Naming molecules

The names of organic molecules are derived from the simplest of organic families: the alkanes.

Counting the carbon atoms

You will need to learn how to name molecules up to five carbons long. There is no easy way to do this - you have got to learn them.

CodeNumber of carbons

Functional groups

  • Alkanes – only single bonds
    • naming: methane, ethane etc
  • Alkenes – contain a C=C
    • naming: ethene, propene etc
  • Alcohols – contain an –OH
    • naming: butanol, propanol etc

Naming more complex molecules [Higher tier – GCSE Chemistry only]

Whilst it is relatively easy to name linear molecules, what happens if there are branches in the chain? What happens if the OH group is attached to the second carbon in the chain?

The following rules enable us to identify any organic compound with a unique name:

  1. Select the longest unbranched chain of carbons. The chain may not be drawn ‘straight’.
  2. Number the chain from the end where branching starts first – or the end where the functional group is.
  3. List the branching groups and functional groups alphabetically, including a number denoting which carbon in the chain they are attached to.

Example: Name the alkene

  • Find the longest chain which includes the double bond: 4 carbons, which is butene.
Structural formula of 3-methylbut-2-ene, with the longest chain of Carbons highlighted.
  • Number the carbon atoms from the side that the double bond is closest to, which is the right hand side.
Structural formula of 3-methylbut-2-ene, with four Carbon atoms labelled 1-4.
  • The double bond is between the first and second carbon of the main chain, so it is but-1-ene.
Structural formula of 3-methylbut-2-ene, with the double bond highlighted
  • Finally, there is a methyl group on the number 2 carbon.
Structural formula of 3-methylbut-2-ene, with the methyl group (CH3) joined to the second carbon highlighted.

Full name: 2-methylbut-1-ene

Any repeat groups within the molecule need the prefix di-(two), tri-(three) or tetra-(four), eg CH3C(CH3)2CH3 – dimethylpropane.

Alcohols and alkenes require numbers midway through the name. The number added is associated with the functional group, eg CH3CH(OH)CH3 is propan-2-ol, CH3CH2CH=CH2 is but-1-ene (so called because the double bond starts on the 1 carbon).