Experiment 10 Post Lab- A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds PDF

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A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds...

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Jacqueline VanEyk Section #: 031 TA: Mohammad Nazmus Sakib Experiment 10 Title: A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds Introduction The objective of this experiment was to use stilbene to prepare stilbene dibromide. Also, the green generation of molecular bromine through oxidation of hydrobromic acid with hydrogen peroxide. The reaction utilized in this experiment is a bromination rection. A bromination reaction is a hydrohalogenation addition reaction, which is the addition of H-X to an alkene where the X can be Cl, Br, or I (Cruz, 2021). In this reaction the HBr is used, and the Br is added to the double bond in the alkene which yields a vicinal dibromide. In this reaction the alkene is acting as a nucleophile because it is electron rich and therefore has a pair of electrons that can be given to the bromine. The bromine in this case is acting as the electrophile and is electron poor. The Br-Br bond is non-polar; however, it is polarizable when it is close to the alkene. So, the partially positive bromine atom is transmitted to the alkene first. This process then creates a ring with the partially negative bromine atom and the bromonium ion. For the reaction to be complete the product must be neutral, so, a backside attack from the bromide ion must occur and opens the ring forming a vicinal dibromide. There are two different bromination reagents that are considered toxic and hazardous. These reagents are carbon tetrachloride and methylene chloride. Both of these reagents are suspected to

be carcinogenic (Weldegirma, 2020). With the two reagents being carcinogenic they are hazardous to anyone involved in the bromination reaction by potentially causing cancer. Bromine is also very hazardous and can cause problems such as, severe burns when in contact with skin, and can be a strong irritant when inhaled. Bromine is also corrosive and is considered an environmental hazard. Being that the two reagents mentioned above are extremely toxic and hazardous when handled another reagent called pyridinium tribromide is used instead. This reagent is less toxic and provides for the “slow release” of bromine into the reaction medium making it safer. However, the fact that pure bromine is still involved, and a lot of waste is produced, the use of pyridinium tribromide as a reagent is not the safest nor the most economical reagent to use. In this experiment the reagents that were used are hydrobromic acid and hydrogen peroxide. These reagents must still be handled carefully but are significantly safer to use than the former. The use of these reagents is considered an application of green chemistry. This is because the use of hydrobromic acid and hydrogen peroxide eliminate the usage of toxic and hazardous reagents. The use of these reagents also improves the atom economy of the reaction by limiting the amount of waste produced. Atom economy is the calculated percent amount of starting materials that are still present in the final product. If the reaction were to produce a large amount of waste and a high chemical yield, then the reaction would not be considered green or safe (Weldegirma, 2020). Thus, atom economy is an important aspect of the reaction to consider.

Mechanism: Preparation of 1,2-dibromo-1,2-diphenylethane

Undesired side reaction of the preparation of 1,2-dibromo-1,2-diphenylethane

Experimental Section Part A: Qualitative Determination of Alkene Compounds Labled 4 test tubes 14. Added 1mL of 30% H2O 2 and 0.2mL of 48% HBr to test tube 4 and shook gently.

Added 3 drops of unknown 1, 1.0mL of ethyl acetate and 5 drops from test tube 4 to test tube 1 and shook.

Observed the color change caused by the mixture in test tube 4 when added to test tubes 1-3 and recorded observations.

Added 3 drops of unknown 2, 1.0mL of ethyl acetate and 5 drops from test tube 4 to test tube 2 and shook.

Added 3 drops of unknown 3, 1.0mL of ethyl acetate and 5 drops from test tube 4 to test tube 3 and shook.

Part B: Synthesis of trans-1,2-dibromo-1,2-diphenylethane Added 200mg of stilbene and 4mL of ethanol to a 10mL round bottom flask, along with a magnetic stir bar.

Found the weight, melting point, % yield, and atom economy of the recrystalized product.

Refluxed the soulution at 100oC and stired for 20min. Added 0.3mL of 48% HBr and 1.6mL of 30% H2O2 to the flask slowly.

Recrystalized the solid in a large test tube using a small amount of ethyl acetate and placed the test tube into a beaker of heated water.

Cooled the solution to room temperature. Added 3mL of sodium bicarbonate to neutralize the pH. Then cooled the solution in an ice bath.

Collected the solid using vacuum filtration and rinsed with DI water. Dryed the collected solid and found the melting point and % yield.

Table of Chemicals Used Chemical Formula IUPAC Name Molar Mass Melting Pt Boiling Pt Hazards

(E)-Stilbene

Ethyl acetate

C14H12

C4H8O2

Transstilbene

Ethyl ethanoate

180.25g/mol 122-126oC 305-307oC

88.1 g/mol -83oC 77oC

Acute oral toxicity, serious eye irritant

Organ toxicity, skin and eye burns, corrosive

Hydrogen Peroxide

Hydrobromic acid

Ethanol

Sodium bicarbonate

H2O2

HBr

C2H6O

NaHCO3

Hydrogen peroxide

Hydrogen bromide

Ethanol

Sodium hydrogen carbonate

34.01 g/mol -33oC 108oC

80.91 g/mol -11oC 122oC

46.07 g/mol -114oC 78oC

84.01 g/mol 50oC 851oC

Acute oral toxicity, flammable, skin irritant

Organ toxicity, severe skin, and eye burns

Oral toxicity, flammable, can cause burns, eye irritant

Not toxic, may cause eye irritation

Results Part A: Qualitative Determination of Alkene Compounds Test Tube 1; Unknown 1 Test Tube 2; Unknown 2 Test Tube Yellow Yellow Color The Unknown that contained the Alkene was: Unknown 3

Test Tube 3; Unknown 3 Clear

Part B: Synthesis of trans-1,2-dibromo-1,2-diphenylethane Started with 0.2g of stilbene Crude 0.114g 216⁰C 30.24% 3.86%

Product Mass (g) Melting Point Percent Yield Atom Economy Calculations:

Pure 0.027g 241⁰C 7.16% 0.915%

Amounts of reactants: 0.3mL 48% HBr, 1.6mL 30% H2O2, 0.2g stilbene Theoretical yield Yield (g) = (0.2g C14H12)

(

1 mol C14 H 12 180.25 g C 14 H 12

)(

)(

1 mol C14 H 12 Br2 340.05 g C14 H 12 Br2 1mol C 14 H 12 1mol C 14 H 12 Br 2

0.377g C14H12Br2 Percent Yield % yield crude = % yield pure =

0.114 g C 14 H 12 Br 2 0.377 g C 14 H 12 Br 2 0.027 g C 14 H 12 Br 2 0.377 g C 14 H 12 Br 2

x 100 = 30.24% x 100 = 7.16%

Atom Economy Amount of HBr used = 0.3mL HBr x Amount of H2O2 used = 1.6mL H2O2 x

1.49 g 1mL 1.44 g 1 mL

= 0.447g HBr = 2.304g H2O2

Total mass of reactants = 0.447g + 2.304g + 0.200g = 2.951g of reactants Atom economy =

Mass of desired products Total mass of reactants

Theoretical atom economy =

0.377 g 2.951 g

x 100

x 100 = 12.78%

)

=

Atom economy for crude = Atom economy for pure =

0.114 g 2.951 g 0.027 g 2.951 g

x 100 = 3.86% x 100 = 0.915%

Discussion In part A of this experiment 3 different unknowns were placed in 3 different test tubes along with ethyl acetate, and a mixture of Hydrobromic acid and Hydrogen peroxide from test tube 4. Test tubes 1-3 were observed once the test tube 4 mixture was added. In both test tubes 1 and 2 the color of the solution changed from clear to yellow when the test tube 4 mixture was added, however, the solution in test tube 3 remained unchanged. The results of part A of this experiment told us that unknown 3 was the unknown that contained the alkene because the hydrobromic acid and hydrogen peroxide mixture did not cause a color change to occur in test tube 3. In part B of this experiment trans-1,2-dibromo-1,2-diphenylethane was synthesized using a greener bromination of stilbene. The mass of the collected crude product was 0.114g, and the percent yield for the crude product was 30.24%. The mass for the collected pure product was 0.027g, and the percent yield for the pure product was 7.16%. The percent yields are relatively low compared to the theoretical. The percent yields could be low for a few different reasons. For example, the process of weighing out exactly 200mg of (E)-stilbene was difficult and the amount that was actually used in the reaction could have been less than the 200mg needed. Also, some of the solid produced after refluxing was complete could have been lost during vacuum filtration. This loss of product would directly impact the percent yield of both the crude and pure products. Once the crude product was collected and dried the melting point was found to be 216⁰ C. The theoretical melting point of trans-1,2-dibromo-1,2-diphenylethane is 241⁰ C. The melting point for the crude product is much lower than the theoretical value due to impurities in the solid. The crude product

was then recrystallized producing a purer product that was then collected through vacuum filtration. The melting point of the pure product was found to be 241⁰ C, therefore, when comparing it to the theoretical melting point it can be assumed that the recrystallized product collected was pure trans-1,2-dibromo-1,2-diphenylethane. The atom economy of this reaction was also calculated. Atom economy is the percent amount of starting materials that are still present in the final product. Atom economy is calculated by dividing the mass of desired products by the total mass of reactants used. In this case the theoretical mass of desired products was calculated to be 0.377g and the total mass of reactants was calculated to be 2.951g. Thus, giving a theoretical atom economy of 12.78%. The atom economy for the crude product was calculated to be 3.86% and 0.915% for the pure product. The experimental atom economies are not consistent with what should be happening. The atom economy should be high to show that the product contains a majority of the starting reactants. Also, a high atom economy means that there is only a small amount of waste being produced. The pure product atom economy was 0.915%, meaning that only 0.915% of the pure product was composed of starting reactants. The experimental atom economies were very low meaning that there was a large amount of waste produced. Conclusion The objective of this experiment was to use stilbene to prepare stilbene dibromide. Also, the green generation of molecular bromine through oxidation of hydrobromic acid with hydrogen peroxide. For part A of the experiment the theoretical background and the experimental results were connected. For, example the unknown that contained the alkene did react with the hydrobromic acid, hydrogen peroxide mixture, meaning that the reaction produced a colorless solution. Also, in part B of the experiment the reaction did produce trans-1,2-dibromo-1,2-

diphenylethane by a greener bromination of stilbene. However, the experimental reaction was not as green as it was supposed to be due to the amount of waste produced given by the low atom economy. Although the reaction was not as “green” as it should have been, the final pure product that was collected had a melting point of 241⁰ C which perfectly matches the theoretical melting point of the product. The experimental data revealed that the unknown that contained the alkene was unknown 3. The data also revealed that some product was most likely lost during vacuum filtration and this can be seen in the calculated percent yields. The techniques used in this lab are used in many different industries. For example, a reflux still may be used in the process of producing alcoholic beverages to ensure that higher boiling point components are returned to the still while lighter elements are routed to a secondary condenser. This is useful in producing highquality alcoholic beverages, while ensuring that less desirable components (such as fusel alcohols) are returned to the still (Citizendium, 2012). Also, the process of recrystallization can be used in any industry where the product collected needs to be purified. Overall, the experiment did accomplish what it set out to do. The unknown that contained the alkene was identified and the synthesis of stilbene dibromide was successful through a greener bromination of stilbene.

References

Weldegirma, Solomon. “Experiment 10: A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds.” Experimental Organic Chemistry Laboratory Manual: CHM 2210L and CHM 2211L, 9th ed., ProCopy, 2020, pp. 55–60. “Reflux (Distillation).” Citizendium, 21 Nov. 2012, en.citizendium.org/wiki/Reflux_(distillation)#:~:text=Reflux%20is%20widely%20used %20in,a%20long%20period%20of%20time.

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For one of my resources, I used my notes from my organic chemistry lecture. My professor is Dr. Cruz....