If the sample has not melted by the time the temperature gets to 190ºC, turn the knob to the Rapid Heat setting. When the sample finally begins to melt, mark the graph as previously indicated. f. When you have determined the approximate melting temperature range for the sample, stop data collection. Store the run by tapping the File Cabinet icon in LabQuest, or choosing Store Latest Run from the Experiment menu in Logger Pro. Discard the capillary tube and sample as directed by your instructor. g. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the next trial. The blue light will turn on indicating that the fan is cooling the Melt Station. 22. Now that you have a rough idea of the melting temperature, a more accurate determination of the melting temperature can be made. Use a previously prepared sample in a capillary tube, as described in Steps 16–17, to determine the melting temperature of the sample: a. Start data collection. b. On the Melt Station, turn the control knob to the Rapid Heat setting. c. Carefully observe the temperature vs. time graph. When the temperature is within approximately 10ºC of the lowest possible melting temperature of your sample, turn the control knob to a temperature setting corresponding to your expected melting temperature.
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Synthesis of Dibenzalacetone by Aldol Condensation
d. Carefully observe your sample. When the solid begins to melt, click Mark to mark the temperature on your graph. When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance. When you are finished with this step, stop data collection. e. Store the run. f. Discard the capillary tube and sample as directed by your instructor. g. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the next trial. 23. Repeat Step 22 until you have determined the melting temperature range of the solid. Record the range in the data table. 24. At the end of the experiment, turn the control knob on the Melt Station to Off.
DATA TABLE Part I Synthesis of Dibenzalacetone Mass of acetone (g) Mass of benzaldehyde (g) Mass of filter paper (g) Mass of filter paper and product (g) Mass of crude product (g) Part II Recrystallization
Mass of filter paper (g) Mass of filter paper and product (g) Mass of recrystallized product (g) Part III Melting Temperature of Dibenzalacetone
dibenzalacetone Measured melting temperature range (°C)
DATA ANALYSIS 1. What is the theoretical yield of dibenzalacetone in your synthesis? What is the actual yield?
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INSTRUCTOR INFORMATION 1. Acetone and ethanol are flammable. Keep away from open flames. 2. Prepare cold distilled water by placing distilled water wash bottles in an ice water bath at the start of the lab period. 3. Prepare the 2.5 M sodium hydroxide solution by adding 100 g of solid sodium hydroxide to enough distilled water for 1 L of solution. CAUTION: Heat is released. The reaction is exothermic. 4. If too much ethanol is added during the recrystallization step, the solvent can be evaporated by excess heating and directing a gentle stream of air at the test tube will facilitate the process. 5. Dry the product thoroughly before performing a melting temperature analysis. 6. The melting temperature range of the product, dibenzalacetone, is 110–112°C. 7. Dispose of hazardous waste appropriately.
HAZARD ALERTS Benzaldehyde: Moderately toxic by ingestion. May cause respiratory tract irritation. Skin and eye irritant. HMIS Classification: Health hazard−3, Flammability−2, Physical hazard−1. Acetone: Fire hazard (flash point −17.0°C). Store in dedicated flammables cabinet. Moderately toxic by inhalation or ingestion. Moderately toxic by ingestion. Vapor causes weakness, fatigue, nausea and headache. Skin and eye irritant. HMIS Classification: Health hazard−2, Flammability−3, Physical hazard−0. Ethanol: Fire risk (flash point 14.0°C). Flammable. Addition of denaturant makes the product poisonous. Store in dedicated flammables cabinet. Skin and eye irritant. Moderately toxic by ingestion and inhalation. HMIS Classification: Health hazard−2, Flammability−3, Physical hazard−1. Sodium Hydroxide: Corrosive solid; skin burns are possible; much heat evolves when added to water; very dangerous to eyes; wear gloves and eye protection when using this substance. HMIS Classification: Health hazard−3, Flammability−0, Physical hazard−2. The hazard information reference is: Sigma-Aldrich Co., 1-800-325-3010, www.sigmaaldrich.com/safety-center/msds-search.html
COMPOUND INFORMATION
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Compound
Chemical formula
Melting temperature (°C)
Molar mass (g/mol)
sodium hydroxide
NaOH
318
40.0
Organic Chemistry with Vernier
Synthesis of Dibenzalacetone by Aldol Condensation
Compound
Chemical formula
Boiling temperature range (°C)
Molar mass (g/mol)
Density (g/mL)
benzaldehyde
C7H6O
178–179
106.12
1.044 (20°C)
acetone
C3H6O
56
58.08
0.791 (25°C)
ethanol
CH4O
78.3
46.07
0.789 (25°C)
SAMPLE RESULT
Melting temperature graph of dibenzalacetone.
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