Uses of Halogenoalkanes

Give example of uses of halogenoalkanes.

• Solvents

• Refrigerants

• Anesthetics

Why are halogenoalkanes used as solvents?

Halogenoalkanes are polar molecules, but are not soluble in water ∴ they are able to mix with polar and non-polar organic compounds.

They are therefore used as degreasing agents and are used as solvents in several organic processes. They are used as dry cleaning fluids.

How are halogenoalkanes used as refrigerants?

When a liquid evaporates, it takes in heat- halogenoalkanes are ideal refrigerants as their boiling points are close to room temperature.

As they evaporate, heat is taken from the surroundings, thereby cooling it down.

Give an example of a halogenoalkane which is used as an anaesthetic.

Trichloromethane- CHCl₃

What are CFCs?

Chlorofluorocarbons (CFCs) are halogenoalkanes that contain both chlorine and fluorine.

What were CFCs used for?

Refrigerants and aerosol propellants.

Why were CFCs banned?

They were responsible for creating holes in the ozone layer, which allows UV rays from the sun to reach the Earth’s surface and cause skin cancer.

Explain how CFCs were responsible for creating holes in the ozone layer.

• In the upper atmosphere, stable CFC molecules encounter UV radiation which break the carbon chlorine bond to form free radicals.

• The C-Cl bond is weaker than the C-F bond (because it is less polar) so, breaks first.

Here is the initiation reaction for trichlorofluoromethane:

CFCl₃ → Cl· + CFCl₂·

• Free radicals then react with an ozone molecule. There are several possible propagation reactions:

Cl· + O₃ → ClO· + O₂

ClO· + O₃ → Cl· + 2O₂

• The chlorine free radical is regenerated setting up a chain reaction, deleting many ozone molecules.

CFCs are gradually being replaced by…

HFCs (hydrofluorocarbons)

These do not contain C-Cl bonds and therefore no chlorine free radicals.