CHE 276 Lab 8 Report- Alkyne Synthesis PDF

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Summary

Organic chemistry lab report: Full marks (edited from feedback to correct any mistakes). Detailed report with pre-lab and post-lab, correct formatting, and calculations....

Description

Experiment 6: Alkyne Synthesis

Nov 2, 2020

TA: Pankaj, Yiran Reaction:

Compounds Compound

Molecular Weight (g/mol)

mmol

Trans-Stilbene

180.24

2.77

Pyridinium tribromide

319.82

3.12

Stilbene dibromide

340.05

2.77

1,2-dibromo1,2-diphenylethane

Boiling Point (°C)

(g)

Density (g/cm3) (mL)

0.5

0.971

1.0

1.73

323.8

0.941

1.61

340.05

323.8

0.5

1.61

Potassium hydroxide

56.10

1327.0

0.25

2.12

Triethylene glycol

150.17

285.0

CH2Cl2

84.93

-

39.6

1,2-diphenylacetylene

178.23

-

170.3

Ethanol

46.07

-

78.37

Ethyl acetate

88.1

-

77.1

Limiting reagent and theoretical yield calculation: The limiting reagent is Trans-Stilbene.

307.0

Amount

3.0

1.13

-

-

1.33

-

-

0.99

2.0 -

-

9.02

Experiment 6: Alkyne Synthesis

0.5g / 180.24g = 0.00277 mols = 2.77 mmol trans stilbene 0.00277 mols x 340.05 g/mol stilbene dibromide = 0.941g stilbene dibromide

Safety Chemical

Safety hazards

Trans-Stilbene

Causes eye irritation Harmful if swallowed May cause skin and respiratory tract irritation

Pyridinium tribromide

Causes severe burns Risk of serious damage to eyes

Stilbene dibromide

Causes eye irritation and possible burns

1,2-dibromo1,2-diphenylethane

Causes eye irritation and possible burns

Potassium hydroxide

Causes severe digestive tract burns and possible death Harmful if inhaled

Triethylene glycol

Skin exposure may cause irritation

CH2Cl2

Redness and irritation may occur with skin exposure

1,2-diphenylacetylene

May cause skin irritation

Ethanol

Flammable

Ethyl acetate

Toxic if inhaled or ingested Highly flammable

References: pubchem.ncbi.nlm.nih.gov

Procedure:

Nov 2, 2020

Experiment 6: Alkyne Synthesis

Nov 2, 2020

A. Bromination of trans-Stilbene Observations: 1. Record mass and dissolve 0.5g of trans-stilbene in 10 mL acetic acid in a 50ml flask that is warming in a 30-35*c sand bath. 2. Add 1.0 g of pyridinium tribromide, swirl to mix. Rinse any solids stuck on the side with acetic acid. 3. Heat for 5 minute, swirl occasionally. 4. Cool reaction with cool water (not ice) and collect crystals with vacuum filtration 5. Wash crystals with ice cold methanol (very small amount), and dry with vacuum. 6. Record mass of product. B. Synthesis of Diphenylacetylene 1. Combine 0.5 g of 1,2-dibromo-1,2-diphenylethane (record an accurate mass), 0.25 g (about 3 pellets) potassium hydroxide, and 3 mL triethylene glycol in a large test tube. Record exact mass. 2. Insert thermometer and wait till reaction reaches 160*c, and heat for 5 more minutes. 3. Use a TLC plate to test the reaction: a. Prepare a TLC plate with lanes for starting material and reaction mixture. b. Use a pipet to dissolve a small amount of mixture with CH2Cl2 in a small vial. c. Develop the plate in ethyl acetate in hexanes and visualize using UV light. 4. Continue heating material until the TLC plate shows the reaction to be complete. 5. Once complete, cool mixture to room temperature, then add 8mL of ice cold water while stirring with a glass rod. 6. Collect product with vacuum filtration, wash with more ice cold water, more vacuum for 10 minutes. 7. Record the mass. (keep some aside) 8. Dissolve crude product in 1-2 mL hot 95% ethanol, and allow to cool to room temperature once dissolved. 9. Cool in ice bath, then collect crystals through vacuum filtration. 10. Transfer to side arm test tube and continue vacuum until crystals are dry. C. Thin Layer Chromatography 1. The three lanes on the TLC plate should be: a. Trans-Stilbene b. 1,2-dibromo-1,2-diphenylethane c. Crude diphenylacetylene d. Recrystallized diphenylacetylene 2. Develop the TLC plate using 10% ethyl acetate in hexanes. 3. Visualize using UV light.

Data and Calculations:

Experiment 6: Alkyne Synthesis

Nov 2, 2020

compound

mass (g)

Rf value

melting point (ºc)

TLC spot

trans-stilbene

0.500

0.596

-

-

1,2-dibromo-1,2diphenylethane

0.141

0.396

-

single spot (clean)

Crude diphenylacetylene

0.037

0.557

-

single spot (clean

Recrystallized diphenylacetylene

0.028

0.534

65

-

Percent yield sample calculation (1,2-dibromo-1,2-diphenylethane): Percent yield = experimental yield / theoretical yield x100 = 0.141g / 0.941g x100 = 14.9% Discussion and Conclusion: The bromination step was somewhat successful. The theoretical yield for the product, 1,2-dibromo-1,2diphenylethane, was 0.941g. However, the experimental yield was only 0.141g. This may have been due to cooling the reaction incorrectly. Instead of using cold water, water that was only slightly cooler than room temperature was used, which may not have cooled the reaction down fast enough to form the crystals. The 1,2-dibromo-1,2-diphenylethane was pure, as the TLC shows one clean spot. The percent yield of the 1,2-dibromo-1,2-diphenylethane was 14.9%. The dehydrobromination step was also only somewhat successful. Since the first reaction did not yield enough product, the reactants for this second reaction were scaled down to match the amount of 1,2dibromo-1,2-diphenylethane used. The reaction itself was successful after testing the reaction twice using TLC, as the spot in the reaction lane was a single, clean spot. The recrystallization was successful, as most of the compound was recrystallized. 0.037g of crude diphenylacetylene produced 0.028g of recrystallized diphenylacetylene. The TLC showed that there was little difference between the Rf values of the crude and recrystallized diphenylacetylene, indicating that the crude diphenylacetylene was pure. The percent yield of the crude diphenylacetylene is 7.49%. The percent yield of the recrystallized diphenylacetylene is 75.6%

Questions: Why does the procedure say not to cool the bromination reaction mixture in ice? (What would happen that is not desired?) If the bromination reaction mixture is cooled in ice, the hot reaction will not cool down slowly

Experiment 6: Alkyne Synthesis

Nov 2, 2020

enough to form crystals. If the process is rushed, the crystals may trap impurities and result in smaller crystals. The use of pyridinium tribromide results in the formation of pyridinium bromide as a byproduct. Where does this end up in our experiment, and why does it not contaminate our product? The pyridinium tribromide is used as a source of bromine in the reaction. The product, pyridinium bromide, crystallizes out of the acetic acid that is added in the reaction. A potential problem with using a very high-boiling solvent like triethylene glycol is that it cannot be evaporated to remove it from the product. How was the triethylene glycol separated from our diphenylacetylene product? Triethylene glycol is soluble in water. Ice water was added to the reaction mixture, allowing the triethylene glycol to dissolve and be separated from the diphenylacetylene (the tan powder). The dehydrobromination step requires a very high temperature, 160˚, but one must be careful not to heat the mixture to 180˚. What might be the reason for this? The boiling point for diphenylacetylene is 170ºc. Heating the mixture past 180ºc can cause the product to boil and evaporate....