RP10: Preparation & Purification of an Organic Solid

1. Synthesis of Aspirin (Acetylsalicylic Acid)

The benchmark organic solid synthesis in OxfordAQA Chemistry is the preparation of aspirin by the esterification of 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride:

Aspirin Synthesis Equation HOC6H4COOH + (CH3CO)2O → CH3COOC6H4COOH + CH3COOH
- Reactants: 2-hydroxybenzoic acid (solid, Mr = 138.0) + ethanoic anhydride (liquid, Mr = 102.0).
- Catalyst: 5 drops of concentrated phosphoric acid (H3PO4) or concentrated H2SO4.
- Products: Aspirin (acetylsalicylic acid, Mr = 180.0) + ethanoic acid (by-product).

Why Ethanoic Anhydride is Preferred Over Ethanoyl Chloride

A classic exam question asks: "State two advantages of using ethanoic anhydride rather than ethanoyl chloride (CH3COCl) in industrial aspirin synthesis."

Safety / Less Corrosive: Ethanoic anhydride does not produce toxic, choking hydrogen chloride (HCl) gas fumes.
Controlled Reaction: It reacts less vigorously and less violently with water/moisture.
Cost: Ethanoic anhydride is cheaper and easier to store safely.

2. Buchner Suction Filtration Protocol

After heating the reaction mixture in a hot water bath and adding ice-cold water to precipitate the crude aspirin, the solid is separated by vacuum filtration:

Apparatus & Operation

Buchner Funnel & Flask: Thick-walled ceramic funnel with a perforated porcelain plate, seated in a side-arm vacuum flask with a rubber collar seal.
Vacuum Source: Connected to a water aspirator or electric vacuum pump to reduce atmospheric pressure underneath the filter plate.
Filter Paper: Circular paper cut to lay completely flat over the perforated plate without creasing up the walls. Moisten with a drop of solvent to form a tight seal before pouring the suspension.

Ice-Cold Washing

Wash the crude crystals on the filter paper with a minimal volume of ice-cold deionised water:

Purpose: Removes soluble acid impurities (excess H3PO4, ethanoic acid by-product) from the crystal surfaces into the filtrate.
Why Ice-Cold: Aspirin is slightly soluble in water; using warm water would dissolve the product and drastically reduce the percentage yield.

3. Recrystallisation from Minimum Hot Solvent

Crude aspirin crystals isolated from the initial reaction mixture contain trapped unreacted starting material and soluble by-products. Purification is achieved by recrystallisation:

Step in Recrystallisation	Specific Practical Technique	Scientific Justification & Examiner Rationale
1. Solvent Selection	Choose a solvent in which aspirin is very soluble at high temperature but virtually insoluble at low temperature (e.g. water/ethanol mix).	Ensures maximum recovery of pure crystalline product upon cooling.
2. Dissolving Solid	Dissolve crude solid in the MINIMUM volume of boiling solvent.	Using excess solvent would keep substantial quantities of product dissolved even when cold, devastating percentage yield.
3. Hot Filtration	Filter hot through fluted filter paper in a pre-warmed glass funnel.	Removes insoluble physical impurities (e.g. dust, porcelain chips). Pre-heating prevents premature crystallization on cold glass.
4. Controlled Cooling	Allow solution to cool slowly to room temperature, then place in an ice bath.	Slow cooling enables pure aspirin molecules to assemble into an orderly crystalline lattice, while impurities remain dissolved in the cold solvent.
5. Suction Filtration	Filter pure crystals using Buchner apparatus and wash with ice-cold solvent.	Separates pure crystals from the mother liquor containing soluble impurities.

4. Drying to Constant Mass

Residual solvent within the crystal lattice falsely inflates the recorded mass, artificially leading to calculated yields exceeding 100%.

Drying to Constant Mass Protocol 1. Place the recrystallised solid on a pre-weighed watch glass.
2. Dry in a desiccator containing anhydrous silica gel (or an oven set safely below aspirin's melting point, e.g. at 50 deg C).
3. Cool and weigh the sample.
4. Return to the desiccator/oven for a further 20 minutes and reweigh.
5. Repeat until two consecutive mass readings agree within +/- 0.01 g (confirming all solvent has evaporated).

5. Melting Point Determination & Purity Assessment

The purity of the dried crystalline organic solid is established by measuring its melting temperature range:

Experimental Protocol

Seal one end of a glass capillary tube in a blue Bunsen flame.
Tap the open end into powdered dry crystals and invert, tapping down so crystals pack into a dense plug ~2-3 mm high at the sealed base.
Insert the capillary into an electrical melting point apparatus (or Thiele tube with liquid paraffin).
Heat steadily until ~10 deg C below expected melting point (138 deg C for aspirin), then heat very slowly (1-2 deg C per minute).
Record the temperature at which the first drop of liquid appears (T1) and the temperature when the solid is completely melted (T2). The melting range is T1 to T2.

Purity Interpretation Criteria

Pure Aspirin: Melts sharply at the exact literature value (138 to 140 deg C) across a narrow range of ≤ 1 to 2 deg C.
Impure Aspirin:
- Melts at a lower temperature than literature value (<138 deg C).
- Melts over a broad, poorly defined range (e.g. 128 to 135 deg C).
Explanation: Foreign impurity molecules disrupt the regular crystalline lattice, reducing the energy required to break intermolecular bonds.

6. Worked Yield Calculation Problem

Worked Example: Calculating Percentage Yield of Aspirin

Problem: In an RP10 synthesis, a student reacts 2.76 g of 2-hydroxybenzoic acid (Mr = 138.0 g mol^-1) with an excess of ethanoic anhydride in the presence of concentrated H3PO4 catalyst. After recrystallisation and drying to constant mass, 2.34 g of pure aspirin (Mr = 180.0 g mol^-1) is obtained.

Calculate the percentage yield of aspirin obtained in this preparation.

Step 1: Calculate moles of limiting reactant (2-hydroxybenzoic acid)

Moles = mass / Mr = 2.76 / 138.0 = 0.0200 mol

Step 2: Calculate theoretical yield of aspirin

1 mol 2-hydroxybenzoic acid → 1 mol aspirin (1 : 1 stoichiometry)
Theoretical moles of aspirin = 0.0200 mol
Theoretical mass = moles * Mr = 0.0200 * 180.0 = 3.60 g

Step 3: Calculate percentage yield

Percentage Yield = (actual mass / theoretical mass) * 100
Percentage Yield = (2.34 / 3.60) * 100 = 65.0%

Final Answer: Percentage Yield = 65.0%

7. Practice Exam Questions

Question 1: Why must crude aspirin crystals be dissolved in the MINIMUM volume of hot solvent during recrystallisation?

A. To ensure the solution is saturated at high temperature so maximum crystals precipitate out upon cooling, minimizing product lost in the filtrate. B. Excess solvent decomposes aspirin into salicylic acid. C. Large volumes of solvent shatter the Buchner ceramic funnel. D. To lower the boiling point of the solvent.

Show Answer & Explanation

Correct Answer: A

Any excess solvent will retain dissolved product even when chilled in an ice bath. Using the minimum boiling volume creates a saturated solution that yields the highest possible recovery of purified crystals.

Question 2: A student determines the melting point of a synthesized sample of aspirin and records a range of 127 to 134 deg C. Pure aspirin has a literature melting point of 138 to 140 deg C. What does this indicate about the student's product?

A. The sample is completely pure because it melted below 140 deg C. B. The sample is impure and/or damp with residual solvent, which disrupts crystal lattice packing, depressing the melting point and broadening the melting range. C. The student heated the melting point apparatus too slowly. D. The atmospheric pressure in the laboratory was too high.

Show Answer & Explanation

Correct Answer: B

Impurities (such as unreacted 2-hydroxybenzoic acid or residual moisture) disrupt the regular intermolecular forces in the crystalline solid, lowering the melting point below literature and broadening the range across 7 deg C.