How to Perform Required Practical 10a: Making Aspirin

Making Aspirin - The Chemistry Behind the Medicine

Ever wondered how aspirin actually works? Aspirin is an ester that blocks pain signals and reduces inflammation, which is why it helps with headaches and can even reduce heart attack risk. However, it can cause stomach bleeding in some people.

The reaction combines salicylic acid with ethanoic anhydride using sulfuric acid as a catalyst. This produces aspirin plus ethanoic acid as a byproduct. The reaction mechanism is called nucleophilic addition-elimination - sounds fancy, but it's just one molecule attacking another and kicking out a leaving group.

Why use ethanoic anhydride instead of ethanoyl chloride? It's much safer! Ethanoic anhydride is cheaper, less corrosive, reacts less violently with water, and won't produce toxic HCl fumes that could harm you.

Top Tip: Remember that this synthesis creates a medicine from simple chemicals - it shows how powerful organic chemistry can be in real life!

The Synthesis Method - Getting Started

You'll start by weighing 2g of salicylic acid into a round-bottom flask and setting up a reflux apparatus. The flask sits in hot water (not directly on the flame), with cold water flowing through the condenser from bottom to top.

Add 5cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid as your catalyst. Always work in a fume cupboard when handling the sulfuric acid - it's seriously corrosive stuff!

The reflux process is crucial here. Heat gently with a half-open Bunsen burner and watch for colour changes - if your mixture starts changing colour, lift it out and swirl it quickly. The condenser ensures any evaporated reactants cool down and return to your mixture rather than escaping.

After 10 minutes of gentle heating (once everything's dissolved), you'll quench the reaction by adding distilled water and cooling in an ice bath. This causes your aspirin to crystallise out as the temperature drops.

Safety Alert: Always turn on the water before lighting the Bunsen burner, and keep that gas valve only half open!

Purification - Getting Clean Crystals

Filtration using a Büchner funnel and vacuum pump removes your crude aspirin crystals. Make sure the filter paper fits perfectly - no gaps around the edges! Moisten it first with ethanol or water so it sticks properly.

The recrystallisation step is where the magic happens. Add just enough ethanol to dissolve your crystals when heated (about 1-2ml), then cool it down slowly. Pure aspirin dissolves in hot ethanol but crystallises out when cold, while impurities either stay dissolved or don't dissolve at all.

Keep the temperature below 75°C during recrystallisation - ethanol boils at 78°C, so you don't want it evaporating away! Use a watch glass to cover your beaker and minimise losses.

Filter your purified crystals again, then weigh them to calculate your yield. Cover with cling film (pierce some holes) and dry in a desiccator to remove any remaining water.

Key Point: Recrystallisation works because aspirin's solubility changes dramatically with temperature, but impurities don't behave the same way!

Testing Purity - Did It Work?

You'll test your aspirin's purity using two methods: melting point determination and thin layer chromatography (TLC). Pure aspirin should melt at exactly 136°C - any impurities will lower this temperature and make it melt over a wider range.

Unreacted salicylic acid is the most common impurity you'll find. If your melting point is too low or covers too wide a range, you know your sample isn't pure enough.

The main safety hazards include corrosive ethanoic anhydride and sulfuric acid, harmful salicylic acid, and flammable ethanol. Always work in a fume cupboard and wear appropriate safety equipment.

Your yield probably won't be 100% - that's completely normal! Some product always gets lost during filtration, washing, and transfer steps. The important thing is getting pure crystals that melt at the right temperature.

Reality Check: Professional chemists rarely get perfect yields either - this practical teaches you real-world synthesis skills, not just theory!