Lab Report 9

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A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds

Introduction— Bromination reactions of alkenes occur when bromine adds across a double bond, forming a molecule with vicinal (and anti-stereochemistry) bromines (Weldegirma, 51). Many of the reactions proceed through a bromonium ion (having a bridged cation) and then move through a backside attack meant to stabilize that positive charge. An example of bromination using the reagents Br 2 and CCl 4 will add bromine, each with anti-stereochemistry, onto each end of the starting alkene. Another example of bromination comes with the reagents Br 2 and H 2 O, since the molecule proceeds through bromination but then experiences hydration, adding an “OH” onto the most substituted end of the previous alkene. A final example of bromination is when Br 2 is used with hv, adding a singular Br in the allylic position to the alkene. Many common bromination reagents, such as carbon tetrachloride and dichloromethane, are extremely hazardous since they are assumed to be cancer-causing. Br2 alone can also cause skin, eye, respiratory tract, and digestive tract irritation and burns, making it a hazardous compound (FisherSci). But the reagent used in this experiment, pyridinium tribromide, allows for a safer and slower reaction progression by generating the bromide itself. Compounds such as bromide can be difficult and unsafe to handle for long periods of time in the lab, giving pyridinium tribromide an advantage as the reagent of choice for the bromination reaction. But pyridinium tribromide doesn’t come without hazards. It can still cause severe skin and eye burns and could cause chemical burns to the respiratory system if inhaled. Bromine is still required for the experiment, and its use isn’t canceled out but moved away from the bromination step of the reaction. Pyridinium tribromide must be handled carefully and cautiously, even if it is considered the safer reagent for bromination.

Green chemistry is defined as the use and design of chemicals and experimental procedures to reduce the generation and pollution of dangerous chemicals. There are 12 principles of green chemistry, which outline a process of making experimental devices “greener.” One of these principles focuses on atom economy, which aims to maximize the usefulness of products measured as a percent. A high atom economy is equivalent to incorporating almost all the starting material into the end product, reducing the amount of chemical waste produced. 44 Main Mechanism— Preparation of 1,2-dibromo-1,2,-diphenylethane The alkene is a nucleophile and attacks bromide, kicking the other bromine ion off as a leaving group. A bromonium ion is formed, the less substituted side is attacked by the bromine ion, and the bridged carbocation is opened. Both bromines end up with anti-stereochemistry, meaning they are on opposite sides of the plane.

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Side Reaction— Experimental Section— Gather four test tubes, label Add 1mL 30% hydrogen peroxide and 0.2mL hydrobromic acid to TUBE 4 Add three drops of unknown one, 1mL ethyl acetate and five drops from TUBE FOUR to TUBE ONE Part One - Definition of Alkene Compounds Add 3 drops of unknown 2, 1mL ethyl acetate, and 5 drops from TUBE FOUR to TUBE TWO Add 3 drops of unknown 3, 1mL ethyl acetate, five drops from TUBE FOUR into TUBE THREE Preparation of Tubes Two and Thre Observe color change of all tubes If clear, it contains an alkene Determine the unknown Set 100 degrees Celsius sand bath on top of a hot plate Observation and Sand Bath Preparation Add 200mg (E)- stilbene to 10mL Add 1.6mL 30% hydrogen Reflux Process and Neutralization Place flask in ice

Table of Chemicals— Chemical Structure— Chemical Name Chemical Structure Ethyl Acetate Sodium Bicarbonate (E)-stilbene Add 4mL ethanol Add stir bar Set up reflux system Heat and stir until solid dissolves Add 0.3mL 48% hydrobromic acid Part Two - Synthesis of Diphenyl Ethane forming cloudy and green solution Reflux for 20m Cool flask to room temp Add ~3mL NaHCO 3 until neutral, check with pH paper beaker for five minutes Collect solid with vacuum filtration Rinse with cold ethanol, dry with drying paper Determine mass, melting point, and % yield Cooling, Drying, and Calculation Calculate theoretical and experimental atom economy of reaction Recrystallize solid with ~1-2mL of ethyl acetate combined with 0.5mL ethanol Calculation and Preparation for Recrystalization Heat until dissolved (2-3m) Perform filtration for 5m Weigh crystals that form Determine the mass and melting point of the product Recrystalization

Ethanol Hydrobromic Acid Hydrogen Peroxide Bromine Trans-1,2-dibromo-1,2-diphenylethane Physical Properties— Chemical Name Molecular Formula Density (g/mL) Solubility in Water Molar Mass (g/mol) Ethyl Acetate C 4 H 8 O 2 0.902 Slight 88.12 Sodium Bicarbonate NaHCO 3 2.16 Moderate 88.00 (E)-stilbene C 14 H 12 0.47 Insoluble 180.23 Ethanol C 2 H 5 OH 0.789 Soluble 46.07 Hydrobromic Acid HBr 1.49 Very Soluble 8.89 Hydrogen Peroxide H 2 O 2 1.13 Miscible 34.01 Bromine Br2 3.12 Slightly 159.81 Trans-1,2- dibromo-1,2- diphenylethane C 14 H 12 Br 2 1.613 Insoluble 340 Chemical Name IUPAC Name Boiling Point (°C) Melting Point (°C) Ethyl Acetate Ethyl Acetate 77 -83.6 Sodium Bicarbonate Sodium Bicarbonate 851 300 (E)-stilbene (E)-stilbene 305 125 Ethanol Ethanol 78.37 -114 Hydrobromic Acid Hydrobromic Acid 122 -11 Hydrogen Peroxide Hydrogen Peroxide 150 -0.43

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Bromine Bromine 58.78 -7.25 Trans-1,2-dibromo- 1,2-diphenylethane Trans-1,2-dibromo- 1,2-diphenylethane 323 241 Chemical Properties— Chemical Name Toxicity Flammability Reactivity Ethyl Acetate Can irritate skin and eyes, and inhalation causes lung irritation High risk of fire hazard Not compatible with oxidizing agents, strong acids, and strong bases Sodium Bicarbonate Can cause skin and eye irritation with contact, can cause lung irritation Minimal risk of fire hazard Not compatible with strong acids, borane oxides, zinc, and calcium oxide (E)-stilbene Causes eye irritation, and may cause respiratory tract infection, causes skin irritation Slight risk of fire hazard Not compatible with strong oxidizing agents Ethanol Causes harm through ingestion, inhalation, or absorption through the skin as well as cracking Serious risk of fire hazard Not compatible with acetyl bromide/chloride, sulfuric acid, potassium, and hydrogen peroxide Hydrobromic Acid Can cause skin and eye irritation, and inhalation causes irritation Minimal risk of fire hazard Not compatible with strong bases, amines, ozone, oxidizing agents, and many organic compounds Hydrogen Peroxide Can irritate skin and eyes; inhalation causes irritation Minimal risk of fire hazard Not compatible with ammonia, iodides, sulfites, reducing agents, strong bases, and oxidizing agents Bromine Can irritate skin and eyes, and inhalation causes lung irritation Minimal risk of fire hazard Not compatible with reducing agents, mercury, phosphorus, titanium, potassium, sodium, amines, and oxidizing agents Trans-1,2-dibromo- 1,2-diphenylethane Harmful if swallowed, can cause burns with skin contact Slight risk of fire hazard Not compatible with strong oxidizing agents

Results— Tube One Tube Two Tube Three Unknown One Unknown Two Unknown Three Light orange Light orange Clear 0.2g (E)-Stilbene Crude Product Recrystallized Product Mass 0.16 grams 0.13 grams Percent Yield 80% 65% Melting Point 245.5 degrees Celsius 241.2 degrees Celsius Theoretical Experimental Atom Economy 90% 41.11% Percent Yield – Crude Product 0.16 g 0.20 g ∗ 100% = 80% Percent Yield – Recrystallized Product 0.13 g 0.20 g ∗ 100% = 65% Atom Economy – Theoretical product molar mass ∑ of molar massof starting materials ∗ 100% 340.06 g [ 180.25 + ( 2 ∗ 80.91 ) + 34.01 ] ∗ 100% = 90% Atom Economy - Experimental theoretical massof product massesof startingmaterials ∗ 100%

0.37 0.16 + 0.21 + 0.53 ∗ 100% = 41.11% Discussion— Unknown number three (in test tube three) is the alkene since it did not change color. Test tubes one and two started clear but ended with a light orange tint, meaning they could not be the alkene. Test tube three began clear but ended clear, containing the alkene. The percentage yield for Part B was 80% for the crude product and 65% for the recrystallized product. Both percentage yields were above the 50% accuracy limit, meaning there was some, but not a ton of, error in the experiment. A source of the reduced mass of the recrystallized product must have been the number of times we had to repeat the recrystallization step since we had to dry two times plus attempt to use the solvent three times. The solid could have been missed on the drying paper or as residue in the flask. The literature value of the melting point was 241 degrees Celsius, and the melting point collected in the experiment ended up being 245.5 degrees Celsius for the crude product and 241.2 degrees Celsius for the recrystallized product. All the melting points were generally close to one another, meaning that the products created in the lab were relatively pure and accurate. Slight deviations may have been from not drying the solid enough after it went through the vacuum apparatus, but I believe this did not greatly impact the melting point value. The theoretical atom economy, a measure in percentage of the usefulness of the products of a reaction, was 90%, and the experimental atom economy was 41.11%. This means that in the experiment performed, 41.11% of the materials in the product were utilized and were not waste products. Deviation of the experimental value from the theoretical value may come from an

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excess amount of waste product formed in one of the many side reactions that could occur during bromination. Conclusion— The theoretical background of this experiment focused on the bromination of an alkene to crease stilbene dibromide and did it through a “green” method. The background results are connected because the accuracy of the mass, melting point, and percentage yield can be used to conclude that this is an accurate way to synthesize stilbene dibromide. The experimental data reveals that the “in situ” formation of bromine is an accurate way to synthesize molecules and that it can be expected to have little waste. Another point the experimental data reveals is that shortcuts can’t be taken in the lab since we used different solvent concentrations than were listed in the lab manual to speed the recrystallization process along possibly. In situ formation is probably the most important technique that was used in the experiment, as it can be used outside of the lab to make complicated chemical drug delivery systems, fabricate electrolytes, and form nanoparticles of metal onto textile fibers (ACS). The experiment did accomplish all its goals. Its primary goal was to use in situ formation to prepare stilbene dibromide from stilbene, and this is what most of the experiment was focused around. Another side goal that was met was to inform students about green chemistry, and what atom economy means for their experiments.

References — Weldegirma, Solomon. “Experiment 9: A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds.” Experimental Organic Chemistry - Laboratory Manual: CHM 2210L and CHM 2211L , University of South Florida, Tampa, FL, 2023, p. 51. FisherSci. “Material Safety Data Sheet - Bromine.” Material Safety Data Sheet - Bromine - FisherSci , https://fscimage.fishersci.com/msds/03340.htm. Montes-Hernandez, German, et al. “In Situ Formation of Silver Nanoparticles (Ag-NPs) onto Textile Fibers.” ACS Publications , 2021, https://pubs.acs.org/doi/10.1021/acsomega.3c00063.