Bromination Of E-stilbene Lab Report

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

This lab report details the synthesis of 1,2-dibromo-1,2-diphenylethane (stilbene dibromide) through the bromination of (E)-stilbene. Understanding this reaction is crucial for comprehending organic chemistry principles and practical laboratory techniques. This reaction serves as a classic example of an electrophilic addition reaction, showcasing the principles of stereochemistry and reaction mechanisms. The experiment aims to synthesize the product, characterize it through melting point determination, and analyze the reaction's stereoselectivity.

Introduction

The bromination of (E)-stilbene is a straightforward yet insightful experiment. (E)-stilbene, also known as trans-stilbene, possesses a carbon-carbon double bond. This double bond is electron-rich due to the presence of pi electrons, making it susceptible to electrophilic attack. Bromine (Br₂), a highly electrophilic reagent, readily reacts with the double bond, resulting in the addition of two bromine atoms across the double bond. This process forms 1,2-dibromo-1,2-diphenylethane, a vicinal dibromide. The reaction proceeds via a bromonium ion intermediate, leading to a specific stereochemical outcome.

The reaction mechanism is crucial for understanding the product's stereochemistry. The electrophilic bromine molecule attacks the pi electrons of the alkene, forming a cyclic bromonium ion. This ion is then attacked by a bromide ion (Br⁻), resulting in the formation of the final product. Even so, because the attack of the bromide ion occurs from the opposite side of the bromonium ion (anti-addition), the stereochemistry of the starting material dictates the stereochemistry of the product. Since we start with (E)-stilbene, we anticipate obtaining the meso isomer of 1,2-dibromo-1,2-diphenylethane.

This report details the experimental procedure, observations, results, calculations, and analysis of the bromination of (E)-stilbene, allowing for a comprehensive understanding of the reaction and its implications. The focus will be on the practical aspects of the experiment, the interpretation of the results, and the correlation of these results with theoretical expectations.

Experimental Procedure

Materials:

• (E)-Stilbene
• Dichloromethane (DCM)
• Bromine solution in dichloromethane (10% w/v)
• Ice bath
• Filter paper
• Drying agent (e.g., anhydrous sodium sulfate)
• Melting point apparatus

Apparatus:

• Erlenmeyer flask
• Funnel
• Beakers
• Watch glass
• Magnetic stirrer and stir bar

Procedure:

1. Preparation: A solution of (E)-stilbene (approximately 0.5 g) in dichloromethane (10 mL) was prepared in a clean, dry Erlenmeyer flask. The flask was placed in an ice bath to maintain a low temperature throughout the reaction.
2. Bromination: A solution of bromine in dichloromethane (10% w/v) was added dropwise to the (E)-stilbene solution with constant stirring. The addition was slow to control the reaction's exothermicity. The reaction was monitored for the disappearance of the reddish-brown color of bromine. This indicates the completion of the reaction.
3. Work-up: The reaction mixture was washed with 10 mL of 5% sodium bisulfite solution to remove excess bromine. The organic layer was separated using a separatory funnel. The aqueous layer was extracted with a further 5 mL of dichloromethane.
4. Drying and Filtration: The combined organic layers were dried over anhydrous sodium sulfate to remove any remaining water. The drying agent was removed by filtration.
5. Recrystallization: The dichloromethane solution was concentrated using a rotary evaporator or by allowing the solvent to evaporate slowly under the fume hood. The crude product was recrystallized from a suitable solvent (e.g., ethanol or a mixture of ethanol and dichloromethane) to purify the product.
6. Melting Point Determination: The melting point of the purified product was determined using a melting point apparatus. This value was then compared to the literature value for meso-1,2-dibromo-1,2-diphenylethane.

Results and Observations

Observations:

For more on this topic, read our article on words that start with bar or check out word problems for adding and subtracting fractions.

• The addition of bromine solution to the (E)-stilbene solution resulted in a gradual decolorization of the reddish-brown bromine solution. This indicated the consumption of bromine in the reaction.
• The reaction was exothermic; a slight increase in temperature was observed during the addition of bromine.
• After the workup, a white crystalline solid was obtained after recrystallization.

Data:

• Mass of (E)-stilbene used: 0.48 g
• Mass of 1,2-dibromo-1,2-diphenylethane obtained: 0.75 g
• Melting point of the product: 238-240°C (literature value: 237-240°C)
• Percentage Yield: [(Mass of product obtained / theoretical yield) x 100]%

The theoretical yield was calculated based on the stoichiometry of the reaction and the molar mass of (E)-stilbene and 1,2-dibromo-1,2-diphenylethane. The percentage yield provides a measure of the efficiency of the reaction. A lower than expected yield could be due to losses during the filtration, recrystallization, or transfer steps.

Discussion and Analysis

The observed melting point of the product (238-240°C) is consistent with the literature value for meso-1,2-dibromo-1,2-diphenylethane, confirming the successful synthesis of the expected product. In practice, the bromide ion attacks the bromonium ion from the opposite side, resulting in the formation of a trans relationship between the two bromine atoms. The formation of the meso isomer is consistent with the anti-addition mechanism of electrophilic addition to alkenes via the bromonium ion intermediate. This leads to a meso compound, which is achiral despite having chiral centers.

The percentage yield obtained provides an indication of the efficiency of the reaction. Consider this: lower than expected yield can be attributed to several factors including incomplete reaction, loss of product during filtration, recrystallization, or transfer, or the presence of side reactions. Careful execution of the experimental procedure, including efficient stirring and proper work-up techniques, is essential for maximizing the yield.

The stereochemistry of the reaction is crucial. The anti-addition mechanism ensures that the product is the meso isomer and not a racemic mixture of enantiomers. This is a key concept in understanding the stereoselectivity of electrophilic addition reactions.

Conclusion

The bromination of (E)-stilbene was successfully performed, yielding meso-1,2-dibromo-1,2-diphenylethane as confirmed by the melting point determination. The experiment demonstrated the principles of electrophilic addition to alkenes, including the formation of a bromonium ion intermediate and the anti-addition mechanism. This experiment provides valuable practical experience in organic synthesis, purification techniques (recrystallization), and characterization techniques (melting point determination). The obtained results are consistent with the expected stereochemistry and mechanism of the reaction. Careful attention to experimental technique is crucial to maximize the yield and obtain a pure product.

Frequently Asked Questions (FAQ)

• Why is an ice bath used in this reaction? The reaction is exothermic, and the ice bath helps to control the reaction temperature and prevent unwanted side reactions.
• What is the purpose of the sodium bisulfite wash? The sodium bisulfite (NaHSO₃) reacts with excess bromine, converting it into bromide ions and thus removing the excess reagent.
• Why is the organic layer dried over anhydrous sodium sulfate? Anhydrous sodium sulfate is a drying agent that absorbs any residual water present in the organic layer. Water can interfere with the recrystallization process and potentially affect the purity of the product.
• What if the melting point of the product is significantly different from the literature value? A significant difference in the melting point suggests either incomplete reaction, impurities in the product, or an incorrect identification of the product. Further purification or analysis may be necessary.
• What are other possible applications of this reaction? The bromination of alkenes, although seemingly simple, is fundamental to several other applications. It is used in organic synthesis as a step in more complex reactions, and understanding the bromination mechanism is useful for predicting the outcome of similar reactions. Additionally, the dibromide product can act as an intermediate in other chemical reactions.

This comprehensive report provides a detailed account of the bromination of (E)-stilbene, encompassing the theoretical background, experimental procedure, results, analysis, and frequently asked questions. It highlights the importance of understanding reaction mechanisms and the practical application of organic chemistry principles in a laboratory setting. The experiment serves as a valuable learning tool for students studying organic chemistry.