Wk6:Purification 2

What is purification?

The physical separation of a desired chemical substance from contaminants.

What are the three non-chromatography purification methods?

Extraction, recrystallisation, distillation.

What is extraction?

Moving a compound from one solvent to another immiscible solvent.

What does immiscible mean?

Two liquids that do not mix (like oil and water).

What are three criteria for a good extraction solvent?

Dissolves the product, does not react, not miscible with water, low boiling point.

Which layer is dichloromethane (DCM) in a water extraction?

Bottom layer (DCM is denser than water).

Which layer is ethyl acetate in a water extraction?

Top layer (ethyl acetate is less dense than water).

What is the difference between extraction and washing?

Extraction moves product into another solvent; washing removes impurities while product stays put.

What is acid-base extraction?

Using acid or base to convert compounds to water-soluble salts for separation.

What happens when you add acid (HCl) to an amine?

The amine becomes a water-soluble ammonium salt and moves into the water layer.

What happens when you add base (NaOH) to a carboxylic acid?

The acid becomes a water-soluble carboxylate salt and moves into the water layer.

What is recrystallisation?

Dissolving a solid in hot solvent and cooling slowly to form pure crystals.

What purity must crude material be for recrystallisation to work?

85–90% pure (only 10–15% impurities).

Why must you use the minimum volume of hot solvent?

So the solution becomes saturated when cooled, allowing crystals to form.

What is hot filtration?

Filtering a hot solution to remove insoluble impurities before crystallisation.

What is distillation?

Separating liquids by boiling and condensing based on different boiling points.

What is simple distillation used for?

When boiling point difference is large (>100°C) or one component is non-volatile.

What is fractional distillation used for?

When boiling points are close (difference <25°C).

What does a fractionating column do?

Provides surfaces for multiple condensation-evaporation cycles, improving separation.

What is vacuum distillation used for?

Purifying liquids that boil above 200°C or decompose at their normal boiling point.

Why does vacuum distillation work?

Lower pressure lowers the boiling point of liquids.

What happens to boiling point under vacuum?

It decreases (liquids boil at lower temperature).

What type of purification is best for a solid crude product?

Recrystallisation.

What type of purification is best for a liquid crude product?

Distillation.

What type of purification is best for a product dissolved in a reaction mixture?

Extraction or washing.

How can you separate a mixture of an amine and a neutral compound?

Add acid – the amine becomes a salt and goes into water; neutral stays in organic layer.

How can you separate a mixture of a carboxylic acid and a neutral compound?

Add base – the acid becomes a salt and goes into water; neutral stays in organic layer.

What is the main limitation of recrystallisation?

Crude material must be relatively pure (85–90%).

What is the main limitation of simple distillation?

Only works if boiling points are very different.