Preparing a Pure Organic Solid (Oxford AQA International A Level Chemistry)

Required Practical 10: Pure Organic Solid Preparation

• Prepare a pure organic solid and test its purity

• To prepare a sample of aspirin

HOOCC₆H₄OH + (CH₃CO)₂O → HOOCC₆H₄OCOCH₃ + CH₃COOH

Apparatus

Preparation of aspirin

• salicylic acid

• 100 cm³ conical flask

• 10 cm³ measuring cylinder

• ethanoic anhydride

• concentrated sulfuric acid

• 400 cm³ beaker

• tripod, gauze and Bunsen burner

• thermometer

• 250 cm³ beaker

• Buchner funnel

• filter paper

• stirring rod

• deionised or distilled water in a wash bottle

• spatula

Recrystallisation and testing the purity

• 25 cm³ measuring cylinder

• boiling tube

• ethanol

• thermometer

• deionised or distilled water in a wash bottle

• 100 cm³ conical flask

• stirring rod

• kettle

• digital mass balance (reading to 2 decimal places)

• thermometer (0 ^oC to 250 ^oC)

• melting point apparatus

• tripod, gauze and Bunsen burner

• rubber rung to attach melting point tube to the thermometer

• melting point tubes

• watch glass

• spatula

Method - preparation

1. 6.00 g of salicylic acid is added to a conical flask along with 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid

2. The mixture is swirled and held in a warm water bath around 60 ^oC for about 20 minutes

3. The flask is then allowed to cool and the contents are added to 75 cm³ cold water in a beaker at which point the aspirin crystallises out

4. The aspirin is recovered using Buchner filtration and left to dry

Diagram

Method - recrystallisation

1. Measure out 15 cm³ of ethanol into a boiling tube using a 25 cm³ measuring cylinder

2. Half-fill a beaker with hot water at a temperature of 75 ^oC

3. Use a spatula to add the crude aspirin to the boiling tube and place the tube in the beaker of hot water

   • Do not scrape the filter paper in this step

4. Stir until the aspirin dissolves into the ethanol

5. Pour this solution containing dissolved aspirin into roughly 40 cm³ of water in a 100 cm³ conical flask and gently warm the contents of the flask until the solution is complete

6. Allow the solution to cool in an ice bath

   • Crystals of aspirin will begin to form

7. Filter the purified solid using a Buchner funnel and allow to dry on the filter paper

8. Record the mass of the dry purified solid

Diagram

Method - melting point test

1. Powder a sample of the prepared sample by crushing with a spatula

2. Fill three melting point tubes with the organic solid to a depth of 0.5 cm

3. Transfer this to the melting point apparatus

4. Heat the oil bath gently and observe the temperature at which the solid melts

5. Record the temperature and repeat with the two other tubes

   • These readings can be taken more slowly as the approximate value recorded from the first reading is approached

6. On the basis of the recorded temperatures, deduce the melting point of your solid

Diagram

Practical tip

• During recrystallisation, scratching the inside of the beaker will help crystals to form

Results

• Calculate the theoretical yield which can be formed from 6.00 g of salicylic acid

• Use this value to calculate the percentage yield

• Compare the melting point recorded to that of the data book value of 136 ^oC

• The proximity of a melting point to the actual data book value can express purity

  • Impurities tend to lower the melting point of a solid

Evaluation

• The proximity of a melting point to the actual data book value can express purity

• Impurities tend to lower the melting point of a solid