For more complex compounds a common name frequently will not exist,
and the systematic nomenclature is the only solution.
The three isomers of pentane differ somewhat in chemical and physical
properties, as the melting and boiling points in the margin of the
preceding page indicate.
In a solid or liquid, hydrocarbon molecules are attracted to one
another by weak van der Waals forces, which vary with the size and
shape of the molecule. Notice that n-pentane is roughly sausage-shaped,
whereas neopentane is a spherical ball.
Neopentane molecules pack better into a crystalline lattice, so
more energy is required to melt the solid; thus its melting point
is the highest of the three isomers. In contrast, in a liquid the
long n-pentane sausages lie in closer contact with one another than
the neopentane spheres do, so intermolecular van der Waals forces
are stronger, and the boiling point of n-pentane is highest of all.
Isopentane is intermediate in behavior. The number of ways of connecting
atoms, and hence the number of isomers, increases astronomically
with the number of carbon atoms. There are 2 butane isomers, 3 isomers
of pentane, 5 hexanes, 9 heptanes, 18 octanes, 35 nonanes, 75 decanes,
and 366,319 different structural isomers of eicosane, C20H42.