Make an organic chemistry practical report (20 marks)

Experiment 3: Preparation of Aspirin (Acetyl Salicylic Acid)

1. Objective
To synthesize aspirin (acetylsalicylic acid) from salicylic acid and acetic acid via an esterification reaction, and to determine its percentage yield and melting point.

2. Materials

• Salicylic acid (1.5 g)
• Acetic acid (6 mL)
• 85% Sulfuric acid or Phosphoric acid (catalyst)
• Distilled water (20 mL + for recrystallization)
• Conical flask
• Water bath
• Ice bath
• Heating plate
• Filter funnel
• Filter paper
• Beakers
• Thermometer
• Weighing balance

3. Procedure

1. Place 1.5 g of salicylic acid into a clean conical flask.
2. Add 6 mL of acetic acid and a few drops of 85% sulfuric acid or phosphoric acid to the flask.
3. Heat the mixture in a water bath at 90 °C for 3 minutes.
4. Remove the flask from the water bath. While the mixture is still hot, add 20 mL of distilled water.
5. Allow the flask to stand at room temperature until crystallization begins.
6. Cool the flask in an ice bath to maximize crystal formation.
7. Filter the crystals using a Büchner funnel or gravity filtration.
8. Recrystallize the crude aspirin by dissolving it in a minimum amount of hot water, then cooling to allow pure crystals to form. Filter and dry the recrystallized product.
9. Weigh the dried, purified aspirin to determine the actual yield.
10. Determine the melting point of the purified aspirin using a melting point apparatus.

4. Reaction Equation
The synthesis of aspirin involves the esterification of salicylic acid with acetic acid, catalyzed by a strong acid.

$$\text{C}_7\text{H}_6\text{O}_3 \text{ (Salicylic Acid)} + \text{C}_2\text{H}_4\text{O}_2 \text{ (Acetic Acid)} \xrightarrow{\text{H}_2\text{SO}_4/\text{H}_3\text{PO}_4} \text{C}_9\text{H}_8\text{O}_4 \text{ (Aspirin)} + \text{H}_2\text{O} \text{ (Water)}$$

5. Calculations

• Molar Masses:

  • Salicylic acid ($\text{C}_7\text{H}_6\text{O}_3$): $138.12 \text{ g/mol}$
  • Acetic acid ($\text{C}_2\text{H}_4\text{O}_2$): $60.05 \text{ g/mol}$
  • Aspirin ($\text{C}_9\text{H}_8\text{O}_4$): $180.16 \text{ g/mol}$

• Moles of Reactants:

  • Moles of salicylic acid: $$\text{Moles} = \frac{\text{Mass}}{\text{Molar Mass}} = \frac{1.5 \text{ g}}{138.12 \text{ g/mol}} = 0.01086 \text{ mol}$$
  • Moles of acetic acid (assuming density of acetic acid $\approx 1.05 \text{ g/mL}$): $$\text{Mass} = \text{Volume} \times \text{Density} = 6 \text{ mL} \times 1.05 \text{ g/mL} = 6.3 \text{ g}$$ $$\text{Moles} = \frac{6.3 \text{ g}}{60.05 \text{ g/mol}} = 0.1049 \text{ mol}$$

• Limiting Reactant: The reaction ratio between salicylic acid and acetic acid is 1:1. Since $0.01086 \text{ mol}$ of salicylic acid is less than $0.1049 \text{ mol}$ of acetic acid, salicylic acid is the limiting reactant.

• Theoretical Yield of Aspirin: Based on the limiting reactant, the theoretical yield of aspirin is:

  $$\text{Theoretical Yield} = \text{Moles of Salicylic Acid} \times \text{Molar Mass of Aspirin}$$ $$\text{Theoretical Yield} = 0.01086 \text{ mol} \times 180.16 \text{ g/mol} = 1.956 \text{ g}$$

  The theoretical yield of aspirin is $\boxed{\text{1.96 g}}$.

• Percentage Yield: To calculate the percentage yield, the actual yield (mass of purified aspirin obtained experimentally) is required.

  $$\text{Percentage Yield} = \frac{\text{Actual Yield (g)}}{\text{Theoretical Yield (g)}} \times 100\%$$

  (Note: Actual yield data is needed to complete this calculation.)

6. Results

• Theoretical Yield: 1.96 g
• Actual Yield: (To be determined experimentally)
• Percentage Yield: (To be calculated after determining actual yield)
• Melting Point of Aspirin: (To be determined experimentally) The expected melting point for pure aspirin is approximately $135 \text{ °C}$. A lower or broader melting point range would indicate impurities.

7. Discussion
The synthesis of aspirin is an esterification reaction. The strong acid catalyst protonates the carbonyl oxygen of acetic acid, making the carbonyl carbon more electrophilic and susceptible to nucleophilic attack by the hydroxyl group of salicylic acid. Heating accelerates the reaction. The addition of water after heating helps to precipitate the aspirin, as aspirin is less soluble in water than salicylic acid. Cooling in an ice bath further enhances crystallization. Recrystallization from hot water is crucial for purifying the crude product, removing unreacted starting materials and byproducts. The percentage yield indicates the efficiency of the synthesis, while the melting point provides an assessment of the purity of the synthesized aspirin.