CHEM 162 Lab 6: A Comprehensive Guide to the Synthesis of Aspirin

CHEM 162 Lab 6: Synthesis of Aspirin focuses on synthesizing aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride. The main objectives are to evaluate the purity and yield of the synthesized aspirin, perform stoichiometry calculations, and compare the melting points of the crude and purified products. This experiment is significant as it demonstrates fundamental organic synthesis techniques and analytical methods used to assess product quality.

Materials and Methods

Sure, here are the materials and methods for the synthesis of aspirin in CHEM 162 Lab 6:

Materials

• Chemicals:

  • Salicylic acid
  • Acetic anhydride
  • Sulfuric acid (catalyst)
  • Distilled water
  • Ethanol

• Equipment:

  • Erlenmeyer flask
  • Beaker
  • Graduated cylinder
  • Pipette
  • Stirring rod
  • Hot plate
  • Ice bath
  • Vacuum filtration apparatus
  • Filter paper
  • Weighing balance
  • Melting point apparatus

Methods

1. Preparation:

   • Weigh out a specific amount of salicylic acid and transfer it to an Erlenmeyer flask.
   • Add acetic anhydride to the flask.
   • Add a few drops of sulfuric acid to act as a catalyst.

2. Reaction:

   • Heat the mixture gently on a hot plate for about 10-15 minutes, ensuring the reaction mixture does not boil.
   • After heating, allow the mixture to cool to room temperature.

3. Crystallization:

   • Add distilled water to the reaction mixture to quench the excess acetic anhydride.
   • Place the flask in an ice bath to induce crystallization of aspirin.

4. Filtration:

   • Set up the vacuum filtration apparatus.
   • Filter the crystallized aspirin using filter paper to separate it from the liquid.

5. Purification:

   • Wash the collected aspirin crystals with cold distilled water to remove impurities.
   • Allow the crystals to dry.

6. Analysis:

   • Weigh the dried aspirin to determine the yield.
   • Measure the melting point of the aspirin to assess its purity.

These steps should help you replicate the synthesis of aspirin in your lab.

Reaction Mechanism

Here’s a concise explanation of the reaction mechanism for the synthesis of aspirin (acetylsalicylic acid):

1. Reactants: Salicylic acid and acetic anhydride.
2. Catalyst: Sulfuric acid (H₂SO₄).

Step-by-Step Mechanism:

1. Activation of Acetic Anhydride:

   • Sulfuric acid protonates the carbonyl oxygen of acetic anhydride, making it more electrophilic.

2. Nucleophilic Attack:

   • The hydroxyl group (-OH) of salicylic acid attacks the carbonyl carbon of the activated acetic anhydride, forming a tetrahedral intermediate.

3. Formation of Aspirin:

   • The intermediate collapses, releasing acetic acid and forming acetylsalicylic acid (aspirin).

4. Quenching:

   • Excess acetic anhydride is hydrolyzed by adding water, producing additional acetic acid.

Chemical Equations:

1. Protonation:

   $$\text{Acetic Anhydride} + \text{H}_2\text{SO}_4 \rightarrow \text{Protonated Acetic Anhydride}$$Acetic Anhydride+H2​SO4​→Protonated Acetic Anhydride

2. Nucleophilic Attack:

   $$\text{Salicylic Acid} + \text{Protonated Acetic Anhydride} \rightarrow \text{Tetrahedral Intermediate}$$Salicylic Acid+Protonated Acetic Anhydride→Tetrahedral Intermediate

3. Formation of Aspirin:

   $$\text{Tetrahedral Intermediate} \rightarrow \text{Aspirin} + \text{Acetic Acid}$$Tetrahedral Intermediate→Aspirin+Acetic Acid

4. Quenching:

   $$\text{Excess Acetic Anhydride} + \text{H}_2\text{O} \rightarrow 2 \text{Acetic Acid}$$Excess Acetic Anhydride+H2​O→2Acetic Acid

This process efficiently converts salicylic acid into aspirin using acetic anhydride and sulfuric acid as a catalyst.

Results

In the CHEM 162 Lab 6 experiment on the synthesis of aspirin, the following results were obtained:

• Yield: The percent yield of aspirin was calculated to be 29%.
• Purity: The melting point range of the synthesized aspirin was determined to be 130°C–133°C, indicating its purity.
• Observations: The aspirin product was crystalline and white in appearance.

Discussion

The synthesis of aspirin in Chem 162 Lab 6 involves reacting salicylic acid with acetic anhydride, catalyzed by sulfuric acid. Here are the key points:

Implications of Results

1. Sources of Error:

   • Impurities: Presence of unreacted salicylic acid or acetic anhydride can affect purity.
   • Measurement Inaccuracies: Errors in weighing reactants or products.
   • Temperature Control: Inconsistent heating or cooling can impact reaction efficiency.

2. Efficiency of Synthesis:

   • Percent Yield: Typically, the percent yield is less than 100% due to side reactions and loss of product during purification steps like recrystallization and filtration.
   • Purity Assessment: Melting point analysis helps determine purity. Pure aspirin has a melting point of 135-136°C. Deviations indicate impurities.

3. Comparison to Theoretical Values:

   • Theoretical Yield: Calculated based on stoichiometry. Actual yield is often lower due to practical limitations.
   • Percent Yield Calculation: (Actual Yield / Theoretical Yield) * 100. A lower percent yield suggests inefficiencies or errors in the synthesis process.

Understanding these aspects helps in refining the synthesis process and improving the quality and yield of the final product.

The Synthesis of Aspirin in CHEM 162 Lab 6

The synthesis of aspirin in CHEM 162 Lab 6 involved reacting salicylic acid with acetic anhydride, catalyzed by sulfuric acid. The results showed a percent yield of 29%, indicating that only about one-third of the theoretical amount of aspirin was produced.

Results and Observations

The melting point range of the synthesized aspirin was determined to be 130°C–133°C, which is close to the expected value for pure aspirin (135-136°C). This suggests that the product is relatively pure.

The observations from the experiment included the formation of a crystalline and white aspirin product. However, the low percent yield and slightly lower melting point range indicate potential sources of error in the synthesis process. These may include impurities present in the reactants or products, measurement inaccuracies, and temperature control issues.

Efficiency and Purity

In terms of efficiency, the percent yield is typically less than 100% due to side reactions and loss of product during purification steps like recrystallization and filtration. The melting point analysis helps determine purity, and any deviations from the expected value indicate impurities in the product.

Improving the Synthesis Process

Comparing the actual yield to the theoretical yield can help identify inefficiencies or errors in the synthesis process. The percent yield calculation is a useful tool for evaluating the success of the experiment and identifying areas for improvement. Overall, understanding these aspects helps refine the synthesis process and improve the quality and yield of the final product.