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6.2.5Unit 6. Practical Skills in Chemistry II

Aspirin Preparation — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 14 of 14 Unit quiz

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Part 1 of 6The Synthesis

6.2.5 Aspirin Preparation

Aspirin is the ester 2-ethanoyloxybenzoic acid.

Equation
  • 2-hydroxybenzoic acid reacts with ethanoic anhydride: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH.
Reaction type
  • The phenol –OH group is acylated, forming an ester link, so the process is esterification using an anhydride.
Catalyst
  • A few drops of concentrated phosphoric acid or sulfuric acid speed up the reaction.
Why not acid
  • Ethanoic anhydride is used instead of ethanoic acid because it reacts to completion rather than giving an equilibrium mixture.

Part 2 of 6Practical Steps

1
Weigh 2-hydroxybenzoic acid into a conical flask, add ethanoic anhydride and a few drops of the acid catalyst in a fume cupboard.
2
Warm the flask in a water bath at about 60 °C for 15 to 20 minutes with occasional swirling.
3
Add cold water carefully to destroy excess anhydride and cool the flask in ice to crystallise the product.
4
Filter the crystals under reduced pressure using a Buchner funnel and wash with ice-cold water.
5
Recrystallise from the minimum volume of hot ethanol and water, filter and dry the pure crystals.

Part 3 of 6Checking Purity

Checking Purity
Melting point
  • Pure aspirin melts sharply at about 135 °C, while an impure sample melts lower and over a wider range.
Iron(III) chloride test
  • Unreacted 2-hydroxybenzoic acid gives a purple colour with neutral iron(III) chloride, because it is a phenol.
TLC
  • A single spot with the same Rf as an aspirin standard indicates good purity.
IR spectrum
  • The pure product shows an ester C=O absorption and no unwanted phenolic O–H.

Part 4 of 6Yield

Percentage yield
  • Percentage yield = actual mass ÷ theoretical mass × 100.
Limiting reagent
  • Use the reagent in smaller molar proportion, which is the 2-hydroxybenzoic acid if anhydride is in excess.
Yield loss
  • Product is lost during transfers, filtration, recrystallisation and because some aspirin stays dissolved.
Safety
  • Ethanoic anhydride is corrosive and a lachrymator, so use gloves, goggles and a fume cupboard.

Part 5 of 6Worked Example

Question

  • 2.00 g of 2-hydroxybenzoic acid (Mr = 138.12) gives 1.80 g of aspirin (Mr = 180.16).
  • Calculate the percentage yield.

  1. Moles of acid = 2.00 ÷ 138.12 = 0.01448 mol.
  2. Theoretical moles of aspirin = 0.01448 mol, giving a mass of 0.01448 × 180.16 = 2.609 g.
  3. Percentage yield = 1.80 ÷ 2.609 × 100 = 69.0%.

Answer Percentage yield = 69.0%

Part 6 of 6Exam Checklist & Tips

6.2.5 – Key Points for the Exam

  • Esterification A reaction forming an ester from an acid derivative and an alcohol or phenol.
  • Ethanoic anhydride (CH₃CO)₂O, an acylating agent that is more reactive than ethanoic acid.
  • Recrystallisation Purifying a solid by dissolving in hot solvent and letting it crystallise on cooling.
  • Percentage yield Actual mass obtained divided by the theoretical maximum, multiplied by 100.

Common Mistakes

  • Using ethanoic acid rather than the anhydride and expecting a high yield.
  • Using too much hot solvent in recrystallisation, which reduces the amount of crystals.
  • Using the mass of ethanoic anhydride to calculate the theoretical yield when it is in excess.
  • Forgetting to add cold water to destroy the excess anhydride before filtering.

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