Synthesis of Aspirin (OCR A Level Chemistry A): Revision Note

PAG 6: Synthesis of an organic solid

• This practical covers a number of key laboratory skills

  • use appropriate apparatus to record a range of measurements

  • use water bath or electric heater or sand bath for heating

  • use laboratory apparatus for a variety of experimental techniques

  • purify a solid product by recrystallisation

  • use melting point apparatus

  • safely and carefully handle solids and liquids

• There are a few choices of experiments that are commonly done in schools which cover these laboratory skills and three which can be done are

  • PAG 6.1: The preparation of aspirin

  • PAG 6.2: The purification of benzoic acid

  • PAG 6.3: The nitration of methyl benzoate

PAG 6.1: Synthesis of aspirin

Method

• Measure 6.0 g of salicylic acid into a conical flask

• Add 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid

• Swirl the mixture and place the flask in a warm water bath at around 60 ^oC for 20 minutes

• Allow the flask to cool

• Pour the contents into 75 cm³ of cold water in a beaker

• Wait for the aspirin to crystallise out

• Recover the aspirin using Buchner filtration and leave it to dry

Recrystallisation and melting point test

• Recrystallise the impure aspirin using ethanol

• Once the solid dissolves, pour the solution into 40 cm³ of cold water

• Recover the purified crystals using Buchner filtration, as before

• The melting point of pure aspirin is 135 ^oC

  • You can assess product purity by comparing to this value

Recrystallisation

• Recrystallisation is used to purify impure solids

• Hot solvent is used to dissolve both the organic solid and the impurities

  • Then as the solution cools the organic solid crystallises out

  • This leaves behind the impurities in the solution

• The key is to use the minimum amount of solvent to dissolve the solid and avoid loss of the product

• If any solid impurities remain in the solution, a hot filtration can be carried out

• Once the solution has cooled down to room temperature and crystallised then the product crystals can be recovered by filtration

• This is faster using Buchner apparatus in which filtration occurs under reduced pressure

• After filtration the product is washed with fresh cold solvent and then allowed to dry on filter paper

Melting point analysis

• The melting point of a solid provides insight into both its identity and purity

• You can match a sample's melting point to a known value to help identify or confirm the product

• A pure solid melts sharply and closely matches the data book value

• Impurities lower the melting point and broaden the temperature range over which the solid melts

  • A pure substance has a sharp, well-defined melting point

  • An impure substance melts over a wide temperature range

• Although techniques can vary depending on the apparatus, key skills include:

  • Correctly preparing melting point tubes

  • Heating the sample slowly to prevent overshooting

  • Repeating measurements to improve accuracy (three runs is typical)

• Ensure the sample is completely dry and finely powdered

  • Crush it using the back of a spatula on filter paper or a tile to remove moisture

• Use the first trial to estimate the melting point range, then repeat using a slower heating rate for precision

Practical skills reminder

• This practical provides an excellent opportunity to develop several assessed skills, including:

  • Accurately measuring solids and liquids

  • Using water baths to heat mixtures safely

  • Purifying solids through recrystallisation

  • Determining melting points to assess product purity

  • Handling, drying, and preparing solid samples correctly