Aspirin is a painkiller made by acylating 2-hydroxybenzoic acid (salicylic acid). The OH group on the benzene ring reacts with ethanoic anhydride, turning it into an ester group. The by-product is ethanoic acid.HOC6H4COOH + (CH3CO)2O → CH3COOC6H4COOH + CH3COOH Ethanoyl chloride would also work, but industry uses ethanoic anhydride because it is:
• cheaper;
• less corrosive and safer to handle;
• less vigorous in its reaction, so it is easier to control;
• less readily hydrolysed by moisture in the air;
• it does not produce corrosive, toxic hydrogen chloride gas.Worked example 1: atom economy. Mr values: 2-hydroxybenzoic acid 138, ethanoic anhydride 102, aspirin 180.atom economy = 180 ÷ (138 + 102) × 100 = 75% The ethanoic acid by-product can be recovered and sold or reused, which improves the economics.Worked example 2: maximum yield. 2.76 g of 2-hydroxybenzoic acid is 2.76 ÷ 138 = 0.020 mol. The ratio is 1 : 1, so at most 0.020 mol of aspirin forms, which is 0.020 × 180 = 3.60 g. The crude product is then purified by recrystallisation.
• cheaper;
• less corrosive and safer to handle;
• less vigorous in its reaction, so it is easier to control;
• less readily hydrolysed by moisture in the air;
• it does not produce corrosive, toxic hydrogen chloride gas.Worked example 1: atom economy. Mr values: 2-hydroxybenzoic acid 138, ethanoic anhydride 102, aspirin 180.