Return to Contents
Basic IUPAC Organic Nomenclature

Basic rules

The IUPAC systematic name of an organic compound can be constructed based on a series of steps and rules:

The steps and rules are summarised below, more details are provided as the cases are encountered.

Principle Functional group
  • The principle functional group is used to define the class the compound belongs to e.g. an alcohol, ROH
  • The principle functional group is the highest priority functional group.
  • The principle functional group is usually given the lowest locant possible.
Longest chain
  • The longest continuous chain containing the principle functional group defines the root name. 
  • Other groups attached to this chain are called substituents.
  • If there are two chains of equal length, then the choice that gives the simplest substituents is chosen.
Numbering
(i.e. assigning locants)
  • The numbers that define the positions of the principle functional group and substituents are called locants.
  • Compounds are numbered from one end of the longest continuous chain.
  • The locants are assigned such that the principle functional group gets the lowest possible locant.
  • If this results in a "tie" then the first point of difference rule is applied so that the first time a difference in numbering occurs, then the method that gives the lower number at this first difference is used.
  • In the event that there is no first point of difference then alphabetisation is used.


 © Dr. Ian Hunt, Department of Chemistry, University of Calgary