Isomer

I

Introduction

Isomer, one of two or more molecules that contain the same atoms but have different arrangements of those atoms. Molecules are groups of atoms bonded together, and a substance made of molecules is called a compound. Two compounds made of isomers may differ in their physical, chemical, and biological properties, even though they contain the same atoms in their molecules. The differences arise from the different arrangement of atoms in the molecules. One compound may be a useful drug, and the other ineffective; or one may be harmless, and the other explosive. Scientists study isomers because these substances each possess unique properties, many of which are essential to living organisms. Other isomers are important in the pharmaceutical, food, and chemical industries. Scientists also study isomers to learn about how the properties of a substance are determined by its molecular structure. Chemists classify isomers as either structural isomers or stereoisomers, depending on how the arrangement of atoms in the molecules differs (See also Atom & Atomic Theory).

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.

III

Stereoisomers

Stereoisomers are molecules with the same chemical formula and the same molecular structure, but with their atoms arranged differently within this structure. Chemists have identified two types of stereoisomers: geometric isomers and optical isomers.

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A

Geometric Isomers

Geometric isomers are molecules with the same atomic organization (geometry), but with atoms placed at different positions of the molecule’s geometric structure. The three isomers of the compound dichloroethene (C₂H₂Cl₂) are geometric isomers:

The two carbon atoms in the dichloroethene molecule are held together by a rigid chemical bond, called a double bond, that doesn’t allow them to rotate (see Chemical Reaction). When two chlorine (Cl) atoms are attached either on opposite sides or on the same side of the double bond, the rigid bond prevents each chlorine atom from rotating from the north position to the south position (and vice-versa) around the carbon atom it is attached to. This means the chlorine atoms can arrange themselves around the double-bonded carbon atoms in three different ways: the configuration on the top (called a cis configuration), the configuration in the middle (called a trans configuration), and the configuration on the bottom (known as 1,1-dichloroethene).

Geometric isomers can have different properties from one another. For example, scientists use the cis isomer of the compound Pt(NH₃)₂Cl₂ as an anticancer drug, while the trans isomer of Pt(NH₃)₂Cl₂ is ineffective against cancerous tumors.

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