Naming organic compounds and isomers

Organic compounds are named by IUPAC rules so that every name stands for exactly one structure. The name is built from three parts: a stem for the number of carbons in the longest chain, an ending for the functional group, and prefixes, with numbers, for any branches or halogen atoms. Compounds with the same molecular formula but different structures are structural isomers.

Key facts

  • A homologous series is a family with the same functional group and general formula, each member differing by CH₂: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH, carboxylic acids CₙH₂ₙ₊₁COOH.
  • Stems give the number of carbons in the longest chain: ⟦meth-⟧ 1, ⟦eth-⟧ 2, ⟦prop-⟧ 3, ⟦but-⟧ 4, ⟦pent-⟧ 5, ⟦hex-⟧ 6, ⟦hept-⟧ 7, ⟦oct-⟧ 8.
  • Endings give the functional group: ⟦-ane⟧ (alkane), ⟦-ene⟧ (C=C), ⟦-ol⟧ (–OH), ⟦-al⟧ (–CHO, always carbon 1), ⟦-one⟧ (C=O inside the chain), ⟦-oic acid⟧ (–COOH, carbon 1). Halogens and alkyl branches are prefixes: ⟦chloro-⟧, ⟦bromo-⟧, ⟦methyl-⟧, ⟦ethyl-⟧.
  • Find the longest carbon chain that holds the functional group; it may bend round corners in a drawing.
  • Number the chain from the end that gives the functional group the lowest number, then the substituents the lowest numbers. Write prefixes in alphabetical order, with ⟦di-⟧, ⟦tri-⟧ for repeats (these do not count for the order): ⟦2-bromo-3-methylbutane⟧, ⟦2,2-dimethylpropane⟧.
  • Structural isomers have the same molecular formula but a different structural formula: chain isomers (different branching), position isomers (the group on a different carbon) and functional group isomers (a different functional group, such as ⟦propanal⟧ and ⟦propanone⟧).

Worked examples

Example 1

Name this alkane.

  1. The longest chain has four carbons: ⟦butane⟧.
  2. A CH₃ branch is a methyl group. Number from the end nearer the branch: it is on carbon 2, not carbon 3.
  3. Put the prefix in front of the stem. ⟦2-methylbutane⟧

Answer: ⟦2-methylbutane⟧

Example 2

Name this alkane. Be careful: the longest chain is not drawn in a straight line.

  1. Count every route from one end carbon to another. The longest runs up through the "branch": five carbons, so ⟦pentane⟧.
  2. One CH₃ group is left over, on the middle carbon of that chain. 3 from either end
  3. So the name is not ⟦2-ethylbutane⟧. ⟦3-methylpentane⟧

Answer: ⟦3-methylpentane⟧

Example 3

Name this compound.

  1. The –OH group makes it an alcohol: ending ⟦-ol⟧. The chain holding it has four carbons: ⟦butan-…-ol⟧.
  2. Number from the end that gives the –OH the lowest number: carbon 2. The methyl group is then on carbon 3.
  3. Write the prefix, the stem and the ending. ⟦3-methylbutan-2-ol⟧

Answer: ⟦3-methylbutan-2-ol⟧

Example 4

Draw and name the four structural isomers of C₄H₁₀O that are alcohols.

  1. Straight chain of four carbons, –OH on an end carbon or a middle carbon. ⟦butan-1-ol⟧, ⟦butan-2-ol⟧
  2. Branched chain of three carbons with a methyl on carbon 2; –OH on an end carbon or on the branch carbon. ⟦2-methylpropan-1-ol⟧, ⟦2-methylpropan-2-ol⟧
  3. ⟦Butan-1-ol⟧ and ⟦butan-2-ol⟧ are position isomers; ⟦butan-1-ol⟧ and ⟦2-methylpropan-1-ol⟧ are chain isomers.

Answer: ⟦butan-1-ol⟧, ⟦butan-2-ol⟧, ⟦2-methylpropan-1-ol⟧ and ⟦2-methylpropan-2-ol⟧