Laboratory Report: Synthesis and Purification of Benzil from Benzoin

The synthesis of benzil from benzoin involves a series of chemical reactions that showcase key principles in organic chemistry. Benzoin, a white crystalline compound, undergoes oxidative cleavage to yield benzil. This laboratory report will detail the experimental procedure, calculations, and results of the synthesis, providing a comprehensive understanding of the underlying chemical processes.

Experimental Procedure:

1. Materials and Apparatus:
   • Benzoin
   • Nitric acid (HNO₃)
   • Sulfuric acid (H₂SO₄)
   • Ice bath
   • Distillation apparatus
   • Filtration setup
   • Crystallization dishes
2. Synthesis of Benzil:
   • Benzoin was dissolved in nitric acid, and the reaction mixture was cooled in an ice bath.
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   • Sulfuric acid was added dropwise, initiating the oxidation of benzoin to benzil.
   • The reaction mixture was then neutralized and filtered to obtain benzil.

Calculations:

1. Molar Mass Calculation:
   • The molar mass of benzoin was calculated using the atomic masses of its constituent elements (C, H, and O).
   • Molar mass of benzoin = (2 * molar mass of C) + (2 * molar mass of H) + molar mass of O.
2. Theoretical Yield Calculation:
   • The balanced chemical equation for the synthesis was used to calculate the theoretical yield of benzil.
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   • Stoichiometry calculations were performed to determine the moles of benzoin reacting and the moles of benzil produced.
3. Percent Yield Calculation:
   • The experimental yield of benzil was determined by measuring the actual mass obtained in the laboratory.
   • Percent yield was calculated using the formula: Percent Yield=(Actual YieldTheoretical Yield)×100%Percent Yield=(Theoretical YieldActual Yield​)×100%

Results:

1. Observations:
   • The color change in the reaction mixture during oxidation indicated the progress of the reaction.
   • The formation of crystals during crystallization was observed.
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2. Quantitative Analysis:
   • The actual yield of benzil was obtained by weighing the crystalline product.
   • The percent yield was calculated and compared to the theoretical yield.

Discussion:

1. Mechanism of Reaction:
   • The oxidation mechanism involving nitric acid and sulfuric acid was discussed, emphasizing the role of each reagent in the conversion of benzoin to benzil.
2. Purity of Product:
   • The purity of the obtained benzil was assessed by analyzing the melting point and comparing it to the literature value.
3. Potential Sources of Error:
   • Possible sources of error, such as incomplete reactions or impurities, were considered to understand variations between theoretical and actual yields.

The synthesis of benzil from benzoin exemplifies fundamental principles of organic chemistry. Through careful experimentation, the laboratory successfully produced benzil and analyzed its properties. The theoretical and actual yields were compared, providing insights into the efficiency of the reaction. This laboratory report serves as a comprehensive guide for students and researchers exploring the synthesis of benzil and its underlying chemical processes.

Additional Information:

Purity Assessment: To assess the purity of the obtained benzil, a melting point determination is recommended. Compare the experimental melting point with the literature value for benzil (94 - 95 °C) to ensure the product's identity and purity. Any deviation from the expected melting point may indicate impurities.

Safety Precautions: Throughout the experiment, prioritize safety precautions due to the use of corrosive substances and the generation of toxic fumes. Work in a well-ventilated hood, wear appropriate personal protective equipment, including gloves, and handle reagents with care to avoid skin and eye contact.

Understanding the Test for Unoxidized Benzoin: Elaborate on the chemical basis of the test for unoxidized benzoin. The reaction involves the formation of a complex between benzil and a product of auto-oxidation of benzoin, resulting in a dark brown or black color. Emphasize the importance of a negative test result, indicating the successful completion of the oxidation process.

Recrystallization Details: If recrystallization is chosen as a purification method, provide additional details on the process. Highlight the importance of keeping the mixture hot until complete dissolution of the solid, adding water dropwise to induce cloudiness, and the subsequent collection of crystals by vacuum filtration.

Calculation of Percent Yield: For a thorough understanding, break down the steps involved in calculating the percent yield. Clearly outline the determination of the theoretical yield and the subsequent comparison with the actual yield obtained in the experiment.

Post-Lab Considerations: Discuss potential post-lab steps, such as reporting the yield and percent yield of benzil. Address any specific instructions from the instructor regarding additional measurements or assessments of last week's product.

Possible Sources of Error: Acknowledge potential sources of error in the experiment, such as variations in reagent concentrations, incomplete reactions, or experimental conditions. Discuss how these factors might contribute to discrepancies between theoretical and actual yields.

Conclusion and Future Work: Summarize the key findings of the experiment, emphasizing the successful synthesis and purification of benzil. Suggest areas for future work, such as optimizing reaction conditions or exploring alternative purification methods.

By incorporating these additional details, the laboratory report provides a more comprehensive and informative guide to the synthesis and purification of benzil from benzoin.