Synthesis Of Acetylsalicylic Acid Lab Report

The synthesis of acetylsalicylic acid, commonly known as aspirin, is a cornerstone experiment in introductory chemistry labs, demonstrating fundamental principles of organic chemistry, including esterification, reaction mechanisms, purification techniques, and yield calculations. This lab report will delve into the procedure, results, and analysis of this essential synthesis.

Introduction: Aspirin and Its Synthesis

Aspirin, or acetylsalicylic acid, is a widely used medication for pain relief, fever reduction, and anti-inflammatory purposes. Its synthesis involves an esterification reaction between salicylic acid and acetic anhydride, catalyzed by an acid, typically sulfuric acid or phosphoric acid. This reaction replaces the hydroxyl group (-OH) on salicylic acid with an acetyl group (-COCH3), forming acetylsalicylic acid and acetic acid as a byproduct.

The chemical equation for the synthesis is:

C7H6O3 (Salicylic Acid) + C4H6O3 (Acetic Anhydride) → C9H8O4 (Acetylsalicylic Acid) + CH3COOH (Acetic Acid)

This experiment offers students a hands-on understanding of organic synthesis, reaction mechanisms, and purification techniques like recrystallization. Furthermore, it provides an opportunity to calculate the percent yield, analyze the purity of the product, and critically assess the experimental procedure.

Objectives of the Experiment

The main objectives of this experiment are:

• To synthesize acetylsalicylic acid from salicylic acid and acetic anhydride.
• To purify the synthesized product through recrystallization.
• To determine the melting point of the synthesized and recrystallized aspirin.
• To calculate the theoretical yield and percent yield of the reaction.
• To analyze the purity of the synthesized product using various tests.

Materials and Methods

This section details the materials used and the step-by-step procedure followed during the synthesis and purification of acetylsalicylic acid.

Materials

• Salicylic acid
• Acetic anhydride
• Sulfuric acid (catalyst)
• Distilled water
• Ethanol (for recrystallization)
• Iron(III) chloride solution (for purity test)
• Erlenmeyer flask
• Beakers
• Graduated cylinders
• Hot plate
• Stirring rod
• Filter paper
• Funnel
• Melting point apparatus

Procedure

1. Reaction Setup:
   • Weigh approximately 2.0 grams of salicylic acid and transfer it to a clean, dry 125 mL Erlenmeyer flask.
   • In a fume hood, carefully add 4.0 mL of acetic anhydride to the flask. Acetic anhydride is corrosive and should be handled with care.
   • Add 5 drops of concentrated sulfuric acid as a catalyst. Swirl the flask gently to mix the contents thoroughly.
2. Reaction:
   • Place the flask on a hot plate set to a moderate temperature (around 50-60°C).
   • Stir the mixture continuously using a stirring rod or a magnetic stirrer.
   • Heat the mixture for 15 minutes, ensuring the salicylic acid dissolves completely.
   • Remove the flask from the hot plate and allow it to cool to room temperature.
3. Precipitation:
   • Slowly add 50 mL of ice-cold distilled water to the flask to precipitate the acetylsalicylic acid. The water hydrolyzes any excess acetic anhydride, converting it to acetic acid.
   • Stir the mixture vigorously to ensure complete precipitation of the product.
   • Cool the flask in an ice bath for 10-15 minutes to maximize the yield of solid acetylsalicylic acid.
4. Filtration:
   • Set up a filtration apparatus using a Buchner funnel and filter paper.
   • Wet the filter paper with distilled water.
   • Carefully pour the mixture into the funnel, ensuring all the solid is transferred.
   • Wash the solid with a small amount of ice-cold distilled water to remove any remaining impurities.
   • Allow the solid to air dry on the filter paper for several minutes.
5. Recrystallization:
   • Transfer the crude acetylsalicylic acid to a clean beaker.
   • Add approximately 20 mL of ethanol to the beaker.
   • Heat the mixture gently on a hot plate, stirring continuously, until the solid dissolves completely.
   • If necessary, add more ethanol in small increments to ensure complete dissolution.
   • Remove the beaker from the hot plate and allow the solution to cool slowly to room temperature.
   • Once crystallization begins, place the beaker in an ice bath to maximize crystal formation.
6. Second Filtration:
   • Filter the recrystallized acetylsalicylic acid using a Buchner funnel and filter paper, as described in step 4.
   • Wash the crystals with a small amount of ice-cold ethanol.
   • Allow the crystals to air dry on the filter paper.
7. Drying and Weighing:
   • Transfer the dried acetylsalicylic acid to a pre-weighed watch glass.
   • Allow the solid to dry completely in a warm oven or desiccator.
   • Weigh the watch glass with the dried acetylsalicylic acid to determine the final mass of the product.
8. Melting Point Determination:
   • Determine the melting point of both the crude and recrystallized acetylsalicylic acid using a melting point apparatus.
   • Pack a small amount of the sample into a capillary tube.
   • Place the capillary tube in the melting point apparatus and slowly increase the temperature.
   • Record the temperature range at which the sample begins to melt and is completely melted.
9. Purity Test (Iron(III) Chloride Test):
   • Dissolve a small amount of salicylic acid and the synthesized acetylsalicylic acid separately in ethanol.
   • Add a few drops of iron(III) chloride solution to each solution.
   • Observe and record any color changes. A violet color indicates the presence of free salicylic acid.

Results

This section presents the data collected during the experiment, including the mass of reactants, the mass of the product, the melting points, and the observations from the purity test.

Data Collection

• Mass of salicylic acid used: 2.0 grams
• Volume of acetic anhydride used: 4.0 mL
• Mass of crude acetylsalicylic acid obtained: [Insert Value] grams
• Mass of recrystallized acetylsalicylic acid obtained: [Insert Value] grams
• Melting point of salicylic acid (literature value): 158-161°C
• Melting point of crude acetylsalicylic acid: [Insert Value] °C
• Melting point of recrystallized acetylsalicylic acid: [Insert Value] °C
• Observation of Iron(III) chloride test for salicylic acid: [Insert Observation, e.g., Intense Violet Color]
• Observation of Iron(III) chloride test for crude acetylsalicylic acid: [Insert Observation, e.g., Pale Violet Color]
• Observation of Iron(III) chloride test for recrystallized acetylsalicylic acid: [Insert Observation, e.g., No Color Change or Very Faint Violet Color]

Calculations

1. Theoretical Yield Calculation:

   • The molecular weight of salicylic acid (C7H6O3) = 138.12 g/mol
   • The molecular weight of acetylsalicylic acid (C9H8O4) = 180.16 g/mol
   • Moles of salicylic acid used = Mass of salicylic acid / Molecular weight of salicylic acid = 2.0 g / 138.12 g/mol = 0.0145 mol
   • Since the reaction is 1:1, the theoretical yield of acetylsalicylic acid is also 0.0145 mol.
   • Theoretical yield of acetylsalicylic acid in grams = Moles of acetylsalicylic acid * Molecular weight of acetylsalicylic acid = 0.0145 mol * 180.16 g/mol = [Calculate Value] grams

2. Percent Yield Calculation:

   • Percent Yield = (Actual Yield / Theoretical Yield) * 100 = (Mass of recrystallized acetylsalicylic acid obtained / Theoretical yield of acetylsalicylic acid) * 100 = ([Insert Value] g / [Insert Value] g) * 100 = [Calculate Value]%

Discussion

The synthesis of acetylsalicylic acid involved the esterification of salicylic acid with acetic anhydride in the presence of sulfuric acid as a catalyst. The reaction produced acetylsalicylic acid and acetic acid. The crude product was then purified by recrystallization to remove impurities and unreacted salicylic acid.

Analysis of Results

• Yield: The percent yield of the reaction indicates the efficiency of the synthesis and purification process. A lower percent yield could be due to incomplete reaction, loss of product during filtration or recrystallization, or impurities in the starting materials.

• Melting Point: The melting point of a pure substance is a characteristic property and can be used to assess its purity. The literature value for the melting point of acetylsalicylic acid is around 135-136°C. The melting point of the synthesized acetylsalicylic acid should be close to this value if the product is pure. A broader melting point range and a lower melting point compared to the literature value indicate the presence of impurities.

• Purity Test (Iron(III) Chloride Test): Salicylic acid reacts with iron(III) chloride to form a colored complex, typically violet. Acetylsalicylic acid does not react with iron(III) chloride unless it hydrolyzes back to salicylic acid. This test is used to detect the presence of unreacted salicylic acid in the product. A positive test (violet color) indicates the presence of salicylic acid, suggesting that the product is not pure.

  The color intensity of the iron(III) chloride test can indicate the level of purity. Ideally, the recrystallized acetylsalicylic acid should show little to no color change with the iron(III) chloride solution, indicating a high degree of purity.

Error Analysis

Several factors could have contributed to errors in this experiment:

• Incomplete Reaction: The reaction may not have proceeded to completion, leaving unreacted salicylic acid in the mixture. Prolonging the reaction time or increasing the temperature slightly could improve the yield.
• Loss of Product During Filtration: Some product may have been lost during the filtration steps. Ensuring that all solid is transferred to the filter paper and minimizing the volume of wash solvent can help reduce this loss.
• Impurities: The starting materials or solvents may have contained impurities that affected the yield and purity of the product. Using high-quality reagents and solvents can minimize this issue.
• Inaccurate Measurements: Inaccurate measurements of the reactants or solvents could have affected the stoichiometry of the reaction and the yield of the product. Careful and precise measurements are essential.
• Hydrolysis: Acetylsalicylic acid can hydrolyze back to salicylic acid and acetic acid in the presence of water. This can be minimized by drying the product thoroughly and storing it in a dry environment.

Improvements to the Experiment

To improve the experiment, the following modifications could be considered:

• Optimizing Reaction Conditions: The reaction time and temperature could be optimized to maximize the yield of acetylsalicylic acid.
• Using a Different Catalyst: Other catalysts, such as phosphoric acid, could be tested to determine if they offer any advantages over sulfuric acid.
• Alternative Purification Techniques: Other purification techniques, such as column chromatography, could be used to obtain a purer product.
• Spectroscopic Analysis: Techniques like infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) could be used to characterize the synthesized product and confirm its identity and purity.
• Quantitative Analysis: Titration methods can be used to quantitatively determine the amount of acetylsalicylic acid in the synthesized product.

Conclusion

The synthesis of acetylsalicylic acid from salicylic acid and acetic anhydride provides valuable insight into organic synthesis techniques and reaction mechanisms. By performing this experiment, students gain hands-on experience in esterification, purification, and characterization of organic compounds. The analysis of the yield, melting point, and purity test results allows for a critical assessment of the experimental procedure and the factors that affect the outcome of the synthesis. While errors and limitations are inherent in any experimental process, understanding and addressing these issues can lead to improvements in future experiments and a deeper understanding of chemical principles. The recrystallization process significantly enhances the purity of the synthesized aspirin, which is confirmed by the melting point and the iron(III) chloride test. The lower the intensity of the violet color (or absence of it) in the iron(III) chloride test, the purer the synthesized aspirin is. Further refinement and optimization of the experimental conditions could potentially increase the yield and purity of the acetylsalicylic acid.

FAQ: Synthesis of Acetylsalicylic Acid

1. What is the purpose of sulfuric acid in the synthesis of aspirin?

   Sulfuric acid acts as a catalyst in the esterification reaction. It protonates the acetic anhydride, making it more susceptible to nucleophilic attack by the hydroxyl group of salicylic acid. This lowers the activation energy of the reaction, speeding up the formation of acetylsalicylic acid.

2. Why is acetic anhydride used instead of acetic acid?

   Acetic anhydride is more reactive than acetic acid in esterification reactions. It reacts more readily with the hydroxyl group of salicylic acid, leading to a higher yield of acetylsalicylic acid. Acetic acid would require a more forcing condition to esterify salicylic acid.

3. What are the hazards associated with this experiment?

   The main hazards are:

   • Acetic anhydride: Corrosive and irritating to the skin, eyes, and respiratory system.
   • Sulfuric acid: Strong acid, corrosive, and can cause severe burns.
   • Ethanol: Flammable.

   Always wear appropriate personal protective equipment (PPE), including gloves, safety glasses, and a lab coat, and work in a well-ventilated area or fume hood.

4. How does recrystallization purify the acetylsalicylic acid?

   Recrystallization is a purification technique based on the difference in solubility of the desired product and impurities in a solvent. By dissolving the crude acetylsalicylic acid in a hot solvent (ethanol in this case) and then allowing it to cool slowly, the acetylsalicylic acid crystallizes out of the solution. Impurities that are more soluble in the solvent remain in the solution and are removed during filtration.

5. What does a wide melting point range indicate?

   A wide melting point range indicates the presence of impurities in the sample. Pure substances have a sharp melting point, typically within a 1-2°C range. Impurities disrupt the crystal lattice, causing the sample to melt over a broader temperature range.

6. Why is it important to use ice-cold water during the precipitation step?

   Ice-cold water is used to decrease the solubility of acetylsalicylic acid, maximizing the precipitation of the product. It also helps to hydrolyze any unreacted acetic anhydride, converting it into acetic acid, which is more soluble in water and can be easily removed during filtration.

7. Can this experiment be scaled up for industrial production?

   The basic principles of the synthesis remain the same, but industrial production involves significant modifications and optimizations to increase efficiency, yield, and safety. Large-scale production uses continuous reactors, automated purification systems, and rigorous quality control measures. Catalysts are also often recovered and reused to minimize waste.

8. What are some common side reactions that can occur during the synthesis?

   One common side reaction is the hydrolysis of acetylsalicylic acid back to salicylic acid and acetic acid, especially in the presence of water. This can be minimized by controlling the reaction conditions and drying the product thoroughly. Another potential side reaction is the formation of diacetylsalicylic acid if the reaction is carried out under harsh conditions with excess acetic anhydride.

9. How does the iron(III) chloride test work?

   The iron(III) chloride test detects the presence of phenols, such as salicylic acid. Salicylic acid reacts with iron(III) chloride to form a complex, which results in a colored solution, typically violet. Acetylsalicylic acid, being an ester, does not react with iron(III) chloride unless it is hydrolyzed back to salicylic acid. The intensity of the color is proportional to the concentration of salicylic acid present.

10. What are some applications of acetylsalicylic acid besides pain relief?

    Besides pain relief, fever reduction, and anti-inflammatory purposes, acetylsalicylic acid is also used as an antiplatelet agent to prevent blood clots. It is commonly prescribed to individuals at risk of heart attack or stroke. Research also suggests potential roles for aspirin in preventing certain types of cancer, but more studies are needed to confirm these findings.