The preparation and purification of an organic solid involves synthesizing the crude product, separating it by vacuum filtration, purifying it through recrystallisation, and determining its melting point to confirm purity. This guide uses the preparation of aspirin from salicylic acid and ethanoic anhydride as the primary example.
Core Specification Link
This practical is part of AQA Required Practical 10 (Preparation of a pure organic solid and liquid) connecting to 3.3.9 Carboxylic acids and derivatives and 3.3.14 Organic synthesis.
Aim
To prepare a pure sample of an organic solid (specifically aspirin from salicylic acid) using recrystallisation, vacuum filtration, and melting point determination to verify its purity.
Equipment List
- Conical flasks (100 cm³ and 250 cm³)
- Measuring cylinders (10 cm³ and 50 cm³)
- Volumetric pipettes (10 cm³) or graduated pipettes
- Buchner funnel and filter paper
- Vacuum filtration flask and pump
- Thermometer (0-200 °C)
- Water bath (electronic or beaker of hot water)
- Capillary tubes (for melting point)
- Melting point apparatus or oil bath (Thiele tube)
- Salicylic acid (2-hydroxybenzoic acid)
- Ethanoic anhydride
- Concentrated phosphoric acid (H₃PO₄) or concentrated sulfuric acid (H₂SO₄)
- Ethanol (for recrystallisation solvent)
- Distilled water and ice
- Watch glasses or petri dishes
Aspirin Synthesis Chemical Equation
\[ \text{C}_7\text{H}_6\text{O}_3 + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{Conc. }\text{H}_3\text{PO}_4} \text{C}_9\text{H}_8\text{O}_4 + \text{CH}_3\text{COOH} \]Salicylic acid (2-hydroxybenzoic acid) reacts with ethanoic anhydride under acid catalysis to produce aspirin (acetylsalicylic acid) and ethanoic acid. Ethanoic anhydride is preferred over ethanoyl chloride because it reacts less vigorously, does not release corrosive HCl gas, and is cheaper.
Experimental Method
Step 1: Synthesis of Crude Aspirin
- Weigh approximately 2.00 g of salicylic acid into a 100 cm³ conical flask.
- In a fume cupboard, carefully add 3.0 cm³ of ethanoic anhydride and 5 drops of concentrated phosphoric acid catalyst. Swirl the flask to dissolve the solid.
- Place the flask into a water bath maintained at 60 °C for 15 minutes.
- Remove the flask and carefully add 20 cm³ of ice-cold distilled water . This decomposes excess ethanoic anhydride and initiates crystallization of the aspirin.
- Cool the mixture in an ice-water bath for 10 minutes to maximize crystal formation.
Step 2: Buchner (Vacuum) Filtration
Vacuum filtration is preferred over gravity filtration because it is faster and leaves the solid crystals significantly drier.
- Set up vacuum filtration. Set up the Buchner filtration apparatus (as shown below).Place a piece of filter paper inside the funnel and wet it with a small amount of cold water to create a tight seal.
- Turn on the vacuum pump . Pour the crystal suspension into the funnel.
- Wash the flask. Wash the conical flask with small volumes of ice-cold distilled water and pour over the crystals.Note: Ice-cold water is used to minimize the dissolving of the product, preventing yield loss.
- Leave the vacuum running for several minutes to pull air through the crystal cake, drying the crude product.
Step 3: Recrystallisation (Purification)
Recrystallisation works on the principle that the desired organic compound is highly soluble in a hot solvent but much less soluble when cold, whereas impurities remain soluble in both hot and cold conditions.
- Dissolve the crude solid in the minimum volume of hot solvent (ethanol) . Why minimum? If too much solvent is used, some product will remain dissolved when cooled, reducing recovery.
- Filter the hot solution through fluted filter paper to remove any insoluble impurities (if present) . Keep the apparatus warm during this to prevent premature crystallization.
- Recrystallise the product. Allow the hot solution to cool slowly to room temperature, then place it in an ice bath.Slow cooling encourages the formation of large, pure crystals, while rejecting impurities from the growing crystal lattice.
- Filter the pure crystals again using the Buchner setup . Wash with a small amount of ice-cold solvent.
- Dry the crystals on watch glasses in a low-temperature oven or desiccator.
Step 4: Melting Point Determination
- Seal one end of a glass capillary tube using a Bunsen burner flame.
- Tap a small amount of dry, purified aspirin into the open end of the capillary tube until you have a sample about 2 to 3 mm deep.
- Place the capillary tube inside the melting point apparatus.
- Heat the apparatus slowly (at a rate of about 1 °C per minute near the melting point).
- Record the temperature at which melting begins and the temperature at which the sample is completely liquid.
Interpreting Melting Point Data
- Pure solids: Melt sharply at a constant temperature within 1 to 2 °C of the literature value (aspirin melts at 136 °C).
- Impure solids: Melt over a wide range of temperatures and at a lower temperature than the literature value. This occurs because impurities disrupt the regular crystal lattice structure, requiring less energy to break molecular forces.
Yield Calculations
Step 1: Calculate moles of limiting reactant (salicylic acid)
\[ \text{Moles} = \frac{2.00\text{ g}}{138.0\text{ g mol}^{-1}} = 0.0145\text{ mol} \]Step 2: Calculate theoretical yield of aspirin
The stoichiometry is 1:1, so 0.0145 mol of salicylic acid theoretically yields 0.0145 mol of aspirin:
\[ \text{Theoretical Mass} = 0.0145\text{ mol} \times 180.0\text{ g mol}^{-1} = 2.61\text{ g} \]Step 3: Calculate percentage yield
\[ \text{Percentage Yield} = \left(\frac{1.85\text{ g}}{2.61\text{ g}}\right) \times 100 = 70.9\% \]The percentage yield is 70.9%.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Ethanoic Anhydride | Corrosive; causes severe burns. Lachrymator (vapours cause tearing and respiratory irritation). | Measure and add inside a fume cupboard. Wear safety goggles and gloves. |
| Concentrated Phosphoric Acid | Corrosive; causes skin burns and eye damage. | Handle with care, wear gloves, and wash splashes immediately. |
| Ethanol | Highly flammable. | No naked flames. Heat using a hot water bath or electric hotplate. |
Sources of Error & Improvements
- Over-washing crystals: Washing the solid in the Buchner funnel with too much water or warm water will dissolve some of the aspirin. Improvement: Wash with minimal portions of ice-cold water.
- Incomplete crystallization: If the mixture is not cooled fully or not left for long enough in ice, some product remains in solution. Improvement: Cool in ice-water for a full 10-15 minutes and scratch the side of the flask with a glass rod to create nucleation sites.
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
2 marksSuggest two reasons why the yield of recrystallised aspirin is less than 100%.
Show Mark Scheme
Any two from:
- Some product stays dissolved in the solvent (filtrate) [1]
- Some product is lost when transferring between containers or during filtration [1]
- The reaction is incomplete or side reactions occur [1]
Exam-style question
2 marksExplain why impure aspirin has a lower melting point than pure aspirin.
Show Mark Scheme
- Impurities disrupt the regular arrangement of molecules in the crystal lattice [1]
- This weakens the forces between the molecules, so less energy is needed to melt it (and it melts over a range) [1]
CPAC Skills Assessed
- CPAC 1: Follows multi-stage protocols to synthesize, filter, and recrystallise organic solids.
- CPAC 2: Sets up and uses vacuum filtration and melting point determination equipment.
- CPAC 3: Identifies and manages chemical hazards (flammable solvents and corrosive anhydrides).
When describing the steps of recrystallisation, you must always include key phrases: "dissolve in the minimum volume of hot solvent", "cool slowly in ice", and "filter under reduced pressure". Omitting these specific qualifiers will lose marks.