Exp 8 Preparation of Diphenylacetylene Lab Report PDF

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Preparation of Diphenylacetylene Introduction The purpose of this week’s lab is to form a carbon-carbon triple bond using two separate reactions. The first reaction involves the halogenation of an alkene, and the second reaction involves dehydrohalogenation of an alkyl halide. An alkene halogenation typically involves the use of bromine as a reagent. However, bromine is highly toxic and corrosive, so this particular lab will substitute bromine with pyridinium hydrobromide perbromide (in glacial acetic acid), which is much safer. The second step will use potassium hydrobromide (in ethylene glycol) and heat to double dehydrohalogenate the intermediate (stilbene dibromide) formed in step one. This will create the final product, diphenylacetylene. Reflux via a condense will be used to react the stilbene dibromide intermediate crystals with KOH in ethylene glycol. Recrystallization and suction filtration will be used to isolate and dry both the intermediate and final product crystals. Melting point determination will be used to confirm identities of each product. The formation of carbon-carbon triple bonds is important in the production of plastics. It can also be important in creating the fuel for an acetylene torch, which cuts and welds steel. Diphenylacetylene, in particular, is used as a building block in organic synthesis. Balanced Equation

Table of Reagents Compound Trans-Stilbene

Br2

KOH





Structure

MW 180.24

MP/BP MP: 124 C

Density

Glacial Acetic Acid

60.05

BP: 117.9 C

1.05

Pyridinium Hydrobromide Perbromide

480.035

MP: 130 C

Sodium Bisulfite

104.06

MP: 150 C

Methanol

32.042

BP: 64.7 C

.79

Stilbene Dibromide

340.058

MP: 232 C

Potassium Hydroxide

56.11

MP: 360 C

Ethylene Glycol

62.07

BP: 197 C

Ethanol

46.07

BP: 78.37 C

Diphenylacetylene

178.24

MP: 62.5 C

Compound Trans-Stilbene

Glacial Acetic Acid

Pyridinium Hydrobromide Perbromide Sodium Bisulfite

Methanol Stilbene Dibromide Potassium Hydroxide

Ethylene Glycol

1.11

Safety Information Causes eye irritation. Harmful if swallowed. May cause skin and respiratory tract irritation. Toxic to aquatic environments. Target organ: eyes. Causes irritation/corrosion to skin and eyes. Avoid ingestion and inhalation. Organ toxicity to skin, eyes, and respiratory tract. Causes skin irritation. Very hazardous in case of ingestion or inhalation Causes irritation to skin and eyes. Inhalation of dust will produce irritation to gastrointestinal or respiratory tract and possibly damage the lungs Causes irritation to skin and eyes. Severe over-exposure can lead to death. Causes irritation to skin and eyes. May cause irritation of the digestive tract and respiratory tract with possible burns. Causes irritation/corrosion to skin and eyes. Inhalation of dust will produce irritation to gastrointestinal or respiratory tract. Severe over-exposure can produce lung damage, choking, unconsciousness or death Causes irritation to skin and eyes. Mutagenic for mammalian somatic cells. The substance may be toxic to kidneys, liver, or central nervous system

Ethanol

Diphenylacetylene

Causes irritation to skin and eyes. The substance is toxic to blood, there productive system, liver, upper respiratory tract, skin, and central nervous system May cause irritation to skin and eyes. May cause gastrointestinal-irritation with nausea, vomiting and diarrhea. May cause respiratory tract irritation if inhaled.

Step One: 1. Use a powder funnel to add 2.0 g of trans-stilbene and 40 mL of glacial acetic acid to a 125 mL Erlenmeyer flask. 2. Put the flask containing the two compounds upon a hotplate and heat gently in order to dissolve the trans-stilbene into the solvent. 3. After the trans-stilbene has been dissolved, add 4.0 g of pyridinium hydrobromide perbromide slowly into the flask. 4. Swirl the contents of the flask to mix them. If any crystals stick to the flask above the solvent line, swirl to rinse them back into the mixture. If this fails, use a miniscule amount of acetic acid to rinse them into the solution. 5. Gently heat the flask for an additional 5 minutes. You should notice dibromostilbene crystals forming. 6. After the heating is completed, cool the flask in an ice bath to further crystal formation. 7. Collect the crystals through suction filtration. 8. Wash the crystals using 10-12 mL of methanol. 9. Dry the crystals again through suction filtration for 5 minutes. Further dry the crystals viaair drying on a watchglass for 10 more minutes. 10. Weigh the dibromostilbene crystals. 11. Dilute the filtrate with water and add an appropriate amount of sodium bisulfate to the solution such that the orange color is removed. 12. Discard filtrate with TA approval.

Step Two: 1. Weigh 1.5 g of potassium hydroxide into a 100 mL round bottom flask. 2. Add 20 mL of ethylene glycol into the flask. 3. Place the flask into a heating mantle. 4. Gently warm the mixture and swirl until the potassium hydroxide has dissolved. 5. Once the potassium hydroxide has dissolved, add the dibromostilbene crystals. 6. Add several boiling chips to the flask and attach a reflux condenser to the apparatus. 7. Reflux the solution for 20 minutes. 8. Once the reflux is complete, remove the condenser and pour the solution into a 250 mL Erlenmeyer flask. 9. Allow the flask to cool to room temperature. The impure product should precipitate. 10. After the solution has settled for 10 minutes, cool the solution in an ice bath for another 5minutes. 11. Collect the product via suction filtration.

12. Using 5-10 mL of cold water to rinse the flask and wash the crystals. Discard the filtrate in the lab sink. 13. Place the crystals into a 100 mL Erlenmeyer flask and use 10-12 mL of warm ethanol. 14. Keep the solution warmed on a hotplate while being mindful to dissolve the product rather than melting it. 15. After the crystals have dissolved entirely, add 0.5-1.0 mL of water to the beaker dropwise. You should see the solution become cloudy. 16. Cool the solution slowly to room temperature. You should see the formation of pure diphenylacetylene crystals. 17. Collect the crystals via suction filtration. Further air-dry the product. 18. Record the weight and melting point of the crystalline product

Experimental Data (Step One) 1. 2. 3. 4. 5. 6.

Starting Weight of trans-stilbene: 2.017 g Starting Weight of pyridinium hydrobromide perbromide: 4.001 g Starting Volume of glacial acetic acid: 40 mL Volume of methanol used to wash the crude product: 13.0 mL Weight of sodium bisulfite used to neutralize the filtrate: ~1.5 g Weight of Recovered Product: 3.341 g

Experimental Data (Step Two) 1. 2. 3. 4. 5. 6. 7.

Starting weight of stilbene dibromide: 3.341 g Starting weight of potassium hydroxide: 1.501 g Starting volume of ethylene glycol: 20 mL Volume of water (ice cold) to wash the crude product: 10 mL Volume of ethanol used to recrystallize the crude product: ~12 mL Weight of purified product (diphenylacetylene): .835 g Melting Point of purified product (range): 60-61 C

Experimental Observations      

Solution took 15-20 minutes at 90 C to completely dissolve trans-stilbene After adding pyridinium hydrobromide perbromide, solution turned milky orange Dibrominated product is a white, fine crystalline solid Impure product is white/yellow in color Recrystallization solution is a pale yellow Final product is white, crystalline solid

Conclusion/Results The diphenylacetylene product collected in this lab produced a relatively low percent yield. The actual yield of the intermediate product was relatively high, which could possibly suggest incomplete drying and solvent left in the product. This also suggests that the loss of the final product primarily occurred in step two of the experiment. Actual yield may have also been low due to general loss of crystals during both suction filtrations, as well as large quantities of crystals being left in glassware and funnels. Percent yield could be easily increased by allotting more time to the experiment. This would allow proper and complete rinsing of all glassware, as well as proper and complete filtration without loss of any product. The melting point range of the

final product was determined to be 60-61 degrees Celsius. The known literature value for the melting point range of diphenylacetylene is 59-61 degrees Celsius. While the experimental melting point range was extremely close to that of the literature value, impurities from dirty glassware, incomplete reflux, and water/ethylene glycol solvent still left in the product could have caused melting point depression. Once again, these are minor issues that could be improved with more time and careful methods. This lab did not introduce any new techniques, but it provided a refresher on reflux, suction filtration, and recrystallization. It also introduced the first multi-step reaction and provided useful information regarding the production of alkynes.

Preparation of Diphenylacetylene Post-Lab Questions: 1. (12pts) Draw a stereospecific skeletal structure for the product(s) formed from the reaction of the following compounds with pyridinium hydrobromide perbromide. Are these compounds chiral? Are they optically active? Explain why or why not:

Part I: This reaction forms this product which is a meso-compound and mesocompounds are optically inactive however while still having chiral carbons the compound itself is not chiral

Part II: Product 1 and Product 2 are enantiomers of one another however this one does have chiral carbons and a chiral center making this optically active

2. (6pts) Provide IUPAC names for each of the dibrominated compounds from question 1. Remember to include the appropriate stereochemistry indicators when necessary: Part I: (1R,2S)-1,2-dibromo-1,2-dicyclopentylethane Part II: (5R,6R)-5,6-dibromo-3,8-diethyldecane 2. (6pts) Provide skeletal structures for the product(s) of the following reactions:

1.

2.

3....