3.1.5 Introduction to Organic Chemistry - Nomenclature

Specification

Students should:
  • know and understand the terms empirical formula, molecular formula, structural formula, displayed formula, homologous series and functional group
  • be able to apply IUPAC rules for nomenclature to simple organic compounds, limited to chains with up to 6 carbon atoms limited in this module to alkanes, alkenes and haloalkanes

Systematic nomenclature

There are millions of different organic molecules. To give every one of them a different name would involve a huge amount of learning, were no logical pattern to be adopted.

The IUPAC developed the systematic naming system (nomenclature) to allow many of the organic compounds to be named in a logical fashion by considering the structural features of the molecule. This systematic nomenclature arrives at the compound name in the following order:

The first step is to identify the longest unbroken chain of carbon atoms. This gives the root of the name:

The next step is to look for unsaturation (multiple bonds between carbon atoms) in the chain. If there are only single bonds then the suffix (the bit at the end) is -ane. If there is a doble bond, then the suffix is -ene, and for a triple bond the suffix is -yne.

The folowing step is to identify any functional groups. This means atoms that are NOT hydrogen. Each functional group gives rise to either a different suffix or, if not important enough, to a pre- prefix. At this stage priorities come into play.

Branching

This means that there are other hydrocarbon (alkyl) groups attached to the main chain. Each is named according to the number of carbons in the side chain adding the suffix yl. The whole part goes as a prefix.

Finally, the position of the heteroatoms, or groups on the chain, or sidechains are numbered (counting from the end of the chain that gives rise to the smallest numbers) to avoid ambiguity. They are named alphabetically.

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