In the industrial setting, salicylic acid is mixed with an excess of ethanoic anhydride to ensure the reaction goes to completion.
The mixture is typically heated gently; the reaction is exothermic, meaning it releases heat as the new chemical bonds form.
After the reaction, the mixture is cooled to allow the aspirin to crystallize, followed by filtration and purification (often via recrystallization) to remove the ethanoic acid by-product and any unreacted materials.
| Feature | Ethanoic Anhydride | Ethanoyl Chloride |
|---|---|---|
| Reactivity | Moderate and easily controlled | High and potentially violent |
| By-product | Ethanoic acid (safe/useful) | Hydrogen chloride (corrosive gas) |
| Cost | Relatively cheap | More expensive |
| Safety | Safer to handle | Dangerous/Highly corrosive |
Identify the Reacting Group: Always remember that in the synthesis of aspirin, it is the hydroxyl () group of the salicylic acid that reacts, not the carboxylic acid () group.
By-product Awareness: If a question asks for the by-product of the industrial synthesis, the answer is ethanoic acid (), not water or .
Structural Recognition: Be prepared to identify the ester functional group in the final aspirin molecule, which is formed during the acylation process.
Reasoning Questions: When asked why anhydrides are used industrially, focus your answer on three pillars: safety (no gas), cost (cheaper reagents), and control (less vigorous reaction).
Wrong Functional Group: A common mistake is drawing the acylation occurring on the carboxylic acid group of the salicylic acid; ensure the remains unchanged.
By-product Confusion: Students often mistakenly list as the by-product because they associate acylation primarily with acyl chlorides. Always check if the reagent is an anhydride.
Reversibility: Do not confuse this with the reversible esterification of carboxylic acids; acylation with anhydrides is generally considered to go to completion more effectively.
