4.3 AlkanesEdexcel IGCSE Chemistry: Revision notes

Alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2. They are described as saturated hydrocarbons because every carbon-carbon bond is a single bond — there are no C=C double bonds, so no more hydrogen atoms could be added to the molecule.

The first five alkanes are methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10) and pentane (C5H12).

For each alkane, the structural formula shows CH3 end groups with CH2 groups between them, e.g. propane is CH3CH2CH3. The displayed formula draws every C-H and C-C bond individually.

The unbranched-chain isomers (straight chains) are named using the stem for the chain length plus the suffix '-ane':

From butane onwards, branched-chain isomers also exist (e.g. methylpropane, C4H10), but only the unbranched-chain names are required here.

Alkanes react with halogens (such as chlorine or bromine) in the presence of ultraviolet radiation, undergoing a substitution reaction. One hydrogen atom on the alkane is replaced by one halogen atom, forming a halogenoalkane and a molecule of hydrogen halide.

For example: CH4 + Cl2 → CH3Cl + HCl (methane + chlorine → chloromethane + hydrogen chloride)

This course only requires mono-substitution (replacement of a single hydrogen atom), even though further substitution is possible in reality.

Alkanes are simple molecular substances held together internally by strong covalent bonds, but held to each other only by weak intermolecular forces. As the carbon chain gets longer:

• relative molecular mass increases
• intermolecular forces become stronger
• boiling point increases

This is why methane, ethane and propane are gases at room temperature, while longer-chain alkanes are liquids, and very long-chain alkanes are solids (as seen in the crude oil fractions).