Halogenoalkanes – A Level Chemistry WJEC Revision

Halogenoalkanes, also known as haloalkanes, are a category of organic compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine, or iodine).
Characterised by general molecular formula CnH2n+1X, where X represents a halogen atom and n represents the number of carbon atoms.
Physical properties include being generally denser than alkanes and having higher boiling points due to additional intermolecular forces.

Classification based on Carbon Atom bonding

Halogenoalkanes can be classified as primary, secondary, or tertiary depending on the type of carbon atom the halogen is attached to.
A primary halogenoalkane has the halogen atom bonded to a carbon that is itself only bonded to one other carbon atom.
A secondary halogenoalkane has the halogen atom bonded to a carbon that is itself bonded to two other carbon atoms.
A tertiary halogenoalkane has the halogen atom bonded to a carbon that is itself bonded to three other carbon atoms.

Chemical Reactions

Halogenoalkanes can undergo nucleophilic substitution reactions, where the halogen atom is replaced by a nucleophile (a species that donates electrons).
They can also undergo elimination reactions, where hydrogen is lost from a carbon atom next to the carbon carrying the halogen atom, leading to the formation of an alkene.
The rate of reaction for primary halogenoalkanes is generally slower than for secondary and tertiary halogenoalkanes. This can be attributed to steric hindrance, which is the restriction of molecular motion caused by the size of the atoms near the reaction site.

Environmental Impact of Halogenoalkanes

Certain halogenoalkanes, particularly chlorofluorocarbons (CFCs), have had a significant environmental impact by causing the depletion of the ozone layer when released into the atmosphere.
The issues with CFCs have led to the development and use of less destructive compounds, such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).

Health Hazards and Safety Measures

Halogenoalkanes can be harmful or lethal if ingested or inhaled, and they can also cause burns and eye damage.
Therefore, working with these compounds requires strict safety measures, especially in laboratories. Protective clothing, gloves, and eye protection are musts when handling halogenoalkanes.