Aspirin Synthesis: Yield, Purity and the Iron(III) Test
A student prepares aspirin by warming 2.00 g of salicylic acid (Mr = 138.1) with 5.0 mL of acetic anhydride (density 1.08 g/mL, Mr = 102.1) and a few drops of concentrated sulfuric acid as catalyst. After cooling, the product is filtered, washed with cold water and dried, giving 1.85 g of crude solid. The crude solid is recrystallised from ethanol–water to give 1.42 g of white crystals melting at 128–134 °C; the literature melting point is 135 °C. A sample of the final product gives a faint purple colour with iron(III) chloride solution. The reaction is: C7H6O3 + C4H6O3 → C9H8O4 + C2H4O2 (Mr of aspirin = 180.2).
1. Identify the limiting reagent and calculate the theoretical yield of aspirin in grams.
Read the worked answer
Moles of salicylic acid: 2.00 / 138.1 = 0.01448 mol.
Moles of acetic anhydride: mass = 5.0 mL × 1.08 g/mL = 5.40 g; moles = 5.40 / 102.1 = 0.0529 mol.
Stoichiometry: 1:1, so the acid is limiting by a wide margin (0.0145 vs 0.053, roughly a 3.6-fold excess of anhydride).
Theoretical yield: 0.01448 × 180.2 = 2.61 g.
Why the excess anhydride matters. It is not there to raise the yield - the acid is already fully consumed - but to drive the equilibrium to the right and to compensate for hydrolysis of the anhydride by water in the flask. This is why the calculation must check the ratio before assuming the larger-mass reagent is in excess: 5.4 g of anhydride is more massive than 2.0 g of acid, yet it is the reagent in excess.
2. Calculate the crude and recrystallised percentage yields, and account for the difference between them.
Read the worked answer
Crude yield: 1.85 / 2.61 × 100 = 70.9%.
Recrystallised yield: 1.42 / 2.61 × 100 = 54.4%.
Where the missing mass went:
- Before filtration: incomplete reaction, product left in the reaction flask, and loss during the wash - aspirin is slightly soluble in cold water, and more so in the ethanol–water used later.
- During recrystallisation: aspirin remains dissolved in the hot mother liquor and is discarded with it; further losses occur on the filter and in transfers. Recrystallisation is a purification step and always costs yield, which is the trade-off being made.
- Side reactions: salicylic acid can form polymeric by-products, and the sulfuric acid catalyst can cause charring, both of which end up as impurities removed on recrystallisation.
So the 16.5-percentage-point gap is the price paid for purity. A yield reported without stating whether it is crude or recrystallised is ambiguous, and 54% after recrystallisation for an undergraduate preparation is reasonable.
3. Interpret the iron(III) chloride result and the melting point. Can the student conclude that recrystallisation improved the purity of the sample?
Read the worked answer
Iron(III) chloride test. Salicylic acid contains a phenol group; phenols form coloured complexes with Fe³⁺, giving a purple colour. Aspirin's phenol has been acetylated, so it does not react. A faint purple therefore indicates that some unreacted salicylic acid survives in the final product - the reaction is incomplete or the wash was inadequate. 'Faint' is a subjective judgement with no calibrated standard, so the test is qualitative only.
Melting point. 128–134 °C is both depressed (below 135 °C) and broad (a 6 °C range). A pure crystalline solid melts sharply, typically within 1–2 °C. Broader and lower means impurity: the impurity disrupts the crystal lattice and lowers the melting point, and the sample melts gradually as composition changes.
Can purity improvement be concluded? No, not from these data. The student reports only the melting point of the final product. Without the crude product's melting point and FeCl₃ result, there is no 'before' to compare with. The observation that the final material is still impure actually shows the recrystallisation was incomplete, not that it helped. The correct conclusion is that the sample is impure to an unknown degree; the comparison needed to demonstrate purification was never made.