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 connects to 3.3.8 Carboxylic acids and derivatives (reactions of acid anhydrides to form esters) and 3.3.13 Organic synthesis (multi-step practical procedures).
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 \(\text{HCl}\) 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 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 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.
- 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 \(0.0145\text{ mol}\) of salicylic acid theoretically yields \(0.0145\text{ mol}\) 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.
Common Exam Questions
1. Why is the crude aspirin dissolved in the minimum volume of hot solvent?
Using the minimum volume ensures that the hot solution is saturated. This guarantees that when the solution cools, the maximum amount of pure product crystallises out rather than remaining dissolved in excess solvent.
2. Suggest why the actual yield of recrystallised aspirin is always lower than the theoretical yield.
Some product remains dissolved in the cold solvent and filtrate; some crystals are lost during transferring between flasks or during filtration; the reaction may not have gone to 100% completion.
3. Explain why the melting point of crude aspirin is lower than that of recrystallised aspirin.
Crude aspirin contains unreacted salicylic acid and other side products. These impurities disrupt the regular packing of the crystal lattice, weakening the intermolecular attractions so that less kinetic energy is needed to melt the solid.
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.