Alkanes (Cambridge (CIE) O Level Chemistry): Revision Note

Alkanes: Properties & Bonding

• Alkanes are a group of saturated hydrocarbons

• The term saturated means that they only have single carbon-carbon bonds, there are no double bonds

• The general formula of the alkanes is C_nH_2n+2

• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases

• Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light in substitution reactions

• Methane is an alkane and is the major component of natural gas

• Methane undergoes complete combustion forming carbon dioxide and water:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

Table showing the displayed formula of the first four members of the alkane homologous series

Displayed formula	Name	Molecular formula
	methane	CH4
	ethane	C2H6
	propane	C3H8
	butane	C4H10

Substitution Reaction of Alkanes with Halogens

• In a substitution reaction, one atom (or group of atoms) is swapped with another atom (or group of atoms)

• Alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation (sunlight is a source of UV radiation)

• This is called a photochemical reaction

• The UV light provides the activation energy, E_a, for the reaction 

• A hydrogen atom is replaced with the halogen atom

• More than one hydrogen atom can be substituted depending on the amount of ultraviolet radiation present

CH₄ + Cl₂ → CH₃Cl + HCl

In the presence of ultraviolet (UV) radiation, methane reacts with chlorine to form chloromethane and hydrogen chloride