Isomers are two or more chemical compounds, with the same numbers and types of atoms in their molecules, which are yet not the same. The various kinds of isomers are categorized and named according to the particular way in which they are different. Most isomers are organic molecules, but there are also inorganic isomers. Isomerism is an important consideration in chemistry; for example, different isomers can have very different effects when used in pharmaceuticals or in biological processes.
Structural isomers are the easiest to recognize. Two or more structural isomers result when the atoms in a chemical formula can be connected in different ways to form different molecules. For example, six carbon atoms, six hydrogen atoms and one oxygen atom can be arranged to form either dimethyl ether (CH3OCH3) or ethanol (CH3CH2OH). Structural isomers are typically very different from each other and have different physical properties, such as boiling and freezing points.
Isomers that are not structural have the same series of connections between their atoms and yet are still different, due to variations in the relative spatial arrangements of those atoms. Nonstructural isomers are known as stereoisomers.
Geometric isomers always occur in pairs and result from the presence of a double bond or a ring structure in a molecule in which two adjacent carbons each have two different groups attached. The molecule with similar groups on the same side of the double bond or ring can be differentiated from its counterpart isomer with similar groups on opposite sides. Geometric isomers will have slightly different physical properties.
Enantiomers are sometimes called optical isomers. Pairs of enantiomers relate to each other as nonsuperimposable mirror images, similar to right and left hands. They are formed when a molecule contains one or more chiral carbon molecules -- a carbon atom with four different atoms or groups of atoms attached to it. Enantiomers have identical physical properties except that they rotate plane-polarized light in opposite directions. Enantiomers sometimes have very different effects when used in medicine; one isomer may be effective, and the other may be ineffective or even toxic. Optical isomers are also seen with certain inorganic molecules known as coordination complexes.
Diastereomers are stereoisomers that are different molecules and yet are not enantiomers; they are nonsuperimposable molecules that are not mirror images. They typically result from a species containing two or more chiral compounds. These isomers have different physical properties. Such molecules occur often in nature. The sugar galactose, for example, is a diastereomer of the sugar glucose.
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