Bromination Of E-Stilbene: Postlab Questions And Analysis

Bromination of E-Stilbene: Postlab Questions and Analysis

This assignment involves analyzing the bromination of (E)-stilbene and related compounds, focusing on calculating theoretical yields, percent yields, stereochemistry, possible by-products, and evaluating the environmental impact of different bromination reactions in the context of green chemistry principles.

Postlab questions and analysis

1. Calculate the theoretical mass of the desired stilbene dibromide product expected to be obtained from the bromination of (E)-stilbene.

2. Calculate the percent yield (% yield) of the stilbene dibromide product from the bromination of (E)-stilbene.

3. Is there any other possible product that could be formed from the bromination of (E)-stilbene or trans-stilbene?

4. Identify the stereoisomeric relationship of the product from the bromination of (E)-stilbene or trans-stilbene. Is it a meso compound, racemic mixture, R-enantiomer only, or S-enantiomer only?

5. An organic chemist has observed that bromination of (Z)-stilbene or cis-stilbene leads to the formation of two possible isomers. Provide the structures and stereochemistry of these two products.

6. Identify the stereoisomeric relationship between the two products formed from bromination of (Z)-stilbene or cis-stilbene. Are they meso compounds, racemic mixtures, or enantiomeric pairs?

7. Compare three different bromination reactions in terms of atomic economy and toxicity, and identify which is most environmentally friendly. Justify your answer.

Analysis and Answer

1. Theoretical Mass of Stilbene Dibromide:

To calculate the theoretical mass of the dibromide product, we need the molar mass of (E)-stilbene and the molar ratio of bromine addition. (E)-stilbene has a molecular formula of C14H12, with a molar mass approximately 180.25 g/mol. Bromination adds two bromine atoms (Br₂), molar mass 159.808 g/mol, resulting in dibromide (C14H12Br2). The molar mass of the product is approximately 180.25 + 159.808 = 340.058 g/mol.

Assuming the experiment starts with a specific amount of (E)-stilbene—say 1.00 g—then the theoretical yield is calculated based on molar ratios:

Moles of stilbene = 1.00 g / 180.25 g/mol ≈ 0.00555 mol.

Since bromination of each mole of stilbene produces one mole of dibromide, theoretical mass = 0.00555 mol × 340.058 g/mol ≈ 1.89 g.

2. Percent Yield:

The percent yield is calculated as (actual yield / theoretical yield) × 100%. If the actual yield obtained from the experiment was, for example, 1.50 g, then:

Percent yield = (1.50 g / 1.89 g) × 100% ≈ 79.37%.

3. Possible Other Products:

Besides the dibromide, partial bromination could lead to monobrominated stilbene derivatives, which retain the remaining double bond and have different stereochemistry. Additionally, depending on reaction conditions, side reactions such as formation of polybrominated compounds or even oxidative by-products could occur. However, under controlled bromination conditions, dibromides are typically the main products of interest.

4. Stereoisomeric Relationship of the Brominated Product:

The addition of bromine across the double bond of (E)-stilbene typically proceeds via a bromonium ion intermediate, leading to anti-addition. As a result, the product is a trans-1,2-dibrominated compound, which is a meso compound due to internal plane of symmetry, or it can be a racemic mixture of enantiomers if chiral centers are generated without internal symmetry. Since the reaction proceeds with anti addition, the stereochemistry of the product is trans-1,2-dibromide, which is usually a meso compound because of the plane of symmetry.

5. Bromination of (Z)-Stilbene (cis-stilbene):

Bromination of cis-stilbene yields two isomers:

- trans-1,2-dibromide, resulting from anti addition across the double bond, where the bromines are added to opposite faces.

- cis-1,2-dibromide, which could occur via syn addition under certain conditions, typically resulting from radical or less stereoselective pathways.

However, in typical electrophilic addition reactions, anti addition is favored, so the major product would be trans-1,2-dibromide. The minor or synthesized product could be the cis-1,2-dibromide, which does not have a plane of symmetry and exists as a pair of enantiomers.

6. Stereoisomeric Relationship of the Two Products:

The trans-1,2-dibromide formed from (Z)-stilbene is a meso compound if symmetrical, whereas the cis-1,2-dibromide exists as a pair of enantiomers. These two are not enantiomers of each other but are stereoisomers—specifically, diastereomers—due to differing configurations at the stereocenters. The trans isomer is a meso compound (if symmetry exists), and the cis isomer is chiral, existing as a pair of enantiomers.

7. Comparing Bromination Reactions in Terms of Atomic Economy and Toxicity:

Reaction 1 involves bromination of (Z)-stilbene with Br₂ in non-polar solvents, which is efficient but involves excess bromine—a halogen that can be toxic and lead to hazardous waste. Reaction 2 uses ethanol with HBr/H₂O₂, which may introduce additional steps and reagents, leading to increased waste and potential hazards. Reaction 3 involves ammonium bromide (NH₄Br), which supplies bromine in a more controlled and potentially less toxic manner, especially if conducted under mild conditions.

Based on green chemistry principles, Reaction 3 is the most atomic economical and least toxic, as it minimizes waste, reduces the use of hazardous reagents, and employs milder reaction conditions. It also offers better atom economy because it incorporates bromine directly from a reagent that produces fewer by-products.

Therefore, Reaction 3 would be considered the best choice for an environmentally friendly bromination process.

References

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