3.9.4Organic Chemistry
Preparing a Pure Organic Solid
5 min read 6 parts Lesson 36 of 55 Virtual lab
Section 1 of 6Preparation of Aspirin
Aspirin is made by acylating salicylic acid with ethanoic anhydride.
1
Weigh a known mass of salicylic acid into a conical flask and add ethanoic anhydride with a few drops of concentrated sulfuric acid as catalyst.
2
Warm the flask in a water bath at about 60 °C for around twenty minutes, swirling to mix.
3
Cool the flask and pour the mixture into cold water, which hydrolyses leftover anhydride and makes the aspirin crystallise.
4
Collect the crude solid by vacuum filtration using a Buchner funnel and wash with a little cold water.
5
Allow the solid to dry and weigh it before purification.
Section 2 of 6Purification by Recrystallisation
Recrystallisation removes soluble impurities from the crude solid.
Dissolve
- Dissolve the crude aspirin in the minimum volume of hot solvent, so that the solution is nearly saturated.
Cool
- Cool the solution slowly, then in ice, so that pure crystals form while impurities stay dissolved.
Filter
- Filter off the crystals with a Buchner funnel, which is faster because suction pulls the liquid through.
Dry
- Dry the product on filter paper or in a warm oven well below its melting point.
Yield loss
- Some product stays dissolved in the solvent, so the yield after recrystallisation is lower than before.
Section 3 of 6Testing Purity
Testing Purity
Melting point
- A pure solid melts sharply at one temperature, and impurities lower the melting point and widen the range.
Method
- Pack a small amount of solid into a capillary tube and heat it slowly beside a thermometer, noting the temperature at which it melts.
Repeat readings
- Repeat several times and use a slow heating rate near the melting point to improve accuracy.
Comparison
- Compare the result with the data book value for aspirin;
- a close match to a narrow range indicates a pure sample.
Section 4 of 6Safety and Evaluation
Safety and Evaluation
Hazards
- Ethanoic anhydride is corrosive and flammable and concentrated sulfuric acid is corrosive, so wear eye protection and gloves.
Heating
- Use a water bath for warming because the organic liquids are flammable and should not meet a naked flame.
Causes of low yield
- Incomplete reaction, losses during transfer, product remaining in solution and loss in recrystallisation all reduce the yield.
Sources of impurity
- Unreacted salicylic acid and traces of ethanoic acid may be present, so washing and recrystallisation are essential.
Section 5 of 6Worked Example
Question
- 5.00 g of salicylic acid (Mr = 138.1) is converted into aspirin (Mr = 180.2) and 3.10 g of pure aspirin is obtained.
- Calculate the percentage yield.
- Moles of salicylic acid = 5.00 ÷ 138.1 = 0.0362 mol.
- The equation C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH shows a 1:1 ratio, so the maximum is 0.0362 mol of aspirin.
- Theoretical mass = 0.0362 × 180.2 = 6.52 g.
- Percentage yield = (3.10 ÷ 6.52) × 100 = 47.5%.
Answer The percentage yield of aspirin is 47.5%.
Section 6 of 6Exam Checklist & Tips
3.9.4 – Key Points for the Exam
- Recrystallisation Purification of a solid by dissolving it in hot solvent and allowing pure crystals to form on cooling.
- Buchner funnel A funnel used with suction to filter solids quickly.
- Percentage yield Actual mass obtained divided by the theoretical mass, multiplied by 100.
- Melting point range The temperature interval over which a solid melts, narrow for a pure substance.
Common Mistakes
- Using a large volume of solvent for recrystallisation, which keeps most of the product dissolved.
- Heating the melting point tube quickly, so the true melting temperature is overshot.
- Claiming a higher melting point means more impurity, when impurities lower the melting point.
- Calculating the theoretical yield from the mass of ethanoic anhydride, which is in excess.