What is a hydrocarbon?
A hydrocarbon is a compound made of only two elements: carbon and hydrogen. Petroleum and natural gas are mixtures of hydrocarbons, and burning them gives most of the energy used for cooking, transport and electricity in Malaysia.
Alkanes and alkenes
Two important families are the alkanes and the alkenes.
- Alkanes have the general formula CₙH₂ₙ₊₂ and contain only single C–C bonds. They are saturated. Examples: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀).
- Alkenes have the general formula CₙH₂ₙ and contain at least one C=C double bond. They are unsaturated. Examples: ethene (C₂H₄), propene (C₃H₆).
Key idea
Alkane = CₙH₂ₙ₊₂ (single bonds, saturated). Alkene = CₙH₂ₙ (one double bond, unsaturated).
Homologous series
Each family is a homologous series – a set of compounds with the same general formula, whose members differ by a –CH₂– unit. Along a series the members show a gradual change in physical properties (boiling point rises as the molecule gets bigger) but have similar chemical properties. The first four alkanes – methane, ethane, propane and butane – are gases; longer chains become liquids and then waxy solids.
Alkanes are fairly unreactive but burn readily, which makes them excellent fuels. Alkenes are more reactive because of the double bond, so they take part in addition reactions and are the building blocks used to make plastics such as polythene. The naming follows the number of carbon atoms, using the prefixes meth-, eth-, prop- and but-.
Example
To tell an alkene from an alkane, add bromine water. An alkene decolourises the orange-brown bromine water quickly because the double bond reacts with it; an alkane does not.
Remember
Complete combustion of any hydrocarbon in plenty of air gives carbon dioxide + water. Too little air gives poisonous carbon monoxide and soot.