Making Aspirin A Level Chemistry

Making Aspirin: A Comprehensive Guide for A-Level Chemistry Students

Aspirin, or acetylsalicylic acid, is a ubiquitous drug used globally as an analgesic (pain reliever), antipyretic (fever reducer), and anti-inflammatory agent. Its synthesis is a classic organic chemistry experiment, providing A-Level students with valuable practical experience in esterification reactions and purification techniques. This article will delve into the process of making aspirin, covering the theoretical background, practical procedure, analysis techniques, and potential safety considerations. Understanding this synthesis reinforces key concepts in organic chemistry, including reaction mechanisms, yield calculations, and the importance of purification methods.

Introduction: Understanding the Esterification Reaction

The synthesis of aspirin involves an esterification reaction, specifically the acetylation of salicylic acid. Salicylic acid contains both a carboxylic acid (-COOH) group and a hydroxyl (-OH) group on the benzene ring. The hydroxyl group is responsible for the undesirable side effects of salicylic acid, such as stomach irritation. By reacting salicylic acid with ethanoic anhydride (acetic anhydride), we replace the hydroxyl group with an acetyl group (-COCH3), forming the less irritating acetylsalicylic acid (aspirin). This reaction is catalyzed by an acid, typically concentrated sulfuric acid.

The reaction mechanism involves nucleophilic acyl substitution. The hydroxyl group of salicylic acid acts as a nucleophile, attacking the carbonyl carbon of ethanoic anhydride. This leads to the formation of a tetrahedral intermediate, which then collapses to form aspirin and ethanoic acid (acetic acid) as byproducts.

Materials and Apparatus: Gathering Your Essentials

Before embarking on the synthesis, ensure you have the necessary materials and apparatus. Safety precautions should always be prioritized. Wear appropriate safety goggles and gloves throughout the experiment. The following are required:

• Chemicals:
  • Salicylic acid (a precise mass, typically around 2g)
  • Ethanoic anhydride (excess, typically around 4ml)
  • Concentrated sulfuric acid (a few drops as a catalyst)
  • Distilled water (for washing and recrystallization)
  • Ice (for cooling)
• Apparatus:
  • Beaker (for the reaction)
  • Measuring cylinder (for measuring liquids)
  • Electronic balance (for accurate mass measurements)
  • Pipette and pipette filler (for accurate liquid measurements)
  • Thermometer (to monitor temperature)
  • Hot plate (for heating)
  • Buchner funnel and flask (for filtration)
  • Filter paper
  • Ice bath
  • Watch glass (for drying the product)

Procedure: Step-by-Step Aspirin Synthesis

The synthesis of aspirin is a relatively straightforward process, but meticulous attention to detail is crucial for a high yield and pure product. Follow these steps carefully:

1. Weighing the Salicylic Acid: Accurately weigh approximately 2g of salicylic acid using an electronic balance. Record the exact mass.

2. Adding Ethanoic Anhydride and Sulfuric Acid: Carefully add 4ml of ethanoic anhydride to the beaker containing the salicylic acid. Then, add a few drops (around 5-10 drops) of concentrated sulfuric acid as a catalyst. Swirl the mixture gently to ensure thorough mixing. Caution: Concentrated sulfuric acid is highly corrosive; handle it with extreme care.

3. Heating and Monitoring the Reaction: Heat the mixture gently using a hot plate, maintaining a temperature between 50-60°C. Monitor the temperature closely using a thermometer. The reaction is exothermic, so the temperature might rise initially. Continue heating for approximately 15-20 minutes, ensuring the mixture remains gently heated but does not boil. Observe any changes in the appearance of the mixture; it should become more homogenous.

4. Cooling and Hydrolysis: Remove the beaker from the hot plate and allow it to cool slightly. Carefully add 50ml of cold distilled water to the reaction mixture. Caution: The addition of water can cause some vigorous fizzing due to the hydrolysis of any remaining ethanoic anhydride. This step converts any unreacted ethanoic anhydride into ethanoic acid.

5. Filtration and Purification: Filter the resulting mixture using a Buchner funnel and flask under reduced pressure (vacuum filtration). This will separate the solid aspirin from the aqueous solution containing ethanoic acid and other impurities. Wash the solid product on the filter paper with cold distilled water to remove any residual impurities.

6. Recrystallization: Transfer the crude aspirin to a beaker and add a minimum amount of hot distilled water (around 50-75 ml) until it just dissolves. The aim is to obtain a saturated solution. Allow the solution to cool slowly to room temperature, and then place the beaker in an ice bath to encourage crystallization.

7. Drying and Weighing the Product: Filter the recrystallized aspirin using the Buchner funnel again. Wash with a small amount of cold water. Allow the product to dry completely on a watch glass. Once dry, weigh the purified aspirin and record the mass.

8. Calculating the Percentage Yield: Calculate the percentage yield of the reaction using the following formula:

Percentage Yield = (Actual yield / Theoretical yield) x 100%

The theoretical yield can be calculated based on the stoichiometry of the reaction and the initial mass of salicylic acid used.

Analysis Techniques: Confirming Aspirin Synthesis

Several techniques can be employed to confirm the successful synthesis of aspirin and assess its purity.

• Melting Point Determination: The melting point of pure aspirin is approximately 135°C. Determining the melting point of the synthesized aspirin and comparing it to the literature value helps assess its purity. A lower or broader melting point range indicates impurities.

• Thin Layer Chromatography (TLC): TLC can be used to compare the synthesized aspirin with a known sample of pure aspirin. Similar Rf values (retention factors) indicate the presence of aspirin in the synthesized product. This technique also helps identify potential impurities.

• Infrared (IR) Spectroscopy: IR spectroscopy provides a "fingerprint" of a molecule, indicating the presence of specific functional groups. The IR spectrum of aspirin should show characteristic peaks corresponding to the carbonyl (C=O) groups in both the ester and carboxylic acid moieties, and the absence of the hydroxyl (O-H) stretch from the salicylic acid starting material.

• Titration: The purity of the synthesized aspirin can be determined through acid-base titration. Aspirin is a weak acid and can be titrated against a standard solution of a strong base, such as sodium hydroxide (NaOH).

Safety Precautions: Handling Chemicals Responsibly

• Concentrated Sulfuric Acid: This is a highly corrosive acid. Wear appropriate safety goggles and gloves when handling it. Add it slowly and carefully to the reaction mixture. In case of spills, immediately neutralize with sodium bicarbonate solution.
• Ethanoic Anhydride: This is an irritant. Avoid inhaling the vapours and wear gloves.
• Disposal: Dispose of all waste materials responsibly according to your school or institution's guidelines.

Frequently Asked Questions (FAQ)

Q: Why is sulfuric acid used as a catalyst?

A: Sulfuric acid protonates the carbonyl oxygen of the ethanoic anhydride, making the carbonyl carbon more electrophilic and thus more susceptible to nucleophilic attack by the hydroxyl group of salicylic acid. This accelerates the reaction rate.

Q: What are the potential side products of this reaction?

A: The main side product is ethanoic acid (acetic acid), formed from the hydrolysis of unreacted ethanoic anhydride. Other impurities might arise from incomplete reaction or side reactions.

Q: Why is recrystallization important?

A: Recrystallization is a purification technique that takes advantage of the difference in solubility of the desired product (aspirin) and impurities at different temperatures. By dissolving the crude product in hot solvent and allowing it to cool slowly, pure crystals of aspirin precipitate out, leaving impurities in solution.

Q: What if my percentage yield is low?

A: A low percentage yield could be due to several factors, including incomplete reaction, loss of product during filtration, or the presence of significant impurities. Carefully review your experimental procedure and techniques to identify potential areas for improvement.

Conclusion: A Rewarding Synthesis

Synthesizing aspirin is a valuable A-Level chemistry experiment that reinforces fundamental concepts in organic chemistry, including reaction mechanisms, purification techniques, and yield calculations. By meticulously following the procedure and applying appropriate analytical techniques, students can gain hands-on experience and produce a relatively pure sample of aspirin. Remember that safety is paramount; always wear appropriate personal protective equipment and follow safe disposal procedures. Understanding this synthesis provides a strong foundation for further exploration of organic chemistry and medicinal chemistry. The process, though seemingly simple, requires precision and highlights the importance of careful experimental design and execution for achieving a successful outcome.