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CP16 prepares aspirin from 2-hydroxybenzoic acid and ethanoic anhydride, then isolates and assesses the solid. The phenolic O–H group is acylated to form an ester while the carboxylic-acid group remains. Identify the limiting reagent and distinguish chemical synthesis from the later purification stages.
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Explain aspirin synthesis from the specified acid and anhydride
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why minimise the hot recrystallisation solvent volume?
To reduce product left dissolved after cooling. A larger cold solvent volume retains more dissolved product at its solubility limit.
After watching 2: Why wash crystals with a small amount of cold solvent?
To remove adhering solution while limiting product dissolution. Soluble surface impurities can be removed with limited loss of product.
What you'll be able to do
CP16 prepares aspirin from 2-hydroxybenzoic acid and ethanoic anhydride, then isolates and assesses the solid. The phenolic O–H group is acylated to form an ester while the carboxylic-acid group remains. Identify the limiting reagent and distinguish chemical synthesis from the later purification stages.
Recrystallisation relies on the product being appreciably soluble in a suitable hot solvent and much less soluble when cold. A good solvent also has a useful behaviour for impurities and does not react with the product. Add the minimum sufficient hot solvent specified by the method. Excess solvent leaves more product dissolved at the final temperature, lowering recovery.
If insoluble impurity is present, an appropriate hot filtration removes it before crystals form; the apparatus and method should limit premature crystallisation. Allow the filtrate to cool so the product crystallises, then use the specified further cooling. Collect crystals by suction filtration using sound apparatus designed for reduced pressure. Wash with a small amount of cold solvent to remove surface solution while limiting product dissolution.
Dry the product by the approved method before weighing or measuring a melting range. A wet crystal mass is not a valid isolated product yield. Handle hot solvent, flammable vapours and vacuum glassware according to the laboratory procedure. A higher crude mass can mean more retained impurity rather than better synthesis.
Report a melting range, from first visible melting to complete melting, with a controlled heating rate near the expected value. An impure solid often melts over a broader range at a depressed temperature, but agreement with a literature value is supporting evidence rather than absolute proof. Sample packing, residual solvent, thermometer calibration and fast heating can distort the range.
Calculate percentage yield as dry recovered product divided by theoretical product, using matching units. Distinguish reaction yield from recrystallisation recovery: recovery compares purified mass with the crude mass processed, which itself may contain impurities. Evaluate whether a lower recovered mass reflects product left in solution or successful removal of unwanted material.
The overall equation is C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. One mole of 2-hydroxybenzoic acid can form one mole of aspirin. The acid-catalysed acylation uses the approved warming arrangement, followed by the specified controlled addition of water and cooling. Water consumes remaining anhydride as well as changing the mixture’s solubility conditions.
Ethanoic anhydride and the acid catalyst are corrosive; reaction and hydrolysis can release heat and irritating vapour. Follow the laboratory’s protection, ventilation, heating and controlled addition method. Do not interpret a written guide as permission to improvise quantities or use an ignition source with flammable solvent.
Unreacted 2-hydroxybenzoic acid can remain as an impurity. Recrystallisation, filtration, washing and drying address product separation, while a melting range and other appropriate evidence assess purity and identity. Residual acid, solvent or starting material can increase a crude mass. A higher crude mass therefore need not mean a better isolated aspirin preparation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A crude product melts at 119–127 °C and a purified sample at 130–132 °C. The expected range is 131–133 °C. What can be concluded?
Purification appears effective; the melting result alone does not prove complete purity.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Practise the sequence in a virtual laboratory, then explain how crystallisation and drying affect yield and purity. Best on a larger screen.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.