Synthesis of aspirin

Summary

In this video, we follow the detailed process of synthesizing aspirin from salicylic acid and acetic anhydride using sulfuric acid as a catalyst. The reaction occurs at 45–50°C, with the product precipitating out of the solution. After filtering and recrystallizing, the aspirin is purified and tested for purity through qualitative FeCl₃ and IR spectroscopy. The video demonstrates the steps of measuring, heating, crystallizing, and purifying the compound, highlighting key techniques like recrystallization, vacuum filtration, and chemical testing, ensuring the synthesis of pure aspirin.

Takeaways

• 😀 The experiment involves synthesizing aspirin using 2.1 grams of salicylic acid and 5 milliliters of acetic anhydride, with sulfuric acid as a catalyst.
• 😀 Temperature control is crucial, with the reaction being held at 45-50°C for five minutes to ensure proper reaction time.
• 😀 Proper mass measurements are important, and any excess salicylic acid should not be returned to the stock bottle to avoid contamination.
• 😀 The reaction solution is heated in a water bath to ensure even temperature, using a thermometer to monitor the temperature throughout the process.
• 😀 A small magnetic stirrer is used to avoid manually stirring the reaction mixture, ensuring better consistency and efficiency.
• 😀 The aspirin product begins to crystallize once the reaction mixture is cooled, but if crystallization doesn’t occur naturally, the process can be assisted by scratching the inside of the flask.
• 😀 Cold water is added to the solution to facilitate the extraction of the aspirin from the flask before filtration, with care taken not to dissolve the aspirin.
• 😀 The crude product is filtered using a vacuum filtration system, followed by a rinse with cold water to remove impurities.
• 😀 A small sample of the crude product is set aside for purity testing before recrystallization.
• 😀 Recrystallization is performed using a combination of boiling ethanol and warm water to dissolve and purify the aspirin, with careful temperature management to prevent solvent evaporation.
• 😀 The purity of the final aspirin is tested using a chemical test (FeCl3), where the absence of a purple color confirms the purity by indicating the absence of phenolic groups typical of salicylic acid.

Q & A

What is the purpose of using concentrated sulfuric acid in the aspirin synthesis?

Concentrated sulfuric acid acts as a catalyst in the reaction, speeding up the esterification of salicylic acid with acetic anhydride without being consumed in the reaction.

Why is it important to maintain the reaction temperature between 45–50°C?

Maintaining the temperature between 45–50°C ensures the reaction proceeds efficiently without decomposing the reactants or products. Higher temperatures could lead to unwanted side reactions.

Why should excess salicylic acid not be returned to the stock bottle?

Returning excess salicylic acid could contaminate the stock, introducing impurities that may affect the accuracy and purity of future experiments.

What is the purpose of using a magnetic stirrer during the reaction?

The magnetic stirrer continuously mixes the reactants, ensuring uniform heating and better interaction between salicylic acid and acetic anhydride for efficient reaction.

How are aspirin crystals initially formed after the reaction?

Aspirin crystals form as the reaction mixture cools to room temperature. If crystallization is slow, scratching the inside of the flask with a glass rod provides nucleation sites to initiate crystal growth.

Why is ice-cold water added before filtration of the crude aspirin?

Ice-cold water helps to solidify the aspirin, making it easier to filter, and minimizes the dissolution of aspirin into the water, preserving yield.

What is the purpose of recrystallizing aspirin, and why is a mixed solvent used?

Recrystallization purifies aspirin by removing impurities. A mixed solvent of ethanol and water is used to dissolve aspirin at high temperature and allow it to recrystallize upon cooling while avoiding side reactions.

How does the FeCl3 test help determine aspirin purity?

FeCl3 reacts with phenol groups to produce a purple color. Pure aspirin has no phenol group, so a lack of purple indicates successful conversion from salicylic acid and high purity.

Why is a small amount of sample used for IR spectroscopy?

Using a thin layer of sample on the IR diamond window ensures that IR light can pass through the sample, providing an accurate spectrum without saturating the detector.

What indicates successful synthesis of pure aspirin in the IR spectrum?

A successful IR spectrum shows the characteristic ester C=O stretch around 1750 cm⁻¹, absence of the phenol O–H stretch from salicylic acid, and other expected peaks corresponding to aspirin's functional groups.

Why is scratching the inside of the glass beaker an effective method for crystal formation?

Scratching provides small rough surfaces that act as nucleation sites, encouraging the dissolved aspirin molecules to start forming solid crystals.

Why is it important to minimize rinsing during filtration of pure aspirin?

Excessive rinsing with water could dissolve some of the purified aspirin, reducing the yield. Only minimal rinsing is done to remove impurities while preserving the product.