Why is cold water added to the aspirin reaction mixture after heating?

Cold water reacts with any excess ethanoic anhydride, converting it to ethanoic acid. The water also makes the aspirin crystallise, because aspirin is only slightly soluble in cold water.

How are crude aspirin crystals separated from the reaction mixture?

The mixture is cooled in ice, and the crystals are collected by filtration under reduced pressure in a Buchner funnel. The crystals are washed with a little ice-cold water.

Describe how crude aspirin is recrystallised.

The crude aspirin is dissolved in the minimum volume of hot solvent, such as an ethanol and water mixture. The solution is cooled slowly, and the pure crystals are filtered, washed with cold solvent and dried.

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Organic Chemistry

Preparing and purifying aspirin

3 things to know
A2 2

Overview

Synthesising aspirin from salicylic acid, filtering and recrystallising the crude product, the iron(III) chloride test, theoretical and percentage yield, melting point purity checks and improving the method.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How is aspirin made in the laboratory?

Key term

Write the equation for the synthesis of aspirin from salicylic acid.

Key term

What type of reaction converts salicylic acid into aspirin?

Questions from this subtopic

Each one with the model answer underneath.

1
How is aspirin made in the laboratory?
Model answer
Salicylic acid is warmed with ethanoic anhydride and a few drops of concentrated sulfuric or phosphoric acid as a catalyst. Aspirin and ethanoic acid form.
2
Write the equation for the synthesis of aspirin from salicylic acid.
Model answer
The equation is C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. Salicylic acid and ethanoic anhydride give aspirin and ethanoic acid.
3
What type of reaction converts salicylic acid into aspirin?
Model answer
The phenol –OH group of salicylic acid is converted into an ester group. The reaction is an esterification, also described as acylation.
4
Why is cold water added to the aspirin reaction mixture after heating?
Model answer
Cold water reacts with any excess ethanoic anhydride, converting it to ethanoic acid. The water also makes the aspirin crystallise, because aspirin is only slightly soluble in cold water.
5
How are crude aspirin crystals separated from the reaction mixture?
Model answer
The mixture is cooled in ice, and the crystals are collected by filtration under reduced pressure in a Buchner funnel. The crystals are washed with a little ice-cold water.
6
Describe how crude aspirin is recrystallised.
Model answer
The crude aspirin is dissolved in the minimum volume of hot solvent, such as an ethanol and water mixture. The solution is cooled slowly, and the pure crystals are filtered, washed with cold solvent and dried.

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