19. The Simple Compounds
                                   of Carbon
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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.

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