Part 2: Required Practical 10: preparing a pure organic solid

Understand aspirin preparation, recrystallisation, suction filtration, melting range and original yield calculations for AQA RP10.

What the solid preparation demonstrates

Required Practical 10 includes preparation of a pure organic solid and a test of purity, as well as preparation of a pure organic liquid. Aspirin is the solid example in the AQA handbook. These notes explain the reasoning; practical work follows the supervised, risk-assessed method supplied by your centre.

Salicylic acid (2-hydroxybenzenecarboxylic acid) reacts with ethanoic anhydride. Its phenolic OH is acylated to form an ester while its carboxylic acid group remains. An acid catalyst and controlled warming promote the reaction. Ethanoic acid is the other product.

Obtain the crude product

Measure the starting material accurately so its amount can be used in the yield calculation. Warm the reaction mixture under the specified conditions; unnecessary prolonged heating can promote unwanted reactions.

During the controlled aqueous work-up, excess anhydride is hydrolysed and the poorly water-soluble product precipitates. Collecting this solid does not yet establish purity: it may contain unreacted material or trapped mother liquor. Follow the order and cooling requirements of the approved method rather than improvising water addition to a reactive mixture.

Recrystallisation: exploit a change in solubility

Choose a solvent or solvent mixture in which the product dissolves well when hot but much less when cold, without reacting with the solvent. Soluble impurities should remain in the mother liquor; insoluble impurities can be removed by hot filtration when that step is needed.

Dissolve the crude solid in the minimum practical volume of hot solvent. Too much solvent leaves more product dissolved after cooling and reduces recovery. In the aspirin handbook example, an ethanol/water mixture is used; the exact solvent choice belongs to the particular preparation.

Allow the solution to cool slowly so crystals grow, then use further cooling if the method calls for it. Collect the crystals by suction filtration. Rinse them with a small amount of suitable cold solvent to remove adhering solution while limiting product loss. Dry them before weighing and measuring the melting range.

Purification trades some recovery for purity. Transfer losses and product left in the mother liquor lower the isolated yield. Wet crystals can give an artificially high mass and a misleading yield, sometimes above 100%.

Why suction filtration needs a seal

A Büchner funnel with correctly fitted filter paper sits in a side-arm flask through a sealing collar. Reduced pressure draws the liquid through; crystals remain on the paper. The seal makes the pressure difference effective. Use appropriate thick-walled equipment and the laboratory vacuum arrangement specified by the teacher.

Test purity with a melting range

Place a small, dry, finely powdered sample in a capillary tube. Heat slowly near the expected melting point and record the range from first melting to complete melting. Repeat with a fresh sample and compare with a reliable value for the intended compound.

A reasonably pure sample usually melts over a narrow range close to the reference value. Soluble impurities typically lower and broaden the range. Do not say that every impurity must behave identically, or that one matching melting point proves identity and perfect purity. Decomposition on heating limits this method.

For example, a sample melting at 128–133 °C against a stated reference of 135–136 °C suggests impurities. A dry sample at 135–136 °C supports improved purity, but additional evidence such as TLC can strengthen the identification.

Worked yield and atom economy

Original calculation: 2.76 g of salicylic acid, Mr 138, reacts with excess ethanoic anhydride. Aspirin has Mr 180 and the molar ratio is 1:1. The amount of salicylic acid is 2.76 ÷ 138 = 0.0200 mol, so the theoretical aspirin mass is 0.0200 × 180 = 3.60 g.

If 2.70 g of dry, purified aspirin is collected, the percentage yield is 2.70 ÷ 3.60 × 100 = 75.0%. Use the limiting reagent; do not assume the larger weighed mass is limiting.

For the balanced anhydride route, the atom economy is 180 ÷ (138 + 102) × 100 = 75.0%. The numerical match to the yield in this example is coincidental. The ethanoic acid by-product accounts for the remaining atoms, even if a process can recover it.

The AQA practical handbook discusses comparing the anhydride and acyl chloride routes. A higher atom economy does not erase other considerations such as corrosive HCl formation, reagent handling, cost or purification.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources