Return to Contents Basic IUPAC Organic Nomenclature

What's in a name?

The IUPAC name of an organic molecule is assembled from components that describe various features and parts of the molecule.

Functional group suffix
This is added to the end of the name based on the principal functional group.

Root
This defines the number of atoms (usually carbon atoms) in the longest continuous chain that contains the principal functional group.

Substituent prefix
Any groups other than the principal functional group appended to the root chain are called substituents, i.e. they have replaced an H atom on that root chain.
Substituents
are added to the beginning of the name and are listed in alphabetical order.

Multiplier
If a functional group or substituent occurs more than once, a simple multiplier (e.g. di, tri, tetra, etc.)  is used to indicate how many times it occurs.

Locants
Locants are numbers (or occasionally letters e.g. N-) that define the position of the principal functional group and substituents.  Typically there needs to be a locant for each functional group and each substituent. The 1993 modifications requires that the locant for the principal functional group is placed before the functional group suffix, e.g. pentan-2-ol, see below. Note that the functional group priority order (see functional group page) only applies if the group is being named as the principal functional group (i.e. it contributes as the functional group suffix), and the priority order does not apply if the group is named as a substituent.

The basic structure of the IUPAC name is shown schematically below :

what's in a name ?

Let's work through the example shown - we will expand on the details in the pages ahead.  
  • the principal functional group is an alcohol, so the suffix  -ol is required
  • the longest continuous chain with the -OH attached is C5 and an alkane so we have a pentane system
  • there are two substituents, both methyl groups, hence dimethyl
  • the -OH is on C2 so we have a -2-ol (preferred by the 1993 modification)
  • the methyl groups are on C3 and C4, so we have a 3,4-dimethyl

 

hence : 3,4-dimethylpentan-2-ol

Note : prior to the 1993 modification, pentan-2-ol could also be written as 2-pentanol



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