Experiment 10 - A Greener Bromoniation of Stillbene and Qualitative Determination of Alkene PDF

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A Greener Bromoniation of Stillbene and Qualitative Determination of Alkene Compounds...

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Experiment 10: A Greener Bromination of Stilbene and Qualitative Determination of Alkene Compounds

Jenna Niles Partner: Amelia Drymon T.A.: Rouxian Gao November 5, 2018

Introduction The objective of the experiment was to prepare stilbene dibromide via in situ generation of bromine, through oxidation of hydrobromic acid with hydrogen peroxide. Bromination of an alkene is the addition of two bromine across a double bond. The mechanism for this reaction involves the alkene acting as a nucleophile while the bromine acts as the electrophile. Under these conditions, the Br-Br bond becomes polarized and breaks a part. The more positively charged bromine is transferred to the carbon-carbon chain where the alkene resided, which forms a cyclic bromonium ion and a bromine ion. The second step involves the bromine performing an SN2 reaction- where the bromide ion attacks the cyclic bromonium ion opening the ring and creating a vicinal dibromide. Typically, bromination uses solvents such as carbon tetrachloride or methylene chloride, both of which are suspected to be carcinogenic. Bromine is a highly volatile and corrosive substance that can cause burns when in contact with skin. Pyridinium tribromide is used as an alternative solvent due to research concluding that it is safer and easier to use, thus it is known as a “green” chemistry. Chemical yield is a tool that uses the chemical and physical properties of products to analyze their efficiency. Chemical yield is calculated by dividing the moles of product obtained by the moles of product possible multiplied by 100. A successful reaction will yield a high chemical percentage of 90-100%. However, the product can contain byproducts or side products which is why atom economy was developed. Atom economy is defined as the percent of atomic weight of all the starting materials appearing in the final product.

Figure 1. Mechanism for 1,2-dibromo-1,2-diphenylethane

Figure 2. Mechanism for Possible Side Reaction during formation of 1,2-dibromo-1,2diphenylethane

Experimental Section Flow Chart Procedure for Qualitative determination of alkene compounds & Synthesis of trans1,2-dibromo-1,2-diphenylethane

4.1mL 30% H2O2, 0.2 mL 48% HBr

3.+ 3 drops of unknown #3, 1 mL ethyl acetate, 5 drops of solution

Test Tube

1.+ 3 drops of unknown #1, 1 mL ethyl acetate, 5 drops of solution

2.+ 3 drops of unknown #2, 1 mL ethyl acetate, 5 drops of solution

+ 4 mL ethanol, 0.2 g (E) stilbene, and a magnetic bar into a vial

After refluxing is complete, allow vial to cool to room temperature, then place into an ice bath fo 5 mins

Table of Contents

Set up reflux apparatus, attach vial, & reflux for 20 mins. over 100◦C sandbath

Collect solid using a Hirshel Funnel. Rinse solid with cold ethanol to remove impurties. Place solid onto filter paper to dry.

-Weigh solid, determine melting point -Calculate percent yield and atom economy

IUPAC Name of Chemical(s) Used/ Chemical Formula Bromine; Br2

Chemical Properties Molecular Weight: 159.808 g/mol

Hydrogen Bromide; HBr

Melting Point: -7.2◦C Molecular Weight: 80.912 g/mol

Peroxide; H2O2

Melting Point: -86.9◦C Molecular Weight: 34.014 g/mol

Stilbene; C14H12

Melting Point:0.43◦C Molecular Weight: 180.25 g/mol

Chemical Properties Dark reddishbrown fuming liquid with a pungent odor.

LCSS Can cause skin burns, and eye damage

Colorless gas with a pungent irritating odor

Toxic if swallowed

Colorless liquid

Explosive; harmful if swallowed

Off-white crystals Harmful if swallowed, can cause serious eye irritation

Melting Point: 124◦C

1,2-Dibromo-1,2diphenylethane; Stilbene Dibromide; C14H12Br2

Molecular Weight: 340.058 g/mol

Clear liquid

Melting Point: 241◦C

Results Table 1. Unknowns with HBr/H2O2

Test Tube

Reaction Time

Reaction Color

Can cause serious burns, harmful if swallowed

1 2 3

Instantly Instantly ~1 minute

Light orange Bright orange Faint yellow

Table 2. Results of the desired Stilbene Dibromide Mass Percent Yield Atom Economy Melting Point

0.823g/0.06 g crystallization 15.9%

9.36% 215◦C/203◦C crystallization Atom Economy =

=

MW of product ∗100 MW of reactants

0.06 g 1,2−dibromo−1,2 diphenylethane ∗100=9.36 % 0.2 g E−stilbene+0.441 g Br 2 Theoretical Yield of 1,2-dibromo-1,2-diphenylethane 0.2g (E)-stilbene x

1mol ( E )−stilbene 1 mol 1,2−dibromo −1,2−diphenylethane =0.00111 mol X 1mol ( E ) −Stilbene 180.25 g( E )−stilbene Actual Yield of 1,2-dibromo-1,2-diphenylethane 0.06 g 1,2-dibromo-1,2-diphenylethane x

1mol 1,2−dibromo −1,2−diphenylethane =0.000018 mol 340.058 g 1,2−dibromo −1,2−diphenylethane Percent Yield % yield =

=

actual yield X 100 theoretical yield

0.000018mol X 100= 15.9% 0.00111 mol

Discussion Of the three test tubes, the unknown sample #3 contained alkene molecules corresponding to test tube #3. This was determined through observation and knowledge regarding the reaction’s characteristics. Test tube 1 reacted instantly and turned a light orange color. Test tube 2 reacted almost instantly and turned a bright orange color. Test tube 3 took approximately a minute to react and turned a very faint yellowish color. This indicated that the unknown sample #3 contained the alkene molecules.

During the synthesis of 1,2-dibromo-1,2-diphenylethane the solution containing (E)stilbene and ethanol was refluxed for 20 minutes. Following this step is the addition of sodium bicarbonate to neutralize the solution. During this step an excess of sodium bicarbonate resulted in a basic solution and thus the experiment was corrupted. The solution was disposed of into organic waste. To obtain results for the calculations a near by lab group shared their results. Due to using the results from a different lab group, it is unclear why the yield was so low. The melting point of the crude was 215◦C, and the recrystallized products melting point was 203◦C, which deviates from the commonly accepted melting point of 241◦C. The low melting point can be attributed to the fact that the mixture was impure. The atom economy was 9.36% due to the low yield of pure product. The small percentage of atom economy and product percent yield can be attributed to the very small amounts of reactants used at the start of the experiment.

Conclusion The experiment was not successful. The 1,2-dibromo-1,2-diphenylethane was not produced properly due to the excess of sodium bicarbonate solution being added to the refluxed

product. However, the alkenes for the unknown samples were successfully identified. Other applications of free radical halogenation included the production of chlorinated methane’s for industrial uses. Overall the experiment was a fail. In the future the steps of this experiment should be followed according to the lab’s procedures.

References

National Center for Biotechnology Information. PubChem Compound Database; CID=93010, https://pubchem.ncbi.nlm.nih.gov/compound/93010 (accessed Nov. 5, 2018). "Radical Allylic Halogenation." https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry/Supplemental_Modules_(Organi c_Chemistry)/Alkenes/Reactivity_of_Alkenes/Free_Radical_Reactions_of_Alkenes/Radical_All ylic_Halogenation (accessed on November 5, 2018) Weldegirma, Solomon. Experimental Organic Chemistry. Laboratory Manual: CHM 2210L. University of South Florida. Fall 2018....