Bromination of trans-Cinnamic Acid: Experiment Report

Abstract

The bromination of trans-cinnamic acid was carried out to determine the stereochemical structure and the mechanism of dibromide formation. The product was identified as erythro-2,3-Dibromo-3-phenylpropanoic acid based on its melting point and infrared spectrum, indicating the involvement of an anti-addition mechanism. This experiment provides insight into the stereochemistry of dibromide formation and its significance in chemical reactions.

Introduction

The bromination of trans-cinnamic acid is a chemical reaction used to investigate the stereochemical structure of the resulting dibromide and the mechanism behind its formation.

Understanding the stereochemistry and mechanism of this reaction is essential for elucidating the behavior of compounds with multiple chiral centers and their reactivity.

Cinnamic acid, a naturally occurring compound, serves various purposes, including flavoring, perfume production, and as a precursor to numerous natural substances. It plays a role in providing color to flowers, butterflies, and fall leaves. The addition of bromine to cinnamic acid is a straightforward chemical transformation that yields valuable insights into stereochemistry.

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The product of the bromination reaction can exist in different stereochemical forms, including erythro and threo isomers. Identifying the stereochemical structure of the product is crucial, as it impacts its properties and reactivity.

Materials and Methods

Materials:

• Trans-cinnamic acid (10.0 mmol)
• Glacial acetic acid (6.0 mL)
• Bromine in acetic acid solution (1.25 M, 8.2 mL)
• Cyclohexenes
• 50 mL round-bottom flask
• Stir bar
• Magnetic stirrer
• Separatory funnel
• Ice water
• Vacuum filtration setup

Methods:

1. 10.0 mmol of trans-cinnamic acid was combined with 6.0 mL of glacial acetic acid in a 50 mL round-bottom flask equipped with a stir bar.
2. A 1.25 M solution of bromine in acetic acid (8.2 mL) was added dropwise to the reaction mixture over a 30-minute period while stirring.
3. After the last addition of the bromine solution, the reaction mixture was stirred at 50°C for 15 minutes.
4. Cyclohexenes were added dropwise to the solution while stirring until it became colorless.
5. The mixture was cooled in ice water until crystallization occurred.
6. The solid product, 2,3-dibromo-3-phenylpropanoic acid, was collected via vacuum filtration and washed with ice-cold water until the acetic acid odor disappeared.
7. The product was dried to measure its melting point.

Results and Discussion

Melting Point

The melting point of the product was determined to be 202°C, which closely matches the literature value of 204°C for erythro-2,3-Dibromo-3-phenylpropanoic acid. This similarity in melting points strongly suggests that the obtained product possesses a stereochemical structure resembling erythro-2,3-Dibromo-3-phenylpropanoic acid.

Infrared Spectrum

The infrared spectrum of the product closely resembled that of the starting material, with characteristic functional groups such as C=O (1700 cm^-1), O-H (3300-2600 cm^-1), and C=C (1680-1640 cm^-1) present. This similarity in functional groups further supports the conclusion that the product is erythro-2,3-Dibromo-3-phenylpropanoic acid.

Mechanisms

Several mechanisms can be proposed for the addition of bromine to an alkene, but in the case of our final product, the anti-addition mechanism is illustrated. The anti-addition mechanism results in the formation of erythro-2,3-Dibromo-3-phenylpropanoic acid, as observed in our experiment.

Conclusion

The bromination of trans-cinnamic acid resulted in the formation of erythro-2,3-Dibromo-3-phenylpropanoic acid, as confirmed by its melting point and infrared spectrum. The close match between the experimental melting point (202°C) and the literature value (204°C) strongly supports this identification. Additionally, the infrared spectrum showed functional groups consistent with the expected product.

The mechanism of the reaction involved an anti-addition mechanism, which is characteristic of the formation of erythro-2,3-Dibromo-3-phenylpropanoic acid. This experiment provides valuable insights into the stereochemistry of dibromide formation and its relevance in chemical reactions involving compounds with multiple chiral centers.

References

1. Experiment is a modified version of an experiment found in: Lehman, J.W. Operational Organic Chemistry: a problem-solving approach to the laboratory course, 3rd ed., Prentice-Hall, Upper Saddle River, New Jersey, 1999.
2. www.sigmaaldrich.com
3. www.chemicalbook.com