In the lab, salicylic acid meets ethanoic anhydride, with a few drops of acid as a catalyst. Warm them together, and aspirin forms.
Ethanoic anhydride is two ethanoyl groups joined through one oxygen atom. Its name tells you where it comes from: anhydride means "without water", and the molecule is two ethanoic acid molecules joined with one water molecule taken out, the same kind of condensation that makes an ester.
Drag the model to turn it. Pinch to zoom, or use the buttons.
Why the anhydride
Salicylic acid's -OH sits on a benzene ring, and an -OH on a ring joins ethanoic acid only slowly. That reaction also gives off water, and water can split the new ester apart again, so the mixture stops part of the way. Ethanoic anhydride reacts much faster. Its leftover half comes off as ethanoic acid, which leaves the ester alone, so the reaction goes almost all the way to aspirin.
| Atom | C₇H₆O₃ + C₄H₆O₃ | C₉H₈O₄ + C₂H₄O₂ |
|---|---|---|
| carbon | 7 + 4 = 11 | 9 + 2 = 11 |
| hydrogen | 6 + 6 = 12 | 8 + 4 = 12 |
| oxygen | 3 + 3 = 6 | 4 + 2 = 6 |
The catalyst is a few drops of concentrated sulfuric or phosphoric acid. Its H⁺ joins the oxygen of one of the anhydride's C=O groups. That oxygen then pulls electrons harder, so its carbon becomes more δ+, and the lone pair on salicylic acid's -OH oxygen reaches it faster. At the end the H⁺ comes off again, ready for the next molecule, as a catalyst does.
In the flask
- Salicylic acid crystals, ethanoic anhydride and the drops of acid go into a flask.
- The flask sits in a bath of hot water for about 15 minutes, and the crystals dissolve as they react.
- Cold water goes in. It turns the leftover ethanoic anhydride into ethanoic acid.
- The flask goes into ice. Aspirin dissolves poorly in cold water, so it comes out of the liquid as white, needle-like crystals.
- The crystals are filtered off, washed with ice-cold water and dried.