Synthesising Aspirin — The Reaction That Founded Modern Pharmaceuticals
For thousands of years, healers knew that chewing willow bark eased pain and reduced fever. The active principle — salicin, a glycoside that the body converts to salicylic acid — was isolated by Johann Buchner in 1828 and synthesised from phenol by Hermann Kolbe in 1859. But salicylic acid has a devastating side effect: it burns the stomach lining, causing nausea, bleeding, and ulcers.
In 1897, Felix Hoffmann at the Bayer company in Elberfeld, Germany, acetylated salicylic acid — replacing the free hydroxyl group with an acetyl group by reacting it with acetic anhydride. The product, acetylsalicylic acid (C₉H₈O₄), retained the painkilling and anti-inflammatory properties but was far gentler on the stomach. Bayer marketed it as 'Aspirin' in 1899, and it became the first mass-produced synthetic drug — launching the modern pharmaceutical industry.
The synthesis is a textbook esterification: salicylic acid (2-hydroxybenzoic acid) reacts with acetic anhydride in the presence of a phosphoric acid catalyst to produce acetylsalicylic acid and acetic acid as a by-product: C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + CH₃COOH. The reaction is fast, clean, and produces beautiful white needle-like crystals — one of the most satisfying syntheses in introductory organic chemistry.
SAFETY WARNING: Acetic anhydride is a corrosive irritant — its vapour causes severe eye and respiratory irritation. Phosphoric acid is corrosive. Work in a fume hood or well-ventilated area. Wear goggles and gloves. The product, aspirin, is a real medicine — but the purity of a lab synthesis is unknown, so NEVER ingest the product.
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