Another possible structure is isooctane, right. These two are structural
isomers of octane, because they have the same number of each kind
of atom, but are arranged in different ways. Isooctane is familiar
from the "octane rating" of gasoline.
Straight-chain gasolines burn suddenly with an audible bang against
the cylinder walls of an engine, which we hear as engine knock.
Branched-chain molecules burn more slowly and quietly. The octane
rating of any gasoline is the percent isooctane in a mixture with
n-heptane that has the same knock behavior.
Pentane has three structural isomers, all of which are shown in
the right margin. Their common names are n-pentane, isopentane,
and neopentane, but these molecules can be used to illustrate the
systematic way of naming organic compounds. "Normal,"
"iso" (meaning an isomer), and "neo" (meaning
new) may suffice for a pentane, but for the 75 different structural
isomers of decane a more orderly method of choosing names is required.
In systematic nomenclature, the longest continuous carbon backbone
that can be traced through the molecule is chosen as the "parent"
compound, and prefixes are added to describe groups branching off
from this backbone.
The carbon atoms along the backbone are numbered from one end.