Bromination Of E Stilbene Lab Report

Bromination of (E)-Stilbene: A Comprehensive Lab Report

The bromination of (E)-stilbene is a classic organic chemistry experiment that demonstrates electrophilic addition reactions and the stereochemistry of vicinal dibromides. This report details the procedure, observations, results, calculations, and discussion of a typical experiment, providing a comprehensive analysis suitable for advanced students.

Abstract

This experiment investigates the stereospecific addition of bromine to (E)-stilbene, resulting in the formation of meso-1,2-dibromo-1,2-diphenylethane. The reaction's mechanism, stereochemical outcome, and the purification techniques employed are examined. The product's identity was confirmed through melting point determination and, potentially, other spectroscopic analyses (if conducted). The experiment successfully demonstrated the anti-addition mechanism of bromine across the alkene double bond, highlighting the importance of stereochemistry in organic reactions.

Introduction

(E)-Stilbene, a symmetrical alkene, undergoes electrophilic addition with bromine. Bromine, a relatively weak electrophile, readily reacts with alkenes due to the presence of the electron-rich double bond. The reaction proceeds via a concerted mechanism, where the bromine molecule attacks the alkene, forming a cyclic bromonium ion intermediate. This intermediate is then attacked by a bromide ion from the opposite side (anti-addition), leading to the formation of a vicinal dibromide. In the case of (E)-stilbene, the product is meso-1,2-dibromo-1,2-diphenylethane, a chiral molecule with a plane of symmetry making it achiral. This experiment focuses on understanding this stereospecific addition and the associated reaction conditions required for optimal yield and purity.

Reaction Mechanism

The mechanism proceeds as follows:

1. Electrophilic Attack: The pi electrons of the (E)-stilbene double bond attack the bromine molecule, forming a three-membered cyclic bromonium ion intermediate. This intermediate is crucial in dictating the stereochemistry of the product.

2. Bromide Ion Attack: A bromide ion then attacks the bromonium ion from the backside, leading to the opening of the three-membered ring. This backside attack is crucial for the anti-addition.

3. Product Formation: The result is the formation of meso-1,2-dibromo-1,2-diphenylethane. The stereochemistry is determined by the anti-addition of the bromide ions.

(Insert a clearly drawn reaction mechanism diagram here with appropriate arrows and structural formulas.)

Expected Product: meso-1,2-Dibromo-1,2-diphenylethane

The product, meso-1,2-dibromo-1,2-diphenylethane, possesses a plane of symmetry bisecting the molecule. This renders the molecule achiral despite having two chiral centers. The symmetry arises from the presence of identical substituents on each carbon atom of the central bond.

(Insert a structural formula of meso-1,2-dibromo-1,2-diphenylethane here.)

Experimental Procedure

Materials

• (E)-Stilbene
• Dichloromethane (DCM)
• Bromine solution in DCM (e.g., 1% w/v)
• Ice bath
• Filter paper
• Buchner funnel
• Aspirator
• Hot plate
• Melting point apparatus

Procedure

1. Dissolution of (E)-Stilbene: Approximately 0.5g of (E)-stilbene was dissolved in a minimum amount of dichloromethane (DCM). The exact amount of DCM used should be recorded.

2. Addition of Bromine Solution: The (E)-stilbene solution was cooled in an ice bath. A 1% solution of bromine in DCM was added dropwise with continuous stirring until the bromine colour persisted. The solution should become colorless, then slightly yellow. This is an indication that the reaction is complete, but you might want to wait a few more minutes before moving on. The precise amount of bromine solution used should be recorded.

3. Precipitation of the Product: After the reaction is complete (bromine colour persists), the product precipitated out of the solution. The reaction mixture was further cooled in an ice bath to enhance precipitation. The amount of precipitation should be observed and recorded.

4. Isolation of the Product: The product was isolated by vacuum filtration using a Buchner funnel. The solid was washed several times with cold methanol to remove any residual impurities. Note that methanol is used in cold form only; otherwise it can dissolve the product.

5. Drying and Recrystallization (Optional): The product was allowed to air dry, then recrystallized from a suitable solvent (e.g., ethanol or a mixture of ethanol and DCM) to further improve its purity. Note: Recrystallization can improve purity and should be carefully evaluated based on observed product appearance.

6. Melting Point Determination: The melting point of the purified product was determined using a melting point apparatus. A sharp melting point range indicates a pure sample. The observed melting point is compared to the reported literature value to confirm the identity of the product.

Results

Observations

• Initial Appearance of (E)-Stilbene: [Describe the appearance of the starting material, e.g., white crystalline solid]
• Change Upon Addition of Bromine: [Describe the color change from colorless to pale yellow to orange/reddish-brown, indicating the bromine is reacting]
• Precipitation of Product: [Describe the formation of a precipitate and its appearance, e.g., white or off-white solid]
• Yield: [Report the actual yield obtained (grams) and calculate the percentage yield using the stoichiometry of the reaction and the starting mass of (E)-stilbene.]
• Melting Point: [Report the experimental melting point range observed for the purified product]. Compare this range to the literature melting point for meso-1,2-dibromo-1,2-diphenylethane (approximately 236-237 °C).

Calculations

• Theoretical Yield: Calculate the theoretical yield of meso-1,2-dibromo-1,2-diphenylethane based on the starting mass of (E)-stilbene.
• Percentage Yield: Calculate the percentage yield using the formula: (Actual Yield / Theoretical Yield) x 100%.

(Provide the full calculations and clearly show your work).

Discussion

Stereochemistry

The experiment demonstrates the stereospecificity of the bromination reaction. The anti-addition of bromine to (E)-stilbene results in the exclusive formation of meso-1,2-dibromo-1,2-diphenylethane. This is because the bromide ion attacks the bromonium ion from the opposite side, leading to the trans-addition of bromine atoms. This confirms the mechanistic understanding of electrophilic addition reactions to alkenes.

Melting Point Analysis

The melting point of the purified product is compared to the literature value. A sharp melting point range close to the literature value indicates a high degree of purity. A broad melting point range or a significant deviation from the literature value may suggest the presence of impurities or incomplete reaction.

Yield Analysis

The percentage yield obtained provides information about the efficiency of the reaction. A low percentage yield may be due to several factors, including incomplete reaction, loss of product during isolation or purification, or side reactions. Consider any possible sources of error that may account for a low yield.

Potential Sources of Error

Several factors can affect the outcome of the experiment:

• Incomplete Reaction: If insufficient bromine is added, the reaction may be incomplete.
• Loss of Product: Product loss may occur during filtration or transfer steps.
• Impurities: The presence of impurities in the starting materials or solvents can affect the yield and purity of the product.
• Inaccurate Measurements: Inaccurate measurements of reactants can affect the stoichiometry of the reaction.

Conclusion

The bromination of (E)-stilbene successfully demonstrated the electrophilic addition of bromine to alkenes and its stereospecific outcome. The formation of meso-1,2-dibromo-1,2-diphenylethane confirms the anti-addition mechanism. The purity and identity of the product were confirmed through melting point determination. The experiment provided valuable insights into reaction mechanisms, stereochemistry, and purification techniques in organic chemistry. Areas for improvement in the experiment could be incorporating spectroscopic techniques like NMR or IR spectroscopy for more definitive product identification and purity assessment.

References

(List any relevant references here, if applicable. This section is important for academic integrity).