Purifying an Organic Liquid

Alcohols

What is the purpose of purifying an organic liquid?

To remove impurities from the desired organic product.

What impurities may contaminate an organic product after a reaction?

Unreacted starting materials, other organic products, inorganic reagents or products, and water.

What five techniques can be used to purify an organic liquid?

Simple distillation, fractional distillation, solvent extraction, drying and boiling temperature determination.

Why is mostly glass apparatus used with organic compounds?

Organic compounds may be flammable or toxic and may attack corks or bungs, increasing the risk of leaks and contamination.

What are ground-glass joints used for?

To connect pieces of glass apparatus tightly without using corks or bungs.

Why is an electric mantle or hot water/oil bath often preferred to a Bunsen burner in organic chemistry?

Many organic compounds are flammable, so avoiding a naked flame is safer.

What is simple distillation?

A method used to separate liquids with very different boiling temperatures by boiling and condensing.

How does simple distillation separate liquids?

The liquid with the lowest boiling temperature evaporates first, passes into the condenser, condenses and is collected separately.

What happens to the liquid with the higher boiling temperature during simple distillation?

It evaporates later.

What is the distillate?

The liquid collected after a vapour has been condensed during distillation.

What is the purpose of the condenser in distillation?

To cool the vapour so that it condenses back into a liquid.

What is the purpose of the thermometer during distillation?

To measure the temperature of the vapour as it enters the condenser.

What does a steady thermometer reading during distillation indicate?

One compound is distilling over.

What does a rise in temperature during distillation indicate?

A different compound is beginning to distil over.

When is simple distillation most suitable?

When the liquids have very different boiling temperatures.

What boiling-temperature difference is ideally needed for simple distillation?

More than about 25°C.

What is an advantage of simple distillation compared with fractional distillation?

It is quicker and easier to set up.

What is a disadvantage of simple distillation?

It does not separate liquids with similar boiling temperatures as effectively as fractional distillation.

What is fractional distillation?

A method used to separate liquids with similar boiling temperatures by boiling and condensing using a fractionating column.

What extra apparatus does fractional distillation use compared with simple distillation?

A fractionating column between the heating flask and still head.

What is usually inside a fractionating column?

Glass beads or pieces of broken glass.

What is the purpose of glass beads in a fractionating column?

They provide a large surface area on which vapours can condense.

What happens to vapour as it travels up a fractionating column?

It repeatedly condenses and re-evaporates.

Why does repeated condensation and evaporation improve separation?

It effectively produces several repeated distillations, giving better separation of the liquids.

When is fractional distillation preferred to simple distillation?

When the liquids have similar boiling temperatures or several compounds need to be separated.

What is the main advantage of fractional distillation?

It separates liquids with similar boiling temperatures more effectively.

What is a disadvantage of fractional distillation?

It takes longer and requires more complicated apparatus than simple distillation.

What is solvent extraction?

A method used to separate a liquid from a mixture by causing it to move from the mixture into a solvent.

What apparatus is used for solvent extraction?

A separating funnel.

What properties should a solvent used for solvent extraction have?

It should be immiscible with the original solvent and the desired organic product should be much more soluble in it.

What does immiscible mean?

Two liquids that do not mix and form separate layers.

Why must the extraction solvent be immiscible with the original solvent?

So two separate liquid layers form and can be separated.

Why should the desired organic product be more soluble in the extraction solvent?

So most of the product moves into the extraction solvent.

What is the first step in solvent extraction?

Place the reaction mixture in a separating funnel and add the chosen solvent.

What should happen when the solvent is added to the reaction mixture?

It should form a separate layer.

What is done after adding the solvent to the separating funnel?

Insert the stopper and gently shake the contents for a while.

What should be done after shaking a separating funnel?

Allow the mixture to settle into two layers.

How are the two layers separated using a separating funnel?

Remove the stopper, open the tap to drain the lower layer into a flask, then pour the upper layer into a separate flask.

Why should the stopper be removed before draining a separating funnel?

To allow air to enter so the liquid can flow freely through the tap.

Why are several small portions of extraction solvent better than one large portion?

Several small extractions remove more of the desired organic product for the same total volume of solvent.

After solvent extraction, what is the desired organic product mixed with?

The extraction solvent.

How can the desired product be separated from the extraction solvent?

By simple or fractional distillation.

Why may an organic liquid need to be dried?

It may contain water because inorganic reagents are often used in aqueous solution.

What is a drying agent?

A substance that removes water from an organic liquid without reacting with the organic compound.

What important property must a drying agent have?

It must not react with the organic liquid being purified.

What are common drying agents used for organic liquids?

Anhydrous calcium sulfate, magnesium sulfate and sodium sulfate.

What does anhydrous mean?

Containing no water.

How do anhydrous drying agents remove water?

They absorb water and form hydrated salts containing water of crystallisation.

What is water of crystallisation?

Water molecules chemically associated with the crystal structure of a hydrated salt.

Why is anhydrous calcium chloride not suitable for drying every organic liquid?

It reacts with some organic compounds and is soluble in alcohols.

How is a drying agent used?

Add it to the organic liquid, swirl or shake the mixture and leave it for a period of time.

How does the appearance of a drying agent change when it absorbs water?

It changes from a powdery appearance to a more crystalline appearance.

How is the drying agent removed after drying the organic liquid?

By decantation or filtration.

What is decantation?

Carefully pouring the liquid away from a solid without transferring the solid.

What is filtration?

Passing a mixture through a filter to separate an insoluble solid from a liquid.

How can the purity of an organic liquid be assessed?

By measuring its boiling temperature.

Why can boiling temperature indicate purity?

A pure substance has a characteristic boiling temperature.

How is boiling temperature used to identify an organic liquid?

Measure the boiling temperature and compare it with an accurate data-book value.

What would suggest that an organic liquid is pure?

Its measured boiling temperature closely matches the accepted value.

What is a limitation of using boiling temperature to identify an organic compound?

Different compounds can have the same or very similar boiling temperatures.

Why might an experimentally measured boiling temperature be inaccurate?

The thermometer may not be sufficiently accurate or the measured value may be affected by experimental error.

Why should you not assume a compound is pure just because its boiling temperature matches a data-book value?

Different organic compounds can coincidentally have the same boiling temperature.

What determines which apparatus can be used to measure boiling temperature?

The volume of liquid available and whether the liquid is toxic or flammable.

What is the overall purification sequence for an organic liquid likely to contain several impurities?

Separate/extract the desired product → wash or separate layers if needed → dry the organic layer → remove the drying agent → distil the liquid → check its boiling temperature.