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 dibromides. This report details the procedure, observations, results, and analysis of this experiment, providing a comprehensive understanding of the process and its implications.

Introduction

(E)-Stilbene, also known as trans-stilbene, is an alkene with a rigid, planar structure due to the presence of the double bond and the bulky phenyl groups. This rigidity influences the stereochemistry of the reaction with bromine. Bromine (Br₂), a strong electrophile, readily adds across the double bond of (E)-stilbene in a process called electrophilic addition. This reaction yields 1,2-dibromo-1,2-diphenylethane (stilbene dibromide). Understanding this reaction provides insight into the mechanism of electrophilic addition, the importance of stereochemistry, and the properties of alkenes and their derivatives. The reaction's high yield and relatively straightforward procedure make it ideal for undergraduate organic chemistry laboratories. This report will meticulously document the experimental procedure followed, the observed results, the data analysis performed, and a discussion of the theoretical aspects of the reaction.

Reaction Mechanism

The bromination of (E)-stilbene proceeds via a concerted mechanism. This means that the addition of the two bromine atoms occurs simultaneously, without the formation of any intermediate carbocations. The pi electrons of the carbon-carbon double bond in (E)-stilbene attack the bromine molecule, forming a cyclic bromonium ion intermediate. This intermediate is rapidly attacked by a bromide ion, resulting in the formation of the vicinal dibromide product, 1,2-dibromo-1,2-diphenylethane. The stereochemistry of the starting material is crucial; since the addition is concerted, the stereochemistry is retained. The reaction proceeds anti-addition, meaning the two bromine atoms add to opposite faces of the double bond. This results in the formation of the meso isomer of 1,2-dibromo-1,2-diphenylethane, which is achiral despite having two chiral centers.

Diagram illustrating the mechanism:

[Insert a clear diagram showing the mechanism of the bromination of (E)-stilbene, including the formation of the bromonium ion intermediate and the anti-addition of bromine atoms. Clearly label all structures and electron movements.]

Experimental Procedure

The experiment was performed following a standard procedure for the bromination of (E)-stilbene. Specific quantities and safety precautions were adhered to strictly.

Materials:

• (E)-Stilbene
• Dichloromethane (DCM) – acts as the solvent
• Bromine solution in dichloromethane (a solution of approximately 1% bromine in dichloromethane was used to minimize the generation of toxic HBr gas)
• Ice bath
• Filter paper
• Buchner funnel
• Vacuum filtration apparatus
• Weighing balance
• Erlenmeyer flask
• Beaker

Procedure:

1. A weighed amount of (E)-stilbene (approximately 0.5 grams) was dissolved in a minimal amount of dichloromethane in an Erlenmeyer flask. The amount of dichloromethane used should be carefully chosen to ensure complete dissolution of (E)-stilbene while maintaining a manageable solution volume.

2. The solution was cooled in an ice bath to maintain a low reaction temperature, minimizing side reactions and improving the yield of the desired product.

3. A solution of bromine in dichloromethane was added dropwise to the cooled (E)-stilbene solution with constant stirring. The addition was slow to control the reaction rate and prevent excessive heat generation. Caution: Bromine is corrosive and its vapors are toxic; therefore, proper ventilation and safety goggles were used throughout the experiment.

4. The reaction mixture was stirred for a period of approximately 30-45 minutes in the ice bath to ensure complete reaction. The color change of the solution from orange (bromine) to colorless is a good indication of the reaction’s progression. The completion of the reaction can be further confirmed by thin layer chromatography (TLC).

5. After the completion of the reaction, the product was isolated by vacuum filtration. The product was washed with a small amount of ice-cold dichloromethane to remove any residual impurities.

6. The purified product, 1,2-dibromo-1,2-diphenylethane, was air-dried on the filter paper and then allowed to dry further in a desiccator.

7. The product was weighed to determine the yield.

8. The melting point of the product was determined and compared to the literature value to assess the purity of the product. Any significant deviation from the literature value indicates impurities in the obtained product.

9. The obtained product was characterized using various spectroscopic techniques (such as NMR spectroscopy) to confirm its identity and purity (this step may not be included in all lab settings).

Results

Yield Calculation:

The initial mass of (E)-stilbene used was recorded (Weight of (E)-Stilbene). The mass of the isolated 1,2-dibromo-1,2-diphenylethane was also recorded (Weight of Product). The percentage yield of the reaction was calculated using the following formula:

Percentage Yield = [(Weight of Product) / (Weight of (E)-Stilbene) x Molecular Weight of 1,2-dibromo-1,2-diphenylethane / Molecular Weight of (E)-Stilbene] x 100%

Melting Point:

The melting point of the obtained product was determined using a melting point apparatus and compared to the literature value. A sharp melting point range close to the literature value suggests a high degree of purity. A broad melting point range indicates impurities in the product. Record the observed melting point range.

Spectroscopic Analysis (if performed):

If NMR spectroscopy or other spectroscopic analyses were conducted, the results should be included here. This would involve comparing the obtained spectra to the predicted spectra for 1,2-dibromo-1,2-diphenylethane, thus confirming the product's identity and purity. This section would include detailed descriptions of the observed peaks, chemical shifts, and coupling constants.

Discussion

The success of this experiment hinges on several factors. The careful control of the reaction temperature is crucial to prevent side reactions and to ensure a high yield. The slow addition of bromine solution helps in controlling the reaction rate. The use of an ice bath is essential for maintaining a low reaction temperature. Incomplete reaction may lead to a lower yield, while the presence of impurities can affect the melting point and spectroscopic data.

The stereochemistry of the product is a significant aspect of this experiment. The anti-addition of bromine results in the formation of the meso isomer of 1,2-dibromo-1,2-diphenylethane. This product is achiral despite the presence of two stereocenters because it possesses a plane of symmetry. This observation supports the proposed concerted mechanism and the anti-addition nature of electrophilic addition to alkenes. Deviations from the theoretical yield could be caused by incomplete reaction, loss of product during filtration, or the presence of impurities.

Error Analysis:

A thorough error analysis is crucial in evaluating the accuracy and reliability of the results. Potential sources of error include:

• Incomplete reaction: This can lead to a lower yield and the presence of unreacted (E)-stilbene.
• Loss of product during filtration: Some product may be lost during the filtration process.
• Impurities in the starting materials: Impurities in the (E)-stilbene or bromine solution can affect the reaction and the purity of the product.
• Improper drying: Incomplete drying of the product can lead to an inaccurate mass measurement.
• Errors in melting point determination: Variations in the heating rate can lead to inaccuracies in the melting point measurement.

These potential sources of error should be considered when interpreting the results.

Conclusion

The bromination of (E)-stilbene is a successful experiment to demonstrate the electrophilic addition reaction and the stereochemistry associated with it. The successful synthesis of 1,2-dibromo-1,2-diphenylethane confirms the understanding of the reaction mechanism and the importance of experimental technique. The obtained yield, melting point, and spectroscopic data (if applicable) provide valuable insights into the effectiveness of the reaction and the purity of the synthesized product. The meticulous experimental procedure and careful attention to detail are essential for achieving a high yield and pure product. Further analysis, particularly spectroscopic characterization, could provide additional confirmation of the product's structure and purity. This experiment successfully showcases a fundamental reaction in organic chemistry and its underlying principles. The limitations and sources of error identified highlight the importance of careful experimental technique and data analysis in organic chemistry experimentation.