Alkanes: nomenclature and isomerism

Hydrocarbons contain only carbon and hydrogen. They are our main fuels (LPG, petrol, diesel) and the starting point for plastics and many other products. They are of three kinds:

• Saturated: only single C–C bonds: the alkanes.
• Unsaturated: with double or triple bonds: the alkenes and alkynes.
• Aromatic: with a benzene-type ring.

Alkanes have the general formula CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈… Every carbon is sp³, with tetrahedral bond angles of 109.5°.

From four carbons onwards, the same formula can have different carbon skeletons: chain isomers. The number grows fast: C₄H₁₀ has 2 isomers, C₅H₁₂ has 3, C₆H₁₄ has 5, C₇H₁₆ has 9, and C₁₀H₂₂ has 75.

Each carbon in a chain is classified by how many other carbons it is bonded to:

• Primary (1°): bonded to one other carbon (or none).
• Secondary (2°): to two.
• Tertiary (3°): to three.
• Quaternary (4°): to four.

An alkyl group (general formula CₙH₂ₙ₊₁) is an alkane with one hydrogen removed: methyl, CH₃–; ethyl, C₂H₅–. Since hydrogens can be removed from different positions, larger groups have isomers: C₅H₁₁ has eight, so there are eight different pentanols.

Naming follows the IUPAC rules: find the longest chain, number it to give the lowest locants, and list substituents in alphabetical order. Names that seem to describe a molecule can still be wrong. '2-Ethylbutane' is not a correct name, because the ethyl group is really part of a longer chain: the molecule is 3-methylpentane. Many compounds also have common names, as a boy called Raj in class may be 'Raju' at home.