Alkane

An alkane is a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂, in which all the carbon atoms are joined by single bonds only. Malay: alkana · Chinese: 烷烃.

Because they are saturated, alkanes are relatively unreactive, and they take part in just two kinds of reaction you need for the exam. The first is combustion: in plenty of oxygen an alkane undergoes complete combustion to carbon dioxide and water, while in a limited supply it undergoes incomplete combustion, giving carbon monoxide, soot and less heat. The second is substitution, in which a halogen replaces one or more hydrogen atoms, but only in the presence of ultraviolet light. Alkanes are named with a stem for the number of carbon atoms, meth-, eth-, prop-, but-, and the ending -ane.

Example. Methane (CH₄) is the first alkane and the main component of natural gas; ethane (C₂H₆) is the second. Both burn cleanly in a good supply of air, and methane reacts with chlorine in ultraviolet light to form chloromethane and hydrogen chloride by substitution.

Confusion to avoid. Do not confuse an alkane with an alkene. An alkane is saturated, fits CₙH₂ₙ₊₂, and does not decolourise bromine water; an alkene is unsaturated, fits CₙH₂ₙ, and decolourises bromine water quickly. Getting the general formula and the bromine-water result the right way round is a common source of lost marks.

The physical properties of the alkanes follow the usual homologous trend, and this is worth knowing alongside the chemistry. The first few members are gases, and as the carbon chain lengthens the compounds become liquids and then waxy solids, with the boiling point rising and the volatility falling at each step. This is why the shorter alkanes are handled as gaseous and liquid fuels while the longer ones turn up in candle wax and lubricating oils. Their lack of reactivity is an advantage here, because a fuel that releases plenty of heat when burned yet sits unreactive in storage is exactly what is wanted.

The alkanes are the first homologous series of the chapter and the unreactive standard against which the reactivity of alkenes, alcohols and acids is judged, so they anchor much of the Carbon Compounds work that SPM Chemistry sets.