A hydrocarbon is a compound made of only carbon and hydrogen atoms. Hydrocarbons whose carbons form a chain, straight or branched, with only single bonds are called alkanes.
Alkanes are named by how many carbon atoms are in their chain. The first part of the name counts the carbons, and the ending -ane says the molecule has only single bonds. The counting parts are meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7 and oct- 8.
| Name | Formula | State | Boils at |
|---|---|---|---|
| methane | CH₄ | gas | −162 °C |
| ethane | C₂H₆ | gas | −89 °C |
| propane | C₃H₈ | gas | −42 °C |
| butane | C₄H₁₀ | gas | −0.5 °C |
| pentane | C₅H₁₂ | liquid | 36 °C |
| hexane | C₆H₁₄ | liquid | 69 °C |
| heptane | C₇H₁₆ | liquid | 98 °C |
| octane | C₈H₁₈ | liquid | 126 °C |
Look at the formulas. Count 2 hydrogens for every carbon, then add 2 more, one for each end of the chain. So an alkane with n carbons has 2n + 2 hydrogens. Chemists write this as CnH2n+2.
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Longer chains cling to each other more, so they boil at higher temperatures. Small alkanes are gases: methane is the main part of natural gas, propane is the bottled gas for barbecues and patio heaters, and butane fills lighters and camping stoves. Alkanes burn well, but apart from that they react with very few things.
Remember ethene and ethyne? The ending tells you about the bonds. -ane means only single bonds, -ene means there is a C=C double bond, and -yne means there is a C≡C triple bond. Only the -ane molecules are alkanes.