Hydrocarbons and names

Hydrocarbons contain only carbon and hydrogen. They're the main part of natural gas, petrol and diesel, and the starting point for most of the chemical industry.

Alkanes have only single bonds. Each carbon carries as many hydrogens as it can (they're saturated), so their formula is CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. The names come from the number of carbons: meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7, oct- 8, non- 9, dec- 10.

Alkenes have a carbon–carbon double bond and end in -ene: ethene (ethylene) C₂H₄, propene C₃H₆. With one double bond the formula is CₙH₂ₙ. Alkynes have a triple bond and end in -yne: ethyne (acetylene) C₂H₂. A number says where the multiple bond starts: but-1-ene has it between carbons 1 and 2, but-2-ene between 2 and 3.

To name a branched alkane, find the longest chain and name it, number it from the end nearer a branch, and put the branches in front with their positions. A CH₃ branch is methyl, a CH₂CH₃ branch ethyl. So a four-carbon chain with a methyl on carbon 2 is 2-methylbutane. Two of the same branch get di- (2,2-dimethylpropane).

This system is the IUPAC naming system (from the International Union of Pure and Applied Chemistry). Older everyday names stick around: ethylene, acetylene, isobutane. Rings get cyclo-: cyclohexane is six carbons in a ring, C₆H₁₂.