Become a fan of h2g2
Introduction to alkanes
Alkanes are hydrocarbons - compounds that contain only carbon atoms and hydrogen atoms. They can be obtained by the fractional distillation of crude oil and their uses include:
- Fuelling cars, boats and planes
- The production of alkenes from long-chain alkanes 1
- The production of candle wax
- Creating new road surfaces
Structure of Alkanes
The only bonds between atoms of an alkane are single covalent bonds. Therefore, alkanes are saturated hydrocarbons: they contain no double or triple bonds.
The general formula of alkanes is CnH2n+2 where n is the number of carbon atoms in the longest carbon chain of the alkane.
Alkanes contain a chain of carbon atoms, all bonded to each other by single bonds. Shown below is the carbon chain of the alkane that contains six carbon atoms. In its organic compounds, carbon must form four bonds with other atoms so any "unused" bonds are also shown.
| | | | | | - C - C - C - C - C - C - | | | | | |
As described above, an alkane contains only atoms of carbon and hydrogen. Therefore, hydrogen atoms are drawn where there is an "unused" bond on the above diagram.
H H H H H H | | | | | | H - C - C - C - C - C - C - H | | | | | | H H H H H HIn general, to draw the structural formula of an alkane:
- Draw a carbon chain of the correct length. Each carbon atom should be joined to its neighbour or neighbours by single bonds.
- Remembering that carbon forms four bonds in its organic compounds, add the correct number of hydrogens to any 'unused' bonds making sure to only attach hydrogen atoms to carbon atoms using single bonds.
An alkane's name is formed using the following rule: The letters ane are added to a prefix which depends on the number of carbon atoms in the longest carbon chain of the alkane.
The prefixes used are the same for all organic molecules. The list below shows the names of the first ten alkanes, with the prefix shown in bold italics. The numbers correspond to the number of carbon atoms in the chain.
Therefore, the alkane with six carbon atoms is hexane (shown above) and the alkane with ten carbon atoms is decane.
Reactions of Alkanes
Alkanes are highly flammable. Carbon dioxide and water are the only products of the reaction when alkanes are burned in excess oxygen. Carbon monoxide is also produced when alkanes are burned in insufficient oxygen. For example, when methane is burned in excess oxygen, carbon dioxide and water are produced.
CH4 (g) + 2O2 (g)→ CO2 (g) + 2H2O (l)The strong single covalent bonds are difficult to break so alkanes are rather unreactive.2 Aside from being flammable, two other notable reactions are:
This is performed on an industrial scale to produce very useful chemicals called alkenes. Alkanes with long carbon chains are not very useful so they are heated in the presence of a catalyst (for example aluminium oxide) to produce an alkane with a shorter carbon chain and an alkene. For example, when decane is heated in the presence of a catalyst, octane and ethene are produced.
C10H22 (l)→ C8H18 (l) + C2H4 (g)
Reaction with Bromine
Alkanes undergo a substitution reaction3 with bromine (Br2 (l)) in strong ultraviolet light. For example, when pentane reacts with bromine, 1-bromopentane4 and hydrogen bromide gas are produced: a bromine atom is swapped with a hydrogen atom.
C5H12 (l) + Br2 (l)→ C5H11Br (l) + HBr (g)