II.

Structural Isomers

Structural isomers are molecules that have the same chemical formula, meaning they have the same atoms in each molecule, but differ in the order in which the atoms are connected. The different order of the atoms produces two completely different molecular structures. The compounds pentane, isopentane, and neopentane—all found in gasoline—are structural isomers of C₅H₁₂:

The different arrangement of atoms gives each compound a distinct set of properties. For example, because pentane molecules are longer than are isopentane or neopentane molecules, pentane molecules become more entangled with each other. These entangled molecules require more energy to separate, for instance, from a liquid into a gas. As a result, liquid pentane has a higher boiling point than that of isopentane or neopentane. In contrast, the spherically shaped neopentane molecules separate from one another more easily, giving liquid neopentane a lower boiling point than that of pentane or isopentane.