Naming alkanes and chain isomers

Methane, ethane and propane each have a single structure. From butane onwards the carbons can be joined in more than one way.

C₄H₁₀ exists as butane (n-butane, b.p. 273 K) and 2-methylpropane (isobutane, b.p. 261 K). C₅H₁₂ exists as pentane (309 K), 2-methylbutane (isopentane, 301 K) and 2,2-dimethylpropane (neopentane, 282.5 K).

Isomers that differ only in the carbon skeleton, straight versus branched, are called chain isomers, a kind of structural isomerism. C₆H₁₄ has five, C₇H₁₆ nine, and C₁₀H₂₂ as many as 75.

The five hexanes are n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane.

A carbon bonded to one other carbon (or none, as in methane) is primary (1°); to two, secondary (2°); to three, tertiary (3°); to four, quaternary or neo (4°). End carbons of a chain are always primary.

An alkyl group (general formula CₙH₂ₙ₊₁) is an alkane minus one hydrogen, such as –CH₃ or –C₂H₅.

IUPAC rules: choose the longest chain, number it so substituents get the lowest locants, and list substituents alphabetically. 'sec' and 'tert' are ignored when alphabetising, but isopropyl is treated as a single word.

To draw a structure from a name, write the parent chain, number it, attach the substituents, then add hydrogens until each carbon has four bonds. For example, '2-ethylpentane' is really 3-methylhexane, because its longest chain has six carbons.