Carbon chains and alkanes

Linear, branched and cyclic chains

Carbon atoms can be linked together in several ways:

• a linear chain: each carbon atom is bonded to no more than two other carbon atoms;
• a branched chain: some carbon atoms are bonded to three or four other carbon atoms;
• a cyclic chain: the chain forms a closed ring, as in cyclohexane.

Alkanes: the simplest family

Alkanes are hydrocarbons (consisting solely of carbon and hydrogen) that contain only single bonds. Their general molecular formula is CnH2n+2.

Name	Molecular formula	Semi-developed formula
Methane	CH₄	CH₄
Ethane	C₂H₆	CH₃-CH₃
Propane	C₃H₈	CH₃-CH₂-CH₃
Butane	C₄H₁₀	CH₃-CH₂-CH₂-CH₃

Nomenclature: the basic rules

The name of a linear alkane combines a prefix indicating the number of carbon atoms (meth-, eth-, prop-, but-, pent-, hex-...) with the suffix -ane.

For a branched alkane:

1. Identify the longest carbon chain: this provides the base name.
2. Number the carbons in this chain to give the branches the smallest possible indices.
3. Name each branch as an alkyl group (methyl, ethyl, etc.) preceded by its position index.

Example: CH₃-CH(CH₃)-CH₂-CH₃ is 2-methylbutane (main chain of 4 carbon atoms = butane, a methyl group on carbon 2).

Common mistake

A common pitfall is to identify the longest ‘straight-line’ chain on the paper, whereas you should follow the bonds, which may form a bent path. Always look for the maximum number of carbon atoms linked end-to-end, regardless of how the molecule is drawn.