As an example, the longest chain that can be traced through the
isopentane molecule contains four carbon atoms, so in systematic
nomenclature this molecule is a butane.
A methyl group (CH3-) branches off
from the second carbon along the chain, so isopentane is called
2-methylbutane. One also could call it 3-methylbutane by beginning
the backbone numbering from the other end, but there is no logical
reason for using this name.
The numbering is begun at the end that leads to the simplest name
with the smallest numbers. Notice that there is no 1-methylbutane
or 4-methylbutane; these molecules simply would be the straight-chain
In systematic nomenclature, neopentane is classed as a propane
because the longest carbon chain contains three atoms. Since two
methyl groups branch off from the second carbon of the chain, the
molecule is called 2,2-dimethylpropane.
Example: What is the systematic name for isooctane, shown at the
top of the preceding page?
Solution: With five carbons as the longest continuous carbon chain,
the molecule is called 2,2-dimethyl-4-methylpentane. In this
example the systematic name clearly is clumsier than the name isooctane,
which is why the latter name ordinarily is used.